Cyclization-cleavage rearrangements

Recently Reported Studies of Organomagnesium Rearrangements Equilibria in Grignard Cyclization-Cleavage Rearrangements... 

The most impressive array of new Grignard cyclization-cleavage rearrangement examples has come from studies on the intermolecular addition of Grignard reagents to alkenes. In 1975, Lehmkuhl and coworkers published extensive descriptions of their work in this area (46, 47). If an allylic Grignard reagent adds to an alkene, the addition product... 

Along with a very wide synthetic application the Cope rearrangement continues to be a subject of intense debates. The key mechanistic question is whether the rearrangement of 1,5-hexadiene derivatives is concerted and passes via a six-electron aromatic transition state, or whether it involves the formation of a diradical intermediate, i.e. a cyclization-cleavage mechanism. In the former case, bond making and bond breaking occur synchronously (a survey of this question has been published210). 

Endo-skeletal rearrangements also take place with 1,6-enynes, bnt the proposed mechanism is just a variation of the exo-single-cleavage rearrangement. Formed by endo cyclization, bicyclo[4.1.0]hept-4-ene derivatives arise in some cyclizations of 1,6-enynes by proton loss and protodemetalation ofthe endo cyclopropylcarbene. " That is the case from 1,6-enynes tethered as sulfonamides and in the intramolecular cyclization of 1,6-enol ethers with alkynes (equation 40). ... 

Cyclization to an organomagnesium compound sufficiently less stable to be present only in small amounts at equilibrium, however, can lead to rearrangement. The organomagnesium-halide reagent either at the left or at the right in Eq. (u) forms a ca. 1 8 mixture of these reagents, by cyclization-cleavage. 

The pyrazole ring is resistant to oxidation and reduction. Only ozonolysis, electrolytic oxidations, or strong base can cause ring fission. On photolysis, pyrazoles undergo an unusual rearrangement to yield imidazoles via cleavage of the N —N2 bond, followed by cyclization of the radical iatermediate to azirine (27). 

FIGURE 6 Speculative mechanism of Crl hydrocarbon biosynthesis from fatty acid hydroperoxides in algae. Homolytic cleavage of the hydroperoxide is assumed to give an allyl radical, which cyclizes to the thermolabile (1S,2R)-cyclopropane. The sequence is terminated by transfer of a hydrogen radical from C(16) to the -X-0 function. The cyclopropane rearranges to (6S)-ectocarpene as shown in Figure 4. 

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