Furans photoadditions

Analogous photoadditions have been observed on irradiation of pyrimidine and purine bases in ethers and amines. Irradiation of 1,3-dimethylur-acil (314) in tetrahydrofuran leads to the formation of 5- and 6-(tetrahydro-furan-2-yl)-5,6-dihydrouracils 315 and 316.261 Similarly, solvent adducts arising by way of initial hydrogen abstraction have been obtained on irradiation of pyrazine derivatives in diethyl ether or tetrahydrofuran.262 The... 

The double bonds in certain heterocyclic compounds, such as furans, Af-acylpyrroles and A-acylindoles are also susceptible to photoaddition of carbonyl compounds to form oxetanes (equation 106) (77JHC1777). A wide range of carbonyl compounds can be used, including quinones, a-diketones, acyl cyanides, perfluorinated aldehydes and ketones and esters. A remarkable case of asymmetric induction in oxetane formation has been reported from optically active menthyl phenylglyoxylate and 2,3-dimethyl-2-butene the oxetane product obtained after hydrolysis of the ester group had an optical purity of 53% (79AG(E)868). 

The photoaddition of benzo[c]furan to a variety of alkenes has been described. With cycloheptatriene the [4 +4] adducts (320) and (321) and the [4 + 6] adducts (322) and (323) are obtained in addition to the photodimer (301) and the photooxidation product 1,2-dibenzoylbenzene. The [4 + 4] adducts may be formed in a concerted manner via an exciplex intermediate in a 7r-7r singlet state. The [4+6] adducts, formally symmetry forbidden,... 

The photoaddition of nitrobenzene to cyclohexene at — 70° results in the formation of the first reported 1,3,2-dioxazole [Eq. (93)].342 A novel dimerization is observed in the photolysis of 2-methoxy-6-phenyltropone (318) to the furan derivative (319) in low yield.343 This is regarded as the first 1,8-dipolar photoaddition to be observed. 

In certain cases benzene will undergo photocycloaddition to oxygen and sulfur heterocycles. The two major photoproducts of irradiation of a mixture of furan and benzene are adducts 356 and 357, arising, respectively, by [ 2 + 2] and [ 4 + 4] cycloaddition processes.306 Irradiation of benzene and 2,2-dimethyl-l,3-dioxol (358) similarly affords adducts 359 and 360, together with dimer 361.307 Thiochromone 1,1-dioxide also undergoes photoaddition to benzene.308... 

There are few new authenticated examples of [ 4 + 4] photoadditions in oxygen- and sulfur-containing heterocycles. 1-Cyanonaphthalene (362), for example, undergoes an addition of this type to furan on irradiation in benzene to give adduct 363 in high yield.309 Addition of 2-cyanonaphthalene... 

Synthesis of oxetanes by photoaddition of benzophenone to furans. Journal of Organic Chemistry, 32, 2918-2920 ... 

Bos and coworkers have developed several applications of alkyne-ketone photoadditions to organic synthesis. 2 Irradiation of a solution of acetone and 1-methylthio-l-propyne gives as a major product the cycloadduct (67) this results from selective oxetane formation due to radical stabilization by the methyl-thio group, followed by electrocyclic ring opening. Similarly, photoreaction of benzil and 1-t-butylthio-1-propyne gave the adduct (68) in 45% yield, which was transformed in 90% yield to the furan (69). 

A recent application of the furan-carbonyl photocycloaddition involved the synthesis of the mycotoxin asteltoxin (147)." Scheme 16 shows the synthetic procedure that began with the photoaddition of 3,4-dimethylfuran and p-benzyloxypropanal to furnish photoaldol (148), which was epoxidized with MCPBA to afford the functionalized product (149) in 50% overall yield. Hydrolysis (THF, 3N HCl) provided the monocyclic hemiacetal which was protected as its hydrazone (150). Chelation-controlled addition of ethylmagnesium bromide to the latent a-hydroxy aldehyde (150) and acetonide formation produced compound (151), which was transformed through routine operations to aldehyde (152). Chelation-controlled addition of the lithium salt of pentadienyl sulfoxide (153) followed by double 2,3-sigma-tropic rearrangement provided (154) as a 3 1 mixture of isomers (Scheme 17). Acid-catalyzed cyclization of (154) (CSA/CH2CI2) gave the bicyclic acetal (155), which was transformed in several steps to ( )-asteltoxin (147). ... 

A direct solution to the problem of enantioselective furan-carbonyl photoproduct synthesis would require a photoaddition that proceeded with enantiofacial selectivity in the aldehyde or furan component and maintained the relative face selectivity that is intrinsic to the reaction. Preliminary work used host-guest chemistry to achieve this objective.In aqueous dioxane, a 1 1 1 inclusion complex of unmodified 3-cyclodextrin, furan and benzaldehyde is formed. Upon irradiation (Hanovia 450 W lamp, Vycor filter), a rapid photoaddition occurs to afford a photoproduct of 10-20% ee. 

