Photochemical cyclization products

For butadiene, two photochemical cyclization products are cyclobutene or bfcyclobutane. 

Three types of volatile products were found to be obtained from citral (neral (10) and geranial (12)) after UV light irradiation at pH 3.5. Aldehydes 3 and 4, photocitral B (5), aldehyde 6, and photocitral A (7) were formed through photochemical cyclization (12-13) (Table I and Figure 1). Both tetrahydrofurans 16 and 17 were obtained by oxidation (4) (Table I). The acid-catalyzed cyclia tion products were furan 2 and alcohols 13-15, 19 and 21 (2, 24-27) (Table I and Figure 2). As shown in Table II, the relative amount of the acid-catalyzed cyclization products 2, 13-15, 19, and 21 was extremely high (83%) under dark conditions. However, the UV light irradiation increased the photochemical cyclization products 3-7 and oxidation products 16 and 17. The amount of products of the former and that of the latter were 38% and 27%, respectively. 

Figure 1. Structures of photochemical cyclization products formed from citral...

The same isomerization also occurs with diarylthiazoles, but when two adjacent phenyl groups are present, even in the final product, a photochemical cyclization gives rise to a polycyclic benzothiazole (Scheme 3) (213,218,219). 

Another example of the analogy between pyrazole and chlorine is provided by the alkaline cleavage of l-(2,4-dinitrophenyl)pyrazoles. As occurs with l-chloro-2,4-dinitrobenzene, the phenyl substituent bond is broken with concomitant formation of 2,4-dinitrophenol and chlorine or pyrazole anions, respectively (66AHC(6)347). Heterocyclization of iV-arylpyrazoles involving a nitrene has already been discussed (Section 4.04.2.1.8(i)). Another example, related to the Pschorr reaction, is the photochemical cyclization of (515) to (516) (80CJC1880). An unusual transfer of chlorine to the side-chain of a pyrazole derivative was observed when the amine (517 X = H, Y = NH2) was diazotized in hydrochloric acid and subsequently treated with copper powder (72TL3637). The product (517 X = Cl, Y = H) was isolated. 

Whereas the condensation of o-iodonitrobenzene with copper acetylides is accompanied by cyclization into isatogens, neither 4-iodo-3-nitro- nor 5-iodo-4-nitro-l,3-dimethylpyrazole gives cyclized products in conditions of acetylide synthesis. Moreover, nitropyrazolylphenylacetylene, as compared with o-nitrotolane, does not undergo thermal, catalytic, or photochemical isomerization to give the fused five-membered rings. 

The thermal, but not the photochemical, decomposition of ferro-cenylsulphonyl azide (14) in benzene gave some intermolecular aromatic substitution product FCSO2NHC6H5 (6.5%) but no intermolecular cyclization product (17). Contrariwise, photolysis of 14 in benzene gave 17 but no anilide 1 ). 

An improved synthesis of 3,4-dihydro-2,l-benzothiazine 2,2-dioxide was reported by Togo and co-workers using photochemical conditions . Treatment of A-alkyl 2-(aryl)ethanesulfonamides 18 with (diacetoxyiodo)arenes under irradiation with a tungsten lamp at 20-30 °C afforded 2,1-benzothiazines 19 and 20. Chemical yields and selectivities were dependent upon the choice of solvents and the reactant s substituents 18 (Table 1). When THF and EtOH were used as solvents, the reactions failed to give the cyclized products, since their a-hydrogen was abstracted by the intermediate sulfonamidyl radical. Compound 20 was obtained as a major product when 1,2-dichloroethane was employed as a solvent. In contrast, in the case of EtOAc as solvent, compound 19 was obtained as the major product. 

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>. 

Phenol esters of a,(3-unsaturated carboxylic acids have an interesting reactivity due to the synthetic utility of the resulting hydroxychalcones (Scheme 19). This aspect will be illustrated in Section IV. However, from the basic point of view, it is worth mentioning that the cis or trans configuration of the olefinic part of the acyl moiety can have a marked influence on the photochemical reactivity of the ester. When para-methoxyphenyl fumarates are irradiated, the normal ortho-rearranged products are obtained. By contrast, irradiation of para-me-thoxyphenyl maleates does not lead to rearrangement. Instead, cyclization products are obtained (Scheme 20). 

