Influence of the protecting groups on the syn/anti stereoselectivity of boron aldol additions with erythrulose derivatives. A theoretical and experimental study

We have investigated a series of aldol additions of protected l-erythrulose derivatives mediated by dicyclohexyl boron chloride. The syn/anti stereoselectivity has been found to depend on the type of protecting groups on the hydroxyl functions at C-3 and C-4. Thus, erythruloses benzylated at these hydroxyl groups gave only syn aldols while the corresponding benzoylated derivatives gave anti aldols under the same reaction conditions. The resident chirality of the enolate promoted a complete internal 1,3-induction, which was syn in both aldol types. Mechanistic proposals are advanced with support of both theoretical calculations and experimental data.