Cyanoketenes. Stereochemistry of the 2-azetidinones resulting from the cycloadditions of cyanoketenes to formimidates

Mechanistic Investigation of the Ring Opening in the Staudinger Cycloaddition Involving Ketenes with Electron‐Withdrawing Substituents

“…Therefore, the 3-substituted b-lactams 3-acetylazetidin-2-one 1a, ethyl 2-oxazetidine-3-carboxylate 1b, and 2-oxazetidine-3-carbonitrile 1c were prepared by reported procedures (Scheme 1). Moore and co-workers confirmed previously that cis-2-oxoazetidine-3carbonitriles are major products in the reduction of 3-halo-2-oxoazetidine-3-carbonitriles with Zn/AcOH as reductant [6c] [12]. Ethyl carboxylate (AE)-1b was synthesized from the reaction of the ethyl half ester of malonic acid and Nbenzylideneaniline in the presence of di(1H-imidazol-1-yl)methanone (CDI) in CH 2 Cl 2 [10].…”

“…Thus, to verify this assumption, we hoped to prepare model b-lactams with p-acceptor substituents at C(3) and to investigate their reactions in the presence of different bases. Although it was reported that 3-chloro-2-oxoazetidine-3-carbonitriles were smoothly reduced to 2-oxoazetidine-3-carbonitriles with Zn/AcOH as reductant [6c] [12], 3-bromo-2-oxoazetidine-3-carbonitrile 4 produced a messy product mixture under the same reduction conditions. trans-3-Acetylazetidin-2-one (AE)-1a was obtained from the reaction of diketene and N-benzylideneaniline (¼ N-(phenylmethylene)benzenamine) in the presence of 1H-imidazole in CH 2 Cl 2 [9].…”

Carbon‐13 and proton nuclear magnetic resonance studies of (E)‐ and (Z)‐ azetidinones

“…The E or Z configuration of the azetidinones was determined either from the coupling constants observed at the H-3 and H-4 vicinal protons (1,3,5,7,9,11,21 3J(HH)truns=2 Hz; 2,4,6,8,10,12,22, 3J(HH)cis f 6 Hz), or from NOE studies, which were made possible by the favourable methyl substituent at C-3 in polysubstituted compounds (13-20; nuclear Overhauser enhancements were of magnitude of 10-15% for Zisomers, while no enhancements were observed for E isomers). Table 1 lists the I3C chemical shift values for the ring carbon atoms, C-2, C-3 and C-4, of the azetidinones, together with the sums of the chemical shift values for C-2, C-3 and C-4, calculated in order to investigate a potential Perlin-Koch type relntionship.…”