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BACKGROUND Strigolactones (SLs) have a vast number of ecological implications because of the broad spectrum of their biological activities. Unfortunately, the limited availability of SLs restricts their applicability for the benefit of humanity and renders synthesis the only option for their production. However, the structural complexity of SLs impedes their economical synthesis, which is unfeasible on a large scale. Synthesis of SL analogues and mimics with a simpler structure, but with retention of bioactivity, is the solution to this problem. RESULTS Here, we present eight new hybrid‐type SL analogues derived from auxin, synthesized via coupling of auxin ester [ethyl 2‐(1H‐indol‐3‐yl)acetate] and of ethyl 2‐phenylacetate with four D‐rings (mono‐, two di‐ and trimethylated). The new hybrid‐type SL analogues were bioassayed to assess the germination activity of seeds of the parasitic weeds Striga hermonthica, Orobanche minor and Phelipanche ramosa using the classical method of counting germinated seeds and a colorimetric method. The bioassays revealed that analogues with a natural monomethylated D‐ring had appreciable to good activity towards the three species and were the most active derivatives. By contrast, derivatives with the trimethylated D‐ring showed no activity. The dimethylated derivatives (2,4‐dimethyl and 3,4‐dimethyl) were slightly active, especially towards P. ramosa. CONCLUSIONS New hybrid‐type analogues derived from auxins have been prepared. These analogues may be attractive as potential suicidal germination agents for parasitic weed control because of their ease of preparation and relevant bioactivity. © 2019 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry., Hybrid‐type strigolactone (SL) analogues were prepared from auxin ethyl ester in a two‐step operation. These SL analogues were bioassayed as germination agents for Striga hermonthica, Pelipanche ramosa and Orobanche minor.

We thank Dr Abdel Gabar Babiker for providing us with highly viable S. hermonthica seeds. We thank Mr Han Rieffe of CRODA, Gouda, The Netherlands, for his valuable help to select the best formulating agent (Atlas G-1086). We are thankful to Xavier Pita, scientific illustrator at King Abdullah University for Science and Technology (KAUST), for the illustrations in Figures 2 and 7. This work was supported by The Bill & Melinda Gates Foundation grant OPP1136424 to SA and by the King Abdullah University of Science and Technology (KAUST).

Strigolactones (SLs) constitute a new class of plant hormones of increasing importance in plant science. The structure of natural SLs is too complex for ready access by synthesis. Therefore, much attention is being given to design of SL analogues and mimics with a simpler structure but with retention of bioactivity. Here new hybrid type SL mimics have been designed derived from auxins, the common plant growth regulators. Auxins were simply coupled with the butenolide D-ring using bromo (or chloro) butenolide. D-rings having an extra methyl group at the vicinal C-3' carbon atom, or at the C-2' carbon atom, or at both have also been studied. The new hybrid type SL mimics were bioassayed for germination activity of seeds of the parasitic weeds S. hermonthica, O. minor and P. ramosa using the classical method of counting germinated seeds and a colorimetric method. For comparison SL mimics derived from phenyl acetic acid were also investigated. The bioassays revealed that mimics with a normal D-ring had appreciable to good activity, those with an extra methyl group at C-2' were also appreciably active, whereas those with a methyl group in the vicinal C-3' position were inactive (S. hermonthica) or only slightly active. The new hybrid type mimics may be attractive as potential suicidal germination agents in agronomic applications.

Strigolactones (SLs) are new plant hormones that play an important role in the control development of plants. They are germination stimulants for seed of parasitic weeds, are the branching factor of arbuscular mycorrhizal fungi and inhibitors for bud outgrowth and shoot branching. Natural SLs contain an annulated system of three rings (ABC scaffold) connected to a furanone (the D-ring) by an enol ether unit. The natural distribution of strigolactones is low, and their synthesis is long and difficult. Therefore, SL analogs are designed to have the same bioactiphore as natural SLs and an appreciable bioactivity. For the design a model is used based on the natural bioactiphore. Typical SL analogs are GR24, Nijmegen-1, and EM1 (derived from ethyl 2-phenylacetate). The synthesis of these SL analogs is reported together with their stability in aqueous solution.

Strigolactones (SLs) are new plant hormones that play an important role in the control development of plants. They are germination stimulants for seed of parasitic weeds, are the branching factor of arbuscular mycorrhizal fungi and inhibitors for bud outgrowth and shoot branching. Natural SLs contain an annulated system of three rings (ABC scaffold) connected to a furanone (the D-ring) by an enol ether unit. The natural distribution of strigolactones is low, and their synthesis is long and difficult. Therefore, SL analogs are designed to have the same bioactiphore as natural SLs and an appreciable bioactivity. For the design a model is used based on the natural bioactiphore. Typical SL analogs are GR24, Nijmegen-1, and EM1 (derived from ethyl 2-phenylacetate). The synthesis of these SL analogs is reported together with their stability in aqueous solution.

