Oxazoline N-oxide mediated [2+3] cycloadditions. Application to a formal synthesis of (+)-carpetimycin A

Cycloaddition between gamma,delta-unsaturated beta-enamino ester 9 and camp hor-derived oxazoline N-oxide 8 afforded a single adduct, 14, Dipolarophile 9 proved to be very reactive despite the substitution on the double bond. Stereoselective sodium cyanoborohydride reduction of the imminium intermedi ate 14a gave rise stereoselectively to beta-amino ester derivative 15a. Oxi dative acidic hydrolysis, oxidation of the resulting aldehyde 18, deprotect ion, and cyclization afforded the beta-lactam 23, a direct precursor of (+) -carpetimycin A.