Stereoselective allylations of erythrulose derivatives under anhydrous conditions

Abstract

We have investigated a number of nucleophilic additions of allylating reagents to several α,α′,β-trioxygenated ketones ( O-protected erythrulose derivatives). Reagents based on lithium, magnesium, copper and titanium gave low to medium 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 gave rise to essentially a single diastereoisomer, the structure of which depended on the type of protecting group. Thus, α,β-di- O-benzylated derivatives experienced almost exclusive addition to the carbonyl Si side, whereas α,β- 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 α-chelates in the additions of 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 again to the reluctance of acetal oxygens to participate in chelated intermediates.