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.