Cycloadditions Made Easy

Engineered reagents with a Lewis acid-base combination facilitate benzene ring construction reactions, according to a report (Angew. Chem. Int. Ed., DOI: 10.1002/anie.201200917). Conventional [4+2] synthesis of substituted benzene rings involves either substitution reactions, which are limited by the dictates of directing groups, or slow cycloaddition reactions that require hours of reaction time at high temperatures. In the new work, a group led by Joseph P. A. Harrity of the University of Sheffield, in England, combined an alkyne with a Lewis acid acceptor and a diene with a Lewis base. The acid and base bring the two reagents together in a complex that yields regiospecific 1,2,3-trisubstituted benzene products in 10 minutes at 40 ºC. The reaction works with a variety of starting materials, including pyridines, azoles, and amides, and in principle can facilitate other directed cycloaddition reactions, such as [3+2]. It also achieves Harrity and colleagues’ goal of synthesizing precursors to benzoxaboroles, which are organic boron compounds of interest for therapeutic use as antibacterial, antifungal, and anti-inflammatory agents.