Spirans. Part VII. Three isomeric grisan derivatives from the oxidative cyclisation of benzylidenebis-2-naphthol

Abstract

N.m.r. studies on the oxidation products [spirans (A) and (B)] of benzylidenebis-2-naphthol show that hypobromite gives (2R*,3S*)-3-phenyl-4,5 : 2′,3′-dibenzogrisa-2′,4′-dien-6′-one (II) and not the (2R*,3R*) stereoisomer reported formerly; the latter compound was obtained from an oxidation with plumbic acetate in pyridine.

A marked difference in the rates at which the related allylic alcohols (A) and (B) are converted by skeletal rearrangement into 1,2 : 5,6-dibenzo-9-phenylxanthen has been confirmed and found to be greater for the acetates. It is now clear that there is steric compression in both series, and the difference is now ascribed to the fact that in the reaction in series (B) the relief from compression comes much earlier than in series (A).

A third isomer, spiran (C), has been encountered and identified analytically as 3-phenyl-4,5 : 4′,5′-dibenzogrisa-2′,4′-dien-6′-one (XVI), a conclusion confirmed by a synthesis of the dimethanesulphonate of the zinc reduction product, (1-hydroxy-2-naphthyl)-(2-hydroxy-1-naphthyl)phenylmethane (XVII). The origin of this isomer has been identified tentatively.