Spirans. Part 15. The effect of ring size upon the regioselective oxidative coupling of heterocyclic phenols

Abstract

The oxidation of 2,3-dihydro-2,2,4,6,7-pentamethylbenzofuran-5-ol (7b) by alkaline potassium hexacyanoferrate(III) gives a complex mixture from which three products have been isolated and identified; all result from oxidation at the 6-methyl group only. Although unidentified products may have resulted from oxidation at the 4-methyl group, the evidence supports the view that the size of the heterocyclic ring determines the regioselectivity. The products are 2,2′,3,3′-tetrahydro-2,2,2′,2′,4,4′,7,7′-octamethyl-6,6-ethylenebisbenzofuran-5,5′-diol (8), which gives the spiran (11), 2,2′,3,3′,7′,8′-hexahydro-2,2,2′,2′,4,4′,7′,9-octamethyl-6H-pyrano[2,3-f]benzofuran-6-spiro-6′(5′H)-benzofuran-5′-one, upon further oxidation, and the related trimer (12), the structure of which is partly disclosed by hydrolysis (by silica) of the acetal grouping. The size of the heterocyclic ring has little effect upon the ease of sigmatropic rearrangement in the spiran (11), but influences the ease with which this dissociates to give a quinone methide and thence the trimer (12).With mild base 4-chloromethyl-2,3-dihydro-2,2,6,7-tetramethylbenzofuran-5-ol (7e) gave a trimer isomeric with compound (12).