Thermische Cyclodehydratisierung von Salicylalkoholen; eine einfache Synthese von 4‐substituierten 2H‐Chromenen

Heating of 1‐(o‐hydroxyaryl)‐2‐propen‐1‐ols (9–13; see scheme 1) in diglyme solution at 147° leads to a 1,4‐elimination of water to yield ω‐vinyl‐o‐quinomethides (b; see scheme 2) as intermediates which cyclise rapidly to form 2H‐chromenes (17–21). 1‐(o‐Hydroxyphenyl)‐5‐hexen‐1‐ol (14) on heating at 147° is transformed into o‐(1,5‐hexadienyl)‐phenol (23). This phenol rearranges at higher temperature (270°) in N,N‐diethylaniline to yield a mixture of 2,4‐propanochromane (25) and cis‐ and trans‐3,4‐propanochromane (cis‐ and trans‐26). The kinetically controlled ratio of these compounds is 2,8:1:2,9. The formation of 25 and 26 can be explained by an intramolecular Diels‐Alder reaction (see scheme 3).