Hetero Diels-Alder reactions in organic chemistry

Abstract

The hetero Diels-Alder reaction is one of the most important methods for the synthesis of heterocycles. In this article an overview is given for the period since 1989 describing the reaction of heterobutadienes such as oxabutadienes, thiabutadienes, azabutadienes, diaazabutadienes, nitroso-alkenes and nitroalkenes as well as of heterodienophiles such as carbonyls, thiocarbonyls, imines, iminium salts, azo- and nitroso compounds. In addition, several other less common hetero Diels-Alder reactions such as cycloadditions of thiaazabutadienes, oxaazabutadienes, dioxabutadienes, dithiabutadienes, oxathiabutadienes, diazaoxabutadienes as well as the use of N-sulfinyl-phosphaalkynes and other dienophiles are mentioned. A main point of discussion is the stereoselectivity of the reactions and the preparation of enantiopure compounds either using dienes and dienophiles carrying a chiral auxiliary or employing chiral Lewis acids. A point stressed is the synthesis of natural products using hetero Diels-Alder reactions leading to carbohydrates, alkaloids, terpenes, antibiotics, mycotoxins, cytochalasans, antitumor agents and several other classes of natural products. Another topic is the use of high pressure in hetero Diels-Alder reactions discussing the influence on the rate constants and the stereoselectivity. Finally, modern developments such as reactions on solid phase, the use of catalytic monoclonal antibodies, transformations in aqueous solution and the microwave activation are described.