Microwave Chemistry Highlights
A short summary of articles published within the last month on microwave-assisted organic synthesis (MAOS)
Tandem Claisen-Mislow-Evans Rearrangement
Matthew J. Pelc and Armen Zakarian from Florida State University have reported on the synthesis of A,G-spiroimine (1) of pinnatoxins (Tetrahedron Lett. 2006, 47, 7519. DOI: 10.1016/j.tetlet.2006.08.083). Key step in this procedure is a cascade sigmatropic route that integrates the Claisen and Mislow-Evans rearrangements. By applying microwave-heating at 170 °C, the reaction time could be reduced to 20 min compared to 15 h under conventional heating at 150 °C.
Synthesis of Functionalized 1-Amido-2-cyclohexenes
An improved protocol for the three-component synthesis of aldehydes, amides and dienophiles (AAD-reaction) was developed by the group of Matthias Beller from the University of Rostock (Tetrahedron 2006, 62, 10962. DOI: 10.1016/j.tet.2006.08.062). Optimization studies revealed that a solventless microwave protocol (150 °C, 20 min) gave a superior yield (90 vs 61%) for the reaction of acetamide (1), crotonaldehyde (2) and N-methyl-maleimide (3) in a 50 times shorter reaction time compared to thermal heating at 110 °C. The addition of acetic acid anhydride (Ac2O) as water removing reagent proved to be beneficial for the reaction. All 10 cyclohexene derivatives are obtained as only one diastereomer, namely as the all-syn product, due to the selective endo addition of the dienophile.
Ru-Catalyst on Raschig Rings for Olefin Metathesis
The groups of Andreas Kirschning (Universität Hannover) and Karol Grela (Polish Academy of Science) have developed the Ru-catalyst 1 for various metathesis reactions (J. Am. Chem. Soc. 2006, 128, 13261. DOI: 10.1021/ja063561k). The Ru-complex is noncovalently (ion exchange) immobilized on glass-polymer composite Raschig rings which has the advantage of easy removal and reactivation after the reaction. By applying microwave heating, the reaction could be enhanced from 2 h to 4 min but with slightly reduced isolated yields. Additionally to the batch protocol, the olefin metathesis was also performed under PASSflow continuous flow conditions, however, a diminished recyclability of 1 (6 cycles vs 2) was observed.
Pd-Catalyzed Asymmetric Allylic Alkylations
The group of Antonio L. Braga from Universidade Federal de Santa Maria, Brazil, has disclosed Pd-catalyzed allylic alkylations of rac-1,3-diphenyl-2-propenyl acetate with malonates (Eur. J. Org. Chem. 2006, ASAP. DOI: 10.1002/ejoc.200600707). By applying chiral β-seleno amides as ligands in combination with microwave heating, reaction times of 2-4 min were possible compared to 24 h at rt with only a minimal loss of enantioselectivity (ee´s up to 94% were obtained). During optimization studies, two methods were found to give high yields (method A and B). For ligands with R2 = Ph and R3 = Bn, i-Bu it turned out that the method which is applied has a significant influence on the ee´s, better values could be reached with method A.