Photocycloaddition 1.3- dipolar

The photochemical behavior of the isomeric 3-methyl-2-phenyl-2-allyl-l-azirine (66) system was also studied. Irradiation of (66) in cyclohexane gave a quantitative yield of azabicyclohexenes (67) and (68). Control experiments showed that (65) and (66) were not interconverted by a Cope reaction under the photolytic conditions. Photocycloaddition of (66) with an added dipolarophile afforded a different 1,3-dipolar cycloadduct from that obtained from (65). The thermodynamically less favored endo isomer (68b) was also formed as the exclusive product from the irradiation of azirine (66b). 

Four-membered heterocycles are easily formed via [2-I-2] cycloaddition reac tions [65] These cycloaddmon reactions normally represent multistep processes with dipolar or biradical intermediates The fact that heterocumulenes, like isocyanates, react with electron-deficient C=X systems is well-known [116] Via this route, (1 lactones are formed on addition of ketene derivatives to hexafluoroacetone [117, 118] The presence of a trifluoromethyl group adjacent to the C=N bond in quinoxalines, 1,4-benzoxazin-2-ones, l,2,4-triazm-5-ones, and l,2,4-tnazin-3,5-diones accelerates [2-I-2] photocycloaddition processes with ketenes and allenes [106] to yield the corresponding azetidine derivatives Starting from olefins, fluonnaied oxetanes are formed thermally and photochemically [119, 120] The reaction of 5//-l,2-azaphospholes with fluonnated ketones leads to [2-i-2j cycloadducts [121] (equation 27)... 

Orfanopoulos et al. studied the photochemical reaction of alkenes, aryalkenes, dienes dienones, and acyclic enones with [60]fullerene to obtain various substituted cyclobutylfullerenes [240,241,243,247], For example, the photocycloaddition of cis- and Irans-1 -(p-mcthoxyphenyl)-1 -propenc 68 to C6o gives only the trans [2 + 2] adducts (Scheme 27), thus the reaction is stereospecific for the most thermodynamically stable cycloadduct. A possible mechanism includes the formation of a common dipolar or biradical intermediate between 3C o and the arylalkene. Subsequent fast rotation of the aryl moiety around the former double bond leads exclusively to the trans-69 [2 + 2] adduct. Irradiation of this product, yielded 90% trans-68,10% cis-68 and cycloreversion products. Thus, a concerted mechanism can be excluded because the photocycloreversion is expected to give the trans-68 as the only product. These results can be explained by the formation of a common dipolar or diradical intermediate. Similarly, cycloreversion products from C6o and tetraalkoxyethylene... 

By using the hypersensitive molecular mechanistic probe 2-(2-methoxy-3-phenylcy-clopropyl)-5-methylhexa-2,4-diene in the 2 + 2-photocycloaddition of [60]fullerene, it was shown that the reaction proceeds via a biradical and not a dipolar intermediate.6 Zirconium-induced cyclodimerization of heteroaryl-substituted alkynes produces tetrasubstituted cyclobutenes with high regio- and stereo-selectivity.7 The ruthenium-... 

The photocycloaddition of alkenes to cyclopropylbenzene (121a) was proposed to involve a dipolar intermediate with the developing positive charge stabilized by the cyclopropyl (equation 15) ... 

Inter- and intra-molecular [3 + 2] photocycloadditions of 1,3-dipolar species generated by elimination of nitrogen from tetrazoles (103) with alkenes (104) have been reported giving pyrazolines (105). Polycyclic azaheterocycles (108) (110) were also synthesized by [3 + 2] tandem photocycloaddition reactions. Intramolecular-proton-transfer mediated photocycloaddition of 3-hydroxyquinolones (111) with alkenes (113) to give [3 + 2] cycloadducts (114). ... 

The photocycloadditions show all the characteristics of concerted reactions, including stereospecificity and regioselectivity. Concerted 1,3-dipolar additions are known to proceed via a two-plane orientation complex. For the case of diphenylazirine and methyl acrylate, there are two possible orientation complexes (26 or 27). The interaction of substituent groups in the syn complex 26 can be of an attractive (tc overlap, dipole-dipole interaction) or of a repulsive nature (van der Waals strain). Both effects are probably negligible in the anti complex 27. The ratio of the two steric courses syn and anti) functions... 

The use of stereoselective 1,3-dipolar cycloadditions to heterocyclic compounds in the synthesis of natural products has been reviewed.1,2-, 1,3- and 2,3-dicyanonaphthalenes, e.g. (25), undergo 3 -I- 2-photocycloadditions with alkenes and 1,3-dienes in the presence of Cu(OAc)2 to produce tricyclic adducts, e.g. (26) (Scheme 10). ... 

Porco et al. reported the synthesis of ( ) methyl rocaglate using [3 + 2] dipolar photocycloaddition reaction of an oxidopyrylium betaine derived from excited state intramolecular proton transfer reaction of 3-hydroxyflavin and methyl cinnamate [144]. Methyl rocaglate was obtained by a base-mediated a-ketol rearrangement followed by hydroxy-directed reduction sequence. They subsequently succeeded in the asymmetric synthesis of methyl rocaglate using functionalized TADDOL derivative (34) (Figure 2.30) as a chiral Bronsted acid (Scheme 2.77) [145]. 

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