The development and growth of plants are regulated by interplay of a plethora of complex chemical reactions in which plant hormones play a pivotal role. In recent years, a group of new plant hormones, namely strigolactones (SLs), was discovered and identified. The first SL, strigol, was isolated in 1966, but it took almost 20 years before the details of its structure were fully elucidated. At present, two families of SLs are known, one having the stereochemistry of (+)-strigol and the other that of (-)-orobanchol, the most abundant naturally occurring SL. The most well-known bioproperty of SLs is the germination of seeds of the parasitic weeds Striga and Orobanche. It is evident that SLs are going to play a prominent role in modern molecular botany. In this review, relevant molecular and bioproperties of SLs are discussed. Items of importance are the effect of stereochemistry, structure-activity relationships, design and synthesis of analogues with a simple structure, but with retention of bioactivity, introduction of fluorescent labels into SLs, biosynthetic origin of SLs, mode of action in plants, application in agriculture for the control of parasitic weeds, stimulation of the branching of arbuscular mycorrhizal (AM) fungi, and the control of plant architecture. The future potential of SLs in molecular botany is highlighted.

BACKGROUND Strigolactones (SLs) are plant hormones that play various roles in plant development. The chemical stability of SLs depends on the solvent, the pH, and the presence of nucleophiles. Hydrolysis leads to detachment of the butenolide ring, and plays a crucial role in the initial stages of the signal-transduction process occurring between the receptor and the SL signaling molecule. RESULTS To date, two different mechanisms have been proposed for SL hydrolysis. Results obtained from kinetic, thermodynamic, and mass spectral data for the reaction between the widely used synthetic SL analog GR24 and seven different nucleophiles demonstrate that the reaction proceeds via the Michael addition?elimination mechanism. CONCLUSION This study provides valuable information on the chemical stability of GR24 in different plant growth media and buffers. Such information is valuable for scientists using GR24 treatments to study SL-regulated processes in plants. ? 2017 Society of Chemical Industry

Parasitic weeds of the genera Striga and Orobanche spp. cause severe yield losses in agriculture, especially in developing countries and the Mediterranean. Seeds of these weeds germinate by a chemical signal exuded by the roots of host plants. The radicle thus produced attaches to the root of the host plant, which can then supply nutrients to the parasite. There is an urgent need to control these weeds to ensure better agricultural production. The naturally occurring chemical signals are strigolactones (SLs), e.g. strigol and orobanchol. One option to control these weeds involves the use of SLs as suicidal germination agents, where germination takes place in the absence of a host. Owing to the lack of nutrients, the germinated seeds will die. The structure of natural SLs is too complex to allow multigram synthesis. Therefore, SL analogues are developed for this purpose. Examples are GR24 and Nijmegen-1. In this paper, the SL analogues Nijmegen-1 and Nijmegen-1 Me were applied in the field as suicidal germination agents. Both SL analogues were formulated using an appropriate EC-approved emulsifier (polyoxyethylene sorbitol hexaoleate) and applied to tobacco (Nicotiana tabacum L.) fields infested by Orobanche ramosa L. (hemp broomrape), following a strict protocol. Four out of 12 trials showed a reduction in broomrape of ≥95%, two trials were negative, two showed a moderate result, one was unclear and in three cases there was no Orobanche problem in the year of the trials. The trial plots were ca 2000 m2 ; half of that area was treated with stimulant emulsion, the other half was not treated. The optimal amount of stimulant was 6.25 g ha-1 . A preconditioning prior to the treatment was a prerequisite for a successful trial. In conclusion, the suicidal germination approach to reducing O. ramosa in tobacco fields using formulated SL analogues was successful. Two other options for weed control are discussed: deactivation of stimulants prior to action and biocontrol by Fusarium oxysporum. © 2016 Society of Chemical Industry.

Strigolactones (SLs) constitute a new class of plant hormones that have received growing interest in recent years. They firstly became known as signalling molecules for host recognition by parasitic plants, and for symbiosis of plants with arbuscular mycorrhizal fungi. Furthermore, they are involved in numerous physiological processes in plants, such as the regulation of plant architecture and the response to abiotic factors. SLs are produced by plants in extremely low quantities, and they may be unstable during the purification process. Therefore, their total synthesis is highly relevant for confirming the structures assigned on the basis of spectroscopic and other physical data. A second important theme in SL research is the design and synthesis of SL analogues that have a simplified structure while still featuring the essential bioproperties. This review summarises the strategy and synthesis of naturally occurring SLs, and the design and synthesis of SL analogues with appreciable bioactivity.

Strigolactones (SLs) constitute a new class of plant hormones that have received growing interest in recent years. They firstly became known as signalling molecules for host recognition by parasitic plants, and for symbiosis of plants with arbuscular mycorrhizal fungi. Furthermore, they are involved in numerous physiological processes in plants, such as the regulation of plant architecture and the response to abiotic factors. SLs are produced by plants in extremely low quantities, and they may be unstable during the purification process. Therefore, their total synthesis is highly relevant for confirming the structures assigned on the basis of spectroscopic and other physical data. A second important theme in SL research is the design and synthesis of SL analogues that have a simplified structure while still featuring the essential bioproperties. This review summarises the strategy and synthesis of naturally occurring SLs, and the design and synthesis of SL analogues with appreciable bioactivity.

