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From a selection of research topics carried out in our laboratory during the last twenty years it becomes apparent that our main target was the discovery of new or improved synthetic methods together with new properties. Our efforts were made with the aim of being of some utility to other fields of research, with particular emphasis to glycobiology and heterocyle-based bioorganic chemistry. We performed new chemistry mainly in the field of carbohydrate manipulations taking as a primary rule the simplicity and efficiency manners. Toward this end, modern synthetic tools and approaches were employed such as heterocyle-based transformations, multicomponent reactions, organocatalysis, click azide–alkyne cycloadditions, reactions in ionic liquids, click photoinduced thiol-ene coupling, and click sulfur–fluoride exchange chemistry. With these potent methodologies in hand, the syntheses of carbohydrate containing amino acids up to proteins glycosylation were performed.1 Heterocyclic Glycoconjugates and Amino Acids2 Triazole-Linked Oligonucleotides: Application of Click CuAAC3 Non-Natural Glycosyl Amino Acids4 Non-Natural Oligosaccharides5 Calixarene-Based Glycoclusters6 Carbohydrate-Based Building Blocks7 Homoazasugars and Aza-C-disaccharides8 Synthesis of Glycodendrimers9 Peptide and Protein Glycoconjugates10 Conclusions

Two orthogonal, metal free click reactions, enabled to glycosylate ubiquitin and its mutant A28C forming two protein scaffolds with high affinity for BambL, a lectin from the human pathogen Burkholderia ambifaria. A new fucoside analogue, with high affinity with BambL, firstly synthetized and co-crystallized with the protein target, provided the insights for sugar determinants grafting onto ubiquitin. Three ubiquitin-based glycosides were thus assembled. Fuc-Ub, presented several copies of the fucoside analogue, with proper geometry for multivalent effect; Rha-A28C, displayed one thio-rhamnose, known for its ability to tuning the immunological response; finally, Fuc-Rha-A28C, included both multiple fucoside analogs and the rhamnose residue. Fuc-Ub and Fuc-Rha-A28C ligands proved high affinity for BambL and unprecedented immune modulatory properties towards macrophages activation., Metal free click reactions used to glycosylate ubiquitin and its mutant A28C afforded two protein scaffolds with high affinity for Burkholderia ambifaria lectin (BambL).

Coupling of sulfonyl fluorides, fluorosulfates and iminosulfur oxydifluorides with proteins gave the corresponding sulfonamide, sulfamate and sulfamide adducts under mild conditions.

The Sulfur(VI) Fluoride Exchange (SuFEx), revived by Sharpless and co-workers from an unrecognized state, is an emerging new click reaction that is based on the high reactivity of sulfonyl fluorides and fluorosulfates with suitable nucleophiles such as silyl ethers and amines. Hence, we comment in this Perspective on the use of SuFEx for the synthesis of a new family of sugar containing sulfonamides from the reaction of a glycosylsulfonyl fluoride with aliphatic amines. We also highlight the applications of SuFEx to multivalent scaffolds such as tetraamino- and tetrafluorosulfonyl-calixarene leading to sulfonamide-linked sugar and iminosugar clusters. Finally, we report on the chemoselective functionalization of bovine serum albumin (BSA) which, owing to the amino group of lysine moieties, reacted with SO2F2 to give a multivalent BSA–SO2F system. The PEGylation of the same protein by coupling with a PEG-fluorosulfate is described as well.

