Synthesis of Naturally Occurring Acetylenes via an Alkylidene Carbenoid Rearrangement

Naturally occurring mosquito larvicidal acetylenes 1 and 2, and analogues 3 and 4, each containing either a 1,3-butadiynyl or a 1,3,5-hexatriynyl moiety, are synthesized via a Fritsch-Buttenberg-Wiechell rearrangement. The alkylidene carbenoid intermediate results from lithium-halogen exchange of a suitable dibromoolefin precursor, and the rearrangement is accomplished under mild conditions. Synthesis of the dibromoolefin precursors to acetylenes 1-4 is easily achieved in three steps from commercially available carboxylic acids or aldehydes, making this procedure a viable alternative to conventional methods for the synthesis of naturally occurring acetylenes.