Diels-Alder cycloaddition/ring-closing metathesis

Interestingly, treatment of the allylic carbonate 23, which had proven problematic in the previous study, under analogous reaction conditions with the copper enolate derived from 24 furnished the a,/9-disubstituted ketone. Subsequent ring-closing metathesis furnished the 1,2-cyclohexenes 25a/25b in 75% overall yield favoring the trans-dia-stereomer 25a (2° 1°=30 1, ds=10 l) [14]. Overall, this reaction provides an alternative approach to an exo-selective Diels-Alder cycloaddition, and indicates that a-substituted enolates are even more tolerant nucleophiles than the unsubstituted derivatives. 

During the present decade, a wide variety of polycyclic carbacephem derivatives have been reported starting from readily available monocyclic /3-lactams, which after transformation in more functionalized compounds and further cyclization yielded different fused carbacephems. Several approaches for the preparation of fused carbacephem derivatives including cycloaddition reactions such as the [2+2], 1,3-dipolar, and Diels-Alder reactions, as well as transition metal-catalyzed reactions such as the Pauson-Khand and ring-closing metathesis (RCM) reactions have been reported in the literature. 

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