B. Alcaide et al., 2,3-DIAZA-1,3-DIENES (AZINES) AS SUBSTRATES FOR THE STAUDINGER REACTION - SYNTHESIS AND REACTIVITY OF N-IMINO-BETA-LACTAMS, Journal of organic chemistry, 59(26), 1994, pp. 8003-8010
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.