2,3-DIAZA-1,3-DIENES (AZINES) AS SUBSTRATES FOR THE STAUDINGER REACTION - SYNTHESIS AND REACTIVITY OF N-IMINO-BETA-LACTAMS

The reaction of aromatic and aliphatic azines with different ketene pr ecursors, such as the acid chloride/Et(3)N system, alkoxychromium(0) c arbenes, and free diphenyl ketene, gives N-imino-beta-lactams in good to excellent yields, with good levels of cis,trans-selectivity. A wide variety of symmetrically-substituted azines derived from aldehydes an d ketones are compatible with the Staudinger reaction. Chiral N-imino- beta-lactams derived from symmetrically or unsymmetrically (mixed) chi ral azines are also obtained in good yields as essentially single enan tiomers (de > 95%). Different reaction intermediates, including hemiam inals, oxadiazols, and hydrazides have been isolated. Free diphenyl ke tene forms Diels-Alder adducts and N-acylazadienes in addition to the previously reported N-imino-beta-lactams, The usual reactivity of the beta-lactam ring is modified in N-imino-beta-lactams by the presence o f the imino group. Thus, beta-hydrazonoesters, N-alkylamino-beta-lacta ms, and NH-beta-lactams can be efficiently obtained by base-catalyzed 2-azetidinone ring opening, catalytic hydrogenation, and ozonolysis, r espectively.