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Microwave Chemistry Highlights
A short summary of articles published within the last month on microwave-assisted organic synthesis (MAOS)

Monday, August 15, 2005
Bimbisar Desai
Karl-Franzens-University Graz

One Pot Solvent Free Synthesis of 2-Aryl and 2-Alkylbenzothiazoles

A recent publication by Edward R. Biehl and co-workers from the South Methodist University, Texas (Heterocycles 2005, 65, 2119. ) describes an improved and eco-friendly synthesis of 2-alkyl and aryl-substituted benzothiazoles. As opposed to conventional heating, under controlled microwave heating the neat reaction of aromatic and aliphatic β-ketoesters and o-aminothiophenols proceeded faster and in higher yields. It was also possible to scale up selected reactions to give multigram quantitites with similar high yields.

Synthesis of Alkenes by McMurry Reaction

Nicolai Stuhr-Hansen from the University of Copenhagen, Denmark (Tetrahedron Lett. 2005, 46, 5491. ) has reported on a microwave-induced high yielding synthesis of alkenes by McMurry coupling of aldehydes and ketones with low-valent titanium. For all the aldehydes and ketones including sulfur end-capped analogues studied, within 10 mins of microwave heating, even at low-energy microwave conditions, complete conversion and above 80% yields of corresponding alkenes were realized. With microwave heating, the corresponding alkenes have been produced much faster with higher yields as compared to conventional heating, and in some cases, without damaging the sensitive tert-butyl sulfur protecting groups.

Rapid and Selective Method for the Synthesis of New Pyrazolo[1,5-a]pyrimidines

A microwave assisted rapid and highly selective method for the synthesis of new pyrazolo[1,5-a]pyrimidines has been demonstrated by Li Ming and co-workers from the Qingdao University of Science and Technology, China (J. Heterocyclic. Chem. 2005, 42, 925. Link). The synthesis of twelve new pyrazolo[1,5-a]pyrimidines has been performed by the reaction of 5-amino-1H-pyrazoles with a variety of enaminones under microwave irradiation. Under controlled microwave heating, the synthesis has shown the same high regioselectivity as in case of conventional heating, however, the yields with microwave heating were improved by 10-20 % and the reaction rates enhanced from 10 to 100 times.

Synthesis of a Library of Complex Macrodiolides by Cyclodimerization of Hydroxy esters

The synthesis of complex macrodiolides involving microwave-accelerated transesterification of chiral, nonracemic, hydroxy esters has been demonstrated by John A. Porco Jr. and co-workers at Boston University, USA (J. Comb. Chem. 2005, 7, in press. ). Dramatic accelerations in cyclodimerizations of monomeric units containing ethers and amino acid subunits have been seen under microwave heating with a fluorous tin oxide catalyst, affording optimal yields. Cyclodimerizations studied under microwave heating indicated an initial dependance of macrodiolide formation on microwave power. Use of a rapid cyclodimerization process under microwave incubation has been demonstrated to prepare a 127 member library of diverse macrodiolides with high levels of macrocyclic ring, stereochemical and functional diversity.

B. Desai, Org. Chem. Highlights 2005, August 15.