Thermal and Photochemical Rearrangement of 3‐Substituted 2‐Methylbenzofuran Epoxides and Their Valence‐Isomeric Quinone Methides

Abstract

The dimethyldioxirane oxidation of the 3-substituted 2-methylbenzofurans 1 [la: 3(E)-styryl, lb: 3-acetoxy, lc: 3-(tert-butyldimethylsilyloxy)] is reported. Only quinone methide 3a, none of the benzofuran epoxides 2a-c, could be detected by 'H-and 13C-NMR spectroscopy at low temperature (-3O"C), which on photoisomerization led to chromene 7a. The benzofuran-3-ones 5b, c and the a-diketone 6c are presumably formed by thermal isomerization of the transient benzofuran epoxides 2b, c and quinone methide 3c.Benzofuran epoxides 2 and their valence-isomeric quinone methides 3, both readily accessible by dimethyldioxirane oxidation of benzofurans 1, have been shown to be very reactive compounds [']. Thus, the epoxides 2 and quinone methides 3 rearrange thermally above -20°C to the allylic alcohols 4 and their tautomeric phenols 4' (Scheme 1 ). Scheme 1 RDue to the fact that the allylic alcohols 4 and their ringopened tautomeric phenols 4' interconvertf'a,q, it has been difficult to establish rigorously, whether these derive from the epoxides 2 or their valence-isomeric quinone methides 3. However, the 1,s-sigmatropic shift in the rearrangement 3 -+ 4 (Scheme 1) requires isomerization of ( 9 -3 to (E)-3, which usually is promoted photochemically. It was our [*I Undergraduate research participant, March 1992. interest, therefore, to examine benzofuran epoxides 2 and their corresponding quinone methides 3 with substituents which cannot rearrange to the allylic alcohols 4 andlor their phenol tautomers 4' (Scheme 1) to provide mechanistic insight into their mode of formation. For this reason, the benzofurans la-c were selected as model compounds, and their dimethyldioxirane oxidation was examined. We report presently on the chemical fate of the transient benzofuran epoxides 2 and their valence-isomeric quinone methides 3. Results and DiscussionThe dimethyldioxirane[21 oxidation of a 92:s mixture of the 3-styrylbenzofurans (EJ-lal(Z)-l a gave exclusively the (E)-styryl-substituted quinone methide (Z)-3a (Scheme 2), which could be isolated at -20°C as a dark-red solid, while benzofuran ( 3 -1 a was not oxidized under these conditions, as revealed by NMR data. At temperatures above 0°C dimethyldioxirane oxidized both, the isomers (@-la and (3-1 a. However, at this elevated temperature decomposition or overoxidation took place, which resulted in a complex mixture, even if one equivalent of dimethyldioxirane was employed. Attempted separation and purification of that mixture by column chromatography (silica gel, ether) at -30°C failed. The most characteristic signals in the I3C-NMR spectrum of quinone methide ( 3 -3 a are the acetyl methyl group at 6 = 29.8, the carbonyl carbon atoms of the cyclohexadienone at 6 = 186.1, and the acetyl group at 6 = 204.8. These data are in accord with those of known quinone methided3"1. The (E) configuration for the styryl group of the quinone methide 3a was ascertained by the coupling constant J = 16.1 Hz of the two styryl protons at 6 = 6.91 (d) and 7.35 (d), which is characteristic for ( E )...