Photochemical reactions cyclization

This approach offers unique opportunities for the generation of multi-functionalized cyclic 2-azadiene systems. A wide variation of the substitution pattern at the positions N-1 and C-6 can be determined by an appropriate choice of the aldehyde and amine. Various substituents can easily be introduced at the C-3 position via addition/elimination reactions on the sensitive imidoyl chloride moiety [24]. Upon reaction with bi-functional reagent, an adequately AT-protected 2(lH)-pyrazinone was elaborated into C-nucleoside analogues (Scheme 8). The desired skeleton and functionalities were obtained by oxidation-cyclization reaction followed by photochemical removal of the protective o-nitrobenzyl group [25]. 

The attacking radical need not always be at carbon. Amidyl radical are known and give cyclization reactions. Aminyl radical cyclizations have been reported. Oxygen radical can be generated under photochemical conditions, and they add to alkenes in a normal manner. ... 

Scheme 5.2. Proposed mechanism of the domino cyclization of photochemically induced reaction of 5-4.

New cyclizations via photochemically generated aminyl radicals have been reported, including further examples of the Hofmann-Loeffler-Freytag reaction.313 Intramolecular addition of an aminyl radical, generated by photochemically induced nitrogen chlorine bond homoysis, is also accompanied by cyclization as illustrated by the conversion of the unsaturated N-chloroamide 378 to the pyrrolidine 379.314 Piperidine formation can also... 

Scheme 9 Photochemical cyclization of diethynylmethanes. Although this reaction is topologically analogous to the cyclization reactions discussed earlier, it does not lead to an aromatic product.

The photochemical reaction can also proceed via the triplet state and in this case no cyclization is observed. Especially when acetophenone is added as a triplet sensitizer, 41 is not formed. Remarkable is the observation that in the presence of anthracene or pyrene as triplet quencher, the yield of the cyclization product 41 was not enhanced and that nitrene insertion into CH bonds of anthracene or pyrene was observed. When the photochemical cyclization reaction was performed with the tosyl azide derivative 42a or the azido nitrile derivative 42b (Scheme 6), only low yields of the tricyclic amide 41 (32% from 42a, 9% from 42b, respectively) were obtained <2001JCS(PI)2476>. 

Cis-stilbene (Zl) also undergoes a conrotatory cyclization reaction into trans-4a,4b-dihydrophenanthrene (DHP, 2), a short living, not isolated product with an absorption in the visible spectrum at 450 nm. In the absence of an oxidizing agent DHP will return to the starting material, by both a thermal and a photochemical ring opening reaction. 

Glycoside-fused dioxathiepins were synthesized by photochemical [1+6] cyclization reaction (Scheme 29). 

The results clearly show that the same kinetic anomeric effects governs these intramolecular cyclization reactions as well as the preceding anomeric hydrogen abstractions described in intermolecular photochemical substitutions. 

A photochemical isomerization of thienyl alkenes has been described (Scheme 2) (92CB2583). In this case, the cyclization reaction plays only a marginal role. 

Nishio et al. <2002HCA2383> performed a photochemical [2+2] cyclization reaction between jV-a 1 koxycarbony 1 -benzo-l,3-oxazol-2-thione 108 and a noncyclic alkene 109. The reaction led to the formation of spiro-thietanes 110, in most cases in high yields (Table 8). They also reported <2003HCA3255> that when in a similar reaction a cycloalkene was used, the reaction led to the formation of compounds 111 and iminothietanes 112. Cyclopentene and indene were used as model cycloalkenes (Scheme 17). 

The polymer-supported distannane 90 was used as a source of stannyl radicals in several radical cyclization reactions, such as the photochemical radical chain addition of f-butyl iodide to acetylenes yielding the Z/E mixture of alkenes or the photochemical cyclization of citronellyl bromide to give menthane in high yields164 (Scheme 44). 

Numerous applications of PET cyclization reactions in organic synthesis appeared over the last years [65], and we have tried to summarize important target compounds in the following Table 1. Since most preparations follow multi-step procedures, we have additionally included the photochemical key-step. The interested reader may refer to the original article for further details. 

Photochemical cyclization reactions of iodinated diphenyl ethers to produce dibenzofuran derivatives have also been studied545. l,3-Diiodo-2-(4 -methoxyphenoxy)-5-nitrobenzene affords 4-iodo-8-methoxy-2-nitrodibenzofuran upon irradiation in acetone. 

This chapter reviews the synthesis of alkaloids and related compounds by the use of photochemical or thermal cyclization reactions of conjugated systems, unconjugated systems structurally related to or derived from what... 

This section covers radical substitution reactions, photochemical additions, cyclizations, and rearrangements, as well as a few other reactions that cannot be classified conveniently under other headings. 

A similar reasoning can be applied to hexatriene to be converted into cyclohexadiene. The only significant change is that the number of tt electrons becomes six corresponding to six conjugated C atoms, and so there is one more node in the frontier orbitals. Accordingly, the symmetries of the HOMO and LUMO at the terminal C atoms are different from those for butadiene, and cyclization occurs by the disrotatory mode or by the conrotatory mode, respectively, for a thermally or a photochemically controlled reaction. The general rule is that the thermal cyclization reactions of a A 7r-electron system will be conrotatory for A =4 and disrotatory for A =4g-f-2 (g = 0,1,2,...). For photochemical cyclizations these relationships are reversed. 

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