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Organoselenium compounds, due to their high structural diversity, special function, and biological activities, have drawn attention in synthetic chemistry. Herein, a novel example of chiral N,N′-di...

Transition states are of central importance in chemistry. While they are, by definition, transient species, it has been shown before that it is possible to “trap” transition states by applying stretching forces. We here demonstrate that the task of transforming a transition state into a minimum on the potential energy surface can be achieved by using hydrostatic pressure. We apply the computational eXtended Hydrostatic Compression Force Field (X- HCFF) approach to the educt of a [2,3]-sigmatropic rearrangement in both static and dynamic calculations and find that the five-membered cyclic transition state of this reaction becomes a minimum at pressures in the range between 100 and 150 GPa. Slow decompression leads to a 70:30 mix of the product and the educt of the sigmatropic rearrangement. Our findings are discussed in terms of geometric parameters and electronic rearrangements throughout the reaction. We speculate that the trapping of transition states by using pressure is generally possible if the transition state of a chemical reaction has a more condensed geometry than both the educt and the product, which paves the way for new ways of initiating chemical reactions.

Dirhodium(II)-catalyzed oxonium ylide formation–[2,3]-sigmatropic rearrangement of 6-allyloxy-2-diazo-3-ketoesters possessing a C-6 substituent is described. The reaction of 6-alkyl- or 6-aryl-substituted 6-allyloxy-2-diazo-3-ketoesters with a catalytic amount of Rh2(S-PTTL)4 proceeded in a chemoselective and stereoselective manner to provide 6-substituted 2-allyl-3-oxotetrahydropyran-2-carboxylates in good yields and with high diastereoselectivities. To demonstrate the utility of this sequential reaction, we conducted the total synthesis of (+)-tanikolide, in which the construction of the δ-lactone skeleton was achieved by employing a 2-iodobenzamide-catalyzed oxidative cleavage of tetrahydropyran-2-methanol.

The first sequentially combined inorganic base promoted N-addition/[2,3]-sigmatropic rearrangement reaction of α-alkylidene pyrazolinones and propargyl sulfonium salts has been reported to construct homoallyl sulfur-containing pyrazolones with moderate to excellent yields. α-Alkylidene pyrazolinones function as N-nucleophilic agents distinguished from the reported C-addition reactions. Propargyl sulfonium salts were first involved in the [2,3]-sigmatropic rearrangement protocol differentiated from the well-established annulation reactions. The excellent regioselectivity, the broad scope of substrates, gram-scale synthesis and convenient transformation embody the synthetic superiority of this cascade process.

A domino process to prepare chiral azabicycles with bridgehead quaternary stereogenic centers is reported., [2,3]-Sigmatropic rearrangement of ammonium ylides represents a fundamental reaction for stereoselective synthesis of nitrogenous compounds. However, its applicability is limited by the scarcity of efficient, catalytic, and mild methods for generating ammonium ylides. Here, we report silver-catalyzed domino generation/[2,3]-sigmatropic rearrangement of ammonium ylides, furnishing chiral azabicycles with bridgehead quaternary stereogenic centers in high enantiomeric purity (up to 99% ee). A combination of density functional theory calculations and experimental studies revealed that residual water in the reaction system is crucial for the mild reaction conditions by functioning as a proton shuttle to assist carbon-silver bond protonation and C2─H deprotonation to generate the ammonium ylide. This reaction has a broad application scope. Besides the diverse substituents, N-fused azabicycles of various ring sizes are also easily accessed. In addition to silver salts, this strategy has also been successfully implemented by using a stoichiometric amount of nonmetallic I2.

