Stereoselective allylations of erythrulose derivatives under anhydrous conditions

We have investigated a number of nucleophilic additions or allylating reage nts to several alpha,alpha ',beta -trioxygenated ketones (O-protected eryth rulose derivatives), Reagents based on lithium. magnesium. copper and titan ium stereoselectivities and did not display any recognizable trend in the s ense of stereoselection. In contrast, reactions involving silicon and tin d erivatives were highly stereoselective and gage rise to essentially a singl e diastereoisomer, the structure of which depended group. Thus, alpha,beta -di-O-benzylated derivatives experienced almost exclusive addition to the c arbonyl Si on the type or protecting group side, whereas alpha,beta -O,O-al kylidene derivatives (dioxolane acetals) yielded the opposite diastereoisom ers as a result of addition to the Re side. These results Suggest the inter mediacy of a-chelates in the additions or silicon and tin reagents to the d i-O-benzylated derivatives. In contrast, the opposite stereoisomers. formed in the reactions of dioxolanes. are believed to be formed through Felkin-A nh transition states, pointing to the reluctance of acetal ox gens to parti cipate in chelated intermediates. (C) 2001 Published by Elsevier Science Lt d.