Although the preparation of anomeric glycosylsulfonyl fluoride was unsuccessful, a tetra-O-acetylated C-glucosylpropanesulfonyl fluoride was synthesized starting from the corresponding thioacetate via sulfonate formation as the key intermediate. This sulfonyl fluoride was a bench-stable product that reacted promptly with primary and secondary alkylamines at 80 °C to give the corresponding sulfonamides in good yield. On the other hand, the same fluoride was inert toward arylamines whereas its precursor, the sulfonyl chloride, showed good reactivity. Another limitation of the acetylated sugar sulfonyl fluoride was its lack of reactivity with a multivalent aminated calixarene, this being due to acetyl transfer from the carbohydrate moiety to the amino groups of the scaffold. Fortunately, the tetra-O-benzylated C-glucosylpropanesulfonyl fluoride, prepared by the same reaction sequence employed for the synthesis of the acetylated analogue, reacted with the tetra-aminopropyl-calix[4]arene to afford the corresponding sulfonamide-linked sugar cluster in high isolated yield. A similar approach to the synthesis of calixarene-based iminosugar clusters was unsuccessful because 1-deoxynojirimycin sulfonyl fluoride derivatives could not be generated. However, a tetra-propylsulfonyl fluoride calixarene, obtained from the free-OH calix[4]arene through a three-step reaction sequence, underwent clean coupling with both C-glucosylpropylamine and N-aminopentyl-1-deoxy-deoxynojirimycin derivatives to give the corresponding tetravalent sugar and iminosugar clusters. This metal-free click reaction may constitute a valuable tool in the arsenal of ligation tools for the synthesis of multivalent carbohydrate architectures.

Protein glycosylation is the most complex post-translational modification process. More than 50 % of human cells proteins are glycosylated, whereas bacteria such as E. coli do not have this modification machinery. Indeed, the carbohydrate residues in natural proteins affect their folding, immunogenicity, and stability toward proteases, besides controlling biological properties and activities. It is therefore important to introduce such structural modification in bioengineered proteins lacking the presence of carbohydrate residues. This is not trivial as it requires reagents and conditions compatible with the protein's stability and reactivity. This work reports on the introduction of lactose moieties in two natural proteins, namely ubiquitin (Ub) and l-asparaginase II (ANSII). The synthetic route employed is based on the sulfur(VI) fluoride exchange (SuFEx) coupling of a lactose tethered arylfluorosulfate (Lact-Ar-OSO2 F) with the ϵ-NH2 group of lysine residues of the proteins. This metal-free click SuFEx reaction relies on the properties of the fluorosulfate employed, which is easily prepared in multigram scale from available precursors and reacts chemoselectively with the ϵ-NH2 group of lysine residues under mild conditions. Thus, iterative couplings of Lact-Ar-OSO2 F to Ub and ANSII, afforded multiple glycosylations of these proteins so that up to three and four Lact-Ar-OSO2 groups were introduced in Ub and ANSII, respectively, via the formation of a sulfamoyl (OSO2 -NH) linkage.

The alkyne hydrophosphonylation is an atom economical method for the synthesis of vinylphosphonates that are compounds of both biological and synthetic relevance. Recent approaches based on the use of metal catalysts have only partially solved the problems of the reaction selectivity as well as of economical and large amount production. We report in this Letter on a metal-free approach that leads to E-vinylphosphonates in high yields. The approach is based on a two-step one-pot synthesis, namely: (a) photoinduced anti-Markovnikov addition of a H-phosphonate to alkynes to give mixtures of E- and Z-vinylphosphonates, and (b) cysteine promoted photoinduced conversion of the crude mixtures of adducts into E-isomers (58–92% overall yields).

Mannose-centered galactoclusters specific for lectin I of Pseudomonas aeruginosa (LecA) were synthesised by a combination of phosphoramidite chemistry and metal-free thiol click chemistry (i.e., thiol addition to acrylamide or nucleophilic displacement of bromine in a bromoacetamide group by a thiol function). These thiol click reactions were performed with microwave assistance in the presence of Et3N and with use of a reducing agent to avoid disulfide formation. Nine tetravalent galactoclusters containing different linkers (aliphatic, oligoethyleneglycol or aromatic) were synthesised with a DNA tag. Their binding to LecA was monitored in a DNA-based glycoarray and compared with that of a galactocluster synthesised by copper-catalyzed azide–alkyne cycloaddition. The results indicated stronger binding of all galactoclusters relative to the monovalent galactoside but slightly weaker binding than that shown by the galactocluster incorporating a triazole ring, due to a favourable interaction of the latter with proline 51 of LecA.