The chemo- and stereocontrolled functionalization of conjugated sulfinyl dienes in a cascade process that involves a Michael-type addition, isomerization of a double bond and a [2,3]-sigmatropic rearrangement is reported. Enantioenriched 1,4-diol and 1,4-aminoalcohol derivatives are obtained in a very straightforward manner. Further functionalization of these structures, including highly stereoselective epoxidation, dihydroxylation and the stereodivergent synthesis of several polyols in a controlled fashion is described, This research was supported by CTQ2016-77555-C2-2-R and MICINN (CTQ2009-07752). We also thank CM for additional support to MU and MEC for a fellowship to I.C

The [2,3]-sigmatropic rearrangement of allylic sulfoxides to allylic sulfenates is a reversible process, generally shifted toward the sulfoxide. In the presence of thiophiles, the sulfenate is trapped, and allylic alcohols are obtained under mild conditions. In most cases, a good transfer of stereochemical information through an ordered transition state is obtained. Furthermore, the ease of coupling this process with other versatile, stereocontrolled reactions has enhanced the usefulness of this protocol. This review aims to provide a comprehensive survey of this rearrangement and its application in the synthesis of natural and bioactive products., This research was supported by MINECO (Spain) (CTQ2013- 46459-C2-02-P, CTQ2016-77555-C2-2-R, CTQ2016-81797- REDC). We are grateful to Dr. Carmen de la Torre (IQOGCSIC) and Dr. Miguel Ángel Sierra (UCM) for encouragement and support

The research leading to these results (K.K.) has received funding from the ERC under the European Union's Seventh Framework Programme (FP7/2007-2013) / ERC grant agreement n° 279850. A.D.S. thanks the Royal Society for a Wolfson Research Merit Award. The tetramisole-promoted catalytic enantioselective [2,3]-sigmatropic rearrangement of quaternary ammonium salts bearing a (Z)-3-fluoro-3-arylprop-2-ene group generates, after addition of benzylamine, a range of β-fluoro-β-aryl-α-aminopentenamides containing a stereogenic tertiary fluorine substituent. Cyclic and acyclic nitrogen substituents as well as various aromatic substituents are tolerated, giving the β-fluoro-β-aryl-α-aminopentenamide products in up to 76% yield, 96:4 dr, and 98:2 er. Postprint

The paper describes a convenient and efficient method for regioselective synthesis of phosphorylated α-hydroxyallenes using an atom economical [2,3]-sigmatropic rearrangement of intermediate propargyl phosphites or phosphinites. These can be readily prepared via reaction of protected alkynols with dimethyl chlorophosphite or chlorodiphenyl phosphine respectively in the presence of a base.

The sigmatropic rearrangement of propargylic sulfinates to allenic sulfones is catalyzed by the copper cation. The Pale Cu 1 -USY zeolite, as well as copper salts, lead to essentially quantitative yield of the corresponding allenic sulfones.

The Rh2(OAc)4-catalyzed oxonium ylide formation–[2,3]-sigmatropic rearrangement of a highly functionalized α-diazo-β-keto ester derived from d -glucose proceeded stereoselectively to give the corresponding tetrahydrofuran-3-one as a single diastereomer in high yield. This reaction was applied to the synthesis of 2-epi-cinatrin C1 dimethyl ester as a key step.

The tandem ylide formation/[2,3]-sigmatropic rearrangement between donor/acceptor rhodium carbenoids and chiral allyl alcohols is a convergent C-C bond forming process, which generates two vicinal stereogenic centers. Any of the four possible stereoisomers can be selectively synthesized by appropriate combination of the chiral catalyst Rh(2)(DOSP)(4) and the chiral alcohol.

α-Amino allenephosphonates were easily prepared in two steps from protected amines, propargyl alcohols, and chlorophosphites. First, ynamides were synthesized from unprotected 1-bromopropargyl alcohols using a copper(II) catalyzed coupling reaction. In the second step, the previously prepared ynamides were transformed directly to allenes through a [2,3]-sigmatropic rearrangement of propargyl phosphites. This efficient method led to the formation of a series of α-amino allenephosphonates with diverse substituents on the amine, the phosphonate, and the allene moieties.

The first asymmetric [2,3]-sigmatropic rearrangement of achiral allylic amines has been realized by quaternization of the amines with an enantiomerically pure diazaborolidine and subsequent treatment with Et3N. The resultant homoallylic amines were obtained in good yields and excellent ee's. The observed diastereo- and enantioselectivities were rationalized by invoking a kinetically controlled process, and support for this model was obtained from an NMR spectroscopic investigation of the chiral Lewis acid-substrate complex. The structure of the Lewis acid-product complex was established by X-ray crystallographic analysis and supported the proposed mechanism.

