Alkenes derivatives, five-membered ring structure

Double bond migration generally competes with ILZ-isomerization and the formation of carbene-derived products, unless it is structurally inhibited as in 2-norbomene (14) and homobrexene (38). It presumably occurs in cyclohexene (27) and cyclopentene (32) but is not observable. In an alkene such as 43, in which the double bond is exocycKc to a five-membered ring, migration is the dominant photoprocess and occurs... 

A very useful class of chiral auxiliaries has been developed for alkenes substituted with a heteroatom. These auxiliaries, attached to the heteroatom, allow for the preparation of enantiomerically enriched cyclopropanols, cyclopropylamines and cyclopropylboronic acids. Tai and coworkers have developed a method to efficiently generate substituted cyclopropanol derivatives using the cyclopropanation of a chiral enol ether (equation 78) . The reaction proceeds with very high diastereocontrol with five- to eight-membered ring sizes as well as with acyclic enol ethers. The potential problem with the latter is the control of the double bond geometry upon enol ether formation. A detailed mechanistic study involving two zinc centers in the transition structure has been reported. ... 

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