Alkyne hydrothiolation – that is, the addition of the SH bond of a thiol across the carbon-carbon triple bond of an unactivated alkyne – belongs to the ample class of hydrofunctionalization reactions whose main feature is their occurrence with total atom economy. Hence this reaction is emerging as a valuable tool for the preparation of sulfur-containing compounds such as vinyl thioethers, which are of interest for their own biological properties and for their use as intermediates in total synthesis. Thus, the first part of this review deals with the various efforts directed towards the preparation of vinyl thioethers in a regio- and diastereoselective manner by metal-catalyzed approaches in recent years. The number of methods employed, with use of a variety of catalysts and reaction conditions, allow the synthesis of vinyl thioethers to be carried out with high efficiency and selectivity. The second part of the article is based on the use of the photoinduced free-radical approach to hydrothiolation, which allows the introduction of two thiol fragments across the carbon-carbon triple bond, thereby leading to the formation of dithioethers. This approach turned out to be especially useful as a ligation tool for the installation of densely functionalized arrays on various scaffolds and complex biomolecular systems. Concluding remarks emphasize the role of the photoinduced free-radical strategy as a complementary tool to copper-catalyzed azide-alkyne cycloaddition.

A hydrolytically stable mimetic of the tumour antigen GM3 lactone is used to decorate multivalent scaffolds. Two of them positively interfere on melanoma cell adhesion, migration and resistance to apoptosis (anoikis). Notably, their ability to hamper melanoma-cells adhesion and reduce the metastatic potential is enhanced when the two scaffolds, presenting a different shape, are used in combination. © 2013 Elsevier Ltd.All rights reserved.

A new polyhedral oligomeric silsesquioxane (POSS) derivative with a periphery of eight PEGylated chains functionalized with terminal propargyl groups was synthesized starting from commercially available octavinyl-POSS. The photoinduced free-radical coupling of this octapropargyl POSS derivative with various sugar thiols enabled the preparation of globular hexadecavalent glycoclusters. Thus, it appears that according to the alkyne hydrothiolation mechanism, two thiyl radicals were added across each triple bond of the POSS scaffold side-chains. The affinities of some of the densely glycosylated clusters towards certain lectins were measured by the Enzyme-Linked Lectin Assay (ELLA). The binding selectivity of Concanavalin A between the hexadecavalent mannosylated and glucosylated clusters was much higher than the selectivity observed for the corresponding octavalent glycoclusters (ref.4). Moreover, the affinity of the N-acetylglucosamine-based cluster towards wheat germ agglutinin (WGA) revealed a remarkable glycoside cluster effect with up to a 9.0 × 105-fold increase in binding compared to monovalent GlcNAc. As a multivalent effect of the same order of magnitude was reported for an octavalent GlcNAc cluster towards the same lectin (ref.4), it is concluded that increasing the number of sugar units around the cubic platform does not lead systematically to an enhancement of binding affinity.

The free-radical hydrothiolation of alkynes (thiol-yne coupling, TYC) unites two thiol fragments across the carbon-carbon triple bond to give a dithioether derivative with exclusive 1,2-addition; this reaction can be used for modification of peptides and proteins allowing glycoconjugation and fluorescent labeling. These results have implications not only as a flexible strategy for attaching two modifications at a single site in proteins but also for unanticipated side-reactions of reagents (such as cycloalkynes) used in other protein coupling reactions.

Hydroamination of unsaturated carbon-carbon bonds is a useful C-N bond forming reaction as it leads to products (amines, imines, enamines) of high synthetic value. While the hydroamination of terminal alkenes and alkynes under metal catalysis has been widely developed, the corresponding reactions with internal substrates remain a challenge. Two recent studies by Buchwald and by Dong which focus on enantioselective hydroamination of internal alkenes and alkynes respectively are of significant importance. [43,44] The results reported broaden the scope of hydroamination as a useful C-N bond forming process and allow acess to chiral amines which are difficult to synthesize by other methods.

Selectins play a key role in the inflammatory cascade. The interaction with their physiological ligands containing the tetrasaccharide sialyl Lewisx (sLex) leads to the recruitment of leukocytes from the vascular system to the site of injury. To facilitate the interaction under the shear stress conditions present in the blood vessel, the conformation of sLex is stabilized via lipophilic inter-residual contacts. sLex and two analogs were synthesized and evaluated for selectin binding, average conformation, and conformational dynamics. We could show that the methyl group in L-fucose is optimally suited to stabilize the sLex core through an interaction with the β-face of D-galactose and thus enables binding to the selectins under shear stress conditions.

