Photoadditions, intramolecular

Recent synthetic applications of the photochemical [2 + 2] cycloaddition of unsaturated sulfones have been noted. Musser and Fuchs84 have effected an intramolecular [2 + 2] addition of a 6-membered ring vinyl sulfone and a five-membered ring vinylogous ester in excellent yield, as part of a synthetic approach to the synthesis of the mould metabolite, cytochalasin C. The stereospecificity of the addition was only moderate, however, and later problems with this synthetic approach led to its abandonment. Williams and coworkers85 have used the facile [2 + 2] photoaddition of 73 and... 

Flegel, M. Lukeman, M. Huck, L. Wan, P. Photoaddition of water and alcohols to the anthracene moiety of 9-(2 -hydroxyphenyl)anthracene via formal excited state intramolecular proton transfer. J. Am. Chem. Soc. 2004, 126, 7890-7897. 

Full details of the [3 + 2] photoaddition of nitrobenzene (296) to cyclohexene (297) to give the 1,3,2-dioxazole (298), stable at -70°C, have now been published.244 An analogous intramolecular cycloaddition has been proposed to account for the novel photocyclization of the nitrouracil 299 to the triazole 300 and the pathway is outlined in Scheme 9.245 A 2-azaallyt radical has been shown to be an intermediate in the photoaddition of benzo-... 

In the presence of a proper second molecule bimolecular photochemical processes occur. Obviously such reactions can also occur in an intramolecular fashion in bifunctional molecules containing both reactive centres. These reactions are a) hydrogen abstraction by the excited molecule if the second molecule (or reactive centre) is a hydrogen donor RH (1.14) b) photodimerisation (1.15) c) photoaddition or photocycloaddition (1.16) d) electron transfer (1.17), if no bonding takes place between the reactants (or reactive centres). ... 

Photoaddition of the enol acetate (48) in hexane resulted in the intramolecular [2 + 2]cycloaddition with the production of a 2 3 mixture of the isomeric tricyclic ketones (49) and (50). (Upon hydrolytic cleavage, the mixture was converted to the compounds (51) and (52) 18,22). 

Intramolecular photoaddition of tertiary amine and styrene moieties has been extensively studied by Aoyama29 and Lewis group28,30,31 (equations 4-8). Equations 4 and 5 show that if the intramolecular additions result in the formation of a five- or six-membered ring, the product yields are excellent. Highly regioselective intramolecular proton transfer is proposed to occur via least motion pathways from the lowest energy... 

The solvent-controlled differential reactivity was also applied to the intramolecular photoaddition of 36 and 37 (equations 11 and 12)45,46. 

Irradiation rraws-2-[3-(7V-methylamino)propyl] stilbene 89 results in the formation of 7V-methyl-l-benzyltetrahydro-2-benzazepine 90 as the only significant primary photoproduct (equation 26), which in turn undergoes secondary photochemical N-demethylation. The final mixture contains 90 (38%) and 91 (25%) at high (>95%) conversion. Intramolecular photoadditions of these (equations 24-26) secondary (aminoalkyl)-stilbenes are highly regioselective processes24. 

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). 

Intramolecular photoaddition of (terminal) allenes or ketenes to a cyclohex-2-enone C —C double bond have been investigated. The former reaction occurs regiospecifically in high yield,2 the latter, wherein the ketene moiety is photochemically generated, proceeds in moderate yield.3... 

The photochemistry of l,5-hexadien-3-ones has been examined by Dauben and coworkers. A relatively rare dependence of the intramolecular enone-olefin photoaddition of several derivatives has been observed. Discuss the mechanistic possibilities in terms of Dauben-Salem-Turro state correlation diagrams. (See Dauben, W. G. Cogen, J. M. Ganzer, G. A. Behar, V., J. Am. Chem. Soc., 1991, 113, 5817.)... 

A series of intramolecular photoadditions of one anthracene nucleus to another resulting in the formation of heterocycles have been reported in certain dianthryl derivatives.273 9-Anthryl-9-anthramide (262), on irradiation in benzene yields the /3-lactam (263) the corresponding carbonate and azo compound undergo analogous cyclizations. 

Intramolecular photoaddition in cis-5,5,6-trimethylhepta-3,6-dien-2-one (284) takes a different course,296 yielding not the oxetane but the two dihydropyrans (285 and 286). This is in contrast to the inter-molecular cycloaddition of a, jS-unsaturated aldehydes to alkenes which affords only oxetanes, and has been accounted for in terms of diradical intermediates (287 and 288) formed from the s-cis conformation (284) of the dienone. The intermolecular equivalent is thought to occur by addition to the s-trans conformation. 

The photoaddition of alcohols and phenols to alkenes has been observed.404 The equivalent intramolecular process results in cyclization and the formation of oxygen heterocycles. Irradiation of 2-allyl-4-(-butylphenol (388) affords 2,3-dihydro-2-methyl-5-<-butyl-benzofuran (389), whereas o-3-methylbut-2-enylphenol (390) gives 2,2-dimethylchroman (391). In both cases, therefore, the addition can be said to occur in a Markovnikov direction. 

Intramolecular photoaddition has been observed in nonconjugated olefins. Thus irradiation of Formula 410 gives the cage structure of Formula 411 (181). Two isomers of the hexachlorocyclopentadiene-cyclooctatetraene adduct give analogous cage structures on irradiation... 

A number of examples of photoaddition to alkynes has been described. Dimethyl acetylenedicarboxylate has been found to add to methyl-substituted 3-benzoylthiophens301 and to thiophen and 2,5-dimethylthiophen302 on irradiation. Benzo[f>]thiophen also undergoes cycloaddition reactions with alkynes,303 and in the case of dimethylacetylene dicarboxylate, product formation has been shown to be wavelength dependent.304 Intramolecular [ 2 -(- 2] cycloaddition has been observed on both direct and triplet-sensitized irradiation of the alkyne (353) and gives the cyclobutene (354)305 the isomer (355) is formed on prolonged irradiation. 

Demonstration of the unique synthetic utility of the [2 + 2] photocycloaddition reaction of enones to alkenes and the success in controlling the stereoselectivity, to some extent, in the intermolecular additions (discussed above) prompted further studies and development of new synthetic applications in the intramolecular photoadditions during the last decade. In most cases that have been studied, the alkene was tethered to the cyclic enone by three carbon units or two carbons and one heteroatom. 

Another example is Crimmins and Gould s114 total synthesis of ( )-laurenene 243 via the intramolecular photoaddition of enone 241, affording an epimeric mixture with very high facial selectivity, followed by subsequent transformations (Scheme 52). 

Very high diastereofacial selectivity was obtained in the intramolecular photoadditions of the chiral unsaturated lactones115 244, 246 and 248 (Scheme 53). 

Scheme 24 Quantum yields of intramolecular photoaddition products from various 5 -(4-trifluoro acety lpheny 1) -1-pentenes.

From 1987 onward, many examples of rearrangements of ortho photocycloadducts to derivatives of cyclooctatriene and subsequent reactions of these compounds have been reported. The same year marks the start of a series of investigations on intramolecular ortho photocycloadditions. This may not be a coincidence, because ortho adducts formed by intramolecular photoaddition seem especially prone to undergo rearrangement, although it has also frequently been observed in adducts arising from intermolecular addition. 

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