Use of functionalised ynamines in a hetero-Diels–Alder approach to dihydronaphtho[1,2-b]pyrans and indeno[1,2-b]pyrans

Abstract

Reaction of the ynamine ester methyl 3-(pyrrolidin-1-yl)prop-2-ynoate 5 with 2-(4-nitrobenzylidene)1-tetralone 1 results in a very poor yield of the chromatographically labile 4-aryl-5, 6-dihydro-4H-naphtho[1,2-b]pyran 8 along with the α-pyrone 10. Increasing the reactivity of the 4π component by using the 2-arylidene indan-1,3-diones 11–13 results in moderate to good yields of the 4-aryl-5-oxo-4H-indeno[1,2-b]pyran-3-carboxylates 14–19. An ynamine nitrile 24, generated in situ, also reacts with 12 and 13, furnishing rather lower yields of the adducts 20 and 21.