Rapid and efficient conversion of sialyl thioglycosides to sialyl esters via NIS/BF3OEt2-promoted glycosylation

A rapid and efficient one-step conversion of sialyl thioglycosides to sialyl esters was disclosed. Under the promotion of NIS and BF₃OEt₂, the glycosylation of per-acetylated sialyl thioglycoside with a set of carboxylic acids provided β-sialyl esters as the major products in good to excellent yields within 5 min. Compared with the long-chain alkyl-, aryl- and α,β-unsaturated acids, complete β-selectivities were observed when the short-chain alkyl acids were selected as the coupling partners. The resultant β-selectivity for the glycosylation of the per-acetylated sialyl thioglycoside with acetic acid was compromised when the 5-N,4-O-oxazolidinone protected sialyl thioglycoside was employed as the coupling partner.     

Synthesis and reactions of 3-carboxytropolone. Conversion to heterocycle-fused troponoid compounds

The haloform reaction of 3-acetyltropolone ( 1 ) afforded 3-carboxytropolone ( 2 ) which was treated with diazomethane to give 2-methoxy-3-methoxycarbonyltropone (3a) and 2-methoxy-7-methoxycarbonyltropone (3b). The tropolone 2 reacted with hydrazine to afford 2-hydrazino-3-hydrazinocarbonyltropone ( 10 ) or 2-hydrazinotropone ( 11 ), depending on the reaction time. The reaction of 2 with phenylhydrazine produced 3-hydroxy-1-phenyl-1,8-dihydrocycloheptapyrazol-8-one (14). The treatment of 2-methoxy-3-methoxycarbonyltropone (3a) with hydrazine or phenylhydrazine gave cyclization products 12 and 15 , respectively. The reaction of 2-methoxy-7-methoxycarbonyltropone (3b) with hydrazine, phenylhydrazine, or methylhydrazine gave 2-hydrazino- ( 13 ), 2-(2-phenylhydrazino)- ( 16 ), and 2-(2-methylhydrazino)-7-methoxycarbonyltropone ( 17 ), respectively.     

Synthesis of angularly fused cyclopentanoids and analogous tricycles via photoinduced ketyl radical/radical anion fragmentation-cyclization reactions

Angular fused tricycles were synthesized through intramolecular tandem fragmentation-cyclization reactions by photochemically induced electron transfer (PET) of tricyclic α-cyclopropyl ketones with an unsaturated side chain at the position γ to the carbonyl group. The reactions resulted in regioselective cleavage of a β-cyclopropyl bond with formation of angular fused tricyclic ring systems via ketyl radical/radical anions as reactive intermediates. In general, triethylamine (TEA) was used as a strong reducing reagent in acetonitrile. The preferred regioselectivity of the cyclization step (exo vs endo) depending on the substitution pattern at the quaternary carbon center (Cβ′) of the tricyclic α-cyclopropyl ketones was investigated. In addition, we also checked a two-step pathway for the synthesis of angular dioxa-triquinanes including photolysis of an allyloxy-substituted cyclopenta[c]furanone derivative and subsequent β-cleavage of the resulted dioxa-[4.5.5.5]fenestrane under reductive PET conditions.     

Synthesis of C-aryl-nucleosides and O-aryl-glycosides via cuprate glycosylation

2'-Deoxy-C-aryl-nucleosides have found increasing importance in studying DNA-DNA and DNA-protein interactions, as unnatural DNA-base pairs, and as oligonucleotide based fluorophores. Access to the required C-aryl-nucleosides is provided by several glycosylation methods. Though useful in many cases, these methods often have drawbacks such as low yields or low economic efficiency. The necessity for intensive optimization of the reaction conditions and/or the use of hazardous and toxic reagents can render C-glycosylation reactions cumbersome. Herein we describe a robust and highly efficient C-glycosylation method. It is shown that aryl cuprates of the Normant-type reliably react with Hoffer's chlorosugar to deliver C-aryl-nucleosides in up to 93% yield. This method may substitute for the previously employed coupling with organocadmium or -zinc species. Peculiar reactivities are reported for C-glycosylation of Gilman-type aryl cuprates which required substantial arene-specific optimization. Interestingly, the glycosylation of Gilman cuprates was found to provide access not only to C-aryl-nucleosides but also to O-aryl-glycosides. The reactions of Gilman cuprates with Hoffer's chlorosugar 1 in the presence of oxygen provided the coresponding O-aryl-2'-deoxyribosides in up to 87% yield without concomitant C-glycosylation.     

Synthesis of glycosyl phosphates from 1,2-orthoesters and application to in situ glycosylation reactions

[reaction: see text] A series of glycosyl phosphates were prepared in high yield by treatment of the corresponding 1,2-orthoesters with dibutyl phosphate. Glycosyl phosphates are efficient glycosylating agents even when used in crude form or when generated in situ. The immunodominant epitope trirhamnoside of group B Streptococcus was prepared to demonstrate the synthetic utility of the method.     

