Photo-induced cyclization

The electron-transfer-induced cyclization of homochrysanthemol proceeds via a flve-membered transition state, from intramolecular substitution at the quaternary cyclopropane carbon, to generate the flve-membered cyclic ethers (69) and (70). In contrast, the intramolecular photo-induced cyclization of chrysanthemol goes via a six-membered transition state involving attack at the terminal vinyl carbon. 

A novel synthesis of 5,6-dihydro-4//-1,2-oxazines (20) is presented via the photo-induced cyclization of y. d-unsaturated oximes (21) see Scheme 4. Irradiation of (21) in the presence of 9,10-dicyanoanthraccnc (DCA) led to the heterocycle (20) only. The proposed mechanism proceeds via the radical cation (22), generated by single-electron transfer (SET) from the oxime (21) to the excited sensitizer (DCA. Cyclization of (22) affords the oxazine (20) after proton transfer to the DCA radical anion (DCA ) and H abstraction.61... 

N-Aik vl-(p-methoxyphenyl)arylamines, such as 76, undergo photo-induced cyclization in the prese ce of protic acids to produce the carbazolones 77. The process is believed to comprisf an initial cyclization, followed by two consecutive proton shifts and a final hydrolysis of the methoxy functionality <02CC270>. 

The photochromism of 1 is achieved by atom rearrangement into the intramolecular oxygen-atom insertion product 2, in contrast to the frequently reported organic photochromic systems based on photo-induced cyclization, cis/trans isomerization, or H atom transfer [1]. The quantum yield of the photoreaction from 1 to 2 at 509 nm in acetonitrile without 02 is 0.14 0.01. In solution, photoreaction of 1 causes the oxidation reaction by atmospheric oxygen, resulting in a mixture of 2 and further oxidation products such as [(RhCp )2((i-CH2)2( i-S03)] and [(RhCp )2 ( r-CH2)2( t-S04)]. In contrast, in the crystalline state, the photochromic system between 1 and 2 is stable and repeatable with essentially 100% interconversion ratio. 

Intramolecular Cyclization Reactions This Section is concerned with photo-induced cyclization reactions of aromatic compounds in which the final product is also an arene. Examples of intramolecular cyclization in which the product is a stable aliphatic system (e.g. intramolecular meta photocycloaddition of ethylenes to benzenes) are considered in Section 3 of this Chapter. The present type of process occurs for a wide variety of systems and may involve arene-arene and arene-ethylene photo-oxidative cyclization or reactions proceeding by loss of halogen acid. For several years now one of the major interests in these processes has been in their use as synthetic procedures and again within the review period there have appeared a number of accounts which illustrate this feature. Stilbene-... 

Intramolecular photo-induced cyclization between aryl and ethylene moieties is a common reaction observed for a variety of molecular structures. The formation of 9,10-dihydrophenanthrene... 

Dihydroisoquinolones (70) are also produced by the photo-induced cycliz-ation of A -alkyl-A -(chloroacetyl)benzylamines (69). The yield is best when R is t-butyl. ... 

The photo-induced cyclizations of the selenol esters (20) and (21) to give (22) and (23), respectively, have been described. Photolysis of benzyl phenyl selenide (24a) or of the phenyl ribosyl derivative (24b) yield the corresponding diselenides (25). The photoreduction of acetophenone by H2Se has been investigated. ... 

R = Me, CH2Ph) with tri-n-butyl tin hydride followed by NaBHjCN reduction affords the 7-exo cyclization products (165 R = Me, CHzPh), even if in only poor to moderate yield <9ITL2829>. In a similar reaction, 2,2-dichloro-iV-methyl-iV-(o-allylphenyl)-acetamide furnishes the 1-exo product, 4-methyl-2,3,4,5-tetrahydro-l/7-benz[Z)]azepine (166) in 49% yield <91JCS(P1)353>. The established photo-induced cyclization of chloracetamides has been applied to a synthesis of the azepino[3,4,5-c,ring system isomeric to that of the clavicipitic acids <88SC671>. 

A Pt macrocycle containing four DTE units has been described. Upon UV irradiation, two photo-induced cyclization reactions occur. The converted closed isomer Pt(c,o,c,o) has been isolated and characterized by H NMR spectroscopy. The high value of the cyclization quantum yield (0.64) is attributed to the enforced antiparallel conformation in the macrocycle [95]. 

Photo-induced cyclization using a norbornene-substituted aminobenz-aldehyde resulted in a minor endoperoxide side product 200, where a 1,6-diradical formed in the reaction can rearrange and be trapped with oxygen to yield the endoperoxide which was characterized by X-ray crystallography (14PCA10487). 

A key basis for modern helicene cyclization chemistry occurred in 1964 when Wood and Mallory effectively prepared phenanthrene via photochemical induced cyclization of stilbene [81]. The new method effectively provided efficient pathways to substituted phenanthrene derivatives, easier access to existing and new helicene structures, as well as sufficient quantities for property studies. In the mid 1960s, Martin and coworkers successfully isolated the previously made hexahelicene as well as new hepta- 99, octa- 100, and nonahelicenes 101 via highly efficient syntheses (Scheme 1.22) [82]. These preparations culminated in photo-induced cyclization of bisarylethylenes with an I2 oxidant and Hg-lamp radiation source. The key improvements over previous efforts included production of a phenanthrene derivative via photo-induced cyclization of intermediate 102 and subsequent conversion to Wittig ylide 103, a common intermediate to the four helicenes. 

Noyori R, Ohnishi Y, Kato M. Photo-induced cyclization of cri,cri-2,8-cyclononadienone via the cis,trans isomer. Tetrahedron Lett. 1971 19 1515-1518. 

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