Photocyclizations enones

Anilinocyclohex-2-enones photocyclize to l-oxo-l,2,3,4,4a,9a-hcxahydro-carbazoles[6]. Similarly, 3-anilinocyclohex-2-cnones give 4-oxo analogues[7]. 

PHOTOCYCLIZATION OF AN ENONE TO AN ALKENE 6-M THYLBICYCL0[4.2.0]0CTAN-2-0NE (Bicyc1o[4.2.0]octan-2-one, 6-methy1-)... 

The direct photocyclization of another interesting acceptor-donor pair, the amine-enone system, has been reported by Mariano [224-226]. Direct irradiation of -(aminoethyl)cyclohexenones 281 leads to the excitation of the conjugated cyclohexenone chromophore. Intramolecular single-electron transfer from the amine donor to the cyclohexenone excited state results in the formation... 

Further research on intramolecular photocyclization of amino enones and amino ketones based on electron transfer has been made by Kraus and Chen [228]. In analogy to the earlier results of Roth and El Raie [229], Kraus and Chen obtain the cyclopropanol derivative 286 as single stereoisomer by direct irradiation of 285. Photolysis of the amino enone 287 does not lead to a three-membered ring product, but only to pyrrolidine 288. The irradiation of the unsaturated keto ester 289 results in the even more unexpected formation of a nine-membered ring product 290. Such remote photocyclizations have rarely been described so far. 

The earliest pioneering work, due to Georgian and Saltzman began with Robinson annulation of 436 and intramolecular [2+2] photocyclization of the bicyclic enone. 

The photocyclization of carvone, 36, to carvonecamphor, 37, is a good example of intramolecular cycloaddition of an enone to an isolated double bond422 ... 

Likewise, the (bis)enone 38 also undergoes both direct and sensitized photocyclization.430... 

Intramolecular [2 + 2] photocyclization reactions of 2- /V-(2-propcnyl)amino]cy-clohex-2-enones (67) are also controlled enantioselectively by carrying out irradiation in inclusion complex with a chiral host compound. When inclusion crystals of 67 with 12 are irradiated in the solid state, optically active 9-azatricyclo[5.2.1.0]-decan-2-ones (68) were obtained in the chemical and optical yields indicated in Table 3 [30],... 

Photocyclization of enones substituted by an a-silylamino group. Irradiation (>320 nm) of these substrates with DCA as sensitizer results in N-heterocycles. 

Intramolecular cycloaddition of the enone (5) affords the adduct (6), which was used as the key intermediate in the synthesis of (7). Photocyclization of the enone (8) affords the tricyclononanone (9) this was used as an intermediate in the synthesis of the fenestrane (10). Irradiation of a mixture of the... 

Entry X Y From stilbene-amine SET system From enone-amine SET photocyclizations From rate constant for acetate promoted deprotonation of diphenylaminium radical (laser spectroscopy)... 

For enone-amine photocyclization reactions the a-CH kinetic acidity seems to be mainly dependent on electronic factors and this has been attributed to the intramolecular nature of proton transfer in the enone-amine zwitterionic radicals (diagram below), which would disfavor overlap of the half vacant p orbital on nitrogen with the a-CH bond [128, 130, 131]. 

Mariano and coworkers have investigated the DCA sensitized photocyclization of silylamino-enone derivatives (Scheme 61) [130, 131, 308]. DCA-sensitized photoreactions of 255 in acetonitrile-methanol mixture gave the substituted piperidines 256 in high yields along with the pyrrolidine ester 257 in the case of 255b. DCA sensitized reaction of 246 resulted in the formation of a diastereomeric mixture (6 1) of hydrosoquinolines [212, 213] and <2 % of the hydrosoindoline, 258. 

A previous report gave an account of the intramolecular photocyclization of the enone (12) to afford the head-to-head adduct (13). The adduct (13) obtained in this way has been used as a starting material for pentalenolaotone G methyl ester (14). While the above reaction only yields a... 

The hydrogen abstraction, in this instance, presumably occurs at the P-carbon of the enone (139). The resultant biradical ring closes to afford the observed products. The ic-ic excited triplet state is involved in the conversion of the enone (141) into the spiroketone (142). The reaction again involves hydrogen abstraction by the ft-carbon followed by cyclization within the biradical. Photocyclization of enamides such as (143) has led to a new synthetic approach to the yohimbine anreserpine type alkaloids. " The photocyclization of the enone (144) provides a route for the synthesis of 3.10-dimethoxyprotoberberines. ... 

Enantioselective intramolecular photocyclization reactions of enones as inclusion crystals with chiral host compounds proceeds very efficiently. Several examples of such reactions are described in this Chapter. 

Additions to Cyclohexenones. Intramolecular additions in cyclohexene systems also provide access to natural products. Thus the photocyclization of enone (27) affords the adduct (28) which is suggested as a route to the nortaxane or taxane skeleton. Intramolecular reaction of the enone (29) affords two products (30, 70%) and (31, 14%). The identity of the major products was determined by X-ray crystallography and is the result of hemiacetalisation of the original (2-f-2)-adduct (32). The minor product (31) is formed from the major by a yie TTo-Aldol reaction. ... 

Toda et al. [73] described photocyclization in the solid state, using various chiral lattices (e.g., 23) as the host for the [2+2] photocycloaddition of an enone teth-... 

The photocyclization of the enone (7) to yield (8) has been used as an approach to the synthesis of the spiro(4,5)decane system. The tetracyclic product (9a) is obtained from the photo-induced intramolecular cyclization of the pyridone (10a) Increase in the chain length involving (10b) resulted in the formation of... 

The unstable cycloadducts (24) and (25) are obtained from the photolysis of the enone ethers (26).A similar approach has been reported by Barker and Pattenden in their study of the photocyclization of enol acetates. Thus the intramolecular photocycloaddition affords the adducts (27) from the mixture of enol acetates derived from (28). An analogous regioselective cycloaddition is encountered in the irradiation of the enol acetates derived from (29) to afford the adduct (30). 

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