New synthetic routes to chiral polyhydroxylated aldehydes from lower terms employing 2-substituted thiazoles as homologating reagents are described. In all cases the thiazole ring serves as a convenient formyl group equivalent.

The macrocyclization of linear d-galacto-2-heptulopyranose-containing oligoketosides has been carried out by intramolecular glycosidation and ring-closing metathesis. The aglycon fragment of the cyclic neglycoconjugates thus formed was an alkylidene or a polyether chain. One of the oligoketoside–crown ethers showed a moderate asymmetric induction in the Cram model phenyl acetate–acrylate addition.

Non-natural axially and equatorially linked C-glycosyl alpha-amino acids (glycines, alanines, and CH2-serine isosteres) with either S or R alpha-configuration were prepared by D- and L-proline-catalyzed (de95%) alpha-amination of C-glycosylalkyl aldehydes using dibenzyl azodicarboxylate as the electrophilic reagent.

Large-scale resolution of racemic monastrol has been carried out by normal-phase mode HPLC on an amylose-based chiral stationary phase. Because monastrol solubility, in media of proper compositions for normal-mode HPLC separations (in terms of retention factors and selectivity), was significantly low and impractical for preparative scopes, racemic monastrol was transformed into the corresponding O-tert-butyldimethylsilyl derivative. The tert-butyldimethylsilyl group was chosen as a suitable derivatizing agent because it induced approximatively a six-times higher solubility and allowed for an almost quantitative recovery of pure monastrol from the derivatization-deprotection sequence. The competitive isotherms of the O-tert-butyldimethylsilyl compounds, measured through frontal analysis, were fitted to competitive Langmuir and four-parameter bi-Langmuir models. The equilibrium dispersive model of chromatography was used for modeling the nonlinear separation of the racemate and to optimize the experimental conditions for collection of highly concentrated fractions of pure (R,S)-O-tert-butyldimethylsilyl compounds, from which significant amounts of the corresponding enantiomers of monastrol (about 100 mg of each enantiomer with 30 runs on an analytical-scale column) were obtained by quantitative back-derivatization.

Nach einer ersten Phase orientierender Studien zur asymmetrischen Organokatalyse, die vornehmlich der Untersuchung wichtiger Modellreaktionen der organischen Synthese galten, sind die Forschungen auf diesem Gebiet nun in einem Stadium angelangt, in dem speziellere Aufgabenstellungen und insbesondere auch stereochemische Probleme gezielt angegangen werden konnen. Dieser Aufsatz diskutiert ausgewahlte Arbeiten aus dem Jahr 2006 und der ersten Halfte von 2007 und soll vier Hauptaspekte hervorheben, an denen sich der gegenwartige Stand und die Perspektiven der Organokatalyse ablesen lassen: 1) chemische Effizienz; 2) Auffinden neuer Substratkombinationen fur die Entwicklung neuer asymmetrischer Synthesen; c) Entwicklung neuer Katalysatoren fur spezielle Anwendungen auf der Grundlage mechanistischer Erkenntnisse; d) Einsatz organokatalytischer Reaktionen in asymmetrischen Totalsynthesen von bisher unzuganglichen Naturstoffen und von bekannten biologisch und pharmazeutisch aktiven Verbindungen.

The first study of a copper(I)-catalyzed azide-alkyne click reaction in ionic liquids (ILs) is reported. The cycloaddition of a sugar azide with a sugar acetylene (CuI, i-Pr2EtN, 80 degrees C) was carried out in 10 ILs as well as in standard molecular solvents (toluene and DMF) to give the 1,4-disubstituted triazole-linked C-disaccharide. The highest yields (84 and 95%) were registered in Ammoeng 110 and [C(8)dabco][N(CN)(2)]. The latter solvent was recycled in four subsequent reactions without loss of the reaction efficiency. Reactions carried out in the absence of the Hünig's base afforded mixtures of 1,4- and 1,5-disubstituted triazole regioisomers.

