Furans photochemical additions

The methylfuronaphthoquinone (33) has been obtained by reductive cyclization of the naphthoquinone (32) with zinc and hydrochloric acid and subsequent oxidation by chromic acid (74CJC88). Furans (35) have been prepared in excellent yields starting with the photochemical addition of an a-diketone to an alkylthioacetylene (34) followed by reductive... 

Hammond and Turro279 recently described a photochemical addition reaction of benzophenone and furan ... 

Photochemical addition of alkenes to conjugated a-diketones leads to hiradicals of type 15 that form allyl carbene intermediates of type 16 which readily isomerize to furnish tetrasuhstituted furans in good yields (Scheme 17) <1996JOC3388>. 

One limitation encountered in synthetic studies of this reaction is the lack of chemoselectivity in the addition of aldehydes to unsymmetrically substituted furans. For example, the photochemical addition of benzaldehyde to 2-methylfuran provides a 1 3 1 mixture of oxetanes resulting from the exo addition of aldehyde to the more and less substituted alkenes, respectively. Ketone photocycloadditions are more selective. For instance, irradiation of 2-butanone and furan forms a 7 1 mixture of regioisomeric oxetanes (167) and (168) (1 1 mixture of epimers). 

Photochemical addition of furan to Pummerer s ketone (121) affords the two (4+2)-cycloadducts (122) and (123). The structures of both of these products has been established by X-ray crystallography. Spectroscopic studies on the enone (121) suggest that the triplet state is highly twisted and the authors suggest that the addition to furan results in the formation of two biradicals which lead to the observed products. No evidence for the involvement of a trans-ground state enone was obtained. ... 

The photochemical reactions of the bicyclic compounds (201 a, c) in acetonitrile-methanol solution in the presence of 1,4-dicyanobenzene have been studied. The irradiation of (201a) affords the mixture of trans cis methoxy derivatives (201b) which are also photochemically reactive under the same conditions. The products from (201a) are formed by the photochemical addition of methanol to the furan (202). This furan is formed by production of the radical cation of (201a) followed by deprotonation affording a doubly benzylic radical. The study was extended to the monocyclic system (203) which on electron-transfer-induced irradiation afforded the acetal... 

The photoaddition of aldehydes or ketones to furan has been reported over the years. Griesbeck and his co-workers have established that the diaster-eoselectivity of the addition of benzaldehyde to furan is 212 1 for the formation of the exo and endo products (98) and (99). The study was extended to the addition of other carbonyl compounds (100) and the ratio of products from these additions is shown under the appropriate structure (101) and (102). L-Ascorbic acid and some of its derivatives (103) also undergo photochemical addition of aromatic aldehydes and ketones. With benzaldehyde and benzophenone the products obtained are the mixture of (104) and (105) with a preference for the formation of the former. The stereochemistry of the addition of the excited state carbonyl compound to ascorbic acid favours the path where the phenyl and the alkoxy groups are cis on the resultant oxetane. Benzaldehyde adds photochemically to the enamine (106) when the mixture is irradiated in acetonitrile solution. Three products are... 

The photochemical addition of biacetyl to indene and furan affords the oxetans (341a) and (341b) as the major products. This result confirms the earlier reports. From the kinetic study of these reactions and from the reaction of... 

Other papers of interest in connection with azetidines include further details of the photochemical formation of azetidine derivatives from 2-cyanopyrroline N-oxide, syntheses of 2- and 3-pyridylazetidines (as nicotine analogues), the photochemical addition of the isoxazoline (7) to benzene, furan, and thiophen, and several examples of [3 + 1] addition of isocyanides to azomethine ylides. ... 

Senboku, H., Kajizuka, Y., Kobayashi, K., Tokuda, M., and Suginome, H., Photoinduced molecular transformations. 160. Furan annelation of 2-hydroxynaphthoquinone involving photochemical addition and radical fragmentation xeclusion of the intermediacy of [2-1-2] cycloadduct in a one-pot formation of furanoquinones by the regioselective 3-1-2 photoaddition of hydroxyquinones with alkenes. Heterocycles, 44, 341,1997. 

A-Substituted pyrroles, furans and dialkylthiophenes undergo photosensitized [2 + 2] cycloaddition reactions with carbonyl compounds to give oxetanes. This is illustrated by the addition of furan and benzophenone to give the oxetane (138). The photochemical reaction of pyrroles with aliphatic aldehydes and ketones results in the regiospecific formation of 3-(l-hydroxyalkyl)pyrroles (e.g. 139). The intermediate oxetane undergoes rearrangement under the reaction conditions (79JOC2949). 

When methyl radicals are generated from diacetyl peroxide they give substitution products with monoalkylfurans, but 2,5-dimethylfuran is hardly attacked (Scheme 41) (73TL637). f-Butoxy radicals generated from t- butyl hydroperoxide and Fe2+ in methanol react with furan to give addition products (64G67), but when generated photochemically from t- butyl peroxide they convert 2-methylfuran into the furfuryl radical, identified by ESR spectros-... 

Pyrones undergo a variety of photochemical reactions. A photochemical reaction of 2-ethyl-3-methyl-4-pyrone in sulfuric acid gave, in addition to the 2-pyrones, the 2-acetyl-5-ethylfuran (Scheme 86). The yield of furan was dependent on the concentration of the acid... 

The allyl ethers of phenols may be converted via a Claisen rearrangement to o-allyl derivatives and thence to the benzo[6]furans. The products of addition of bromine to the o-allylphenols give good yields of the heterocycle on prolonged heating with alkali, but the photochemical rearrangement of O-allylphenol is not so effective. 

Examples of photochemically driven carbon-carbon bondforming reactions emanating from the radical cations formed by photoinduced electron transfer include the addition of diphenyl-ethylene to furan, eq. 26 (80),... 

Cycloaddition reactions with furans remain a source of new compounds. On photochemical excitation of benzofuro-annulated oxanorbomadiene 42 in the presence of dipolarophiles Ill-addition compounds are obtained. The formation of 43 constitutes one of the rare examples of the cycloaddition of a nitrile as a dipolarophile <95LA1503>. 

The photocycloaddition of benzophenone to furan 87a was originally described by Schenck et al. [79]. Additionally to the 1 1 adduct 88 also two regioisomeric 2 1 adducts 89a,b were isolated [80], the structure of 89a was revised by Toki and Evanega [81]. All prostereogenic carbonyl addends when photochemically added to furan showed regioselectivities >99 1 in favor of the bicyclic acetal product (Sch. 25). 

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