Broomrapes (Orobanche and Phelipanche spp) and witchweeds (Striga spp) are the two most devastating parasitic weeds causing enormous losses in agriculture. They have a strong dependence on germination stimulants that are released by the host plants. Knowledge of the structure and bioactivity of these stimulants play an essential role in designing control methods for these weeds. The seeds of the parasites are very minute in size and remain viable in the soil for up to 20 years. Reduction of the weed seed bank has been suggested as an attractive option. Suicidal germination is achieved by applying the germination stimulants to the soil prior to planting crop seedlings or sowing crop seeds, Orobanche seeds germinate in absence of the host and as a consequence the germinated seeds die due to lack of nutrients from the host plants. This approach requires a very strict protocol and hence difficult to perform at field levels. Recently, we proposed a novel concept for weed control by decomposing the germination stimulants in soil prior to action. In this paper, this concept is further substantiated by two crucial experiments, namely a study of the phototoxic effect of the reagents used and a demonstration the actual quenching of the germination of seeds of parasitic weeds by applying borax and/or thiourea. The results demonstrate that timely decomposition of stimulation is indeed a feasible method for weed control.

Strigolactones are signaling molecules that play a role in host recognition by parasitic plants of the Striga, Orobanche and Phelipanche genera which are among the most detrimental weeds in agriculture. The same class of molecules is also involved in the establishment of the symbiosis of plants with arbuscular mycorrhizal fungi. In addition, strigolactones are being shown to be involved in an increasing number of physiological processes in plants, such as the regulation of plant architecture and the response to abiotic factors such as nutrient availability and light. Important advances in knowledge about the structure determination, occurrence, biological function and physiological and biochemical regulation of the strigolactones have been revised. This review presents the complete collection of available spectroscopic data of correct structures of strigolactones, the occurrence in plant kingdom, as well as germination and hyphal branching activities, that are of high importance to the scientific community that is investigating these novel plant hormones. Moreover, two new structures of strigolactone members are proposed.

Strigolactones (SLs) are new plant hormones with varies important bio-functions. This Letter deals with germination of seeds of parasitic weeds. Natural SLs have a too complex structure for synthesis. Therefore, there is an active search for SL analogues and mimics with a simpler structure with retention of activity. SL analogues all contain the D-ring connected with an enone moiety through an enol ether unit. A new mechanism for the hydrolysis SL analogues involving bidentate bound water and an α,β-hydrolase with a Ser-His-Asp catalytic triad has been proposed. Newly discovered SL mimics only have the D-ring with an appropriate leaving group at C-5. A mode of action for SL mimics was proposed for which now supporting evidence is provided. As predicted an extra methyl group at C-4 of the D-ring blocks the germination of seeds of parasitic weeds.

In this paper 50 years of organosulfur chemistry have been reviewed on the occasion of the 25th International Symposium on the Organic Chemistry of Sulfur in 2012.

Strigolactones (SLs) are important new plant hormones that receive much attention in current plant science. SLs are produced by many plants and are exuded by the root system. SLs are, amongst others, germination stimulants for seed of parasitic weeds. Naturally occurring SLs invariably contain three annelated rings, the ABC-scaffold, connected to a butenolide (the D-ring) via an enol ether unit. The synthesis of natural SLs requires many steps, therefore there is a continuous search for SL analogues with a simpler structure but with retention of bioactivity. In this study modified D-ring variants are investigated, especially analogues having a methyl group at C-2 instead of a hydrogen. For these analogues the ABC-scaffolds of GR24 and Nijmegen-1 were used. The coupling reaction proceeds profoundly better with chlorobutenolides than with the corresponding bromides. Bioassays reveal that the introduction an extra methyl at C-2 does not influence the germination activity, which is relevant for gaining insight in the mode of action of SLs.

Main conclusion The key step in the mode of action of strigolactones is the enzymatic detachment of the D-ring. The thus formed hydroxy butenolide induces conformational changes of the receptor pocket which trigger a cascade of reactions in the signal transduction. Abstract Strigolactones (SLs) constitute a new class of plant hormones which are of increasing importance in plant science. For the last 60 years, they have been known as germination stimulants for parasitic plants. Recently, several new bio-properties of SLs have been discovered such as the branching factor for arbuscular mycorrhizal fungi, regulation of plant architecture (inhibition of bud outgrowth and of shoot branching) and the response to abiotic factors, etc. To broaden horizons and encourage new ideas for identifying and synthesising new and structurally simple SLs, this review is focused on molecular aspects of this new class of plant hormones. Special attention has been given to structural features, the mode of action of these phytohormones in various biological actions, the design of SL analogs and their applications.

