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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...
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...
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 )...
The oxidation of the benzofurans 1a–f (tetrahydrobenzofurans 1g, h) with excess m‐CPBA is reported. The in situ generated, highly reactive benzofuran epoxides 2a–f and their quinone methides 3a–f (cis‐ene diones 3g, h) afford the labile tautomeric peroxy esters 5 and 5′ by nucleophilic addition of the peroxy acid. On elimination of m‐chlorobenzoic acid, the peroxy esters 5/5′ of the benzofuran derivatives 1a–f rearrange thermally to the keto esters 6 by CC cleavage or to the spiro epoxides 7 by CO cleavage. The latter undergo thermal isomerization to the 1,3‐benzodioxoles 8 and Diels‐Alder cycloaddition to the corresponding dimers 9. Independently, the keto esters 6 and the 1,3‐dioxoles 8 were synthesized by thermolysis of the dioxetanes 11. The tautomeric m‐CPBA adducts 5/5′ of the persistent ene diones 3g, h, derived from the tetrahydrobenzofuran derivatives 1g, h, rearrange as well to the spiro epoxides 7g, h. In contrast to the benzofuran derivatives 6a–f, the keto enol ester 6h suffers Baeyer‐Villiger rearrangement with another molecule of m‐CPBA to form the ene diester 10h.
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