title: Ti-catalyzed straightforward synthesis of exocyclic allenes.
abstract: Exocyclic allenes constitute useful building blocks in organic synthesis and have recently been identified as key intermediates in the synthesis of natural products. Here the first general method for the most straightforward synthesis of exocyclic allenes reported to date is presented. This method is based on the Barbier-type cyclization of propargyl halides catalyzed by titanium; a safe, abundant, and ecofriendly metal. The reaction proceeds under mild conditions compatible with different functional groups and provides good yields of five-, six-, and seven-membered carbocycles and nitrogen-containing heterocycles bearing an exocyclic allene group. Experimental evidence supporting the proposed reaction mechanism is also provided. Moreover, this procedure can be carried out in an enantioselective manner by using chiral titanocene(III) catalysts. The utility of this method has been proved in the synthesis of the natural alkaloid stemoamide.
authors: Dpto. Química Orgánica, Facultad de Ciencias, Universidad de Granada, Campus Fuentenueva s/n, 18071 Granada (Spain), Fax: (+34) 958248437.
authors: Juan
authors: J
authors: Muñoz-Bascón
authors: Carmen
authors: C
authors: Hernández-Cervantes
authors: Natalia M
authors: NM
authors: Padial
authors: Míriam
authors: M
authors: Alvarez-Corral
authors: Antonio
authors: A
authors: Rosales
authors: Ignacio
authors: I
authors: Rodríguez-García
authors: J Enrique
authors: JE
authors: Oltra
keywords: allenes; cyclization; enantioselectivity; heterocycles; synthetic methods; titanium