The chemistry of sulfines (thione S-oxides) is briefly reviewed with focus on highlights and milestones. Sulfines can conveniently be prepared by oxidation of a large variety of thiocarbonyl-containing compounds. Alkylidenation of sulfur dioxide with α -silyl carbanions is an attractive method of preparation as it allows access to sulfines that cannot be obtained by the oxidation method. The reaction of activated methylene compounds with thionyl chloride is a highly versatile method of preparation. The scope of the reaction with thionyl chloride is extended by using silyl enol ethers and silyl ketene acetals as starting materials. The cycloaddition reaction of sulfines with 1,3-dienes is widely used for trapping purposes. The Diels–Alder type cycloadducts are useful compounds in synthesis. Dipolar cycloaddition reactions are of interest as well as the addition reaction of alkyllithium regents. O-Alkylation of aminosulfines gives iminosulfenates which are of interest for further synthetic elaboration.

The single step synthesis of a newly designed series of strigolactones (SLs) from cyclic keto enols is described. The germinating activity of these SL analogues towards seeds of the parasitic weeds Striga and Orobanche spp. is reported. The first of these SL analogues are derived from the hydroxyl γ-pyrones kojic acid and maltol, the second type from hydroxyl α-pyrones, namely, 4-hydroxy-6-methyl-2H-pyran-2-one and 4-hydroxy-coumarin and the third type from 1,3-diketones, namely, 1,3-cyclohexane-dione (dimedone) and tricyclic 1,3-dione. All keto enols are coupled in a single step with the appropriate D-ring precursor in the presence of a base to give the desired SL analogues. All SL analogues are acceptably biologically active in inducing the germination of seeds of Striga hermonthica and Orobanchecernua. Most interesting are the analogues derived from 4-hydroxy coumarin and dimedone, as they have a remarkably high biological activity towards the seeds of parasitic weeds at relatively low concentrations, comparable with that of the general standard stimulant GR24.

Strigolactones are important signaling compounds in the plant kingdom. Here we focus on their germination stimulatory effect on seeds of the parasitic weeds Striga and Orobanche spp. and more particularly on the design and synthesis of new active strigolactone analogs derived from simple cyclic ketones. New analogs derived from 1-indanone, 1-tetralone, cyclopentanone, cyclohexanone and a series of substituted cyclohexanones (including carvone and pulegone) are prepared by formylation of the ketones with ethyl formate followed by coupling with a halo butenolide. Both enantiomers of the analog derived from 1-tetralone have been prepared by employing a homochiral synthon for the coupling reaction. For three other strigolactone analogs the antipodes have been obtained by chromatography on a chiral column. All analogs have an appreciable germinating activity towards seeds of Striga hermomonthica and Orobanche crenata and O. cernua. Stereoisomers having the same configuration at the D-ring as in naturally occurring strigol have a higher stimulatory effect than the corresponding antipodes. The analogs obtained from 1-indanone and 1-tetralone have an activity comparable with that of the well known stimulant GR 24. Analogs derived from 2-phenyl-cylohexanone, carvone and pulegone also have a good germinating response. The results show that the working model for designing new bioactive strigolactones is applicable.

The synthesis of some 3-O-long-chain alkyl ethers of D-glucitol, D-mannitol, and mixtures thereof, from readily available di-O-isopropylidene derivatives of D-glucose and D-fructose is described. Some physical characteristics including critical-micellar-concentration (CMC) values, Krafft temperatures, thermotropic and lyotropic liquid-crystal behaviour are also reported.

The preparation of sulfines by oxidation of thioketones is described. Thiobenzophenone-S-oxide (I) and 4,4′-dimethoxythiobenzophenone-S-oxide (II) were obtained by oxidizing the corresponding thioketone with less than one equivalent of monoperphthalic acid. The oxidation of thiaxanthione appeared to give three types of sulfines, viz., IV, V, and VI, the formation of which was rationalized. Factors controlling the stability of sulfines are discussed and also the mechanism of the oxidation of the moiety with peracid. The alternative route to sulfines, viz., elimination of hydrogen chloride from sulfinyl chlorides, is described for thiofluorenone-S-oxide, thiobenzophenone-S-oxide and 2-methoxynaphthalene-1-thial-S-oxide. In the latter case the originally published yield of 18% was increased to 63%. In addition to this sulfine the sulfinyl sulfone XI was isolated as a by-product which loses sulfur dioxide on heating giving the sulfoxide XII.

