Pyruvic acid acetals (l-carboxyethylidene acetals) linked to aldohexopyranosyl residues are known to occur in a number of polysaccharides. These include agarl, and exocellular2 and capsular polysaccharides'. Pyruvic acid is most often linked to O-4 and O-6 of such hexoses as o-glucose, D-mannose, D-galactose, and Z-acetamido-2-deoxy-D-glucose, and is also found linked to such vicinal atoms as O-3 and O-4 of D-galactose and L-rhamnose, and O-2 and O-3 of o-glucuronic acid.
Pyruvic acid acetals of hexopyranosides were first synthesized by Gorin and Ishikawa4 by acetalation with 1-acetoxy-2-propanone in the presence of acid, followed by platinum-oxygen oxidation. However, the total yields from these two steps were not satisfactory for preparative purposes s. In the study described in this communication, the method6 of acetal formation between a ketone and a trimethylsilyl ether of a diol in the presence of trimethylsilyl triflate was successfully applied to synthesis of the pyruvate acetals of several hexopyranosides, using tert-butyldimethylsilyl ethers of diols. Among them, the 4,4-acetals of a-D-mannopyranosides and the 3,4-acetals of a-D-galactopyranosides were synthesized for the first time.
The carbonyl function in a carboxylic ester can form the acetal, giving the orthoester, as proved in the synthesis of glycosylideneglycoses', including such orthosomycin antibiotics as* destomycin C However, in acetalations with pyruvate, only the keto function reacts, to give the desired pyruvate acetals as depicted in Scheme 1.
Methyl 2,3-di-0-benzyl-o-D-hexopyranosides of the &co (3) manno (S)+, and galacto (7) configurations and methyl 2,6-di-O-benzyl-~-D-galactopyranoside (IO) were silylated with beak-butylch~orodimethylsilane and imidazoie in N,N-di-*To whom correspondence should be addressed. +The diol5 was prepared in 75% yield from the known methyl 4,6-di-0-ally)-ar-D-mannopyranoside7 by conventional benzyfation and 0-deallylation.