Photocyclization with iodine

Thiophen Analogues of Helicenes.— The photocyclization with iodine as oxidant of 1,2-di(diheteroaryl)ethenes, in which the heteroaryl group is derived from benzo[fe]thienyl or from tricyclic systems such as (434) or (440), is the key step in the synthesis of heterohelicenes. The ethylenes in turn are prepared from the aldehydes and the chloromethyl derivatives via phos-phonium salts or phosphonates through the Wittig reaction. The Bestmann method was also used for the synthesis of symmetrically substituted ethylenes from phosphonium periodates. Thus from (443) the heterohexahelicene (444) was obtained, (445) gave (446), and the hetero-heptahelicene (448) was obtained from (447). The undecahelicene (450) was prepared from (449). - These syntheses illustrate the fact that heterohelicenes are more easily available than helicenes, as the necessary aldehyde is prepared by metalation of (448) with butyl-lithium followed by reaction with iV-methylformanilide. The heptahelicene (452) was obtained by a double photocyclization of (451). 

Photocyclizations of mono- and distyryl substituted polynuclear arenes in the synthesis of helicenes have been reported by El Abed et al. (Chapter 4). In this work, irradiation of toluene solutions of (17 R = H, OMe, CN, Me, OH) and (18 R, = H, OMe, Me) with iodine in the presence of propylene oxide... 

Beccalli et al. reported a new synthesis of staurosporinone (293) from 3-cyano-3-(lH-indol-3-yl)-2-oxo propionic acid ethyl ester (1464) (790). The reaction of 1464 with ethyl chlorocarbonate and triethylamine afforded the compound 1465, which, on treatment with dimethylamine, led to the corresponding hydroxy derivative 1466. The triflate 1467 was prepared from 1466 by reaction with trifluoromethanesulfonic anhydride (Tf20) in the presence of ethyldiisopropylamine. The palladium(O)-catalyzed cross-coupling of the triflate 1467 with the 3-(tributylstannyl)indole 1468 afforded the vinylindole 1469 in 89% yield. Deprotection of both nitrogen atoms with sodium ethoxide in ethanol to 1470, followed by photocyclization in the presence of iodine as the oxidizing agent provided the indolocarbazole 1471. Finally, reductive cyclization of 1471 with sodium borohydride-cobaltous chloride led to staurosporinone (293) in 40% yield (790) (Scheme 5.248). 

During the past two decades the photocyclization of substituted diaryl ethylenes (analogous to A) has assumed considerable synthetic importance. In such cases photocyclization is carried out simultaneously with dehydrogenation (e.g., by iodine), the aim being the one step formation of the fully aromatic system ... 

A number of l-aryl-2-thienylethylenes have been photocyclized in the presence of an oxidizing agent (usually iodine) to polycyclic aromatic compounds. Representative examples are given in Table 1. The mechanism, as with the conversion of stilbene to phenanthrene, probably involves conversion of the trans-alkene to the c/s-form, cyclization to the dihydro isomer, and oxidation of the latter to the fully aromatic compound. The yield of the cyclized product seems to decrease when the ethylene is attached to the /3-position of the thiophene. 

The second special case is formed by the photoreactions of 2,4-dinitro-6-(phenyliodonio)phenolate (266) with several nucleophiles759. Upon irradiation of this fairly stable zwitterion in methanol, 6-methoxy-2,4-dinitrophenol is formed in 65% yield. Photoreaction with pyridine affords 2,4-dinitro-6-pyridiniophenolate (85%) and irradiation in the presence of phenyl isothiocyanate in acetonitrile affords a mixture of two stereoisomeric 2-phenylimino-5,7-dinitro-1,3-benzoxathioles (269) (71%) which could not be separated. The reaction starts with attack of the nucleophile on the positively charged iodine atom, leading to an iodinane (267) and proceeds by expulsion of iodobenzene. The mechanism is illustrated for phenylisothiocyanate, a case in which the substitution product (268) can undergo further photocyclization to a benzoxathiole derivative (269) (equation 196). 