One of the key steps used in a new synthesis of the bis(tetrahydrofuran) moiety of Asteltoxin (94) is the photoaddition of the propanal (95) to 3,4-dimethylfuran, yielding the adduct (96). This cycloaddition is a common outcome of the irradiation of aldehydes or ketones with furans. An analogous adduct (97) results from the photoreaction of butyl glyoxalate with 2-methylfuran. Two other products [(98) and (99)] are also formed, the first of which is presumably the result of ring opening of the isomeric oxetane (100), while (99) is produced by a hydrogen abstraction radical coupling pathway. 

A further report of the photocycloaddition reactions of benzothiophenes has been made. The two photoproducts (199) and (200) from the benzophenone-sensitized cycloaddition of bromoethene to the 2,3-dichloro-derivative (201a) have been demonstrated to have the stereochemistry shown.The photoaddition of cis-dichloroethene to (201a) is stereoselective and affords predominantly the adduct (202). When 2,3-dibromobenzothiophene (201b) is used with bromoethane, a different reaction path is followed, yeilding (203). Linear products are also obtained with dichloroethene and (201b). [2 + 2] Photocycloaddition of furan or thiophene to the iso-oxazolines (204) affords the adducts... 

Photoinduced reactions of ethylenes and acetylenes with five-membered heteroarenes continues to be of interest to a number of groups. Mukai and co-workers have shown that 3-aryl-2-isoxazolines (65) undergo [2 r + 2 r] photoaddition to furan and thiophene to give the novel adducts (66) and (67), respectively. The reaction provides a further example of the rarely observed... 

Alcohols can undergo photoaddition to alkynes, and the products from acetylenic esters are Y-hydroxy-rra//j-a,j3-unsaturated esters or the unsaturated lactones derived from the cis isomers by cyclization (equation 21). Reaction with acetylenic ketones provides a route to furans (equation 22) , and there is n.m.r. evidence for... 

The photoaddition of benzophenone to the vinyl sulphides (54) affords a mixture of the oxetanes (55) in yields ranging from 12 - 79X dependent on the alkene used. - Photoaddition of aldehydes to 2,3-dihydrofurans has been studied. The addition affords oxetanes of the type shown by (56) where addition of benzaldehyde affords two isomers. Addition to the furan derivative (57) was also studied. In this instance the addit ion of aldehydes afforded two types of oxetane illustrated by (58) and (59). ... 

The photoaddition of a furan and an aldehyde can serve as a photochemical version of a stereoselective aldol reaction, since the photoadduct can be viewed as a protected aldol, as indicated in Scheme 23 (Schreiber et al., 1983). 

The intramolecular (4n + 4ti) photoaddition of furan-pyran-2-one bichromo-phores [e.g. (38) to give (39)] is potentially useful as an approach towards the fusicoccane/ophiobolane skeleton (Chase et al.), and the formation of (40) from the irradiation of (41) constitutes the first example of a (47t + 4ji) photocycloaddition of an enone to the benzene ring (Kishikawa et al.). Wender et al. continue to report elegant new applications of the intramolecular meta photocycloaddition of 5-phenylpent-l-ene derivatives and have described the synthesis of cis,cis,cis, traMi-[5,5,5,5]-fenestrane (42) using the photoreaction of (43) as a key step. Park... 

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... 

Photoaddition of the ynone (47) to 1,1-diethoxyethene gave the oxetan (48) (50%), by a conventional (2+2)-cycloaddition route, as well as the furan derivative (49) (18%). The formation of this latter product arises by way of the carbene (50), itself a product of (3- -2)-addition of the ynone to the alkene. Intramolecular trapping of the carbene... 

Substituted furans, which are equally difficult to prepare by direct substitution into the uran nucleus, can be obtained by a variety of novel cycllzation reactions, or by acid-catalyzed isomerization of the adducts (oxetanes) obtained by photoaddition of furan with aldehydes. ... 

Details are now presented of studies on the benzene-furan system, in which a [2 + 2]cycloadduct (43) and a [4 + 4]adduct (44) are the major products adduct (44) can be converted thermally or photochemically into (43). This photoaddition is the first example involving two monocyclic aromatic compounds benzene and thiophen give products in only very low yield on irradiation. 

Reactions involving excited states of benzene include the photoaddition of benzene to furan,41 to olefins,42 and to amines,43 the photochemical chlorination of benzenes,44 the photo-oxidation of benzene,48 and the photo-reactions of tetrafluoroiodobenzene.48 These papers will be discussed in detail in Part 1 of this volume. The gas-phase structures of benzvalene and hexamethylprismane and hexamethylbenzene have been evaluated.47... 

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