Photolytic cyclization

The a, -unsaturated acyltelluride also takes part in the photolytic cyclization. 

Photolytic cyclization of the phenolic bromo compounds 278 and 279 led to ( + )-oxocrinine (275) (methylenedioxy instead of methoxy-hydroxy) and ( )-oxomaritidine (275), known intermediates to crinine and maritidine (277) (59, 60). Oxocrinine was also obtained by photolysis of the isomeric bromophenol 280 (61). 

A similar photolytic cyclization has been achieved, with concomitant oxidation, of the iV-formyl-enamine (85) in the presence of hydrogen iodide to produce (86), no evidence being found of non-oxidative cyclization. The enamine with the (Z)-configuration is obtained directly from the 3,4-dihydroisoquinoline by the action of formic acid and acetic anhydride.95... 

Photolytic cyclization of the chloroacetamide 129 afforded the azaazu-lenone 130 in 25% yield (72JOC3691). Irradiation of a benzene solution of the pyrrolizine 131 and dimethyl acetylenedicarboxylate (DMAD) gave a 26% yield of the 1 1 photoadduct 132, which on heating at 180-200°C gave 61% of the azaazulene 133 (72JCS(P1)2517). 

Similar photolytic cyclizations which are usually carried out in methanolic hydrogen chloride solution can yield either benzimidazoles or the iV-oxidcs, depending on the natures of the tertiary amino group and the ring substituents [ 161 ]. Undcr more vigorous reaction conditions (or in the presence of reducing agents), AyV-dialkyl(or aralkyl)-o-nitroanilines are converted into benzimidazoles (61) rather than the iV-oxides (Scheme 2.1.25). 

Hydrazones of 4-chloro-5-hydrazino-3(2//)-pyridazinones undergo photolytic cyclization (Equation (29)) to give pyrazolo[3,4- /]pyridazinones <84JHC1249>. 

The titled hydrochloride salt was synthesized by the photolytic cyclization of 2,3,3-trichloroallyl N-(a-methylbenzyl) dithiocarbam-ate. The neutralization of 3 with a base afforded the free base (66-67). When a pure sample of the above dithiocarbamate was stored in a sealed glass bottle at 25-30 for a period of three years, the identical cyclization reaction occurred to give 3>. 

The Nazarov cyclization of vinyl aryl ketones involves a disruption of the aromaticity, and therefore, the activation barrier is significantly higher than that of the divinyl ketones. Not surprisingly, the Lewis acid-catalyzed protocols [30] resulted only in decomposition to the enone derived from 46,47, and CO. Pleasingly, however, photolysis [31] readily delivered the desired annulation product 48 in 60 % yield. The photo-Nazarov cyclization reaction of aryl vinyl ketones was first reported by Smith and Agosta. Subsequent mechanistic studies by Leitich and Schaffner revealed the reaction mechanism to be a thermal electrocyclization induced by photolytic enone isomerization. The mildness of these reaction conditions and the selective activation of the enone functional group were key to the success of this reaction. 

Cyclizations of dihydroxystilbene 256 using 4 mol % of chiral ruthenium complexes under photolytic conditions were investigated by Katsuki et al. (Scheme 65) [167]. Coordination of alcohols/phenols to Ru(IV) species generates a cation radical with concomitant reduction of metal to Ru(III). Cycli-zation of this oxygen radical followed by another cyclization provides the product 257. Catalyst 259 provided 81% ee of the product in chlorobenzene solvent. Optimization of the solvent polarity led to a mixture of toluene and f-butanol in 2 3 ratio as the ideal solvent. Substituents on the phenyl rings led to a decrease in selectivity. Low yields were due to the by-product 258. 

