Organoaluminum Induced Cyclization of Unsaturated Aldehydes

Abstract

The pronounced solvent as well as temperature effects on the course of trialkylaluminuminduced cyclization of unsaturated aldehydes were observed. Thus, unimolecular decomposition of the 1: 1 complex of trimethylaluminum-citronellal at -78°C to room temperature led to the formation of an acyclic methylated compound, isopulegol as a cyclization-deprotonation product, and/or a methylated cyclization product depending on the choice of solvents. The acyclic compound was obtained predominantly in hexane, while isopulegol was produced exclusively in 1, 2-dichloroethane. Furthermore, the methylated cyclization product was formed with the highest selectivity using excess trimethylaluminum at low temperature. In contrast with trimethylaluminum, the 1: 1 complex of other trialkylaluminum-citronellal decomposed upon warming to room temperature to furnish a reduction product, citronellol as a major product. Another aldehyde showed a similar variation in reactivity under the apove conditions.