Photochemical cyclization fragment

A laser flash study of the photoreactions of hexan-2-one and 5-methylhexan-2-one has provided evidence for the existence of the triplet 1,4-biradicals produced by the y-hydrogen abstraction typical of Norrish Type II reactivity. The photochemical behaviour of the alkanone, nonan-5-one, in urea inclusion compounds has been studied. In solution, irradiation of nonan-5-one yields hexan-2-one, propylene, and two cyclobutanols. In the clathrate, the fragmentation products were essentially the same but only one cyclobutanol was observed. The cyclization fragmentation ratio was established as 0.67, compared with 0.32 in methanol. The authors suggest that the CIS-cyclobutanol has less stringent rotational requirements and that it is this isomer (43) which is formed in the clathrate. 

The photochemically induced fragmentation of esters 1 to vinylcarbenes by loss of ethene gives the corresponding cyclopropene in up to 40% yield after cyclization. For example, ethyl 3-(l-methylcyclopropyl)but-2-enoate gave ethyl 2,3-dimethylcycloprop-2-cnecarboxylate (R = = Me, R = H) in 40% yield but normally as one component of a mixture of several... 

A (2 + 2)-photoadduct is formed on irradiation of 2,3-dimethylmaleic anhydride with 3,4-dimethyl-1-phenylphosphole. " The photocycloaddition of maleic anhydride derivatives (124) to the alkene moiety in (125) affords the adducts (126) by a straightforward (2 + 2)-addition. The (2 + 2)-photoadducts (127) are readily produced by the dimerization of the corresponding maleic anhydrides in ethyl acetate solution.The intramolecular photochemical cyclization of the succinimide derivative (128) is a key step in the new synthetic approach to the tetracyclic fragment of neotuberostemonine. ... 

The fragmentation/cyclization ratio is determined by the relative orientation of the respective molecular orbitals, and thus by the conformation of diradical species 2. The quantum yield with respect to formation of the above products is generally low the photochemically initiated 1,5-hydrogen shift from the y-carbon to the carbonyl oxygen is a reversible process, and may as well proceed back to the starting material. This has been shown to be the case with optically active ketones 7, containing a chiral y-carbon center an optically active ketone 7 racemizes upon irradiation to a mixture of 7 and 9 ... 

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. 

Likewise, benzyldihydroisoquinolinium derivatives can be used in a photochemical synthesis of tetrahydroisoquinolines. Thus, 2-(2-trimethyl-silylmethylphenylmethyl)-3,4-dihydroisoquinoliniun perchlorates have been successfully cyclized, as in the synthesis of the protoberbine alkaloids (+)xylopinine and (+)stylopine. The reaction proceeds via SET from the xylyl donor to the iminium moiety, fragmentation of the benzylsilane radical cation and carbon-carbon bond formation in the intermediate diradical. The synthesis is rather general and the yields compare favorably with those obtained from related substrates via a dipolar cycloaddition methodology [298] (Sch. 27). 

Emetamine (10) and (9-methylpsychotrine (7) were among many other fragmentation and/or oxidation products obtained by photochemical or thermal decomposition of emetine (1) (91). Battersby et al. (92) synthesized ( )-10 from their emetine precursor ( )-29 (36) through cyclization of the corresponding / -3,4-trimethoxyphenethylamide. 

Tandem fragmentation-cyclization of bicyclic ketones connected to unsaturated side chains have been used to construct hi-, tri-, and spirocyclic ketones [334], Cossy et al. have shown that depending upon the substitution pattern, ketyl radical anions obtained from photochemically induced electron transfer from amines to cyclopropylketones lead either to the formation of 3-substituted cycloalkanones or to ring expanded products (Scheme 78) [335],... 

The photochemical behavior of cyclobutanone (IS) contrasts sharply with that of other ketones. Cyclobutanone undergoes a cleavage also from the (n, r ) state, with subsequent fragmentation to ketene and olefin, decarbonylation to cyclopropane or cyclization to oxacarbene (16), whose concerted formation has also been proposed on the basis of stereochemical observations (Stohrer et al., 1974). In contrast, cyclohexanone cleaves exclusively from the triplet state and undergoes disproportionation reactions. The photochemical activity of cyclobutanone persists even at low temperatures (77 K) where cyclohexanone is photostable. 

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