Reaction of highly substituted dithiobenzoates 1 (a-e) with chloramine-T in methanol at 20°C gives, in high yields, thione S-imides 2 (a-e) to which the E-geometry was assigned. Heating of these compounds in methanol for 1 hr causes thermodynamic isomerization to the Z-isomers 3 (a-e). Similarly, 2,4,6-trimethyl-thiobenzophenone gives the corresponding E- and Z-S-tosylimides 6 and 7. Irradiation of thione S-imides 2, 3, 6 and 7 leads to extrusion of sulfur yielding N-tosylimines 4 and 8. The geometrical configurations were based on ASIS experiments. From the 1H-NMR spectra of 2 and 3 the preferred conformation of the thione S-imides have been deduced (S-trans conformation for the E-isomer 2 and a folded S-cis conformation for the Z-isomer 3). It will be shown that in order to obtain stable thione S-tosylimides from dithiobenzoates and thiobenzophenones, steric hindrance in the direct environment of the thiono function is a prerequisite.

The synthesis of sulfines (thione oxides) (2, 10 and 17) derived from three chiral dithio esters (1, 8 and 16) bearing a hydrogen at the α-carbon atom is described. The sulfine bearing an amido group at C-α (2), does not racemize and proved to be stable over a long period. When C-α in the sulfine (10) bears an acetoxy group the optical activity remained unchanged for a period of two months; after longer storage a rearrangement to a mixture of dithioperoxy ester 12 and S-methyl ester 11 took place. With an alkyl group at C-α, the sulfine 17 undergoes a fast racemization and rearranges slowly to a mixture of S-ethyl ester 14, dithioperoxy ester 18 and S-vinyl thiosulfinate 19.

The aggregation behaviour of a synthetic C4 phospholipid, containing two phosphate groups and two stearoyl groups, has been studied. The surfactant formed different types of aggregates in water depending on the concentration, the temperature and the treatment of the samples. Tilted bilayers were found at high surfactant concentrations, whereas at low surfactant concentrations intercalated bilayers could be identified. The intercalated structures could be converted into very small vesicles upon sonication. Tube-like aggregates were formed after storage at low temperature, which coagulated into thread-like superstructures.

Aziridine-2-carboxylic esters 1 were converted into bicyclic boroxazolidones 3 in a three-step sequence. First, the ester groups were hydrolyzed either by sodium hydroxide or with lithium hydroxide in methanol. The salts thus obtained were acidified with acetic acid or through a weakly acidic ion-exchange resin. Treatment of the zwitterionic aziridine-2-carboxylic acids 2 with triethyl-borane in hexane gave boroxazolidones 3 in high yields. Boroxazolidone 3g, obtained from optically active 1g was reconverted to the starting material by 8-hydroxyquinoline without any loss of optical activity.

Aziridine-2-carboxylic esters 1 were converted into 2H-azirine-2-carboxylic esters 4 in two steps. Treatment of 1 with tert-butyl hypochlorite in ether gave smooth N-chlorination. Reaction of N-chloroaziridines 2 with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) or 1,4-diazabicyclo[2.2.2]-octane (DABCO) gave 2H-azirine-2-carboxylic esters 4 in moderate yields, with concomitant formation of aziridines 1. Aziridines 1 could readily be N-brominated with N-bromosuccinimide. Treatment of N-bromoaziridines 3 with base, however, did not lead to azirine formation. N-Halogenated aziridines 2 and 3 exist as mixtures of invertomers, which slowly interconvert in solution.

Optical antipodes of 4-alkyl-γ-lactones 3 have been prepared by photochemical rearrangement of optically active α,β-epoxy diazomethyl ketones 1 in ethanol to give 4-hydroxy-alkenoates 2, followed by reduction of the alkene bond and subsequent lactonization. The required epoxy diazomethyl ketones 1 were obtained via the following sequence of reactions: alkylation of 2-propyn-l-ol, subsequent reduction to the alkenols 6, Sharpless epoxidation to 2,3-epoxy alcohols 7, oxidation to glycidic esters 8 and finally conversion to diazo ketones 1. The enantiomeric purities range from 84 to 100%.

The preparation of the chlorosulfines RC(Cl)=S=O where R is C6H5 and C6H5S, by oxidation of the corresponding thioacyl chlorides, is described. Reaction of these chlorosulfines with thiophenol and potassium thiocyanate leads to substitution of the chlorine atom giving phenylthio- and thiocyanosulfines, respectively. The displacement of the chlorine atom in chloro-phenylsulfine by the phenylthio group occurs stereospecifically: the geometrical configuration of the chlorosulfine is predominantly retained in the product. A mechanism for these displacement reactions is proposed.

