Photochemical and Thermal Rearrangement of Acyl‐Substituted Quinone Methides, Accessible by Dimethyldioxirane Oxidation of Benzofurans and Subsequent Valence Isomerization of the Resulting Epoxides

Abstract

Quinone methides / ( Z / E ) Photoisomerization / Norcaradiene / Benzocycloheptafuran / Thermal rearrangement / Xanthenes, 9-acetylThe photoisomerization of quinone methides 3 to benzocycloheptafuran 4 and the thermolysis of the latter to xanthenes 5 are reported. The quinone methides 3 are accessible by dimethyldioxirane oxidation and subsequent valence isomerization of the resulting benzofuran epoxides 2. On irradiation ( h > 400 nm), the quinone methides 3 rearrange by cyclization to the corresponding norcaradiene, and ring enlargement affords the benzocycloheptafurans 4. Thermolysis of the cycloheptatrienes 4 leads to the xanthenes 5, first by cycloreversion to the norcaradienes, followed by electrocyclization to the chromenes and tautomerization of the latter. The new cycloheptatrienes 4 and xanthenes 5 were fully characterized.As a part of our work on the oxidation of benzofurans by dimethyldioxirane, we reported"] on the reversible valence isomerization between the 2,3-dimethylbenzofuran epoxide A and its corresponding quinone methide B. Unequivocal evidence for the equilibrium reaction A e B was obtained by irradiation of a nearly equimolar mixture of the quinone methide B and its epoxide A to afford the corresponding benzoxetene (Eq. l)[lfl. When the latter was allowed to stand at -20°C for prolonged times, the same ratio of benzofuran epoxide A and its quinone methide B, as initially started with, was obtained by cycloreversion of the benzoxetene. Other examples of quinone methide photochemistry are rare in view of their propensity to dimerize and, consequently, their inherent lack of persistence. Nevertheless, Bos reported on the photoisomerization of a persistent quinone methide, which was stabilized by bulky substituents, to a mu 'BU benzocycloheptafuran (Eq. 2)[21. As a plausible intermediate, the corresponding 1,3 diradical has been suggested, which after cyclization to the norcaradiene and valence isomerization affords the cycloheptatriene. On the other hand, Padwa investigated the photodecarbonylation of a 2-benzofuranone to the transient quinone methide, which on further photolysis gave the corresponding xanthene (Eq. 3) [31. n In this paper we report on the photochemical behavior of the 3-phenyl-substituted quinone methides 3a, b (Scheme 1). Epoxidation of 3-phenylbenzofurans 1 a, b by dimethyldioxirane afforded the epoxide 2a and the quinone methide 3b as confirmed by NMR spectroscopy ["]. In view of the reversible valence isomerization 2 a + 3a, the concentration of 3a is high enough in the sample of 2a for effective photolysis. Indeed, 3a in the colored solution of 2a shows an absorption with a A, , , , , = 411 nm and a strong tailing up to 575 nm, while 3b starts absorbing at 570 nm with no absorption maximum below this wavelength.Irradiation of 2a and 3b in acetone at -30°C and h > 400 nm afforded essentially quantitatively ('H NMR) the benzocycloheptafurans 4a, b. In contrast to the 2,3-dimethylbenzofuran derivatives of Eq. 1 [lfl, no benzoxetenes were detected. The acid-and base-sensi...

Visible light-driven dearomative ring expansion of (aza)arenes to access dihydrofuran-based polycyclic compounds

“… 28 Although this synthetic issue was subsequently solved by Adam and co-workers using dimethyldioxirane oxidation of benzofurans, the requirement for specific tetra -substituted olefins remains unavoidable. 29 In addition, we hypothesized that the exothermal aromatization process might be another crucial point for the success of these chemical transformations. It is worth mentioning that cycloheptatrienes are prone to undergo 4π electrocyclization after absorption of photon energy, 30 which further compromises the yield and represents a challenge for chemoselectivity.…”