Microwave Chemistry Highlights
Fluorous Suzuki-Type Coupling Reactions
A recent publication by Wei Zhang and co-workers from Fluorous Technologies (Org. Lett. 2004, 6, 1473. DOI: 10.1021/ol0496428) describes a new strategy to improve the efficiency of Suzuki coupling reactions by combining rapid microwave synthesis with fluorous separation techniques (F-SPE). Aryl perfluorooctylsulfonates 1 as precursors for Suzuki-type couplings were readily prepared from phenols and commercially available perfluorooctylsulfonylfluoride. Subsequent Suzuki reaction with aryl boronic acids in the presence of a suitable Pd catalyst provided the desired biaryls 2 in high yield. Work-up simply involved filtration of the reaction mixture through a fluorous solid-phase extraction cartridge (F-SPE).
Resin-Bound Dienophile Scavengers
Xiaoguang Lei and John A. Porco Jr. of Boston University have demonstrated (Org. Lett. 2004, 6, 795. DOI: 10.1021/ol036502+) the usefulness of a thermally stable polymer-supported anthracene derivative (i.e. 3) for scavenging dienophiles under microwave conditions. This strategy was successfully used to rapidly sequester reactive dienophiles from reaction mixtures containing flavonoid Diels-Alder cycloadducts (e.g. 5) prepared by microwave-assisted Diels-Alder cycloaddition from flavonoid dienes (e.g. 4).
One-Pot Imidazole Synthesis
A simple, high yielding synthesis of 2,4,5-trisubstituted imidazoles of type 6 from 1,2-diketones and aldehydes in the presence of ammonium acetate was recently reported by Scott E. Wolkenberg and co-workers from Merck (Org. Lett. 2004, 6, 1453. DOI: 10.1021/ol049682b) . Utilizing microwave irradiation, alkyl-, aryl-, and heteroaryl-substituted imidazoles are formed in very high yields ranging from 76-99%. Further microwave-assisted alkylation of 2,4,5-trimethylimidazole with benzyl chloride in the presence of base led to the alkaloid lepidiline B in 43% overall yield.
Preparation and Reactions of Cyclic Thioureas
Eric Wellner and co-workers from Active Biotech AB have made extensive use of microwave chemistry in the preparation of cyclic thioureas and guanidines (J. Org. Chem. 2004, 69, 1571. DOI: 10.1021/jo030338m) . Utilizing MAOS it was possible to assemble all intermediates and target molecules without any need of activation or protecting groups, cutting reaction- and workup times to a minimum.