Synthesis of conformationally restricted serine derivatives through ruthenium(II)-catalyzed ring closing metathesis.

Stereoselective synthesis of alpha-amino-beta-hydroxy acids where the alpha-carbon of the amino acid is incorporated into a five-, six- or seven-membered ring is described. The stereoselective control at the chiral auxiliary carbon results from stepwise alkenylation and aldol formation using metalated species of the chiral auxiliary (R)-2,5-dihydro-3,6-dimethoxy-2-isopropyrazine. Ring closing metathesis was effected by ruthenium(II)-catalysis. The spiro-cycloalkene intermediates were transformed into 1-amino-2-hydroxycycloalkene-1-carboxylic acid derivatives by mild acid hydrolysis. (C) 1997 Elsevier Science Ltd.