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Although the use of cyclopropanes in synthesis has been an area of very considerable interest for many years, much less attention has been drawn to the potential and actual uses of cyclopropenes. This may perhaps be a reflection of the perceived difficulty in handling such species, but probably also results from the rather limited routes to cyclopropenes, and in particular those bearing other functional groups, compared to the variety available which lead to the saturated analogues. However, the development of more flexible routes to cyclopropenes makes available not only the exploitation of those reactions which are peculiar to the unsaturated system but also, through stereocontrolled addition to the double bond, provides more efficient routes to cyclopropanes of particular stereochemistry. This review will attempt to identify the progress which has been made in the synthesis of cyclopropenes, to show the rich variety of their synthetic transformations, and to point to areas where future progress may occur. It will not cover purely structural or spectroscopic aspects of cyclopropene chemistry, which were dealt with in most excellent fashion by Closs 1), and in several later reviews 2). Nor will the photochemical or metal induced reactions be dealt with in great detail, the former area having been covered in reviews by Padwa3) and the latter in an article in the present series by Binger and Buch4). [ABSTRACT FROM AUTHOR]

Syntheses and ring opening reactions of vicinally donor-acceptor-substituted cyclopropanes are treated in this review. Special attention is directed to methyl 2-siloxy cyclopropanecarboxylates which display an particularly rich and versatile chemistry leading to synthesis of polyfunctional compounds, certain carbocycles, and heterocyclic systems. Other classes of donor-acceptor-substituted cyclopropanes are also regarded emphasizing new developments. Although preparative aspects are strongly accentuated, interesting mechanistic features, characteristic for this type of three-membered rings, will also be discussed. [ABSTRACT FROM AUTHOR]

Substituted on the same carbon by an electron donating group and an adjacent multiple bond, cyclopropanes provide building blocks of unprecedented synthetic potential. They are readily available along different routes involving: cyclopropanone hemiacetals, oxaspiropentanes, alkylidenecyclopropanes, 1-heterosubstituted lithiocyclopropanes, α-enone silyl enol ethers, 1,3-dichloroacetone and 1-hydroxycyclopropylcarbonyl derivatives as main sources. First of all, they undergo acid-induced C3→C4 ring expansion to four-membered rings, in particular to the 2-vinylcyclobutanone system, which is an efficient precursor of C5, C6 and C8 homologous rings by subsequent acid- and base-induced, thermal, or photolytic ring enlargements. On the other hand, they undergo thermal C3→C5 ring expansion providing cyclopentanone enol ethers or derivatives with high chemo-, regio- and stereoselectivity. [ABSTRACT FROM AUTHOR]