Claisen rearrangement of 1-hydroxy-3-(3-methylbut-2-enyloxy)xanthones

Abstract

Claisen rearrangements of 1-hydroxy-3-(3-methylbut-2-enyloxy)xanthone (6a) and its 7-methoxy-(6b) and 6-methoxy-(6c) derivatives at 200–210°in vacuo yield mixtures of the corresponding dealkenylated xanthones (7a–c), angularly condensed 4,4,5-trimethyl-4,5-dihydrofuro-derivatives (8a–c), and their linear isomers (9a–c). These results show that normal Claisen rearrangement occurs in both the available ortho-positions, viz. 2 and 4, and is followed by spontaneous cyclisation to give linearly and angularly condensed dihydrofuro-derivatives. However, 1-hydroxy-5-methoxy-(3-methylbut-2-enyloxy)xanthone (6d) afford, besides the dealkenylated xanthone (7d), only linearly condensed dihydrofuroxanthones (9d) and (14).