Nonproteinogenic amino acids:: an efficient asymmetric synthesis of (S)-(-)-acromelobic acid and (S)-(-)-acromelobinic acid

An efficient synthesis of (S)-(-)-acromelobic acid (1) and (S)-(-)-acromelobinic acid (2) is described via asymmetric hydrogenation protocol. Asymmetric hydrogenation of dehydroamino acid derivative 23 using (R,R)-[Rh(DIPAMP)(COD)]BF4 catalyst followed by removal of the protective groups afforded (S)-(-)-acromelobic acid (1) in >98% ee. The key intermediate 23 was prepared from citrazinic acid (8). The dehydroamino acid derivative 33 required for the synthesis of (S)-(-)-2 was prepared from 2,5-lutidine (27), which upon hydrogenation using (S,S)-[Rh(Et-DuPHOS)(COD)]BF4 catalyst afforded (S)-(+)-34 in 93% yield and >96% ee. Removal of protective groups in (S)-(+)-34 afforded (S)-(-)-acromelobinic acid (2) in good overall yield. (C) 2002 Elsevier Science Ltd. All rights reserved.