The synthesis and degradation of benzyl 4,6- O-benzylidene-2,3-dideoxy-3- C-ethyl-2- C-hydroxymethyl-α- d-glucopyranoside and -mannopyranoside

Abstract

The use of carbohydrates for establishing, by synthesis, the absolute configuration of branched aliphatic alcohols is demonstrated by the synthesis and degradation of carbohydrate derivatives that contain two branch points. Benzyl 4,6- O-benzylidene-2,3-dideoxy-3- C-ethyl-2- C-hydroxymethyl-α- d-glucopyranoside ( 23) and -mannopyranoside ( 24) were formed from benzyl 2,3-anhydro-4,6- O-benzylidene-α- d-mannopyranoside ( 17) by a reaction sequence that involved ring-opening with ethylmagnesium chloride, oxidation, epimerisation, methylenation, and hydroboronation. The gluco isomer 23 was converted into (+)-( R)-2,3-bisacetoxymethylpentyl acetate ( 1) by sequential hydrogenolysis, borohydride reduction, periodate oxidation, borohydride reduction, and acetylation. The synthesis of 1 provides confirmatory evidence for the absolute configuration of the alkaloid pilocarpine ( 2). Unidentified products, and not the expected free-sugars, were obtained by acidic hydrolysis of methyl 4,6- O-benzylidene-2,3-dideoxy-3- C-ethyl-2- C-hydroxymethyl-α- d-glucopyranoside ( 8) and -mannopyranoside ( 9). Convenient syntheses of benzyl α- d-glucopyranoside derivatives are described.