Transition states double-three-membered-ring

Photochemical or thermal extrusion of molecular nitrogen from ot-diazocarbonyl compounds generates a-carbonylcarbenes. These transient species possess a resonance contribution from a 1,3-dipolar (303, Scheme 8.74) or 1,3-diradical form, depending on their spin state. The three-atom moiety has been trapped in a [3 + 2] cycloaddition fashion, but this reaction is rare because of the predominance of a fast rearrangement of the ketocarbene into a ketene intermediate. There are a steadily increasing number of transition metal catalyzed reactions of diazocarbonyl compounds with carbon-carbon and carbon-heteroatom double bonds, that, instead of affording three-membered rings, furnish hve-membered heterocycles which... 

A third one-step addition reaction to C=C double bonds that forms three-membered rings is the epoxidation of olefins with percarboxylic acids (Figure 3.14). Suitable percarboxylic acids must, however, not be (too) explosive. Thus, aromatic percarboxylic adds are preferable. Until recently one epoxidized almost exclusively with mefa-chloroper-benzoic acid (MCPBA). An alternative has become magnesium monoperoxyphthalate (MMPP). In the transition state of this type of epoxidation, four electron pairs are shifted simultaneously (which is a record in this book except for the Corey-Winter elimination in Figure 4.42). 

The effect of substitution is also evident in the epoxidation of verticillene (21)153 at the 7,8-double bond giving 22. Once again, the conformation of the transition state for the preferred mode of attack is, ideally, puckered. The additional three-carbon bridge prevents total ring inversion additional substitution of the double bonds limits conformational mobility. Thus, even for this 14-membered ring system high diastereoselectivity is predicted. Experimental evidence so far, however, indicates only that no second diastcrcomcr of the 7,8-cpoxidc 22 has been isolated from the reaction mixture. 

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