Photocycloadditions intramolecular

Whereas the cycloaddition of arylazirines with simple alkenes produces A -pyrrolines, a rearranged isomer can be formed when the alkene and the azirine moieties are suitably arranged in the same molecule. This type of intramolecular photocycloaddition was first detected using 2-vinyl-substituted azirines (75JA4682). Irradiation of azirine (54) in benzene afforded a 2,3-disubstituted pyrrole (55), while thermolysis gave a 2,5-disubstituted pyrrole (56). Photolysis of azirine (57) proceeded similarly and gave 1,2-diphenylimidazole (58) as the exclusive photoproduct. This stands in marked contrast to the thermal reaction of (57) which afforded 1,3-diphenylpyrazole (59) as the only product. 

Numerous examples of intermolecular and intramolecular photocycloaddition to heterocyclic systems (including the dimerization of individual heterocycles) have now been reported. Two types of cycloaddition can readily be effected photochemically, namely, [n2 + 2] and [ 4 + 4] additions. Although concerted suprafacial additions of this type are allowed photochemical processes, in reality many cycloadditions occur via diradicals, zwitterions or exciplexes. 

A novel intramolecular photocycloaddition involving vinylogous amides and allenes led to an interesting type lb entry to functionalized pyrroles <060L4031>. For example, photolysis of allene 11 provided fused pyrrole 12 via a [2+2] cycloaddition and retro-Mannich reaction. 

The unsaturated aldehyde in (2.2) very often undergoes intramolecular photocycloaddition with formation of an oxetane (cf, chapter 4.3.6). The... 

The second reaction mode is rearrangement of the ketocarbene to a ketene. In the presence of a C—C double bond this species reacts further via an intramolecular photocycloaddition (cf. chapter 4.3.3), as shown in (2.23) 238). 

The addition of the 2-substituted 2-cyclopentenone in (4.57) to ethylene affords an intermediate which is converted to tricyclo[4.2.0.01,4]octane 469a). A similar broken window compound was also obtained by an intramolecular photocycloaddition of a 2-cyclopentenone (4.58)469b). 

Dithioamides behave similarly186 (equation 131) but, in the presence of steric hindrance in addition to the thiocarbonyl groups, the photoaddition will become regioselective187 (equation 132). Intramolecular photocycloaddition will lead to the formation of strained polycyclic thietanes188 (equation 133). Hydrogen abstraction by the thioamide functional group is also possible189 (equation 134). 

One of the exciting areas that has gained importance over the recent decade is the photochemical cyclization of non-conjugated dienes in the presence of species that can act as templates. One such species that has been used is copper(I) salts. The earliest example of the use of copper salts in the intramolecular photocycloaddition of non-conjugated dienes is that described for cycloocta-1,5-diene. When this is irradiated in the presence... 

The Dauben-Walker approach has yielded the smallest and most strained fenestrane known to date Following the intramolecular Wadsworth-Enunons cyclization of 443 which also epimerizes the butenyl sidechain to the more stable exo configuration, intramolecular photocycloaddition was smoothly accomplished to provide 444. Wolff-ELishner reduction of this ketone afforded the Cj-symmetric hydrocarbon 445. Application of the photochemical Wolff rearrangement to a-diazo ketone 446 p,ve 447. 

Section 8.11.6.2.1) <2007ARK(viii)7>, and to the /ftt-amino effect <2006RCB384>. The intramolecular photocycloaddition of novel sulfur-substituted l,3-dioxin-4-ones (Equation 98) has been described <2006JA9040>. 

Besides the l-(but-3-enyl)allyl alcohols discussed above, 2- and 3-(pent-4-enyl)allyl alcohols also undergo clean photobicyclization in the presence of copper trifluoromethanesulfonate as catalyst. However, no intramolecular photocycloaddition product could be isolated from irradiation of a homologous 2-(hex-5-enyl)allyl alcohol.8... 

A novel entry to decahydrocyclopentacyclooctene derivatives via the intramolecular photocycloaddition of fused a,/3-unsaturated y-lactones has been developed (80CC1011). Irradiation of the butenolide (153) in acetone solution gave both the fused and bridged photoadducts (154) and (155) (2-3 1). The major adduct was hydrolyzed, oxidized and esterified to afford (156). Reductive cleavage of the unsaturated keto ester (156) with lithium in ammonia afforded a five-component mixture of a,/3- and /3,y-unsaturated esters. Equilibration with 0.1M sodium methoxide in methanol converted the mixture into a single a,j8-unsaturated ester (157 Scheme 34). This annelative two-carbon ring expansion method may find application in the synthesis of ophiobolin and ceroplastol sesterterpenes. 

