[2+3] Carbo- and Hetero-Cyclic Annulation in the Design of Complex Molecules.1

Vinylcyclopropanes, vinyloxiranes, and vinylaziridines were prepared by either a [2+3] protocol involving the additions of the dienolate of ethyl 2-bromocrotonate to enones, aldehydes, and some imines, or by a [4+1] protocol that utilized the intramolecular cycloadditions of diazoketones or azides to 1,3-dienes. The resulting strained 3-membered rings provided, through their energy contents, the necessary driving force for the corresponding rearrangments to annulated cyclopentenes, dihydrofurans, and pyrrolines respectively. The applications of these processes are highlighted in the syntheses of natural products containing five-membered rings. Retigeranic acid, ipomeamarone, and pyrrolizidine diols are the targets of these applications. An approach to the taxane skeleton is also advanced and is based on the ease with which endo vinylcyclopropanes undergo the divinylcyclopropane Cope rearrangement.