Small Ring Compounds in Organic Synthesis VI

Since the pioneering work by Sarel and co-workers on the iron carbonyl p- moted transformation of vinylcyclopropanes and related compounds [1],a - riety of transition metal complexes have been examined to achieve effective - tivation of the vinylcyclopropane-cyclopentene rearrangement which usually requires pyrolytic conditions. These reactions have been applied to natural product synthesis in some cases and have already been reviewed in several - cellent articles [2¿4]. Contrary to the well-established chemistry of the vinylcyclopropanes, the corresponding reactions of alkynyl- and propadienylcyclopropanes have not, until recently,received much attention.We present here a summary of the recent efforts towards the development of transition metal promoted transformation of these molecules with a brief survey of the corresponding thermal reactions. 2 Rearrangement of Alkynylcyclopropanols Unlike the well-known chemistry of the vinylcyclopropane-cyclopentene - arrangement, there is no general method for the rearrangement of alkyn- cyclopropane to cyclopentene derivatives.One specific example is the pyrolysis of 1-ethynyl-2-methylcyclopropane to methylenecyclopentene and other c- pounds [5].At 530°C, 1-ethynyl-2-methylcyclopropane (1) undergoes a [1,- hydrogen shift to give hexa-1,2,5-triene ( 2),which further isomerizes to met- lenecyclopentenes 3 and 4 in 38 and 29% yield,respectively (Scheme 1).