Ring closure via microwave-assisted intramolecular OMs displacement by a γ-OBn group (O-nucleophilic attack) in protected polyhydroxylated N-Boc-thiazolylalkyl amines afforded C-furanosides (37-81%) featuring a chiral thiazolylmethylamino side chain, which, upon thiazole to carboxylate (through aldehyde) transformation, furnished enantiopure C-furanosyl glycines.

The first paper on the hydrophosphonylation of terminal alkenes was published in 1958 by Stiles and coworkers. Afterwards various papers described organometal-catalyzed and free-radical reactions leading to linear anti-Markovnikov adducts and/or branched Markovnikov products. In 1996 Han and Tanaka reported the first example of alkyne hydrophosphonylation catalyzed by a palladium complex. Further studies using other metal catalysts registered poor selectivity as mixtures of adducts were obtained in most of the cases examined. The first example of alkyne hydrophosphonylation by H-phosphonates under free-radical conditions leading to Z- and E-vinylphosphonates in a 1 : 1 ratio was reported by our group. Nevertheless, Z- to E-isomerization took place upon irradiation in the presence of a thiol.

The chromatographic behavior of a set of racemic dihydropyrimidines (DHPMs) has been evaluated on two polysaccharide-derived chiral stationary phases under normal phase conditions. One of these is coated, the other chemically immobilized. The outstanding solvent compatibility of the immobilized chiral stationary phase (CSP) permits the use of solvents such as ethyl acetate (EtOAc) that are unsuitable for coated supports, for which traditional 2-propanol:hexanes mixtures have been employed. Drastic changes in the chromatographic retention and resolution of DHPMs and, in general, in the separation performances have been observed for the two systems. From a practical point of view, EtOAc has been proven to be a better choice for the separation of this important class of compounds. By comparing molecules different in specific positions of their scaffolds, hypotheses concerning the role of individual chemical groups on retention and selectivity have been done. These effects have been quantified, in some cases, in terms of standard Gibbs energy variations. Even if no chromatographic measurements have been made under nonlinear conditions, clear indications of the potential use of immobilized chiral adsorptive media operated with EtOAc:hexanes mixtures for preparative separations of DHPMs have been evidenced.

A method for the construction of substituted pyranoses by means of the hetero-Diels–Alder (HDA) reaction of ethyl vinyl ether with 1-oxabuta-1,3-dienes bearing a thiazolyl ring at C-2 is described. The cycloaddition with 1-(thiazol-2-yl)-2-penten-1-one (2) occurred with good endo/exo selectivity to give cis- and trans-3,4-dihydro-2H-pyrans 3a and 3b in a ca. 9:1 ratio and 91% overall yield. The elaboration of 3a through the conversion of the thiazole ring into the formyl group and reduction of the latter to alcohol, followed by hydroxylation of the double bond through hydroboration–oxidation led to the ethyl 2,3-dideoxypyranoside 8. The asymmetric version of this synthetic sequence started from the HDA cycloaddition of the same alkene with the chiral oxabutadiene 10 bearing the D-galacto-pentopyranosid-5-yl moiety at C-3. This reaction afforded a mixture of the four diastereomeric cycloadducts—3,4-dihydro-2H-pyrans 11a,b and 12a,b—in 97% overall yield. The reaction was moderately endo and face selective. A high level of endo selectivity (96%) was obtained by the use of catalytic Eu(fod)3. The elaboration of the endo cycloadducts 11a and 12a by the same synthetic sequence as that developed for 3a (i.e. thiazolyl-to-formyl conversion and hydroxylation of the double bond) gave the uncommon C-disaccharides 15 and 16 featuring two directly linked pyranose rings.

