Bioactive Heterocycles from Domino Wittig-Pericyclic Reactions

Heterocycles present the core of many biologically or pharmaceutically interesting compounds. A new concept that addresses crucial issues of heterocycle synthesis such as selectivity of the actual ring closure step and tolerance of other functional groups, employs the cumulated ylide keteneylidenetriphenylphosphorane Ph3P=C=C=O. Featuring a unique combination of ylidic and ketene properties in a dipolar electronic structure, it reacts with derivatives of carboxylic acids bearing additional OH, NH-, SH- or CH-acidic groups. This reaction proceeds by a domino addition-Wittig olefination giving rise to the formation of the corresponding heterocycles with five- to seven-membered rings. Further pericyclic steps such as Diels-Alder cycloadditions, Claisen rearrangements, ene reactions, and various combinations thereof may ensue with the newly installed C=C bond. In some cases, the outcome of these processes is fully controllable by merely adjusting the external conditions. For example, allyl and cinnamyl α-hydroxycycloalkanoates could be selectively converted either to Claisen rearranged 3-allyltetronic acids or to Claisen-Conia rearranged 3-(spirocyclopropyl)-dihydrofuran-2,4-diones. The former furnished anti-HIV-active 5-spiro-3-(α-cyclopropylbenzyl)-tetronic acids upon Simmons-Smith reaction and the latter could be stereoselectively ring-opened with alcohols, amines and water to give herbicidal 3-(β-synalkoxy / amino)tetronic acids. Congenerous esters with di- or trialkyl substituted allyl residues undergo a formal [2,3]-sigmatropic rearrangement also proceeding via 3-(spirocyclopropyl)dihydrofuran-2,4-diones but leading to 3- exo-alkylidene-5-spirodihydrofuran-2,4-diones which in turn rapidly autooxidize to give potentially antimalarial spirotricyclic hemiketal endoperoxide lactones as products of an overall seven-step cascade. This review takes stock of these and other recent developments in the field of domino Wittig-pericyclic synthesis of bioactive heterocycles. Keywords: bioactive heterocycles, wittig reactions, wittig-pericyclic reaction, domino wittig reaction, wittig olefination