Thiourea-catalyzed diels-alder reaction of a naphthoquinone monoketal dienophile

Abstract

A variety of organocatalysts were screened for the catalysis of the naphthoquinone monoketal Diels-Alder reaction. In this study we found that Schreiner's thiourea catalyst 10 and Jacobson's thiourea catalyst 12 facilitate the cycloaddition of the sterically hindered naphthoquinone monoketal dienophile 3 with diene 4. The use of thiourea catalysis allowed for the first time the highly selective synthesis of the exo-product 2a in up to 63% yield. In this reaction a new quaternary center was built. The so formed cycloaddition product 2a represents the ABC tricycle of beticolin 0 (1) and is also a valuable model substrate for the total synthesis of related natural products. © 2013 Kramer and Bräse; licensee Beilstein-Institut.