Cyclopen tenes rearrangement

An improved version of the Carroll reaction, the ester enolate Carroll rearrangement, was reported in 1984 by Wilson and Ptice. Dianions of allylic acetoacetates, generated by treatment with 2 equiv. of LDA at -78 °C in THF, were rearranged at room temperature or 65 C to yield >keto acids in 40-80% yield (equation 12). In the course of a synthesis of the sesquiterpene isocomene, Snider and Beal used this method for the rearrangement of acetoacetate (73), prepared in 83% yield from reaction of cyclopen-tene (72) with diketene and a catalytic amount of DMAP (Scheme 11). The ( )-isomer of ketone (74) is obtained stereospecifically, since there is a severe steric interaction between the methyl groups in the Carroll rearrangement transition state leading to the (Z)-isomer. 

Lewis acid catalysis of the vinylcyclopropane rearrangement has been reported for vinylcy-clopropane 3, a key intermediate in the synthesis of the plant hormone antheridogen-An. Most recently, a report has appeared describing a highly stereoselective, diethylaluminum chloride promoted rearrangement of vinylcyclopropanes 5 to cyclopentenes 6. The cyclopen tenes appear to be formed directly, in some cases as a consequence of the rhodium-promoted cyclopropanation of enol ethers (see Section 2.4.3.1.3.). ... 

Treatment of a vinylcyclopropane substituted with two geminal electron-withdrawing groups on the three-membered ring with catalytic amounts of a palladium(O) complex gives a zwitte-rionic 7r-allyl complex, which reacts regioselectively but not stereoselectively to form a vinyl-cyclopen tene 5 13. This method represents a mild alternative to the thermal vinylcyclopropane rearrangement. 

In this review, we will concentrate on the E-Z-photoisomerization and subsequent (photo)chemical transformations of cycloalkenes caused by direct and sensitized excitation. Valence isomerization, rearrangement, and fragmentation, observed upon irradiation of cyclopropene, cyclobutene, and cyclopen-tene, are only briefly described, as they have been extensively reviewed elsewhere. - - - Although the photochemistry of acycKc alkenes will not be included in this chapter, we will refer to the photobehavior of conjugated cycloalkenes, such as cycUc dienes, enones, and styrenes, as far as it relates to the cycloalkene functionahty. As the photochemical formation of labile (E)-cycloalkenes is one of the major topics in this chapter, the physical properties of the (E)-isomers will be tabulated and discussed in detail. Recent studies on enantiodifferentiating photoisomerization of cycloalkenes are summarized in one section in view of the rapidly growing interest in asymmetric photochemistry. ... 

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