Cyclopropene synthesis

Triafulvene 69 was synthesized by an interesting route starting with diphenyl cyclopropenium cation and lithio ethyl acetate72. Although widely used in calicene chemistry (see later) this reaction principle — like the Wittig reaction — did not find general application in methylene cyclopropene synthesis. 

TRANSITION METAL CATALYZED REACTIONS OF CYCLOPROPENES SYNTHESIS OF 1197 VINYLCYCLOPROPANES... 

The most common source of a,j5-unsaturated carbenes for cyclopropene synthesis is from the decomposition of 3H-pyrazoles which are, in turn, available from the addition of a diazo compound to an alkyne. On photolysis these compounds suffer ring cleavage to an a>diazoalkene prior to nitrogen loss and carbene formation. In this way cyclopropene 27 is available from either 25 or 26 by way of common diazo and carbene intermediates. ... 

A number of reviews on cyclopropene synthesis have been published. 

Not all vinylcarbenes will cyclize to the cyclopropene. For example, those with an acyl substituent on the carbene-bearing center preferentially undergo a Wolff rearrangement, but the most significant limitations to the scope of this synthetic route arise from the problems of making the required 3//-pyrazoles. Many routes exist, but as far as cyclopropene synthesis is concerned, the 3/f-pyrazoles have been made (1) by addition of diazoalkanes to alkynes (2) from dihydropyrazoles (3) from cyclization of a,/l-unsaturated tosylhydrazones and finally (4) by modification of the structure of other 37/-pyrazoles. 

Cyclopropenes.— The synthetic routes to and the properties of cyelopropenes, based on the 1974 literature, have been reviewed, and the addition of alkoxycarbenoids to alkynes has already been discussed (p. 15). The intermediates in the photodecomposition of 3.ff-pyrazoles have been further examined and routine applications of this method for cyclopropene synthesis have appeared. ... 

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