The development of new methods for the preparation of C-glycosyl beta-amino acid libraries with chemical and stereochemical diversity levels was investigated and the results are described herein. Two complementary one-pot three-component Mannich-type and Reformatsky-type synthetic strategies have been developed for the construction of chiral 3-amino propanoate fragments (eventually bis-substituted at C-2) directly linked to the anomeric carbon of pyranose and furanose residues. Both methods involved as the initial step the coupling of a sugar aldehyde to p-methoxybenzylamine but differed in the nucleophile (a d(2) synthon equivalent) which was successively added: a ketene silyl acetal (Mannich route) or a bromozinc enolate (Reformatsky route). Individual C-glycosyl beta-amino esters were isolated as single 3R diastereoisomers in fair to excellent yield (60-90%) and their structure assigned by NMR spectroscopy (Riguera protocol) supported by X-ray crystallography. A tentative explanation of the observed stereochemical outcome based on transition-state models is provided. A preliminary study on the synthesis of alpha,alpha-difluoro C-glycosyl beta-amino acids via a more traditional Reformatsky route is also reported.

[Reaction: see text]. A collection of 13 unsymmetrical ketones, each one featuring a sugar (d-glucosyl, d-galactosyl, d-mannosyl, and l-fucosyl) and an aglycone moiety (phenyl, 2-thiazolyl, TMS-ethynyl, allyl, and 1-propenyl) was prepared by a uniform route based on the use of benzothiazole as a carbonyl group equivalent. Succinctly, C-glycosylbenzothiazoles readily prepared by addition of 2-lithiobenzothiazole to sugar lactones and deoxygenation, were subjected to a one-pot reaction sequence involving N-methylation of the heterocyclic ring by MeOTf, treatment of the N-methylbenzothiazolium salt with a Grignard reagent, and HgCl(2)-promoted hydrolysis of the benzothiazoline thus formed. The resulting ketones were isolated in yields varying from 35 to 80%. Treatment of the sugar ketones with various organometals containing the phenyl, 2-thiazolyl, TMS-ethynyl, or ethynyl group as a substituent afforded chiral tertiary alcohols. These addition reactions were highly stereoselective as observed by crude NMR analysis and isolation of a single epimer in high yield in each case examined. However, because of the complexity of the reagents involved, the stereochemical outcome of these reactions appears to be difficult to rationalize by simple classical steric models, thus, ab initio studies taking into account the role of the sugar fragment are advisable. An interesting synthetic elaboration of a propargylic alcohol containing the thiazole ring into a propargylic alcohol bearing the formyl and carboxylate groups is reported.

A general access to a novel class of sugar α-amino acids composed of iminofuranose and iminopyranose residues anomerically linked to the glycinyl group through an alkyl chain is described. A set of eight compounds was prepared by the same reaction sequence involving as an initial step the Grubbs Ru−carbene-catalyzed cross-metathesis (CM) of various N-Cbz-protected allyl C-iminoglycosides with N-Boc-vinyl- and N-Boc-allyloxazolidine. The isolated yields of the CM products (mixtures of E- and Z-alkenes) varied in the range 40−70%. Each mixture was elaborated by first reducing the carbon−carbon double bond using in situ generated diimide and then unveiling the N-Boc glycinyl group [CH(BocNH)CO2H] by oxidative cleavage of the oxazolidine ring by the Jones reagent. All amino acids were characterized as their methyl esters. The insertion of a model C-iminoglycosyl-2-aminopentanoic acid into a tripeptide via sequential carboxylic and amino group coupling with l-phenylalanine derivatives was carried out as a demo...

Chiral 3,4-dibenzyloxy-5-hydroxymethyl-2-thiazolylpyrrolidines under Mitsunobu conditions (R-OH, Ph 3 P, DIAD in THF, 80 °C) afforded the corresponding R-protected pyrrolidines and 2-deoxy-piperidines in different ratios depending on the stereochemistry of the starting pyrrolidine and the nature of the acid R-OH. A mechanistic scheme is proposed involving the formation of an aziridinium ion as an intermediate. A piperidine derivative obtained in 74% yield was converted in four steps into the title allonojirimycin.

