Spirans. Part 12. Stereochemical control in the oxidative cyclisation of bisnaphthols

Abstract

Almost all oxidising agents cyclise benzylidene-1,1′-bis-2-naphthol (1a) to the stereoisomers (2a) and (3a) of phenylnaphtho[2,1-b]furan-2(1H)-spiro-1′(2′H)-naphthalen-2′-one. A few are stereospecific. Hypobromite and persulphate give (2a), whereas iodine oxyacids and (diacetoxyiodo)benzene are specific for (3a). Radicaltype oxidants such as hexacyanoferrate(III) and 2,4-di-t-butyl-6-phenylphenoxyl (4) give mixtures. Horseradish peroxidase behaves like hexacyanoferrate(III). The presence in the benzylidene residue of methoxy, nitro, fluoro, and other groups has little effect upon the isomer ratios, which are believed to be determined mainly by the bulk of the benzylidene group and the limits it imposes upon conformations during the oxidation processes. Oxidation by (diacetoxyiodo)benzene occurs through a cyclic intermediate (9) and gives the less-hindered (3a); oxidation of hypobromite occurs not directly but by exchange through an aryl hypobromite (Scheme) which halogenates its opposite naphthol nucleus and then substitutes the halogen, with inversion, thus leading to the more-hindered isomer (2a).