Photochemical irradiation

Radical cyclization is compatible with the presence of other functional groups. Treatment of XCH2CON(R)-C(R )=CH2 derivatives (X = Cl, Br, 1) with Ph3SnH and AIBN led to formation of a lactam via radical cyclization. " Cyclization of N-iodoethyl-5-vinyl-2-pyrrolidinone led to the corresponding bicyclic lactam, " and there are other examples of radical cyclization with molecules containing a lactam unit " or an amide unit. Radical cyclization occurs with enamines as well. Photochemical irradiation of A,A-dialIyl acrylamide leads to formation of a lactam ring, and in this case thiophenol was added to generate the phenylthio derivative. Phenylseleno N-allylamines lead to cyclic amines. co-Iodo acrylate esters cyclize to form lactones. " ... 

Photochemical irradiation of dimethyl and diethyl sulphoxides yields the corresponding sulphone in the presence of air and a photosensitizer such as methylene blue in yields up to 99% . Sulphoxides are also oxidized when they act as traps for persulphoxides, the intermediate formed on reaction of a sulphide with photochemically generated singlet oxygen - , equation (9). Isotope studies have shown that such reactions proceed through a linear sulphurane intermediate . Persulphones also react with sulphoxides in a similar manner , equation (10). 

Azatriquinacene 39 has been successfully synthesized in three steps starting from azatriquinane 37. Azatriquinane 37 on treatment with sulfuryl chloride under photochemical irradiation gave nonachloride 89 which on reduction with tri- -butyltin hydride provided the 2,3,5,6,8,9-hexachloroazatriquinane 198 in 53% yield. This on treatment with lithium in tert-butyl alcohol gave completely dehalogenated azatriquinacene 38 in an acceptable 32% yield (Scheme 84) <2000JOC7253>. 

Fig. 14.4 Apparatus for electrodeless photochemical irradiation. A. antenna, B. transmitter, C-j. capacitor, C2. variable capacitor,...

Photochemical irradiation of a,-unsaturated ketones in the presence of Mc3SiOP(OR)2 (R = Me, Et) gave phosphonosilylation products of 1,4-conjugate additions which are hydrolysed to the phosphono ketones (165). The envisaged initiation step is electron transfer to the triplet excited state of the enone to generate a radical ion pair. 

Dinitrogen pentoxide reacts with alkanes in carbon tetrachloride at 0 °C via a radical mechanism to give nitration products which can include nitrate esters.Reactions of alkanes with dinitrogen pentoxide in nitric acid are complex and of little synthetic value. 1-Adamantyl nitrate is one of the products obtained from the photochemical irradiation of a solution of adamantane and dinitrogen pentoxide in methylene chloride. ... 

Aldosterone Aldosterone, llj3,21-dihydroxypregn-4-en-2,18,20-trione (27.2.4), is synthesized from 21-0-acetylcorticosterone, which when reacted with nitrosyl chloride in pyridine gives the nitrite 27.2.1. When photochemically irradiated, this compound is transformed to the oxime 27.2.2, which is hydrolyzed by nitrous acid and forms the semiacetal 27.2.3, which is an acetate of the desired aldosterone. Alkaline hydrolysis of the acetyl group of this compound leads to the desired aldosterone (27.2.4) [33]. 

A -3,4-cis-THC has now been found in Cannabis sativa (Phenotype III) other papers reporting the characterization of compounds from Cannabis sativa concern conclusive identification and synthesis of cannabinodiol, " which is known to result from the photochemical irradiation of cannabinol (Vol. 7, p. 51), and the identification of A -tetrahydrocannabinolic acid " and (+)-cannabitriol (263) " (263) and the corresponding C-2 ethyl ether may be epoxide-derived. " ... 

Thieno[3,4-3]thiophenes 500 were prepared from thiophen-3-yl-substituted [l,3]dithiol-2-ones 496 in good yields <2003JOG7115>. The photochemical irradiation of 496 leads to thieno[3,4-f]dithiines 497 via a unique ring cleavage reaction and subsequent reductive treatment of dithiines 497 proceeds to give 2,3-dihydrothieno[3,4-/ ]thiophenes 498 and 499. Heating of 499 in the presence of further iodomethane leads to the formation of the thieno[3,4-/ ]-thiophenes 500 (Scheme 63). 

Photochemical irradiation of (i-Pr3Si)3SiH (14) with light of 254 nm in either 2,2,4-trimethylpentane or pentane leads to the elimination of f-Pr3SiH and the generation of bis(triisopropylsilyl)silylene (/-Pr3Si)2Si (15). Silylene 15 can also be generated by the thermolysis of the same precursor 14 at 225 °C in 2,2,4-trimethyl-pentane (Scheme 14.11). Reactions of 15 include the precedented insertion into an Si H bond, and additions to the ti bonds of olefins, alkynes, and dienes. 

The epoxide is also formed from the reaction of C60 with dimethyldioxirane, and the 1,3-dioxolane (figure 9) is also obtained (Elemes et al. 1992). Photochemical irradiation of C60 in hexane had previously been shown to result in the addition of 2-5 oxygens, and also various methylene groups (the results depend upon the fullerene purity) (Wood et al. 1991). In this work, two oxygens were found to add to C70. 

Stereoselective radical reduction or allylation.2 Radical reduction of (1-alkoxy-a-halo esters such as 1 shows marked onri-stereoselectivity with Bu3SnH (AIBN) at 50°, which is markedly improved when conducted under photochemical irradiation at —78°. Similar but even higher stereoselectivity obtains in reduction of the tetrahy-drofuran derivative 2. 

So far, no 5-substituted penem has been reported. Two possible absolute configurations exist for the bridgehead carbon atom C-5, (R) remaining the most encountered one as Sf.SV-cpimcrs are inactive <1980MI239>. However, the epimerization reaction from 5(R) to 5(S) can be readily performed via photochemical irradiation in ethyl acetate... 

An arylation reaction of potassium thioamide enethiolates was achieved in good yield by treatment with aryl iodides in the presence of FeBr2 or with photochemical irradiation, perhaps through an SRN1 mechanism [122]. 

Photochemical irradiation of 4-hydrazinoisoxazolo[5,4- ]pyridines affords pyrazolo[4,3-c]py-ridines via cleavage of the N—O bond to give nitrene intermediates which then evolve into the final products by an intramolecular attack of the hydrazine group <88H(27)1899>. For example, irradiation of 6-chloro-4-hydrazino-3-methylisoxazolo[5,4- ]pyridine (592) with a water-cooled low-pressure mercury immersion lamp (6W) gave l-amino-6-chloro-3-methylpyrazolo[4,3-c]pyridin-4-one (593) (60%) (Scheme 76). 

Benzofurazans show greater thermal stability but may be cleaved photochemically. Irradiation of benzofurazan in benzene and in methanol gives the azepine (26) and the urethane (27), respectively in the presence of triethyl phosphite (Z,Z)-l,4-dicyanobuta-1,3-diene is formed. The proposed mechanism (Scheme 3) involves nitrile oxide, oxazirene, acyl nitrene and isocyanate intermediates, and is supported by spectrophotometric studies (76HCA2727) and by trapping of the nitrile oxide as its isoxazole cycloadduct with DMAD (75JOC2880). 

Photochemical irradiation of y,S-epoxyeucarvone above 327 nm yields the diketone (164) and the bicyclic ketone (165) from the expected biradical Lewis-acid rearrangement of y,S-epoxyeucarvone gives the diketone (164) and the lactone (166).258... 

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