A novel and versatile strategy for the synthesis of heterocyclic alpha-amino acids has been described. The use of components (aldehyde or beta-ketoester) bearing a masked glycinyl moiety in Biginelli and Hantzsch cyclocondensations allowed access to the 4-dihydropyrimidinyl-alpha-glycines, 4-dihydropyrimidinyl-alpha-alanines, 4-pyridyl-alpha-alanines, and 2-pyridyl-alpha-alanines classes. Dihydropyrimidinyl-amino acids were obtained as a mixture of diastereoisomers due to the formation of the stereocenter at C4 of the dihydropyrimidinone ring. Individual stereoisomers were isolated as pure compounds and their structures were assigned with the aid of X-ray crystallography and chiroptical properties. The enantiomeric purity of a representative selection of the above amino acids was greater than 96% as verified by derivatization to the corresponding Mosher's amides and subsequent (1)H and (19)F NMR spectroscopy. Incorporation of the 4-pyridyl-alpha-alanine derivative into a peptide chain is also described.

The first paper on the hydrophosphonylation of terminal alkenes was published in 1958 by Stiles and coworkers. Afterwards various papers described organometal-catalyzed and free-radical reactions leading to linear anti-Markovnikov adducts and/or branched Markovnikov products. In 1996 Han and Tanaka reported the first example of alkyne hydrophosphonylation catalyzed by a palladium complex. Further studies using other metal catalysts registered poor selectivity as mixtures of adducts were obtained in most of the cases examined. The first example of alkyne hydrophosphonylation by H-phosphonates under free-radical conditions leading to Z- and E-vinylphosphonates in a 1 : 1 ratio was reported by our group. Nevertheless, Z- to E-isomerization took place upon irradiation in the presence of a thiol.

The introduction of carbohydrate moieties at either the lower or upper rim of calixarenes provides a special class of glycoconjugates that are referred to as glycocalixarenes or calixsugars. The most commonly employed scaffolds are calix[4]arenes, but substrates with larger cavities such as calix[8]arenes are also occasionally employed. The assembly of calixsugars is carried out with the use of different carbon–carbon and carbon–heteroatom bond-forming reactions so that the carbohydrate units are linked to the calixarene scaffolds through a variety of tethers. The ligation means employed are (1) the Mitsunobu reaction, (2) O-glycosylation, (3) S-alkylation, (4) amide bond formation, (5) palladium-catalyzed coupling, (6) Wittig olefination, (7) Cu(i)-catalyzed azide–alkyne cycloaddition, (8) nucleophilic substitution in tosyltetrazoles, (9) isothiocyanate–amine addition and (10) thiol–ene coupling. In addition to the synthetic efforts, molecular recognition studies have also been carried out, mainly towards plant and human lectins.

The use of glycosylated reagents in Hantzsch-type cyclocondensation reactions leading to C-glycosylated dihydropyridines (DHPs) has been investigated. A three-component approach with an anomeric sugar aldehyde (galacto, manno, and ribo derivatives), a β-keto ester, and an aminocrotonate afforded C(4)-glycosylated DHPs in high yield (70–90%). A two-component cyclocondensation approach based on different glycosylated β-amino acrylates (sugar enamines) and an enone derived from the Knoevenagel condensation between benzaldehyde and ethyl acetoacetate was followed for the preparation of C(6)-glycosylated 4-phenyl-substituted DHPs in fair yields (60–70%). The latter compounds were obtained as mixtures of diastereoisomers owing to the asymmetric induction of the sugar moiety in the formation of the C(4)-stereogenic center of the DHP ring. The diastereomer excess of the major products varied from 30 to 60%. The structure of selected compounds was determined by X-ray crystallography and by chiroptical measurements. The two-component cyclocondensation method was also employed for the preparation of a C(6)-ribofuranosyl-containing analogue of the well-known hypotensive agent nifedipine.

The introduction of a formyl group at the anomeric center of 2,3,5-tri-O-benzyl furanoses and substitution of the ring oxygen with a basic nitrogen atom (aminohomologation) was carried out via stereoselective addition of 2-lithiothiazole to N-benzyl, N-furanosylhydroxylamines (masked N-benzyl sugar nitrones), followed by reductive dehydroxylation of the resulting open-chain adducts, and then ring closure via intramolecular displacement of the free hydroxy group by the amino group and unmasking of the formyl group from the thiazole ring. The resulting formyl aza-C-glycosides were transformed into 2,5-dideoxy-2,5-imino-hexitols (pyrrolidine homoazasugars) by reduction of the formyl to the hydroxymethyl group and removal of the O- and N-benzyl groups by hydrogenolysis. This reaction sequence was applied to four furanoses (D-arabino, D-ribo, D-lyxo, L-xylo) to give the hydroxy- and amino-free homoazasugars, including the natural product 2,5-dideoxy-2,5-imino-D-mannitol, in 17% overall yields (six steps). The formyl aza-C-glycosides proved to be valuable intermediates for the synthesis of more complex derivatives. In fact, these sugar aldehydes were employed in Wittig-type coupling reactions with galactose and ribose phosphoranes to give bis-glycosylated alkenes, which upon reduction of the double bond were transformed into methylene isosteres of (1--6)- and (1--5)-linked disaccharides in which one of the two sugar moieties was an azasugar (aza-(1--x)-C-disaccharides).