Like the cyclization of the parent system, the photocyclization of hetero analogues of norbornadiene was reported many years ago. Recent interest in this area will serve to illustrate the potential of the conversions. Thus the irradiation of the oxanorbomadiene system 259 brings about conversion to the oxepine 260138. The formation of 260 presumably arises by quadricyclane 261 formation, followed by secondary photolytic ring-opening. Other research has shown that irradiation of the oxanorbomadiene 262 follows the same reaction mode and it undergoes (2 + 2)-cycloaddition to the quadricyclane derivative 263. Apparently, in this instance, cycloreversion affords the ylide 264 that can be trapped by suitable addends, giving the adducts illustrated in Scheme 3139. 

The oxidative cyclization of Ar,Ar-diarylamines to carbazoles has been achieved by thermal or photolytic induction [7, 75]. However, the yields for this transformation are mostly moderate. Better results are obtained by the palladium(II)-mediated oxidative cyclization of Ar,Ar-diarylamines (Scheme 27). Oxidative cyclization by heating of the Ar,Ar-diarylamines 76 in the presence of a stoichiometric amount of palladium(II) acetate in acetic acid under reflux provides the corresponding 3-substituted carbazoles 77 in 70-80% yield [118]. The cou-... 

Photolytic. Irradiation of trifluralin in hexane by laboratory light produced a,a,a-trifluoro-2,6-dinitro-A-propyl-jo-toluidine and a,a,a-trifluoro-2,6-dinitro-p-toluidine. The sunlight irradiation of trifluralin in water yielded a,a,a-trifluoro-A, 7 -dipropyl-5-nitrotoluene-3,4-diamine, a,a,a-trifluoro-A/ ,A/ -dipropyltoluene-3,4,5-triamine, 2-ethyl-7-nitro-5-(trifluoromethyl)benzimidazole, 2,3-dihydroxy-2-ethyl-7-nitro-l-propyl-5-(trifluoromethyl)benzimidazoline, and 2-ethyl-7-nitro-5-(trifluoromethyl)benzimidazole. 2-Amino-6-nitro-a,a,a-trifluoro-p-toluidine and 2-ethyl-5-nitro-7-(trifluoromethyl)benzimidazole also were reported as major products under acidic and basic conditions, respectively (Crosby and Leitis, 1973). In a later study, Leitis and Crosby (1974) reported that trifluralin in aqueous solutions was very unstable to sunlight, especially in the presence of methanol. The photodecomposition of trifluralin involved oxidative TV-dealkylation, nitro reduction, and reductive cyclization. The principal photodecomposition products of trifluralin were 2-amino-6-nitro-a,a,a-trifluoro-jo-toluidine, 2-ethyl-7-nitro-5-(trifluoromethyl)benzimida-zole 3-oxide, 2,3-dihydroxy-2-ethyl-7-nitro-l-propyl-5-(trifluoromethyl)benzimidazole, and two azoxybenzenes. Under alkaline conditions, the principal photodecomposition product was 2-ethyl-7-nitro-5-(trifluoromethyl)-benzimidazole (Leitis and Crosby, 1974). 

Considerable effort has been expended on elucidating the mechanism of the photocyclizations. Di-p-tolylamine can be cyclized to 3,6-dimethylcar-bazole either photolytically in petrol or thermally at 880°C. Each process was viewed as electrocyclic, proceeding via cis and trans versions of 312 (R = Me, = H) produced by dis- or conrotatory processes, respectively. ... 

Other synthetic routes to the carbapenem ring system include photolytic Wolff rearrangement (73JCS(P 1)2024), aldol condensation (78JA313), addition-cyclization (80JOC1135) and /3-lactam formation (78JOC4438,79TL4359). An excellent review of carbapenem synthesis has recently appeared (82H(17)463). 

The Ic pattern for carbazole synthesis arises primarily in the dehydrogenative cyclization of diphenylamines. This cyclization can be accomplished photolytically, with an oxygen sensitive intermediate having been detected (equation 58) (81JA6889). Preparative work with substituted diphenylamines has resulted in rather variable yields, depending upon particular substituents. This variability may be the result of alteration in the dominant photochemical process with substituent changes. 

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