Synthesis of conjugated δ-lactams using ring-closing metathesis

Addition of allyl magnesium or metallyl magnesium bromide to the N-benzyl imines of benzaldehyde and cyclohexanone, followed by acylation with acryloyl or metacryloyl chloride provided the corresponding α,β-unsaturated amides. Ring-closing metathesis of the latter with ruthenium catalyst PhCHRuCl2(PPh3)2 in the presence of Ti(OiPr)4 provided excellent yields of the corresponding conjugated δ-lactams with both disubstituted and trisubstituted CC bonds. Some specific trisubstitution patterns, however, as well as tetrasubstituted CC bonds, were not obtained. In these cases, even the use of a second generation, imidazolylidene-substituted ruthenium catalyst at high temperature did not lead to success.