Synthesis of enantiomerically pure enones (2-benzyloxypyran-3-ones) derived from pentoses.

The useful synthons sugar enones (2-benzyloxypyran-3-ones) derived from pentoses have been prepared starting from 2-acetoxyglycals or benzyl pentopyranosides. The glycals were glycosylated with benzyl alcohol in the presence of a Lewis acid (SnCl or InCl) to give enantioenriched enones (ee = 80-90%). Under catalysis with InCl, benzyl 2-enopyranosides gave also the enones (ee = 87%). On the other hand, enantiomerically pure enones were synthesized via an improved straightforward and high yielding sequence (70% overall) from benzyl pentopyranosides. However, the yields of both, the glycosylation of glycals as well as some specific reactions of the sequence from glycosides, were lowered when a p-nitro substituent was introduced into the benzyl group. These routes became impractical in the case of p-acetamidobenzyl derivatives, because of the large extent of decomposition. Therefore, alternative sequences have been developed for the synthesis of 2-(p-acetamidobenzyloxy)pyran-3-ones.