Die thermische Umlagerung von 2‐(Buta‐1′,3′‐dienyl)‐phenolen in 2‐Alkyl‐2 H ‐chromene; Beispiele für aromatische [1,7 a ]‐ und [1,5 s ]sigmatropische Wasserstoffverschiebungen

2‐(1′‐ cis ,3′‐ cis ‐)‐ and 2‐(1′‐ cis ,3′‐ trans ‐Penta‐1′,3′‐dienyl)‐phenol ( cis, cis ‐ 4 and cis, trans ‐ 4 , cf. scheme 1) rearrange thermally at 85–110° via [1,7 a ] hydrogen shifts to yield the o ‐quinomethide 2 (R CH 3 ) which rapidly cyclises to give 2‐ethyl‐2 H ‐chromene ( 7 ). The trans formation of cis, cis ‐ and cis, trans ‐ 4 into 7 is accompanied by a thermal cis, trans isomerisation of the 3′ double bond in 4 . The isomerisation indicates that [1,7 a ] hydrogen shifts in 2 compete with the electrocyclic ring closure of 2 . The isomeric phenols, trans , trans ‐ and trans, cis ‐ 4 , are stable at 85–110° but at 190° rearrange also to form 7 . This rearrangement is induced by a thermal cis, trans isomerisation of the 1′ double bond which occurs via [1, 5 s ] hydrogen shifts. Deuterium labelling experiments show that the chromene 7 is in equilibrium with the o ‐quinomethide 2 (R CH 3 ), at 210°. Thus, when 2‐benzyl‐2 H ‐chromene ( 9 ) or 2‐(1′‐ trans ,3′‐ trans ,‐4′‐phenyl‐buta1′,3′‐dienyl)‐phenol ( trans , trans ‐ 6 ) is heated in diglyme solution at >200°, an equilibrium mixture of both compounds (∼ 55% 9 and 45% 6 ) is obtained.