Stereoselective allylations of erythrulose derivatives under anhydrous conditions

We have investigated a number of nucleophilic additions or allylating reagents to several alpha,alpha ',beta -trioxygenated ketones (O-protected erythrulose derivatives), Reagents based on lithium. magnesium. copper and titanium stereoselectivities and did not display any recognizable trend in the sense of stereoselection. In contrast, reactions involving silicon and tin derivatives were highly stereoselective and gage rise to essentially a single diastereoisomer, the structure of which depended group. Thus, alpha,beta -di-O-benzylated derivatives experienced almost exclusive addition to the carbonyl Si on the type or protecting group side, whereas alpha,beta -O,O-alkylidene derivatives (dioxolane acetals) yielded the opposite diastereoisomers as a result of addition to the Re side. These results Suggest the intermediacy of a-chelates in the additions or silicon and tin reagents to the di-O-benzylated derivatives. In contrast, the opposite stereoisomers. formed in the reactions of dioxolanes. are believed to be formed through Felkin-Anh transition states, pointing to the reluctance of acetal ox gens to participate in chelated intermediates. (C) 2001 Published by Elsevier Science Ltd.