The highly diastereoselective base-promoted intramolecular cyclization of a variety of enantiopure sulfinyl dienyl amines provides novel sulfinyl tetrahydropyridines that are readily converted to 3-hydroxy tetrahydropyridines via sigmatropic rearrangement. The influence of N- and C- substituents on the process has been studied. Procedures to shorten the sequence such as the tandem cyclization followed by [2,3]-sigmatropic rearrangement, as well as cyclization of the free amine, under Boc- or ArSO- deprotection conditions have been examined. Good to excellent levels of selectivity are generally observed for the reported transformations (dr: 75/25 to >98/2). A novel protocol to access substituted amino dienyl sulfoxides is also reported., This research was supported by MINECO (CTQ2013-46459- C2-2-P) and (CTQ2009-07752) cofunded by FEDER. We also thank CSIC for a contract to RHB (201080E101) and CSIC and Fondo Social Europeo for a JAE-doc contract to CS. LI thanks Comunidad de Madrid (Spain) for support (BIPEDD-2: S2010-BMD-2457). We are grateful to Dr. Carmen de la Torre (IQOG-CSIC) and Dr. Miguel Ángel Sierra (UCM) for encouragement and support.

A ring-contractive and highly diastereoselective [2,3]-sigmatropic rearrangement occurs when N-methyl-1,2,3,6-tetrahydropyridine is treated with sub-stoichiometric amounts of copper or rhodium salts, in the presence of ethyl diazoacetate, giving ethyl cis-N-methyl-3-ethenyl proline (4).

The rearrangement of a range of N-benzyl-O-allylhydroxylamines to the corresponding N-allylhydroxylamines upon treatment with n-BuLi in THF, followed by reduction to the corresponding N-allylamines, is described. Mechanistic studies of the transformation are consistent with an intramolecular [2,3] sigmatropic rearrangement.

The scope of the NCS-mediated amination/[2,3]-sigmatropic rearrangement of enantioenriched allylic selenides has been expanded to provide access to three new product classes. The use of N-protected amino acid amides provides a novel strategy for accessing peptide chains containing unnatural vinyl glycine amino acid residues. Also reported is the use of amino acid esters, allowing the diastereoselective synthesis of N,N-dicarboxymethylamines, a motif found in a number of pharmaceuticals. Furthermore, use of a range of N-aromatic and N-heteroaromatic amines allows the formation of enantioenriched N-arylamino acids, a motif found in a number of synthetically and biologically interesting compounds.

The tandem ylide generation and rearrangement or cycloaddition sequence triggered by copper (I) or dirhodium (II) complex-catalyzed decomposition of α-diazo carbonyl compounds has recently found numerous applications in organic synthesis. While exceptionally high levels of enantiocontrol in C-H insertions and cyclopropanations have already been achieved using welldesigned chiral metal catalysts, the development of an enantioselective version of this process has not been so straightforward as it was believed until several years ago that the reactions of ylides would occur from the free ylide detached from the catalyst rather than from the metal complexassociated ylide. The rapid recent developments in this area reviewed here have shown that high levels of asymmetric induction are possible with rearrangements of oxonium ylides and with 1, 3-dipolar cycloadditions of carbonyl ylides where the chiral metal catalyst can remain bound to the ylides in the product-forming step.

The reaction of a series of ketones with dimsylsodium in dimethyl sulfoxide resulting in the formation of γ-unsaturated thiols was studied experimentally. [2,3]-Sigmatropic rearrangements of β-unsaturated sulfinyl carbanions are involved at the key step of those transformations. DFT computations at the B3LYP/6-31+G* level indicated that such rearrangements, as well as some typical [2,3]-sigmatropic rearrangements, e.g., thermal rearrangements of allyl sulfoxides and Wittig rearrangements of sulfur ylides and lithiated allyloxy methyl anions, are concerted and moderately synchronous processes. Negative (diatropic) nucleus-independent chemical shifts (NICS) and high diamagnetic susceptibility exaltations indicate that the transition structures of these [2,3]-sigmatropic migrations are aromatic.

