Cyclizations photochemical

The ergosterol system mentioned earlier falls in this category. Before the system is discussed, it is worthwhile to point out that, in general, substitution of even a simple group such as an alkyl seems to change the details of the photochemical primary processes in a startling manner among the dienes and trienes. Thus, in solution, 1,3,5-hexatriene cannot be converted to 1,3-cyclohexadiene photochemically, but 2,4,6-octatriene does cyclize photochemically to (rans-5,6-dimethyl-l,3 Cyclohexadiene under the same conditions. ... 

Indoles and Carbazoles. - Formation. 2-Arylindoles (132) are formed by intramolecular Wittig reaction of the phosphonium salts (131). The hydroxamic acids PhN(OH)COCH2COR (R = alkyl or aryl) cyclize in boiling toluene to mixtures of indoles (133) and 3-isoxazolones (134). Irradiation of a solution of o-iodoaniline and the potassium enolate of acetone affords 2-methylindole. The enamino-ketone (135) cyclizes photochemically to 1,2-dimethylindole (136) with elimination of acetaldehyde/ The styrene derivative (137), obtained by the action of Meerwein s acetal, Me2NCH(OMe)2, on o-nitrotoluene, yields 1-hydroxyindole on treatment with zinc/ Azidobenzocyclobutanes (138 R = Me, Ph, or CH2Ph) are converted into indoles (133) by the action of concentrated sulphuric acid/ ... 

Five-membered ring systems with two adjacent phenyl groups can often be cyclized photochemically to give fused phenanthrene products. This type of reaction is reported for 2,3-diphenylbenzo[6]thiophens (127) if a primary... 

Electrolysis of the nickel(II) seco-corrin (75) under strictly anhydrous conditions gave the corrin complex (78), thereby demonstrating that the electrochemical simulation of the photocyclization is feasible moreover, the nickel(II) seco-corrin could not be cyclized photochemically (See Section 11 A.)... 

Benzothiepin (281), obtained from (282) by the action of a rhodium catalyst, has been cyclized photochemically to the cyclobutene (283), which can also be obtained directly from (282) by the action of heat, silver perchlorate, or radiation. For the thermal conversion of (282) into (283), deuterium-labelling studies ruled out an intermediate of type (284), in which rotatioh of the four-membered ring could occur, and the most likely mechanism appears to be a + 2j ring-opening to (281a) followed... 

The bicyclic ketone (96) has been cyclized photochemically to the cyclobutanol (97) 66 jjjg bicyclo[2,2,l]hexane (98), which is prepared from (97) by a Favorski ring contraction, has also been cyclized photochemically to the norcubane derivative (99). 

The a,P-epoxy ketone (320) cyclizes photochemically to the cyclobutanols (321) and (322). The unmethylated analogue (323) does not ycUze but instead rearranges to the P-diketone (324), and this is the more common pathway in the photochemistry of a,P-epoxy ketones. The difference in the reaction pathways caused by the introduction of a methyl group is attributed to steric factors. 

Hexatrienes can also cyclize photochemically through anti-Hiickel... 

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