C. Frechou

Asymmetric synthesis of polyhydroxylated natural products I. Efficient preparation of L-arabinitol

Abstract A new approach to sugar synthesis is demonstrated through the synthesis of L-arabinitol. Reduction of allylic β-ketosulfoxides followed by hydroxylation of the double bond in the resulting allylic β-ketosulfoxides are the two important asymmetric steps involved in the process.

Stereospecific hydroxylation of chiral allylic β-hydroxysulfoxides: Applications to the asymmetric synthesis of optically active vicinal triols

Abstract Chiral allylic β-hydroxysulfoxides 2 have been hydroxylated by the osmium tetroxide catalyzed reaction. The reaction can be highly stereoselective depending on the nature of the substituant linked to the double bond and the configurations of the sulfoxide and hydroxylic groups. The diastereoselectivity can be as high as 90%.

A new route to C-glycosyl compounds. Wittig-type reaction promoted by zinc

In recent years, methods for the preparation of C-glycosyl compounds have become increasingly important in synthetic organic chemistry. These compounds are useful as subunits for the synthesis of biological active products’ and as potential enzyme inhibitors’. Significant attention has been focused on the development of new routes to prepare functionalized C-glycosyl compounds that are synthetic precursors of more complex C-glycosyl compounds. The starting materials can be furanose or pyranose carbohydrates, the main problem being the stereoselective introduction of a functionalized carbon atom at C-l. A widely used procedure to obtain carbon-carbon bonding at the anomeric position of carbohydrates was first reported by Zhdanov ef al. 3 This two-step procedure involves the formation of an unsaturated, open-chain intermediate, followed by a cyclization leading to the expected C-glycosyl compound. The first step is a Wittig reaction with protected furanose and pyranose hemiacetals. When stabilized ylides are used4, subsequent cyclization of the unsaturated intermediate may occur by treatment with bases or, sometimes, spontaneously. But when the Wittig reaction takes place with unstabilized ylides, the cyclization can be obtained by the iodoor mercuro-cyclization process’. It should be noted that C-glycosyl compounds can be synthesized in the same way when phosphonates are used instead of phosphorane

Réaction d'Ylures du soufre sur des sucres réducteurs: Application à la synthèse d'hydroxyméthyl C-glycosides.

. Lastly, although stereoselective routes exist for both C-aand /I-glycosyl compounds, they mainly lack a general application. Stereoselectivity indeed depends on carbohydrate structures, ylides, and solvents. In the case of 2,3,4,6-tetra-O-benzyl-a,/?-D-glucopyranose (l), Nicotra et al.’ have observed an abnormal reaction of (carbethoxymethylene)triphenylphosphorane leading solely to a diene. The same elimination by Wittig reaction was observed for 3,4,6-tri-O-acetyl-2-O-benzyl-a&D-glucopyranose and partial elimination for 2,3,4,6-tetra-U-acetyl-a&D-glucopyranose. These abnormal reactions led Allevi et af.6 to use a Wittig-Horner reaction with 1 and sodium triethylphosphonoacetate

Réactions de type Wittig sur des aldoses partiellement protégés: Synthèses d'aldooctènoates et octènonitriles de stéréochimie E acycliques

Abstract Treatment of reducing carbohydrates with unstabilized sulfur ylides affords epoxy alcohols. When the reaction is not accompanied by elimination of a leaving group at C-3, spontaneous cyclisation occurs, and hydroxymethyl C-glycosyl compounds are obtained.

Synthèse directe d'azotures de glycosyle

Abstract Mono or di-protected aldoses react with methyl bromoacetate or bromoacetonitrile, in presence of tri-n-butylphosphine and zinc, to give stereoselectively with good yields the corresponding E unsaturated Wittig adducts.

Synthèses de C-glycosides : utilisations d'un dérivé de l'arsenic

Azidation of some unprotected aldoses with the Ph3P-N-chlorosuccinimide-LiN3 system leads regioselectively to glycosyl axides ; 1,2-trans compounds are obtained stereoselectively.

Réaction d'aldoses partiellement protégés avec des ylures d'arsenic stabilisés: synthèse de dérivés insaturés E acycliques et de dérivés C-glycosyle

Abstract Reaction of arsenic ylides with pyranose and furanose carbohydrates mainly leads to E-olefinic derivatives. Cyclisation of the olefinic derivatives obtained from 2,3,4,6-tetra-O-benzyl-D-glucose and D-galactose, promoted by ZnBr2, gives C-glycosyl compounds with high stereoselectivity. One the other hand, reaction of the corresponding arsonium salt, in presence of zinc, with a furanose derivative gives directly and stereoselectively the β-C-glycosyl compound.

Bromation régiosélective d'itols non protégés

Abstract The reaction of partially protected aldoses with (carbomethoxymethyl)triphenylarsonium bromide and zinc in toluene gave E α,β-unsaturated acyclic esters. When intramolecular transest erification occurred, bicyclic 1,4-lactone derivatives were formed concurrently. Otherwise, using these non alcaline conditions, olefination did not favour the formation of C -glycosyl derivatives. On the other hand, when the reaction was performed with (carbomethoxymethylene)triphenylarsorane in toluene, followed by the addition of n -butyllithium, bicyclic derivatives were obtained rapidly in good yields. Moreover, when cyanomethyltriphenylarsonium bromide was used in place of (carbomethoxymethyl)triphenylarsonium bromide, the corresponding E aldooctenonitriles were produced in satisfactory yields.

Reduction of chiral .beta.-hydroxy sulfoxides: application to the synthesis of both enantiomers of 4-substituted butenolides

Abstract Hydrobromide acid in acetic acid or 1-bromocarbonyl-1-methyl-ethylacetate allows the transformation of unprotected pentitols and hexitols into linear 1,5 and 1,6-dibromo derivatives in high yields.