Reduction of aryl and vinyl radicals to the corresponding organosamarium species by SmI2 can be attained by using benzene-HMPA as a solvent system. Thus, SmI2-mediated Barbier- and Grignardtype coupling reactions between ketones and aryl, vinyl, or alkynyl iodides proceed via organosamarium species in benzene-HMPA. Reductive dehalogenation of 1, 1-dihaloalkenes by SmI2 in benzene-HMPA generates alkylidenecarbenes, which undergo either 1, 5-C-H insertion giving cyclopentenes or 1, 2-shift giving alkynes. On the other hand, SmI2 was found to be useful to effect 2, 3-rearrangement under mild conditions. Metalated ethers undergoing Wittig rearrangement are regioselectively generated by either an intramolecular 1, 5-hydrogen transfer of a vinyl radical, generated by a single electron transfer from SmI2 to γ-haloallyl ethers, or a net two-electron reduction of diallyl acetals with the liberation of an allyloxy samarium by SmI2 in acetonitrile. 2, 3-Rearrangement of allylic sulfonium ylides can be effected by the reaction of allylic sulfides with samarium carbenoid, generated from CH2I2 and SmI2 in THF.

Tricyclic benzothiazepinium salts (5) were prepared by [2+ +4] polar cycloaddition of thionium intermediates (4A), generated from corresponding α-chloro sulfides (4) and dienes in the presence of silver perchlorate. X-Ray analysis of 5a revealed that the configuration of the thiazepinone skeleton and the dihydrothiopyran ring is cis-fused.Reactions of benzothiazepinium salts (5) with NaBH4 or NaH afforded 3, 6-epithiobenzazocinone derivatives (9) in high yields by [2, 3]-sigmatropic rearrangement of an ylide intermediate (11). The stereochemistry of epithiobenzazocinone (9a) was determined by the nuclear Overhauser enhancement (NOE) technique and finally by X-ray analysis of the sulfoxide (10) derived from epithiobenzazocinone (9a) by m-chloroperbenzoic acid (MCPBA) oxidation. Alkylation of epithiobenzazocinone (9a) afforded 3-alkyl-3, 6-epithiobenzazocinones (12) with retention of the configuration at C-3. Dihydrothiopyran derivatives (13) were obtained in good yields by SmI2 reduction of benzothiazepinium salts (5).

Chemical building blocks: ?-Sulfenylated imines containing a quaternary center, an ubiquitous subunit and excellent building block, were achieved by rhodium-catalyzed denitrogenative [2,3] sigmatropic rearrangements of N-sulfonyl-1,2,3-triazoles with allylaryl(alkyl) sulfides. Coupling of this methodology with a copper-catalyzed cycloaddition reaction provides a platform for the direct regioselective functionalization of alkynes (see scheme; TC=thiophenecarboxylate; Ts=p-toluenesulfonyl). � 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Asymmetric synthesis using organosulfur and organoselenium compounds has been extensively studied since the discovery of Andersen synthesis of chiral sulfoxides and the first isolation of chiral selenoxides, respectively. Recent advances in this field were reviewed in the following three topics; 1) synthesis of chiral sulfoxides, 2) synthesis of chiral selenoxides, 3) asymmetric induction via chiral selenoxides.

Rhodium-catalyzed reactions of tertiary propargylic alcohols with methyl aryl- and styryldiazoacetates result in tandem reactions, consisting of oxonium ylide formation followed by [2,3]-sigmatropic rearrangement. This process competes favorably with the standard O-H insertion reaction of carbenoids. The resulting allenes are produced with high enantioselectivity (88-98% ee) when the reaction is catalyzed by the dirhodium tetraprolinate complex, Rh(2)(S-DOSP)(4). Kinetic resolution is possible when racemic tertiary propargylic alcohols are used as substrates. Under the kinetic resolution conditions, the allenes are formed with good diastereoselectivity and enantioselectivity (up to 6.1:1 dr, 88-93% ee), and the unreacted alcohols are enantioenriched to 65-95% ee. Computational studies reveal that the high asymmetric induction is obtained via an organized transition state involving a two-point attachment: ylide formation between the alcohol oxygen and the carbenoid and hydrogen bonding of the alcohol to a carboxylate ligand. The 2,3-sigmatropic rearrangement proceeds through initial cleavage of the O-H bond to generate an intermediate with close-lying open-shell singlet, triplet, and closed-shell singlet electronic states. This intermediate would have significant diradical character, which is consistent with the observation that the 2,3-sigmatropic rearrangement is favored with donor/acceptor carbenoids and more highly functionalized propargylic alcohols.

