Microwave Chemistry Highlights
A short summary of articles published within the last month on microwave-assisted organic synthesis (MAOS)
Ru-Catalyzed Aryl Azide-Alkyne Cycloaddition
The synthesis of 1,2,3-triazoles via Ru-catalyzed aryl azide-alkyne cycloadditions was described by the group of Valery Fokin from The Scripps Research Institute (Org. Lett. 2007, 9, 5337. ). In comparison to the Cu(I)-catalyzed azide-alkyne cycloaddition where 1,4-disubstituted triazoles are obtained, the Ru-catalyzed version produces 1,5-regioisomers of 1,2,3-triazoles. [Cp*RuCl]4 proved to be superior in activity than the original Cp*RuCl(PPh3)2 catalyst and in combination with microwave heating higher yields, cleaner products and shorter reaction times were accomplished. By-product formation and lower yields were noticed under conventional heating conditions in an oil bath.
One-Pot Suzuki-Miyaura/Etherification Reaction
In the course of the synthesis of quinazolinone TGF-β RI inhibitors 5, Hong-yu Li and co-workers from the Lilly Corporate Center in Indianapolis have disclosed the synthesis of key intermediate 3 via a one-pot three-component Suzuki-Miyaura/etherification sequence (Tetrahedron 2007, 63, 11763. ). In order to prevent side reactions in the Suzuki coupling of boronic acid 1 and quinazoline 2 (in particular Suzuki reaction at the 2-chloro position of 2), ethylene glycol was employed for the etherification step at the 2-position of 2. In addition, product purification was improved by performing the reaction sequence in one pot. The final products 5 were obtained by amination of scaffold 4, again using microwave heating.
Synthesis of Potential β-Sheet Nucleators via Suzuki Coupling
Vincenzo Santagada and his group from Universitá di Napoli “Federico II”, Italy, have reported on the synthesis of biphenyls 3 that incorporate an amino acid moiety with the carboxyl and amino function in ortho position (Tetrahedron 2007, 63, 12779. ). These biphenyl scaffolds were synthesized in order to develop new compounds that are able to induce β-hairpin folding in peptides. Biphenyls 3 were obtained in a single step by the Suzuki coupling of boronic acids 1 and aniline (n = 0), benzylamine (n = 1) or phenethyl amine (n = 2) bromides 2 in aqueous DMF as solvent and 30 min microwave heating at 150 °C.
Asymmetric Heck Reactions using Phosphine-Thiazole Ligands
Pher Andersson and co-workers from the Department of Biochemistry and Organic Chemistry in Uppsala have made investigations toward the impact of the ligand structure on the asymmetric Heck coupling of 2,3-dihydrofuran with various triflates (Adv. Synth. Catal. 2007, 2595. ). With all employed phosphine-thiazole ligands 1-6 the regioselectivity was excellent whereas the enantioselectivity and the reaction rate were dependent on the substitution patterns of the ligands. For the coupling with aryl triflates bulkier substituents in the 2-position of the ligands enhanced the enantioselectivity and reaction rate: ligand 6 proved to be the best regarding ee´s (94-98%) and reaction time (1 h) whereas with ligand 4 low conversions (28-30%) and ee´s (77-80%) were obtained. By employing microwave heating the reactions proceed faster with constant high enantioselectivities.