An intramolecular photocycloaddition of unspecified quantum efficiency has been reported for l,5-hexadien-3-one.429 Again the product may be postulated to result from closure to a biradical with a five-membered ring. 

The tetrahydro-2-quinolone (212) affords a very small yield of 1,2-cycloadduct on irradiation with diphenylacetylene in methanol.189 The two major products are the 1,4-dimer and the pentacyclic derivative (213), arising presumably by intramolecular photocycloaddition... 

Powdered single crystals of 1 (ca. 20 mg) were sandwiched between two Pyrex cover glasses and placed in a polyethylene bag, which was irradiated with a 400-W high-pressure mercury lamp for 6 h (3 h for each side of the sample) in an ice-water bath. Out of seven carboxamides examined, la, le and lg showed intramolecular photocycloaddition in solid state to give the [4+4] cycloadducts 2a, 2e, and 2g in an almost quantitative yield after complete conversion, and If gave 2f in 9% yield. 

Photocycloaddition proceeds between allylsilane and A-methylphthalimide to yield a mixture of [2 + 2] and [4 + 2] adducts along with the allylated product291. Intramolecular cycloadditions of the vinylsilanes with the cyclopentenone moieties in 200 furnish good yields of cyclic products stereoselectively (equation 164)292. In the presence of 1,4-dicyanonaphthalene, diallylsilane 201 undergoes an intramolecular photocycloaddition reaction in an aromatic solvent to give a four-membered ring product (equation 165)293. 

The effect of substituents on the stereoselectivity of the intramolecular photocycloadditions of alkenes to cyclohexenones was systematically examined by Becker and coworkers84 who obtained high stereofacial selectivity in compounds 283a-c. However, small changes in the position, geometry or steric effect of the substituents have dramatically affected the selectivity, indicating the complexity in predicting the stereoselectivity in such system (Scheme 61). 

Systematic study on the diastereofacial selectivity in the intramolecular photocycloaddition of alkenes to chiral dioxinones was recently reported by Haddad and coworkers129 on compounds of type 298. Preferred pyramidalization in the direction of the less exposed side (the axial methyl at the acetal center) described in structure 298b, and first bond formation at this position (found to be the case in dioxinones 143 and 146, Scheme 31), are essential features for obtaining selective photocycloadditions of alkenes to chiral dioxinones from this side, leading to the kinetically favored products. In such cases the preferred approach is not necessarily from the more exposed side (Figure 6). 

General and stereoselective synthesis of spiroethers and less thermodynamically stable spiroketals have recently been developed by Hadded and coworkers129,130. The key step is the intramolecular photocycloaddition of chiral dioxinones of type 305 to dihydropyrones. Subsequent fragmentation of the produced four-membered ring provides, after oxidative enlargement of the cyclic ketone, the thermodynamically less stable spiroketal 310 (R = H) as was demonstrated on photoproduct 308 (Scheme 66). 

Scheme 27 Effect of substituents meta to the tether on intramolecular photocycloaddition of para-alkenyloxyacetophenones.

Therefore, if ratios are given, these must be considered with great care, especially if irradiation times were long. Ortho adducts formed by intramolecular photocycloaddition are even less stable than those from intermolecular reactions. Only from pentafluorophenyl prop-2-enyl ether [111], 2-methyl-6-(4-fluo-rophenyl)hex-2-ene [112], and l-(2-methoxybenzyloxy)-3-methylbut-2-ene [113] have primary ortho adducts reportedly been isolated. The many secondary... 

Scheme 40 Intramolecular photocycloaddition of ( )-6-(2-methoxyphenyl)-5,5-dimethyl-2-hexenenitrile.

This chapter deals primarily with the intermolecular and intramolecular photocycloaddition of unsaturated compounds to aromatic rings and the inter-... 

The photoreactions of mono- and di-cyanonaphthalene derivatives with al-lyltrimethylsilane have been reported by Mizuno and Albini, independently [136,428,433,436], The product ratios of photocycloadducts via exciplexes and the allylated products via electron transfer were dependent on the solvent polarity and additives. The intramolecular photocycloaddition of the reductive allylated products efficiently gives tricyclic compounds in high yields [428,433,436] (Scheme 23). 

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