Photocyclization—

Photocyclization of an enamide bearing an o-methoxyl group takes place at the site occupied by the methoxyl to afford a substituted pyridone after work-up.  

This approach has been used by Ninomiya, Ishii, and their co-workers in the synthesis of such aromatic benzophenanthridine alkaloids as dihydro-nitidine and dihydroavicine.  

A corollary is that chelilutine, sanguirubine, and sanguilutine, which are analogs of chelirubine, also incorporate a trioxygenated ring D, and indeed possess the structures indicated below.  

Ninomiya and co-workers have recently succeeded in a total synthesis of corynoline and 5-hydroxycorynoline using enamide photocyclizations. The main challenge was to obtain a cis B/C ring fusion, and this was achieved by immediate hydrogenolysis of the crude photocyclization product 2, which was obtained in 20% yield from the enamide 

The above sequence was extended to a preparation of 6-epicorynoline. Performic acid oxidation of deoxycorynolone, followed by treatment with alkali, afforded a mixture of diols 3, epimeric at C-5. Lithium aluminum hydride reduction of diols 3, followed by palladium hydrogenolysis of the benzylic C-5 hydroxyl, gave rise to 6-epicorynoline. It will be noted that in this instance performic acid oxidation, followed by hydrolysis, furnished C-6 beta hydroxyl derivatives.  

Norrish Type II photocyclizations are the result of intramolecular hydrogen abstraction by an excited carbonyl group followed by cyclization of the resulting biradical. Hydrogen abstraction from the y-carbon atom is preferred (1,5-hydrogen transfer). The introduction of a heteroatom into the 

Intramolecular (5-hydrogen abstraction (1,6-hydrogen transfer) is often a competing process and is preferred in the absence of a suitably positioned y-hydrogen atom. 2-Alkenyl-3-hydroxytetrahydrofurans (458) and (459) 

Thiones, in general, do not undergo similar photoreactions. An exception to this behavior has been found in the thiolactone (483), which on irradiation in methanol is converted to the ether (484).422 Photocyclization of a different type has been observed in the thione (485), leading to the formation of the thiophen (486).423 Details of the mechanism of this and related photo-cyclizations424 are not completely clear. 

A wide range of other photocyclizations resulting in the formation of oxygen and sulfur heterocycles have been reported. Only those conversions that are generally applicable and that have some preparative value will be considered in this section. Flavanone (487) is obtained on irradiation of the 2 -hydroxychalcone (488)425 such transformations are best carried out in 

Photocyclization, arising by intramolecular hydroxyl addition to a carbon-carbon double bond, occurs in the norbornene derivatives (489) and yields the cyclic ethers (490).428 This is in marked contrast to the inter- 

Experimental data on photocyclization of stilbenes and related indolic compounds such as bisphenylazostilbene were reported in Ref [5]. The stereoselectivity of the formation of macrocyclic stilbenes as well as the regioselectivity of their photocyclization are strongly influenced by the length of the connecting alkanediyl chain [6]. Photochemical cyclization reaction in cis-3,3, 5,5 -tetramethoxystilbene was investigated [7]. The reaction occurred to give dihydrophenanthrene-type 

Stilbenes. Applications in Chemistry, Life Sciences and Materials Science. Gertz Lilditenshtein Copyright 2010 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 978.3.S27-32388.3 

Photophysics and photochemistry of ring-fluorinated stilbenes have been investigated. 

The absorption and emission spectra, E—Z photointerconversion, photocyclization of the Z-isomers, and transient triplet—triplet (T—T) absorption spectra of five 

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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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