Organoaluminum induced cyclization of unsaturated aldehydes

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.