Stepwise oxidation of p-tolyl p-methoxydithiobenzoate (1) gives the sulfines (thione oxides) AnC(=S=O)STol (2), AnC(=S=O)SOTol (3) and AnC(=S=O)SO2Tol (4) (An = p-CH3OC6H4 and Tol = p-CH3C6H4). Reaction of 2 with p-toluenethiol (TolSH) in the presence of Et3N furnishes AnCH(STol)SSTol (5). The sulfine 4 yields with TolSH/Et3N 5 and AnCH2SO2Tol (6), with EtSH/Et3N AnCH(SEt)SSEt (7) and with t-BuSH/Et3N the sulfone 6. The sulfine 3 gives with TolSH/Et3N 5 and with t-BuSH/Et3N 6. In the presence of ZnCl2 all three sulfines give rise to deoxygenated products, viz. dithioesters like 1 and AnCSSEt (8). In the absence of a catalyst, sulfine 2, with TolSH and EtSH, undergoes deoxygenation to dithioesters. Sulfine 4 gave with EtSH the dithioester 8 as the principal product, whereas with TolSH besides 1 and 5 the compound AnCH(SO2Tol)SSTol (9) was obtained. From sulfine 3 products were obtained that point to a disproportionation into 2 and 4 prior to the reaction of thiol with the sulfine function. In the discussion, a rationale is presented for the reactions observed. It is shown that nucleophilic attack of thiol or thiolate anion may occur at the sulfine sulfur as well as the sulfine carbon atom. In the presence of Et3N both reaction modes take place, whereas in the presence of ZnCl2 attack at the sulfur seems to dominate.

Ring-opening reactions of 3-arylaziridine-2-carboxylic esters with various nucleophiles are described. Racemic methyl 3-(4-methoxyphenyl)-, 3-phenyl- and 3-(4-nitrophenyl)aziridine-2-carboxylate (1a, 1b and 1c, respectively) were selected as substrates. In the absence of acid, no ring opening occurred. On treatment with ethereal hydrogen chloride, 1a gave a mixture of diastereomers 2a, whereas 1b and 1c gave mixtures of regioisomers 2b/3b and 2c/3c, respectively. Boron-trifluoride etherate catalyzed reaction of 1a with benzenethiol and indole also resulted in the formation of diastereomeric ring-opened products 4a and 6a, respectively, due to the electron-releasing properties of the p-methoxy group. Only C3 attack was observed. Under the same conditions, 1b and 1c gave a clean SN2-type ring opening, leading to diastereomerically pure products 4b, 4c, 6b and 6c. Reaction of 1b with acetic acid gave 7 in an SN2-type ring opening at C3, followed by an ON acyl shift. Treatment of enantiopure (+ )-(2S,3R)-1b with benzenethiol, indole and acetic acid gave the corresponding enantiomerically pure β-functionalized α-amino acid derivatives. Functionalization of 1b at nitrogen strongly increased the reactivity: N-acylaziridine 8 isomerized to trans-oxazoline 11 when treated with boron trifluoride etherate in acetonitrile.

The addition of chlorine and bromine to 2-methoxynaphthalene-1-thial-S-oxide (II) yields the dichloride and dibromide (IVa and b). The dichloride gives upon treatment with triethylamine 2-methoxythionaphthoyl chloride-S-oxide (V). Similarly, thiobenzaldehyde-S-oxide (prepared in situ) was converted into cis- and trans-thiobenzoyl chloride-S-oxide (scheme B). Tetrachloro-ortho-quinone reacted with xanthione-S-oxide (IX) and thiofluorenone-S-oxide (I) yielding 1:1 adducts (X and XI) in which the α-dioxo part of the quinone had acted as diene. The reduction of the sulfine I with triphenylphosphine gave initially the thioketone, which reacted further with the phosphine as outlined in scheme D. The same reaction was carried out with the thial-S-oxide II. The acid catalyzed hydrolysis of thioketone-S-oxides gave rise to the formation of the corresponding ketones (scheme E).

3-Aryl-3-azido-2-hydroxypropanoic esters, prepared from the corresponding 3-aryl-oxirane-2-carboxylic esters by ring opening with sodium azide, were reduced with tin(II) chloride dihydrate in methanol to give 3-amino-3-aryl-2-hydroxypropanoic esters in good yields. Under these conditions, halogen substituents in the aromatic rings were not affected. The nitro group, however, was partially reduced to the amino group. Treatment of aliphatic oxirane-2-carboxylic esters with acetonitrile in the presence of boron trifluoride etherate led to regiospecific formation of 2,4-dialkyl-2-oxazoline-5-carboxylic esters, resulting from reaction of the nitrile at C3. Acidic hydrolysis of these oxazoline-5-carboxylic esters gave the corresponding 3-(acylamino)-2-hydroxy carboxylic esters. With these two complementary methods, both aryl- and alkyl-substituted β-amino α-hydroxy acid derivatives are accessible.

Oxidation of methyl (phenylsulfonyl)dithioacetate (1) with m-chloroperbenzoic acid gave the sulfine C6H5SO2CH2C(SO)SCH3 (2). Deprotonation of 2 with TlOEt and subsequent S-alkylation of the thallium(I) salt with methyl iodide gave (phenylsulfonyl)ketene dimethyl dithioacetal moxonide (5). When other bases were used in the deprotonation step deoxygenated products, viz. C6H5SO2CHC(SCH3)2 were obtained. Evidence is presented that oxygen is lost by an intramolecular redox reaction of an initially formed vinylsulfenic ester. The new ketene dithioacetal monoxide 5 reacts with malonate anions in an additon-elimination reaction in which the phenylsulfonyl group serves as the leaving group.

