Cyclopentene, 1-methylcyclopropanation

Unlike the well-known chemistry of the vinylcyclopropane-cyclopentene rearrangement, there is no general method for the rearrangement of alkynyl-cyclopropane to cyclopentene derivatives. One specific example is the pyrolysis of l-ethynyl-2-methylcyclopropane to methylenecyclopentene and other compounds [5]. At 530°C, l-ethynyl-2-methylcyclopropane (1) undergoes a [1,5]-hydrogen shift to give hexa-l,2,5-triene (2), which further isomerizes to methy-lenecyclopentenes 3 and 4 in 38 and 29% yield, respectively (Scheme 1). 

The thermal rearrangement of vinylcyclopropane to cyclopentene was uncovered in I96090 91. That vinylcyclopropanes, like other cyclopropanes, may undergo cis, trans iso-merizations was inferred in 1964 when trans-l-vinyl-2-methylcyclopropane was thermally converted to mostly (4Z)-1,4-hexadiene, a product formed at much lower temperatures from cw-1-vinyl-2-methylcyclopropane92. The reversible interconversion of the cis and trans isomers of l-vinyl-2-d-cyclopropane (equation 2) was reported soon thereafter, in 196793"96. Additional examples, including cases showing both geometrical isomerization and enantiomerization processes, soon followed. 

Use of chiral cyclopropylethynyllithium derivatives permits the elegant selective synthesis of labeled chiral vinylcyclopropanes, for stereochemical studies of the thermal vinyl cyclopropane-cyclopentene rearrangement237. Thus, reductive elimination of (1 S,trans)-(2,2-dibromoethenyl)-l-methylcyclopropane with BuLi in pentane, followed by hydrolysis of the lithium acetylide, afforded (15,/ram)-2-ethynylmethylcyclopropane (equation 157). 

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