Studies on the biosynthesis of clavulanic acid. Part 4. Synthetic routes to the monocyclic β-lactam precursor, proclavaminic acid

Abstract

Aldol condensation of 3-substituted propionaldehydes with the lithium enolate of ethyl or benzyl (2-oxoazetidin-1-yl)acetate yielded derivatives of proclavaminic acid. The proportion of the threo diastereoisomer in the aldol product could be increased by thermodynamically controlled equilibration with 1,5-diazabicyclo[4.3.0]non-5-ene. In the case of benzyl 5-azido-3-hydroxy-2-(2-oxoazetidin-1-yl)valerate the diastereoisomers were separated and the threo diastereoisomer was resolved by enzymatic hydrolysis of the ester by subtilisin Carlsberg [EC 3.4.21.14]. Catalytic reduction of the unhyorolysed threo enantiomer yielded (2S, 3R)-5-amino-3-hydroxy-2-(2-oxoazetidin-1-yl)valerate which had spectroscopic properties identical with those of natural proclavaminic acid and which was a substrate for clavaminic acid synthase. Two crystalline, derivatives of (2S, 3R)-proclavaminic acid were prepared for X-ray analysis.