Intramolecular Diels–Alder additions to 2-benzopyran-3-ones; endo-selective additions and some reactions of the adducts

Abstract

Unlike o-quinodimethanes lacking a bridge between the termini of the diene system e.g. 18a (n= 3 or 4), the 2-benzopyran-3-ones 10a, 10b and 10c undergo endo-selective intramolecular Diels–Alder addition of the connecting chain to give cis-BC fused ring systems of type 13. The adduct 13b is converted into the ester 21 with methanolic HCl and into the ketol 36 by LiAlH₄ reduction followed by oxidation (MnO₂). The ketone 31 is prepared from 21 by epoxidation followed by BF₃·Et₂O-catalysed rearrangement; with NaOMe–CD₃OD it fails to undergo cis-trans isomerisation and fails to incorporate deuterium at C-8 probably due to the overwhelming acidity of the C-6 protons. The cis-dihydronaphthalene 26 obtained from 21 gives cis-27 with KOBu^t–(CD₃)₂SO showing the greater thermodynamic stability of the cis-fused ring system in this case.