Baylis-Hillman adducts such as 55 and 56 derived from 2-nitrobenzaldehydes were shown to function as useful precursors to functionalized (1H)-quinol-2-ones and quinolines. Treatment of 55 and 56 with iron and acetic acid at 110 °C afforded 57 and 58, respectively <02T3693>. A variety of other cyclization reactions utilized in the preparation of the quinoline scaffold were also reported. An iridium-catalyzed oxidative cyclization of 3-(2-aminophenyl)propanols afforded 1,2,3,4-tetrahydroquinolines <02OL2691>. The intramolecular cyclization of aryl radicals to prepare pyrrolo[3,2-c]quinolines was studied <02T1453>. Additionally, photocyclization reactions of /rans-o-aminocinnamoyl derivatives were reported to provide 2-quinolones and quinolines <02JHC61>. Enolizable quinone and mono- and diimide intermediates were shown to provide quinolines via a thermal 6jt-electrocyclization <02OL4265>. Quinoline derivatives were also prepared from nitrogen-tethered 2-methoxyphenols. The corresponding 2-methoxyphenols were subjected to a iodine(III)-mediated acetoxylation which was followed by an intramolecular Michael addition to afford the quinoline OAc O... 

Reduction of the cis lactam 61 with LAH afforded the cis amine 60, which was also obtained from either oxidative photocyclization of the enamide 56 or nonoxidative photocyclization of the bromoenamide 56, followed by successive reductions. Oxidative photocyclization of the enamide 56 in the presence of iodine afforded the corresponding dehydrolactam 59 in good yield, which is a useful intermediate for further conversion to various aromatized benzo[c]phenanthridines, the basic structure of many aromatic alkaloids (19,20). 

The combination of oxygen and iodine is the most satisfactory system found for the photocyclization of stilbenes to phenanthrenes. " Thus a mixture of trans-stilbene (0.01 m.) and iodine (0.005 m.) in cyclohexane on irradiation with a mercury... 

An approach to the saturated amine 200 was reported by Tse and Snieckus (66) in 1976 (Scheme 34). An intermediate imide, prepared from 3,4-(methylenedioxy)-j8-phenethylamine (195) and maleic anhydride 196, was iodinated to furnish 197. Grignard addition of methyl magnesium iodide followed by dehydration afforded 198, which underwent photocyclization to afford the tricyclic system 199. Successive hydrogenation and reduction of 199 provided the fully saturated amine 200, from which the Dolby-Weinreb enamine 43 is easily obtained on oxidation with mercuric acetate. 

The NH and neighbouring 2-CH of imidazole are annulated when the compound is heated with cyclopropyl aryl ketone and PPA in decalin or heated without a solvent. When the 2-position is blocked, cyclization at C-S of the imidazole occurs [3276]. 3 -lndole adds on to DMAD at room temperature to form a pyridoindole in moderate-to-good yield [3394]. 2-Styrylbenzimidazole is photocyclized in good yield in methanol containing iodine [284S]. 

Two classes of photocyclization reactions of indoles are well exemplified in the literature. One is based on a 6-ti electrocyclic closure of trienes to cyclohexadienes where the indole C2-C3 bond forms part of the triene. The other involves photolysis of m-haloalkylindoles whereupon the halogen-substituted carbon becomes attached to an indole ring position. The first example of the former reaction was reported by Szmuszkovicz [75], who found that UV light irradiation of 2,3-bis-(para-methoxyphenyl)indole in the presence of iodine gave the carbazole 71 (Scheme 37). By analogy with the photocyclization reaction of other 1,2-diarylethenes [76], this reaction presumably proceeds via conrotatory closure of the indole singlet excited... 

A novel photocyclization of tosylstilbenes was employed to effectively generate a densely functionalized tricyclic compound. Thus, irradiation of the tosylstilbene illustrated below with a medium pressure mercury lamp in the presence of iodine and an excess of propylene... 

As both symmetrical and unsymmetrical stilbenes can be readily synthesized by employing Wittig [126], Heck [127], or McMurry [128] reactions, various graphene-type molecules have been prepared via photolytic cyclodehydrogenation, some of which will be discussed at this point A typical example of photocyclization is the irradiation of the [2.2.2]paracydo-phane 96 in the presence of an oxidant, such as iodine, leading to the corresponding coronene derivatives 97 in high yield. This offers a versatile method for the preparation of new coronene derivatives with different functionality and substitutional symmetry (Scheme 13.26) [129]. 

In analogy to stilbenes, their aza-analogs (stilbazoles) were found to undergo photocyclization to azaphenanthrenes upon irradiation with ultraviolet Hght, both in the presence and absence of iodine [133, 134]. This method was later adopted to construct larger heteroatom-containing PAHs, such as 104-106, from different heterocycHc precursors (Scheme 13.29) [135-137]. 

In the 1960s and 1970s, the synthesis of PAHs by photocyclization was frequently investigated. For example, benzoperylene (119) can be obtained from several precursors under light irradiation with/without iodine (Scheme 8.60). 

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