Microwave Chemistry Highlights
A short summary of articles published within the last month on microwave-assisted organic synthesis (MAOS)
Catalyst- and Base-Free Suzuki-Type CouplingJie Yan and co-workers from Zhejiang University of Technology, China, have reported on Suzuki-type couplings in water without the use of catalyst and base (Eur. J. Org. Chem. 2006, 2060. ). Both symmetrical and unsymmetrical biaryls could be synthesized by coupling of sodium tetraphenylborate with hypervalent iodonium salts or iodanes under mild reaction conditions.
Synthesis of 1,4-Benzoxazine Scaffolds
A general protocol for the synthesis of 2-alkyl-3,4-dihydro-3-oxo-2H-1,4-benzoxazines 2 has been established by the group of Wei-Min Dai from the Hong Kong University of Science and Technology (Tetrahedron 2006, 62, ASAP. ). The first step was a reductive N-alkylation of substituted 2-aminophenols with aromatic aldehydes to obtain the N-alkylated anilines 1. Regioselective O-alkylation of 1 with 2-bromoalkanoates furnished acyclic intermediates which showed spontaneous cyclization to 1,4-benzoxazines 2. Higher temperatures (180-200 °C) for the second step were necessary when electron-withdrawing groups were present at the C6 position (R1 = NO2, Cl).
One-Pot Synthesis of 1,2,4-Oxadiazoles
Mehdi Adib and co-workers from University of Tehran have disclosed a three-component one-pot reaction of nitriles, hydroxylamine and aldehydes to give 1,2,4-oxadiazoles (Tetrahedron Lett. 2006, 47, 2965. ). First the nitrile and hydroxylamine were reacted at 100 °C for 1 min, subsequent addition of aldehyde and irradiation at 150 °C for 3 min under solvent-free conditions gave oxadiazoles in excellent yields.
Synthesis of Tetracyclic β-Carbolines
The synthesis of thiohydantoin fused tetrahydro-β-carbolines has been investigated by the group of Chung-Ming Sun at the National Chiao Tung University in Taiwan (J. Comb. Chem. 2006, 8, 141. ). A soluble polymer-supported strategy using polyethylene glycol (PEG) resin combined with a traceless cyclative cleavage step was applied to achieve products 1 in high stereoselectivity. By employing microwave heating the thermodynamically stable trans diastereomers were obtained preferentially.