The asymmetric Diels-Alder reactions of (R)-1-(p-tolylsulfinyl)-1,3-bu- tadienes (3a-c) with N-methylmaleimide (NMM) have been explored. The cycloadditions are stereospecific: only one endo adduct is formed, the π-facial diastereoselectivity being controlled by the sulfoxide both in thermal and catalytic conditions. The in situ cycloaddition/sulfoxide sulfenate [2,3]-sigmatropic rearrangement starting from chiral (3a-c) in the presence of an excess of NMM, which acts as thiophilic agent, provides a convenient one-step access to enantiomerically pure and all- cis functionalized cyclohexenols (-)-5a-c

Several β-sulfoxy-substituted acetylenic carbinols were prepared by the addition of thiyl radicals and oxygen to conjugated enynes. The products obtained are derived from thiyl radical attack at the olefinic bond to generate a propargylic radical. Capture of this radical by oxygen followed by hydrogen transfer from thiophenol gives a hydroperoxide intermediate, which undergoes oxygen transfer by both intra- and intermolecular pathways. The resultant β-phenylsulfinyl propargylic alcohols proved to be versatile intermediates for the preparation of several different classes of compounds. The [2,3]-sigmatropic sulfinate to sulfoxide rearrangement was found to give 1,4-bis(phenylsulfonyl)1,3-butadienes, α,β-unsaturated phenylsulfoxy ketones, and β-phenylsulfonyl α-allenic sulfoxides

Studies directed at the amine exchange reaction of vinamidinium salts followed by sodium borohydride reduction to secondary and tertiary allylic amines are described. The tertiary allylic amines were alkylated and subjected to base mediated rearrangement to yield a variety of highly functionalized tertiary homoallylic amines.

Chiral allenamides are prepared with high levels of enantiomeric purity by [2,3]-sigmatropic rearrangement of propargylic sulfimides. The required branched propargylic sulfides are prepared by an enantioselective organocatalytic aldehyde alpha-sulfenylation followed by Corey-Fuchs alkynylation.

An efficient enantioselective approach for preparation of quaternary centers from proline 5 building block has been developed through [2,3]-sigmatropic Stevens rearrangement co-catalyzed by ionic liquid. The sigmatropic rearrangement was stereospecific because the [2,3]-migrations were restricted to the same face, and the stereoselectivity arose from the previous N-alkylation step in 8. The approach showed better yields than described in literature. The use of 1-butyl-3-methylimidazolium hexafluorophosphate showed an improvement in the yields of the Stevens rearrangement due to a possible stabilization and/or activatation of zwiterionicspecies in solution by the ionic liquid. Several indolizidine moieties were synthesized from quaternary proline derivative (S)-5. Uma abordagem enantiosseletiva eficiente para a preparação de centros quaternários a partir da prolina 5 foi desenvolvida através do rearranjo [2,3]-sigmatrópico de Stevens, co-catalisado por líquido iônico. O rearranjo sigmatrópico foi estereoespecífico porque as migrações-[2,3] foram restritas à mesma face e a estereosseletividade surgiu na etapa preliminar da N-alquilação em 8. O método mostrou melhores rendimentos do que os descritos na literatura. O uso de hexafluorofostato de 1-butil-3-metilimidazólio mostrou uma melhora nos rendimentos do rearranjo de Stevens devido a possível estabilização e/ou ativação das espécies zuiteriônicas em solução, pelo líquido iônico. Diversas indolizidinas foram sintetizadas a partir do derivado (S)-5 da prolina quaternária.

The [2,3]sigmatropic rearrangement of α-substituted N,N-dimethylmethallylamine N-oxides results in a highly stereoselective formation of trans olefins oxygenating at the allylic position, and provi...