Treatment of amino sulfines (thioamide S-oxides) 1 with triethyloxonium tetrafluoroborate, followed by sodium carbonate, leads to α-imino sulfenates 3. Reaction of 3 with primary and secondary amines gives α-imino sulfenamides 5. Treatment of amino sulfines with sodium hydride, followed by an alkyl halide, leads to the formation of α-imino sulfoxides 6, which are extremely sensitive towards hydrolysis. With ketenes, the imines 6 undergo a [2 + 2] cycloaddition reaction to give β-lactams 8 in low yield.

Optically active oxirane-2-carboxylic esters, prepared from allylic alcohols employing the Sharpless epoxidation, were treated with sodium azide. The azido alcohols obtained were subsequently converted into aziridine-2-carboxylic esters by reaction with triphenylphosphine, in good yields and with high optical purity. Various racemic oxirane-2-carboxylic esters were subjected to the same sequence of reactions.

The hydrolysis of a series of norbornane-type carboxylic esters catalyzed by pig liver esterase (PLE) has been investigated. It was found that an exo-eer function (syn to the C7 methylene group) is hydrolyzed with high preference. Enantioselective hydrolysis can be accomplished when a vicinal carbonyl-containing function (eer, formyl, acetyl) is present in a trans position with respect to the exo eer. The regiospecificity of the enzymatic hydrolysis has been used for the separation of exo/endo mixtures of the cycloadducts derived from cyclopentadiene and alkene esters. The regiospecificity and enantioselectivity of the enzymatic hydrolysis of norbornane-type esters were analyzed in terms of Tamm's substrate model and also by Griengl's method.

Carba analogues 1, 14, 15 and 16 of Strigol were synthesised in which the vinyl ether oxygen is replaced by methylene. Compound 1 was prepared by the Wittig reaction of ylid 2 with aldehyde 3. Compounds 14, 15 and 16, for which this approach failed, were prepared by reaction of aldehyde 20 with lactones 17, 18 and 19, respectively, using the method of Tanaka et al.35,36, or with phosphonates 32 and 33, in a Wittig-Horner reaction. Subsequent oxidative elimination of the thioether moiety gave the desired products.

The synthesis of thiophenes starting from allenyl sulfones, via intermediate formation of α,β-unsaturated sulfines, is described. The allenyl sulfones were synthesized by a [2,3]-sigmatropic rearrangement of appropriately substituted prop-2-ynyl sulfinates. Treatment of the allenyl sulfones with n-butyllithium and chlorotrimethylsilane gave α-silylated allenyl sulfones in almost quantitative yield, which, on heteroconjugate addition of organolithium reagents, gave the desired α-silyl carbanions. These reacted with sulfur dioxide to give the α,β-unsaturated sulfines, which underwent in-situ rearrangement to 2-sulfonylthiophenes. The yield of the thiophenes depends on the organolithium reagents used. The use of exocyclic allenyl sulfones was more troublesome, and a thiophene was obtained for only one example.

Heteroconjugate addition of various alkyllithium reagents to α-silylallenephosphonates 3b,c has been accomplished in high yields. The formation of thiophene-2-phosphonates 5, by heteroconjugate addition of organolithium to 3bc, followed by treatment with sulfur dioxide, proceeds in reasonable yields via the intermediate formation of α,β-unsaturated sulfines 4. An alternative synthesis of the heteroconjugate addition product diethyl 1-(trimethylsily)-2,3-dimethylbut-2-ene-1-phosphonate 6a from 2,3-dimethylbut-2-enyl bromide has also been described.

The asymmetric phase-transfer-catalysed (PTC) Darzens condensation of aldehydes and ketones with the chiral reagents (S)-( - )-N-(chloromethylsulphonyl)-2-(methoxymethyl)pyrrolidine (2), (S)-( - )-tert-butyl N-(chloromethylsulphonyl)prolinate (4) and ( + )-O-methyl-N-(chloromethylsulphonyl)ephedrine (6), using triethylbenzylammonium chloride (TEBA) as the phase-transfer catalyst, to give optically active diastereomeric α,β-epoxy sulphonamides (oxiranesulphonamides), is described. Of the three chiral reagents used, 2 was found to be the most effective, imparting an asymmetric induction of up to almost 50%, as assessed by Eu(fod)3-resolved 1H NMR spectra. The stereochemical course of the asymmetric synthesis is discussed.

An efficient procedure for the glycosidation of D-fructose and D-glucose catalyzed by iodine is described. The reaction yields mainly alkyl glycofuranosides. Treatment of inulin under similar conditions leads to inter-glycosidic bond cleavage and to formation of alkyl D-fructofuranosides. The reaction conditions are particularly mild and relatively selective.