As early as 1969, Block and Corey (73) observed the photochemical transformation of divinyl sulfide 37 to a mixture of isomeric cyclization products (39 and 40) (Scheme 5.13). A pathway involving the cyclic thiocarbonyl ylide 38 was proposed. A theoretical study concerning the nature of intermediates in the photochemical cyclization of divinyl sulfides was recently carried out (74). Several examples are known in which the vinyl group is part of an aromatic system. 

Photochemical excitation of cyclic a-amino ketones (41 and 43) leads to the formation of bicyclic azetidines and azetidinols by abstraction of a hydrogen y to the ketone followed by cyclization. Production of (42) (72CC1108) and (44) (75TL2463) is believed to occur as a singlet state reaction. 

The photochemical conversion of ( )-l-(3-indolyl)-2-(pyridyl)ethylenes into their Z isomers is followed by photocyclization. Single products are obtained from the 2- and 4-pyridyl compounds, but the two expected isomeric products are obtained from the 3-pyridyl derivatives. Oxidation of the cyclized products yields the fully aromatized systems (75JHC455, 80T1209). 

Mould Metabolites.—A second synthesis of deoxybrevianamide E has been reported 300 in essence, it is similar to the earlier synthesis,306 the major difference being simply the order in which the various stages are effected. Photochemical oxidation of deoxybrevianamide E (34a) provides the first satisfactory laboratory synthesis of brevianamide E (34b), which was obtained in 42% yield, together with its stereoisomer, the alternative cis-fused cyclization product. The stereochemistry depicted in (34b) rests on a comparison of the n.m.r. spectra of these two stereoisomers.300... 

By phenolic oxidation of the amide 183 using K3[I e(CN)6] the two coupling products 184 and 185 in 5 and 10% yields, respectively, were obtained. Conversion of 184 into 147 and 148 proceeded in the usual manner (<3S). The important intermediate 184 was also prepared from the bromoamide 186 by photochemical cyclization (39). 

The fragmentation of nitrate esters to carbon radicals under thermal or photochemical conditions has been exploited by Batsanov and co-workers in the stereoselective construction of 8-lactones [95JCS(P1)1281]. Treatment of 205 with tin hydride in refluxing benzene gave the 6-exo-trig cyclization product 206 in good yield and high stereoselectivity. 

A similar synthesis of tricyclic fused-ring thietanes by photochemical cyclization of N-substituted mono- and dithioglutarimides has been reported (Equations 17 and 18) <2000H(53)2781>. The carbonyl compound 64a and thiocarbonyl derivatives 65 were obtained in moderate yields. An analogous route was used for thiosuccinimides, where the use of a dithioglutarimide derivative resulted in the formation of a bicyclic side product 65. 

Substitution of the acetate group at the C-3 position of the /3-sultam 105 can occur by reaction with silyl enol ethers in the presence of zinc iodide or zinc chloride. When the diazo compound is used, after desilylation with tetrabutyl-ammonium fluoride (TBAF), photochemical cyclization gives the bicyclic /3-sultam 106 as a mixture of two cis/ fra -diastereoisomers. When silyl enol ethers derived from cyclic ketones are used, the substitution product is stabilized by a retro-Michael-type reaction leading to open-chained sulfonamides 107 (Scheme 31) <1997LA1261>. 

The photochemical strategy, first applied by Ito and Tanaka in their synthesis of erybidine (1) (33), was later extended to the preparation of laurifine (5), lau-rifonine (4), and laurifinine (6). These three 2,11,12-trisubstituted dibenzazo-nines were prepared employing the photochemical cyclization of amide 16 as the key step (Scheme 3) irradiation of 16 in methanolic sodium hydroxide solution gave a mixture of 17,18, and 19 from which the major compound was separated and transformed into the above-mentioned derivatives (26). The cyclized product 18 was later used as a precursor in the synthesis of neodihydrothebaine (7) (34). 

Dysazecine has been synthesized by a photochemical route. Irradiation of 116 gave the reduced derivative 117 (17%) and the cyclized products 118 (13%) and... 

There are a large number of photochemical cyclizations of aromatic compounds that lead initially to polycyclic, non-aromatic products, allhough subsequent rearrangement, elimination or oxidation occurs in many instances to form aromatic secondary products. The archety pe for one major class of photocyclization is the conversion of stilbene to phenanthrene by way of a dihydrophenanthrene (3.60). 

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