Alkynes ketocarbenoids

Ethyl diazopyruvate, under copper catalysis, reacts with alkynes to give furane-2-carboxylates rather than cyclopropenes u3) (Scheme 30). What looks like a [3 + 2] cycloaddition product of a ketocarbenoid, may actually have arisen from a primarily formed cyclopropene by subsequent copper-catalyzed ring enlargement. Such a sequence has been established for the reaction of diazoacetic esters with acetylenes in the presence of certain copper catalysts, but metallic copper, in these cases, was not able to bring about the ring enlargement14). Conversely, no cyclopropene derivative was detected in the diazopyruvate reaction. 

The reaction of ketocarbenoids with alkynes is a direct method for the synthesis of functionalized cyclopropenes.1 122 123 Until quite recently copper catalysis was generally used and the reactions proceeded in fairly moderate yields, except with terminal alkynes, which failed to generate cyclopropenes due to competing C—H insertions.1 This limitation could be circumvented, however, by using trimethylsilyl derivatives. This approach is illustrated in the synthesis of (131), the unsaturated analog of 1-aminocy-clopropanecarboxylic acid, the biosynthetic precursor to ethylene in plants (Scheme 27).124 The initial... 

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