Photocycles

Figure C3.2.17. Diagram of a liposome-based artificial photosynthetic membrane showing the photocycle that pumps protons into the interior of the liposome and the CFqF j-ATP synthase enzyme. From [55],...

The photocyclization of iV-vinylanilines is an e.xarnple of a general class of photocyclizations[l]. If the vinyl substituent has a potential leaving group or the reaction is carried out so that oxidation occurs, the cyclization intermediate can aromatize to an indole. 

For example, ot-anilino-P-ketoesters photocyclize to indole-2-carboxylate csters[2]. 

Anilinocyclohex-2-enones photocyclize to l-oxo-l,2,3,4,4a,9a-hcxahydro-carbazoles[6]. Similarly, 3-anilinocyclohex-2-cnones give 4-oxo analogues[7]. 

Fig. 20. The photocycle of bacteriorhodopsia with its iatermediates including lifetimes and absorption wavelengths ia nm ia parentheses (158).

Methacrylates also undergo 2 + 2 photocyclization reactions with a variety of substrates (35,36). 

A dehcate stereochemical balance is clear in the high yield photocyclization of one cyclophane configuration (47) to a cage compound (48), whereas its spatial relative (49) leads only to higher molecular weight product (38). 

A second synthesis of cobyric acid (14) involves photochemical ring closure of an A—D secocorrinoid. Thus, the Diels-Alder reaction between butadiene and /n j -3-methyl-4-oxopentenoic acid was used as starting point for all four ring A—D synthons (15—18). These were combined in the order B + C — BC + D — BCD + A — ABCD. The resultant cadmium complex (19) was photocyclized in buffered acetic acid to give the metal-free corrinoid (20). A number of steps were involved in converting this material to cobyric acid (14). 

A rare type of reaction involves the photocyclization of the 5-(unsaturated acyl)aminopyrimidines (168) to reduced 7-methylpyrido[3,2-Similar reactions are known involving related 4-aminopyrimidine derivatives, leading to pyrido[2,3-[Pg.221]

The only other type noted concerned the photocyclization of the unsaturated pyrazine amide (425) to (426) with subsequent selenium dioxide aromatization (72CPB2264). 

Ethylene, /3-(dimethylamino)-nitro-in pyrrole synthesis, 4, 334 Ethylene, dithienyl-in photochromic processes, 1, 387 Ethylene, furyl-2-nitro-dipole moments, 4, 555 Ethylene, l-(3-indolyl)-2-(pyridyl)-photocyclization, 4, 285 Ethylene, l-(2-methyl-3-indolyl)-l,2-diphenyl-synthesis, 4, 232 Ethylene, (phenylthio)-photocyclization thiophenes from, 4, 880 Ethylene carbonate C NMR, 6, 754 microwave spectroscopy, 6, 751 photochemical chlorination, 6, 769 synthesis, 6, 780 Ethylene oxide as pharmaceutical, 1, 157 thiophene synthesis from, 4, 899 Ethylene sulfate — see 2,2-dioxide under 1,3,2-Dioxathiolane... 

Pyridines, [(l-methyl-2-indoloyl)amino]-photocyclization, 4, 204 Pyridines, [(l-methyl-2-pyrroloyl)amino]-photocyclization, 4, 204 Pyridines, nitramino-synthesis, 2, 343 Pyridines, nitro-N-oxides... 

PHOTOCYCLIZATION OF AN ENONE TO AN ALKENE 6-M THYLBICYCL0[4.2.0]0CTAN-2-0NE (Bicyc1o[4.2.0]octan-2-one, 6-methy1-)... 

Fi re 12.6 Schematic diagram Illustrating the proton movements in the photocycle of bacteriorhodopsin. The protein adopts two main conformational states, tense (T) and relaxed (R). The T state binds trans-tetinal tightly and the R state binds c/s-retinal. (a) Stmcture of bacteriorhodopsin in the T state with hflus-retinal bound to Lys 216 via a Schiff base, (b) A proton is transferred from the Schiff base to Asp 85 following isomerization of retinal and a conformational change of the protein. 

Z-Stilbene also undergoes photocyclization to 4a,4b-dihydrophenanthrene via an electrocyclization. ... 

Photocyclization and subsequent dehydrofluorination yield specifically fluor-inated arenes [177] (equation 46)... 

The photocyclization of N-aryl enamines derived from cyclic or acyclic ketones proceeds under mild conditions to produce 2,3-dihydroindole derivatives (178b). The stereochemistry of the products is predominantly trans, which follows from a photochemical electrocyclic process which should take place in a conrotatory manner (178c,I78d). However, the presence of some cis products is not as easily explained. 

FIGURE 10.25 The photocycle of light-adapted halorhodopsin (liR), shown in the presence and absence of chloride. The superscripts indicate the maxima of the difference spectra between IrR and the intermediates. 

Photochemical syntheses have also been attempted for the preparation of in-dolocarbazoles. Thus when the N,N -diphenyl-p-phenylenediamines 188a-b underwent photocyclization in diethyl ether, formation of low yields of the products... 

Photocyclization of A -(4-phenyl-4-pentenyl)monothiobarbiturate (327) afforded a mixture of l,2,3-trimethyl-9-phenyl-l,2,3,6,7,8-hexahydro-4//-pyrido[l,2-u]pyrimidine-2,4-dione and tricyclic nitrogen bridgehead compound 328 (96H(42)117). 

Electron transfer initiated photocyclization of a methanolic solution of 90 followed by catalytic hydrogenation gave a mixture of benzoindolizines 91 and tetrahydroquinoline. Hydrogenation is necessary to stabilize one of the proposed products (82TL919, 83JA1204) (Scheme 17). 

Photocyclization of 2,4,6-triphenylpyridinium salts 64 by irradiating by 313 nm gave 7-substituted 2,4-diphenyl-6,7-dihydropyrido[2,l-u]phthalazi-nium salts 62 (94MI1). 

Essentially the same procedure may be used to produce mixtures of cyclodimers from isoprene,4 1,3-cyclopentadiene,4 and 1,3-cyclohexadiene.7 Separation of all products is somewhat difficult in most cases but has always been possible by preparative vapor phase chromatography. Despite the problems that may be involved in separation of desired products in some instances, photocyclization frequently is the method of choice for preparation of 1,2-dialkenylcyclobutanes if they can be made major products of photoreactions. Starting materials are readily available, and the preparations are easily carried out on the scale described. There is little doubt that the method is the best for preparation of trans-1,2-divinyleyclob u tane. 

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