The first synthesis of the structurally unique marine natural product antheliolide A (1) has been accomplished by the pathway outlined in Scheme 1. The sequence is stereocontrolled and has led to the synthesis of (+/-)-1, and ent-1 as well as 1. The route contains a number of noteworthy or novel steps including (1) formation of the mixed acetal 7, (2) the diastereoselective bicyclization to form 8 in which stereocenters are correctly established at each carbon of the four-membered ring, (3) the chain extension 8 --11, (4) the efficient closure of the nine-membered ring of 12, (5) the mild oxidative cleavage sequence 14 --17, and (6) the successful and quick formation of the last three rings of 1 from aldehyde 17 via 18. The synthesis of 1 has also resulted in the clarification of its absolute configuration, which had not been determined previously.

Reaction of branched allylic sulfides with the N-Boc-oxaziridine 1 results in [2,3]-sigmatropic rearrangement of the intermediate allylic N-Boc-sulfimides with a high level of chirality transfer. The first example of formation of a quaternary stereocenter using this transformation is reported.

meso-Tetrakis(p-tolyl)porphyrinatoruthenium(II) carbonyl, [Ru(II)(TTP)(CO)], can effect intermolecular sulfonium and ammonium ylide formation by catalytic decomposition of diazo compounds such as ethyl diazoacetate (EDA) in the presence of allyl sulfides and amines. Exclusive formation of [2,3]-sigmatropic rearrangement products (70-80% yields) was observed without [1,2]-rearrangement products being detected. The Ru-catalyzed reaction of EDA with disubstituted allyl sulfides such as crotyl sulfide produced an equimolar mixture of anti- and syn-2-(ethylthio)-3-methyl-4-pentenoic acid ethyl ester. The analogous "EDA + N,N-dimethylcrotylamine" reaction afforded a mixture of anti- and syn-2-(N,N-dimethylamino)-3-methyl-4-pentenoic acid ethyl esters with a diastereoselectivity of 3:1. The observed catalytic activity of [Ru(II)(TTP)(CO)] for the ylide [2,3]-sigmatropic rearrangement is comparable to the reported examples involving [Rh(2)(CH(3)CO(2))(4)] and [Cu(acac)(2)] as catalyst. Similarly, cyclic sulfonium and ammonium ylides can be produced by intramolecular reaction of a diazo group tethered to allyl sulfides and amines under the [Ru(II)(TTP)(CO)]-catalyzed reaction conditions. The subsequent [2,3]-sigmatropic rearrangement of the cyclic ylides furnished 2-allyl-substituted sulfur and nitrogen heterocycles in good yields (90%). By employing [Ru(II)(TTP)(CO)] as catalyst, the cyclic ammonium ylide [2,3]-sigmatropic rearrangement reaction was successfully applied for the total synthesis of (+/-)-platynecine starting from cis-2-butenediol.

Boron trifluoride and BBr(3) mediated [2,3]-sigmatropic rearrangements of allylic alpha-amino amides have been developed affording secondary amines in good yields. (E)-Crotyl and (E)-cinnamyl alpha-amino amides 2b and 2c exhibit excellent syn-diastereoselectivity upon rearrangement with either Lewis acid. The allylic amine 2a forms upon treatment with BF(3) or BBr(3) a five-membered heterocylic complex in which a single halide anion has been displaced by the carbonyl oxygen atom. The structures of the Lewis acid-amine complexes were elucidated using NMR spectroscopy. A plausible reaction mechanism, based on DFT calculations, is presented. Thus, BF(3)- or BBr(3)-complexed allylic amines 2 are shown to preferentially proceed, after deprotonation, via an endo transition state.

