Benzylidenation of l-Sorbose

Abstract

Abstract The acid-catalyzed benzylidenation of l-sorbose (I) was studied. The reaction by Reichstein’s procedure gave a mixture of two diastereomeric 2,3:4,6-di-O-benzylidene-α-l-sorbofuranoses (III and IV), which were then successfully separated by preparative thin-layer chromatography. The use of slightly acidic conditions yielded l,3:4,6-di-O-benzylidene-β-l-sorbofuranose (XI), l,2:4,6-di-O-benzylidene-α- (XVII and XVIII) and-β-l-sorbofuranoses (XIX and XX), 1,2-O-benzylidene-α-l-sorbopyranoses (XXI and XXII), and 1,3-O-benzylidene-l-sorbose (XXIII). The absolute configurations at the acetal center of the five-membered benzylidene rings were assigned. A reaction sequence (I→(XXI, XXII\ightleftarrowsXXIII)→XI→IV→III) comparable with that of the ethylidenation of I was established.