Toluene photocycloaddition with

Photocycloaddition of allene to cyclohexenone (341) gave the (3,y-enone (342), which reacted with vinyl magnesium bromide to produce the tertiary alcohol (343) in 79% yield. When the compound (343) was treated with KH and 18-crown-6 in THF at room temperature for two hours and quenched with aq. NH4C1, the cyclobutene (344) was obtained. The thermal ring opening of the cyclobutene (344) proceeded in toluene in a sealed-tube at 180 °C for twelve hours to give a readily separable 5 1 mixture of the civ-olefin (345), and the trans-olefin (346) respectively in 95 % yield. Moreover, (345) could be converted to a mixture of (346) and (345) in the ratio of 10 1 by irradiation. The compounds (345) and (346) possess the skeleton of the germacranes (347), (348) and (349) 122). 

In contrast to the [2-1-2]-photocycloaddition which is a widely used method to generate four-membered rings, Koreeda and Zhang used a thermal rearrangement key step to construct the tricyclic framework of kelsoene [23]. In earlier work, it was shown that upon treatment with base, y-keto-p-toluene-sulfonate rac-32 is converted to a 44/56 mixture of bicyclo[3.1.1 ]heptanone rac-33 and bicyclo[3.2.0]heptanone rac-34 (Scheme 10) [24-26]. 

Ohashi et al. [128] found that the yields of ortho photoaddition of acrylonitrile and methacrylonitrile to benzene and that of acrylonitrile to toluene are considerable increased when zinc(II) chloride is present in the solution. This was ascribed to increased electron affinity of (meth)acrylonitrile by complex formation with ZnCl2 and it confirmed the occurrence of charge transfer during ortho photocycloaddition. This was further explored by investigating solvent effects on ortho additions of acceptor olefins and donor arenes [136,139], Irradiation of anisole and acrylonitrile in acetonitrile at 254 nm yielded a mixture of stereoisomers of l-methoxy-8-cyanobicyclo[4.2.0]octa-2,4-diene as a major product. A similar reaction occurred in ethyl acetate. However, irradiation of a mixture of anisole and acrylonitrile in methanol under similar conditions gave the substitution products 4-methoxy-a-methylbenzeneacetonitrile (49%) and 2-methoxy-a-methylbenzeneacetonitrile (10%) solely (Scheme 43). 

Stable spirocyclic aminothietanes 25 (R = Me or H), obtained by photocycloaddition iV-alkoxy- and iV-aryloxycar-bonylbenzoxazole-2-thiones with alkenes upon reflux in toluene, were transformed into iminothietanes 26 (path a) and/or 2-substituted benzoxazole 27 (path b) (Scheme 2) < 1999J(P1) 1151, 2002HCA2383>. 

The formation of trans-products is observed to a lesser extent in the reaction of 3-alkoxycarbonyl-substituted cyclohexenones, in the reaction with electron-deficient alkenes and in the reaction with olefinic reaction partners, such as alkynes and allenes, in which the four-membered ring is highly strained (Scheme 6.11). The ester 26 reacted with cyclopentene upon irradiation in toluene to only two diastereomeric products 27 [36]. The exo-product 27a (cis-anti-cis) prevailed over the endo-product 27b (cis-syn-cis) the formation of trans-products was not observed. The well-known [2 + 2]-photocycloaddition of cyclohexenone (24) to acrylonitrile was recently reinvestigated in connection with a comprehensive study [37]. The product distribution, with the two major products 28a and 28b being isolated in 90% purity, nicely illustrates the preferential formation of HH (head-to-head) cyclobutanes with electron-acceptor substituted olefins. The low simple diastereoselectivity can be interpreted by the fact that the cyano group is relatively small and does not exhibit a significant preference for being positioned in an exo-fashion. 

There are few reports of photocycloaddition reactions of di- and poly-ynes which involve more than one of the alkyne groups, but the production of toluene or other alkylbenzenes in the irradiation of butadiyne with propene or other terminal alkenes (equation 60) is one such process. ... 

Very few asymmetric [4+2] photocycloadditions have been described in the literature [11]. However, reported examples are usually found to proceed with good levels of selectivity. The intramolecular [4+2] photocycloaddition of the diaryl-ethene derivative (114) was found to proceed smoothly with good diastereoselec-tivity (de = 86.6% in toluene at -40 C) (Scheme 27) [155]. When the dielectric constant was increased slightly, the de was found to increase this was also the case as the temperature was lowered. The mechanism of the asymmetric induction is described in detail. 

Further examples of mixed [2 + 2] photocycloadditions, in which various chiral auxiliaries were attached to the alkene. are shown for the reactions of 3-methyl-2-cyclohexenone with the chiral enoates 6a-c75. Irradiation of a toluene solution gave the cis-anti-cis, head-to-head adducts as inseparable mixtures of diastereomers 7a-c and 8a-c. Reduction and spontaneous lactonization of the adduct mixture with sodium borohydride in refluxing methanol led to the formation of the lactone 9 as an unequal mixture of enantiomers. 

In addition to the examples with alkene substitution, photocycloadditions of enones containing optically active ester substituents were investigated. Irradiation of a toluene solution of cy-clopentene and the chiral enone esters 13a-c gave two types of diastereomeric adducts, cis-anti-cis 14a-c and cis-syn-cis 15a-c77. By varying the size of the chiral auxiliary R the cyclobutane ring junctions syn- vs. anri-adducts) as well as the degree of asymmetric induction is altered. It is possible to obtain high enantiomeric excesses (e.g., 79% for 15a). 

The 5,6-double bond in uracils is involved in several [2 + 2] photochemical reactions. The cyclo-octapyrimidine-2,4-dione (338) is formed by photocycloaddition of the A-substituted 6-chlorouracil (336) to benzenes in the presence of an acid, preferably TEA (Scheme 57). The reaction is assumed to proceed by orr//o-addition, with a cyclobutane (337) as intermediate. Without acid, 6-phenyluracil is formed in low yield. In toluene as solvent and participating reagent, a regioisomeric mixture with the methyl group in the 6-, 7-, 8-, or 10-position was formed. In fluorobenzene, all of the regioisomers... 

The dimesitylbotyl borepin (169) underwent [2 + 2] photocycloaddition to give (169). A cast firm of (169) from a toluene solution showed photochromic behaviour with a colour change from colourless to deep blue. On the other hand, the bis-boron bridged stilbene (170) upon 254 nm irradiation afforded the novel boron-containing heterocyclic compound (171). ... 

Upon irradiation with X > 300 nm, the 4-alkenyloxy-2-quinolones 23 and 25 cyclized in an intramolecular [2+2]-photocycloaddition diastereoselectively to the crossed and straight cycloadducts 24 and 26, respectively. In the presence of 2.6 equivalents of host compound 21 or ent-21, high enantioselectivities were achieved in a nonpolar solvent at low temperatures." " At -60°C in toluene, 24 and 26 were obtained with >90% ee (Scheme 10). Higher temperatures (84% ee at -15°C, 39% ee at 30°C) and more polar solvents (4% ee at 30°C in acetonitrile) significantly reduced the enantioselectivity under otherwise identical conditions. 

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