Synthesis of elongated cavitands via click reactions and their use as chemosensors

A very efficient modular reaction protocol was developed to attach various functionalities to a rigid cavitand scaffold. In this way, aryl, iodoaryl, benzyl, pyrrolidylmethyl groups, as well as a polyethylene-glycol chain were attached to the ‘triazol-level’ of the cavitand. The palladium-catalyzed ethynylation of the iodoarene moieties, followed by the copper(I)-catalyzed azide–alkyne cycloaddition produced novel cavitands with significantly elongated binding pockets. The dimensions of these molecules are calculated to be at least 9 Å×18 Å, which place them amongst the largest unimolecular hosts obtained by pure covalent synthesis. A cavitand-based click conjugate is used for selective complexation of Cu²⁺ and Fe³⁺, providing significant fluorescent quenching.     

Intramolecular polar addition reactions of active methylene moieties to aryl-substituted alkenes via photoinduced electron transfer

The synthesis of cycloalkane derivatives with ring sizes of 5-7 was achieved in moderate yields by photoirradiation of aqueous acetonitrile solutions, containing compounds that consist of active methylene moieties tethered to aryl-substituted alkenes along with sodium hydroxide and 9-cyanophenanthrene. When the substrate contains ethyl cyanoacetate as an asymmetric active methylene moiety, the reaction proceeds in a highly diastereoselective manner.     

Synthesis and evaluation of antiviral activity of L-acosamine and L-ristosamine nucleosides of furanose configuration.

Mercuric-catalyzed hydrolysis of acetylated L-rhamnal 1 gives an alpha,beta-unsaturated aldehyde 2. 1,4-Addition of DBU-phthalimide salt with concomitant acetyl shift resulted in L-ribo and L-arabino isomers of 5-O-acetyl-2,3,6-trideoxy-3-phthalimido-hexofuranose 3 and 4. After acetylation at the anomeric center, coupling with silylated thymine resulted in three new nucleosides, with L-acosamine and L-ristosamine of furanose configuration as the carbohydrate moiety. The target compounds have been evaluated for their antiviral activity against HIV and HSV-1.     

硫代糖苷在糖化学合成中的应用

介绍了近几年来有关硫代糖苷在一些复杂寡糖及其缀合物合成中的应用。     

Regio- and stereoselective synthesis of 2-deoxy-C-aryl glycosides via palladium catalyzed decarboxylative reactions

An efficient and versatile approach for the synthesis of 2-deoxy-C-aryl glycosides is reported. This strategy is based on a palladium-catalyzed decarboxylative Heck coupling reaction of benzoic acids and glycals. A wide variety of glycals and benzoic acids have been screened, and all these reactions could afford the desired C-aryl glycoside products in moderate to good yields with exclusive regio- and stereoselectivities.     

Synthesis of amidomethyltrifluoroborates and their use in cross-coupling reactions

Amidomethyltrifluoroborates were successfully synthesized in a one-pot fashion and used in cross-coupling reactions with a wide variety of aryl and heteroaryl chlorides.     

Synthesis of densely functionalized cispentacin derivatives through selective aziridination and aziridine opening reactions: orthogonally protected di- and triaminocyclopentanecarboxylates

A novel substrate-directed synthetic route to a series of highly functionalized, orthogonally protected di- or triaminocyclopentanecarboxylate derivatives with multiple chiral centres from an unsaturated bicyclic β-lactam has been accomplished by applying stereoselective ring C–C double bond aziridination with chloramine-T and phenyltrimethylammonium tribromide, followed by regioselective aziridine opening with different N,O nucleophiles and hydrides. The functionalization strategy was successfully extended for access to enantiomerically pure orthogonally protected triaminocarboxylates.     

Synthesis of heterocyclic compounds via Michael and Hantzsch reactions

Heterocycles represent the largest diversity of organic compounds with signifcant chemical, biomedical, and industrial applications. They exist in numerous natural products, dyes, and as scaffolds in diverse drugs and related pharmaceutically active substances. Substantial attention has been paid to develop various elegant methods to synthesize heterocycles. Among different strategies, Michael and Hantzsch reactions are considered as effective approaches for construction of heterocycles and their corresponding fused derivatives. This review covers this area especially in the last 10 years. The heterocyclic systems reported in this review are classified according to the kind of the heterocyclic systems.     

Synthesis of sugar-derived spiroaminals via lactamization and metathesis reactions

A series of sugar-derived spiroaminals has been synthesized by utilizing cross metathesis, ring closing metathesis and lactamization reactions as key steps from 1-C-alkylated glycosyl azides and important correlations in the spectral data between spiroaminals and their respective anomers are reported.     

Approaches to intramolecular sialylation. 2. Synthesis of per-O-acetylated thioglycosides of N-acetylneuraminic acid containing an unprotected carboxy group

Various approaches to the synthesis of per-O-acetylated thioglycosides of N-acetylneuraminic acid (Neu5Ac) containing an unprotected carboxy group starting from the corresponding methyl esters were comparatively studied. One-step demethylation of methyl thioglycoside (LiI, Py, reflux) proceeded inefficiently in contrast to the analogous smooth reaction of phenyl thioglycoside. An indirect route to derivatives with a free carboxy group involving saponification followed by acetylation of hydroxy groups proved to be more efficient for methyl -thioglycoside of Neu5Ac.