Selective Cyclodimerization and Cyclotrimerization of Acetals Bearing Electron-Withdrawing Groups Catalyzed by Lewis Acids

Acetals like 3,3-diethoxypropionate bearing electron-withdrawing groups were found to undergo cyclodimerization and cyclotrimerization in the presence of Lewis acids to give coumalates and 1,3,5-trisubstituted benzenes. The selectivity of these products depended on the Lewis acids employed. For instance, ethyl coumalate was obtained from ethyl 3,3-diethoxypropionate in high selectivity under the influence of d-block Lewis acids like FeCl3, whereas triethyl 1,3,5-benzenetricarboxylate was obtained in preference to ethyl coumalate under the influence of lanthanoid Lewis acids like GdCl3. Various coumalates were synthesized by the FeCl3-catalyzed cross-cyclodimerization of acetals with active methylene compounds. From 4,4-dimethoxy-2-butanone, however, 1,3,5-triacetylbenzene, which is difficult to prepare regioselectively by conventional methods, was formed in quantitative yield under the influence of AlCl3. This reaction would provide a very convenient route to 1,3,5-triacetylbenzene. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)