Heating a mixture of [60]fullerene, N-methylglycine (sarcosine), and a sugar aldehyde in refluxing toluene resulted in the formation of a complex mixture of products from which the fulleropyrrolidine monocycloadduct was isolated in 14, 10, and 12% yield for formyl C-galactopyranoside, formyl C-glucopyranoside, and formyl C-mannofuranoside, respectively.

Formyl C-glycosides, prepared via the thiazole-based formylation of sugar lactones in three steps, are readily condensed with N-benzylhydroxylamine and hydroxylamine to give the corresponding glycosyl nitrones and oximes respectively. The latter imino derivatives are the precursors to nitrile oxides via the N-bromosuccinimide method.

Alkene and alkyne functional polyester-based dendrimers of generation 1 to 4 have been prepared and reacted under free-radical conditions with 2-acetamido-2-deoxy-1-thio--D-glucose (GlcNAc-SH). As ...

The total synthesis of SPP-100 and its C-5 epimer involves the construction of the β-amino alcohol segment via addition of the Grignard reagent derived from 3-aryl-2-isopropyl-1-chloropropane to the nitrone functional group installed at C-4 of the pseudoephedrine spiroanellated γ-butyrolactone derivative.

The addition reaction of diethyl phosphite to O-silylated N-benzyl nitrones derived from chiral α-alkoxy and N-Boc α-amino aldehydes has been studied as a stereoselective carbon phosphorus bond forming process for the synthesis of polyhydroxylated α-amino and α,β-diamino phosphonates. Key intermediates are the corresponding N-hydroxy α-amino phosphonates.

A new entry to (1→6)- and (1→5)-aza- C -disaccharides featuring the pyrrolidine ring as the imino sugar component is described via Wittig condensation of polyhydroxylated 2-formylpyrrolidines with galactopyranose 6-phosphorane and ribofuranose 5-phosphorane, respectively.

The title C-pentagalactoside featuring β-(1→6) ethylene bridges between the d -galactopyranose units has been prepared through four consecutive cycles comprising the Wittig reaction of a formyl C-galactoside building block with sugar phosphorane substrate followed by the introduction of a methylphosphonium group at the nonreducing end; the target product was obtained by hydrogenolysis of the tetraene pentasaccharide.

The coupling of α- or β- d -linked lithium C -glycosyl acetylides with N -Boc d -serinal acetonide followed by the reduction of the triple bond, deoxygenation, and oxidative cleavage of the oxazolidine ring afforded α- and β-anomer pairs of C -glycosyl α-aminopentanoic acids ( gluco , manno , galacto , and 2-acetamido- galacto ).

A highly effective method for the synthesis of formyl C-glycosides is described via addition of 2-lithiothiazole or 2-lithiobenzothiazole to sugar lactones, deoxygenation of the resulting ketols, and releasing of the formyl group from the heterocyclic ring. The synthetic utility of these sugar aldehydes is demonstrated by the development of synthetic methods to more elaborate C-glycosides. These include various (1-6)-C-disaccharides and some higher oligomers up to a C,C,C,C-pentasaccharide; C-glycosyl amino acid isosteres of N-glycosyl asparagines; the methylene isostere of β-Δ-galactosyl ceramide; linear and cylic (2-1)-ketoside oligomers. An alternative synthesis of ethylene-bridged glycosyl asparagine isosteres is illustrated by the use of ethynyl C-glycosides as starting materials.