Nucleophilic ring-opening reactions of 3-alkylaziridine-2-carboxylic esters are described. Without ring activation at nitrogen, ring opening could only be accomplished with ethereal hydrogen chloride. Introduction of electron-withdrawing activating groups at the nitrogen atom was necessary. Three types of activating groups were used, viz. acyl, alkoxycarbonyl and sulfonyl groups. In the presence of a Lewis-acid catalyst, ring opening with two nucleophiles, namely indole and benzenethiol, could be accomplished. SN2-type attack at C3 was observed exclusively. With Bronsted acids (hydrogen chloride, formic acid, acetic acid), ring opening could also be effected. 3-Formyloxy derivatives could also be prepared by reaction of the activated aziridines with N,N-dimethyl-formamide in the presence of boron trifluoride etherate. Reaction with sodium azide caused some difficulties; complex product mixtures were usually obtained. N-Sulfonylaziridine-2-carboxylic esters gave a mixture of the two possible regioisomers, resulting from attack at C2 and C3. Treatment of N-sulfonylaziridine-2-carboxylic esters with trimethylsilyl azide, however, gave products exclusively ring-opened at C2. On reaction of the activated aziridines with acetonitrile in the presence of boron trifluoride etherate, a ring-expansion reaction was observed, leading to imidazolines. On standing, these heterocyclic compounds slowly hydrolyzed to α,β-diamino carboxylic acid derivatives.

Cyclopropanation of tricyclo[5.2.1.02,6]deca-2(6),8-dien-3-one using dimethylsulfoxonium ylide gave a highly strained annulated cyclopropane in 68% yield with complete exo-face selectivity. Nucleophilic epoxidation gave a strained epoxide in 68% yield, again completely exo-face selective. Surprisingly, using methanol as the co-solvent in this epoxidation yielded a disubstituted tricyclodecenone in 85% yield instead of the epoxide. This result can be explained by a Payne-type rearrangement of the initially formed epoxide.

Tricyclo[5.2.1.02,6]deca-2(6),8-dien-3-one contains a highly strained central double bond due to geometrical constraints imposed by the tricyclic skeleton which does not allow optimal sp2 hybridization at the C2 and C6 bridgehead positions. Michael addition of various nucleophiles (alkoxides, cyanide, and malonate) under protic conditions resulted in an exclusive exo-facial selectivity. This preference can be explained by steric and electronic factors. Michael additions using lithium dialkylcuprates resulted in predominant formation of endo-products, but also some exo-products were obtained. These exo-products arising from endo-approach may be the result of coordination of the cuprate with both the enone moiety and the olefinic C8–C9 bond. Michael additions to tricyclo[5.2.1.02,6]dec-2(6)-en-3-one, which lacks this C8–C9 double bond showed exclusive exo-facial selectivity to give exo-products. Besides these additions were all considerably slower than those to tricyclo[5.2.1.02,6]deca-2(6),8-dien-3-one proving significant electronic participation of the C8–C9 double bond in reactions with this substrate.

Alkylation at C 6 of tricyclo[5.2.1.0 2,6 ]deca-4,8-dien-3-one (R=H) was achieved by treatment of 6-bromotricyclo[5.2.1.0 2,6 ]deca-4,8-dien-3-one with lithium dimethylcuprate and subsequently with an appropriate electrophile. The best results were obtained in THF as the solvent. A wide range of alkyl halides, bromo ketones and esters, and acetyl chloride resulted in C 6 -tricyclo[5.2.1.0 2,6 ]deca-4,8-dien-3-ones in moderate to good yields. This alkylation reaction proceeds via a C 6 -carbanionic Cu intermediate, which is likely stabilized by the enone olefinic bond. 6-Bromo- endo -tricyclo[5.2.1.0 2,6 ]dec-8-en-3-one, which lacks this double bond, behaves differently. Treatment with lithium dimethylcuprate leads to dehydrobromination to give tricyclo[5.2.1.0 2,6 ]deca-2(6),8-dien-3-one in high yield.

Parasitic plants cause devastating losses to crop yields in several parts of the world. The root parasites, Striga and Orobanche species, use chemical signalling molecules that are exuded by the roots of plants in extremely low concentrations, and that can induce germination of the seeds of these parasites, to detect the vicinity of a suitable host. The majority of the so far identified germination stimulants belong to the strigolactones. It was recently discovered that this class of compounds can also induce hyphal branching in the symbiotic arbuscular mycorrhizal fungi, a process involved in root colonisation. The elucidation of the structure of new strigolactones is hindered by their low abundance and instability. In the present paper, we have used existing knowledge on the structure of strigolactones and combined it with recently obtained insight in the biosynthetic origin of these signalling compounds. This enabled us to postulate structures for strigolactones that have been isolated but for which so far the structure has not been elucidated, but also to propose structures of strigolactones that may be discovered in the future. Considering the strongly increased importance of the strigolactones, we expect that more groups will look for these compounds and also in systems so far not exploited. This could lead to the discovery of new strigolactones for which we expect the present biogenetic considerations will facilitate identification and structure elucidation.