Quinones photocycloaddition

The photocycloaddition of arylazirines with a variety of multiple bonds proceeds in high yield and provides a convenient route for the synthesis of five-membered heterocyclic rings. Some of the dipolarophiles include azodicarboxylates, acid chlorides, vinylphosphonium salts and p-quinones. 

Many other photocycloadditions with quinones have been reported/14 ... 

A very remote secondary H/D isotope effect has been measured for the 2 + 2-cycloaddition of TCNE to 2,7-dimethylocta-2,fran -4,6-triene. The reaction of nitric oxide with iV-benzylidene-4-methoxyaniline to produce 4-methoxybenzenediazonium nitrate and benzaldehyde is thought to proceed via a 2 + 2-cycloaddition between nitric oxide and the imine double bond. A novel mechanism for the stepwise dimerization of the parent silaethylene to 1,3-disilacyclobutane involves a low-barrier [1,2]-sigmatropic shift. Density functional, correlated ab initio calculations, and frontier MO analysis support a concerted 2 + 2-pathway for the addition of SO3 to alkenes. " The enone cycloaddition reactions of dienones and quinones have been reviewed. The 2 + 2-photocycloadditions of homochiral 2(5H)-furanones to vinylene carbonate are highly diastereoisomeric. ... 

The photocycloaddition of 3-acetyl-2(3//)-oxazolone 84 with 9,10-phenanthrene-quinone 244 gives the spirooxetane 245 regioselectively as a major cycloadduct. Irradiation of 3,4,5-trisubstituted-2(37/)-oxazolones 247 in the presence of 244 results in the predominant formation of dioxane derivatives 248. When duroquinone 249 is irradiated in the presence of the 3,4,5-trisubstituted-2(37/)-oxazolone 247a,... 

If the triplet energy of the carbonyl compound is below that of the diene, triplet-triplet transfer will become inefficient and photocycloaddition may occur. For example, the triplet energy of 1,4-benzo-quinone is about 50 kcal mole - V41 and photocycloaddition to dienes can indeed occur.42 The products are spiro-pyrans 13 (not vinyl oxetanes), which may arise via the allylic radical intermediate 12. 

Oxetane Formation—The Patemo-Bnchi Reaction. A large number of carbonyl compounds, primarily aldehydes, ketones, and quinones, form oxetanes by photocycloadditions to olefins.61-63 In general, it is observed that (/) carbonyl compounds which have low-lying (77, ) triplet states and which are photoreduced in isopropyl alcohol form oxetanes most readily, and (2) oxetane formation takes place when energy transfer from the carbonyl compound to the olefin is unfavorable because of the relative location of their triplet levels.64,65 Hence, oxetanes are most readily formed from simple olefins and allenes63,66 but are seldom formed from dienes.67 An extensive review by Arnold63 covers the mechanism and scope of this reaction. 

Photochemical reactions of quinones with allenes have also been studied and in some cases cyclobutane formation occurs, although in competition with products derived from attack of the allene on the carbonyl oxygen. Thus, photocycloaddition of tetramethyl-l,4-benzoquinone with 1,1-dimethylallene affords the four-membered carbocycle 6 in good yield.12... 

The intermolecular [2 + 2]-photocycloaddition of para-tetrahydronaphthoqui-nones has been applied by Ward et al. to the synthesis of cyathin diterpenes [52], An example is represented by the total synthesis of ( )-allocyathin B3 (46), during the course of which the diastereoselective [2 + 2]-photocycloaddition of allene to substrate 44 served as one of the pivotal steps (Scheme 6.17) [53]. The addition delivered a mixture of regioisomers (r.r. = 80/20), from which compound 45 was separated. The facial diastereoselectivity was perfect due to the concave shape of the quinone. 

The photochemical addition of 2H-azirines to the carbonyl group of aldehydes, ketones and esters is completely regiospecific (77H143). Besides the formation of the isomeric oxazolines 18 from 3 and ethyl cyanoformate, there is also formed the imidazole 19 from addition to the C = N in the expected regioselective manner. Thioesters lead to thiazolines 20, while isocyanates and ketenes produce heterocycles 21 (Scheme 4). The photocycloaddition of arylazirines with a variety of multiple bonds proceeds in high yield and provides a convenient route for the synthesis of five-membered heterocyclic rings. Some of the dipolarophiles include azodicarboxylates, acid chlorides, vinylphospho-nium salts and p-quinones. 

It is possible to form cyclobutene adducts from a quinone which has a lowest (n,n ) excited state by a sequence involving protection of the quinone as a Diels-Alder adduct with anthracene (equation 95) . The enedione which undergoes photocycloaddition with alkyne reacts at the C=C bond rather than at the C=0 bond (see also equation 48). 

