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dichroi'sm after derivatisation.A pyranose pathway degradation leads to a 1,2,3-pentane triol, configuration of which can be confirmed by circular It is well established that L-glycero-D-manno-heptopyranose is an important component of the core region of many polysaccharides of Gram negative bacterial. In order to synthesize this compound, various groups have realized many suitable nucleophilic additions on conveniently protected D-manno-hexodialdopyranoside. Thus, K. Dziewiszck and A. Zamojski2 described CI extension of D-mannopyranose by treatment of methyl 2,3,4-tri-0-benzylu-D-mannohexodialdo-l,5-pyranoside with hydrogen cyanide; the reaction was not stereospecific. A great stereospecificity was observed in this series only with organomagnesium compounds as nucleophilic reagents : G.J.P.H. Boons and coL3 have obtained a stereospecificC1 extension of D mannose using isopropoxydimethylsilylmagnesium bromide on the same aldehyde as reported also by Garegg and coL4 A Cp stereospecific extension of D-mannose was also described by Chapleur and coL5 using vinyl magnesium bromide on methyl 2,3-O-isopropylidene-4-O-benzylu-D-mannohexodialdo-l,5-pyranoside.The unique compound obtained in each of these experiments can be transformed in L-glycero-Dmannoheptopyranose. Its respective configuration was proved by filiation with diethyl dithioacetal of L-glycero-Dmannoheptose6 or by NMR spectroscopy of an adequate bis cyclic aceta15. We have prepared stereospecifically the methyl 2,3,4-tri-O-benzyl-7,8-dideoxy-L-glycerou-Dmanno-oct-7-ynopyranoside 2 with a good yield (90%) by reaction of ethynylmagnesium bromide at -1 0" in tetrahydrofuran on methyl-2,3,4-tri-O-benzyla-D-mannohexodialdopyranoside 1. This new compound could also be transformed in L-glycero-D-mannoheptopyranose. Here we report the procedure used for the determination of its configuration, because this procedure could be applied to compounds resulting of a C2 homologation of any alkyl hexodialdo-l,5-pyranoside.Catalytic hydrogenation of the methyI-2,3,4-tri-Obenzyl-7,8-dideoxy-glycero-a-D-manno-oct-7-ynopyranoside 2 with palladium on carbon (10%) in ethanol gave methyl-7,8-dideoxy-L-glycero-~z-D-mannopyranoside 3. Periodic oxydation of 3 led to a dialdehyde 4, which was not separated but visualized on thin layer chromatography. This aldehyde was reduced to the complex triol 5 which after acid hydrolysis gave the pentane triol 6. The yield was 80% from the starting material 2 (Scheme 1).The pentane triol 6 was successively converted to 1 -anthroyl ester 7, then to 2,3 bis paramethoxycinnamoyl ester 8 as described by Nakanishi'. The global yield of these reactions was 60% after purification of the products by chromatography and preparative thin layer chromatography. Careful analysis showed that a unique product resulted from each esterification*. The triester was characterized by its NMR spectrum and its elementaryanalysis. It showed a characteristic circular dichroi'sm curve (CD) (Scheme 2) proving a 2R,3S configuration for the pentane triol 6; carbon 3 ...