Thermische Cyclodehydratisierung von Salicylalkoholen; eine einfache Synthese von 4‐substituierten 2<i>H</i>‐Chromenen

Abstract

Abstract 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 2 H ‐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).