The photocycloaddition of tran -piperylene to the quinone (485) yields the oxetan (486). From a study of this and other dienes e.g., cyclopentadiene and cyclohexa-1,3-diene), it was concluded that the addition involves an exciplex where both the singlet and the triplet states are involved. 

A detailed account of the (2 + 2)-photocycloaddition reactions of homo-quinones has been published. The addition reactions afford regio and exo.endo isomers of tricyclic diones. The major product is thought to arise from the more stable triplet 1,4-biradical intermediate.A review has reported on the addition of alkenes and alkynes to the enone system of homobenzoquinones. Dibromo-9-(3-oxo-3-phenylprop-l-ynyl)-[9 -fluoren-9-yl-3-fluoren-9-ylidene-1-phenylpropenone is photochromic in the crystalline state. 

Intramolecular [2 + 2] photocycloaddition of the quinone moiety of quinopimaric acid (48) to the isopropyl-substituted double bond leads to a polycyclic cage compound." 2-Hydroxynaphthoquinones undergo a formal [3 + 2] photocycloaddition with different cyclic alkenes, delivering chiral products (49). In the presence of a chiral template, a limited enan-tioselectivity is observed." ... 

An extensive review of 2 + 2-photocycloadditions of dienones and quinones has been published. The photocycloaddition of heterocyclic 2,3-diones (18) with electron-rich alkenes in the presence of the photosensitizer benzophenone yields 2-1-2-cycloadducts (19) and (20) with high regio- and stereo-selectivity (Scheme 1) Time-resolved spectroscopy has shown that in the photochemical cycloaddition between stilbenes and quinones, the (singlet) ion-radical pair [S ", Q+ ] is the primary reaction intermediate and therefore establishes the electron-transfer pathway for this typical Paterno-Biichi transformation. A kinetic study of the Paterno-Biichi cycloaddition of stilbene to chloranil shows that solvent polarity and donicity control the formation as well as the reaction path of the ion-radical. The photoirradiation of chloranil with... 

The chemical selectivity in the photocycloaddition of ethylene (78) to benzene (77) was discussed. Diels-Alder cycloadducts of 1,2-benzo-quinones (82) with benzynes generated from (83) were converted to naphthalene derivatives (85) by photoinduced double decarbonylation of (84) (Scheme 22). ... 

The photocycloaddition of triplet benzophenone to norbornene was originally reported by Scharf and Korte. The photoproduct 101 that is formed in high exo-selectivity could be thermally cleaved to the 5,e-unsaturated ketone 102, an appHcation of the carbonyl-olefin metathesis (COM) concept. The 1,4-biradical formed in the interaction of norbornene with o-dibenzoyl-benzene was trapped in an intramolecular fashion by the second carbonyl moiety. A highly regioselective reaction of triplet benzophenone was reported with 5-methylenenorborn-2-ene, with preferential attack toward the exo CC double bond. A number of publications have discussed the photocycloaddition reactions of triplet carbonyl compounds to norbornadiene and quadricyclane, as weU as the competition between the Paterno-Biichi reaction and the sensitized norbornadiene/quadricyclane interconversion. Oxetane formation has also been reported for the photoreaction of biacetyl and para-quinones with benzvalene. ... 

Styrene derivatives are commonly used addends in the photocycloaddition studies of 1,4-quinones. With Z- and -anethole, 1,4-benzoquinone (BQ), 1,4-naphthoquinone (NQ), and 9,10-anthraquinone in acetonitrile solvent yield spiro-oxetanes in which the trans-isomer (e.g., 4 from naphthoquinone) predominates. The process has been studied in detail by CIDNP techniques from which it is deduced that product formation proceeds from triplet radical ion pairs to the triplet biradical, and that there is no significant contribution from direct conversion of exciplex intermediates into the biradicals. Spiro-oxetane formation between simple alkenes and BQ generally has low regioselectivity but this is markedly improved with alkylidene cyclohexanes (Figure 87.3) such that the major isomer can be used as a new access to useful synthetic building blocks. For the BQ/homobenzvalene 5 system, however, where the difference in stability between the intermediate biradicals can be expected to be considerably less, the selectivity ratio for the spiro-oxetanes 6 and 7 is reduced to 3 1, respectively, and the addition to NQ yields only the cyclobutane derivative 8. Quadricyclane and norbornadiene undergo the same photocycloaddition reaction to BQ, affording the oxolane 9 and the spiro-oxetane 10. Evidence from CIDNP... 

Goez, M. and Frisch, 1., Photocycloadditions of quinones with quadricyclane and norbornadiene... 

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