Highly diverse organic reactions with hydroxylamine-based compounds have been developed during past ten decades or more and are now playing an important role in organic synthesis in a myriad of contexts. Nevertheless, we expect that there still remains useful and promising functionality for the compounds of this class. Hence, we have been making extensive efforts to disclose hidden potential of such compounds. In this report we will present some interesting and powerful organic transformations becoming available as the results of such our activities, which covers the following five novel processes or reactions : (1) silicon-tether strategy for controlling the regioselectivity and diastereoselectivity of intramolecular nitrone cycloadditions; (2) intramolecular Diels-Alder reactions employing hydroxamate tethers; (3) novel [2, 3] -sigmatropic rearrangement for carbon-nitrogen bond formation; (4) dicobalt octacarbonyl-promoted rearrangement of 4-isoxazolines to acylaziridines; (5) novel ene-like cycloisomerization reaction of nitrile oxides with a tethered allyltrimethylsilyl group.

A vanadium-catalyzed method for the oxidation of prochiral aryl, allylic selenides with tandem [2,3] sigmatropic rearrangement has been developed. This protocol has been screened on a series of substrates to test for its generality and effectiveness. The applicability of this methodology to the synthesis of enantiomerically enriched allylic alcohols has been studied on a series of chiral oxazole-containing systems with a diastereomeric ratio (d.r.) of up to 85 : 15. The chiral transfer observed in the allyl alcohol products is the result of a net 1,9- and/or 1,10-induction. Finally, the first example of a selenium–oxygen nonbonding interaction in oxazole-containing selenide appears to have been observed via X-ray crystal analysis.

The iron(II)-catalyzed Bach reaction of tert-butoxycarbonyl azide (BocN(3)) and allyl sulfides has been extended to include propargyl sulfides, which give N-allenylsulfenimide products. Using 10 mol % dppeFeCl(2) as catalyst the reaction proceeds at 0 degrees C with a number of different propargyl sulfides in 31-73% isolated yield. The reaction is limited by product instability toward catalyst and termination of the catalytic cycle by excess BocN(3). N-Allenylsulfenimide 2b smoothly undergoes catalytic hydrogenation and a Diels-Alder reaction with cyclopentadiene.

The iron(II)-catalyzed Bach reaction of tert-butoxycarbonyl azide (BocN(3)) and allyl sulfides has been extended to include propargyl sulfides, which give N-allenylsulfenimide products. Using 10 mol % dppeFeCl(2) as catalyst the reaction proceeds at 0 degrees C with a number of different propargyl sulfides in 31-73% isolated yield. The reaction is limited by product instability toward catalyst and termination of the catalytic cycle by excess BocN(3). N-Allenylsulfenimide 2b smoothly undergoes catalytic hydrogenation and a Diels-Alder reaction with cyclopentadiene.

Catalytic asymmetric sulfur ylide [2,3]-sigmatropic rearrangement of carbenoids generated from aryldiazoacetates has been investigated with a number of chiral Rh(II) and Cu(I) catalysts, and moderately high enantioselectivity (52-78% ee) can be achieved with Cu(MeCN)(4)PF(6)/2,2'-isopropylidenebis[(4S)-4-tert-butyl-2-oxazoline].

Addition of germanium dichloride dioxane complex to α,β-unsaturated acetals gives β-dichloroalkoxygermane enol ethers via a net [2,3]-sigmatropic rearrangement

The synthesis and characterization of four new steroidal spin labels, viz., 3'-[[(pregn-5-en-3 beta-ol-20S-yl)methyl]oate]- 2',2',5',5'-tetramethylpyrrolidine-1'-oxyl,3'-[[(pregn-4-en-3- one-20S-yl)methyl]oate]-2',2',5'.5'-tetra methyl pyrrolidine-1'-oxyl, 3 "[(3 beta-acetoxypregn-5-en-20-one-16 alpha-yl)prop-2'xi-ol-3'xi-oate]-2 ",2 ",5 ",5 "-tetramethyl pyrrolidine-1 "-oxyl and 3 "-[(pregn-5-en-3,20-dione-16 alpha-yl)prop-2'xi-ol-3'xi-oate]-2',2',5',5'-tetramethylpyrrolidine-1'-oxyl has been described which involves functionalization of the parent hormone at C-20 and C-16. The key step to all the products was the condensation of 3-carboxyproxyl with the derivatized synthons. (C) 1999 . .

A thioxanone-based [2,3]-sigmatropic rearrangement strategy which delivers Cβ-chiral pent-4-enoic acids with excellent Cβ-induction is reported