Microwave Chemistry Highlights
Tandem Cyclization/Claisen Rearrangements
The base-catalyzed intramolecular cyclization of appropriately substituted 4-alkyn-1-ols, followed by in situ Claisen rearrangement was investigated by Ovaska and co-workers (Synlett 2004, 2579. ) . The tandem cyclization-Claisen rearrangements were best carried out in DMF or phenetole as solvent in the presence of 10 mol% MeLi base. In most cases, the resulting cycloheptanoid ring systems were produced in high yields in a matter of minutes upon microwave irradiation at 150-200 °C. Some of the reactions were also performed under solvent-free conditions providing similar isolated product yields. Several other bicyclo[5.3.0]decane ring systems were also available from relatively simple acetylenic alcohols using this strategy.
Preparation of Primary Thioamides
Bagley and co-workers have described the preparation of primary thioamides by treatment of nitriles with ammonium sulfide in methanol solution (Synlett, 2004, 2615. ) . While the reaction with electron-deficient aromatic nitriles proceeds at room temperature, other aromatic and aliphatic nitriles require microwave heating at 80-130 °C for 15-30 minutes to furnish the thioamides in moderate to high yields. This protocol avoids the use of hydrogen sulfide gas under high pressure, proceeds in the absence of base and provides thioamides usually without the need for chromatographic purification.
A simple, efficient, and high-yielding synthesis of quinazolin-4-ylamines and thieno[3,2-d]pyridin-4-ylamines based on the condensation of appropriately functionalized N'-(2-cyanophenyl)-N,N-dimethylformamidines and primary amines has been reported by Han and co-workers (Org. Lett. 2004, 6, 4775. ) . Optimization of the reaction parameters resulted in the use of acetonitrile/acetic acid as solvent mixture and utilization of 1.2 equivalents of the corresponding amine. In general microwave heating to 160 °C for 10 minutes provided excellent product yields.
An in-depth look at the field of microwave chemistry was published in a
recent issue of the ACS journal Chemical and Engineering News.
C. O. Kappe, Org. Chem. Highlights 2005, January 15.