Effective asymmetric synthesis of the key chiral building blocks of 20(S)- and 20(R)-camptothecins

An effective diastereoselective synthesis of (S)- N,N-diethyl-2-formyl-2- (methoxymethoxy)butanamide and (S)-2-formyl-2-(methoxymethoxy)butanoic acid ethyl ester, which are two key chiral building blocks for the synthesis of 20(S)-camptothecins, has been developed by employing an asymmetric bromolactonization using (R)-proline. The (R) compounds were also synthesized to obtain 20(R)- camptothecin.