Vinylcyclopropane- cyclopentene rearrangement stereoselectivity

The presence of additional unsaturation in vinylcyclopropane (56) was invoked as an explanation for its facile nickel(0)-catalyzed rearrangement to vinylcyclopentene (57) in excellent yield (Scheme 12). The geometry of the diene in (56) turned out not to be important as the ring opening led to the equilibration of ( )- and (Z)-isomers prior to the final clostire. The reaction was therefore stereoselective, but cyclopentene (57) suffered, in some cases, isomerization to (58) under the reaction conditions. 

Palladium-catalyzed rearrangements of dienylcyclopropanes have also been reported. Vinylcyclopropane (64) gave cyclopentene (65) without regard to the ( /(Z) composition of the diene (equation 27). Similar results with respect to (E) and (Z) mixtures were also observed in the more-substituted case of (66) with moderate stereoselectivities (3 1) observed upon reclosure (equation 28). It has been suggested that the palladium-catalyzed rearrangement is in fact a nucleophilic-like opening followed by reclosure. Such transformations are discussed in Section 8.1.3.4 in the context of the stepwise opening of activated vinylcyclopropanes with iodide and other nucleophiles. 

Further studies on the scope and stereochemical course of the oxyanion-accelerated vinylcyclopropane reairangement were reported in 1981. This paper introduced a general [4+1] annulation strategy for the synthesis of cyclopentene derivatives in which the anion-accelerated VCP rearrangement functions as the key step. In this report, the accelerated version of the vinylcyclopropane rearrangement was also shown to proceed with remarkably high stereoselectivity, in further contrast to the thermal process. 

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.). ... 

An interesting rearrangement, which appears to be anion-accelerated, takes place in the enol thioether, anion-terminated vinylcyclopropanes of type 14. ° The rearrangement proceeds at — 78 C and is reasonably stereoselective with regard to the final cyclopentene products (syn selectivity 16 1). Regioisomers are encountered in the formation of the dihydrothiopyran cycloaddition adducts 13 in several instances. The mechanism of this rearrangement appears to involve the enol thioether anion in accord with the well-documented donor acceptor principles " and may be related to similar rearrangements observed with trimethylsilyl enol ether terminated vinylcyclopropanes under fluoride ion or Lewis acid catalysis." " ... 

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