Additions to Cyclohexenones and Related Systems

Over the years (+)-grandisol (41) has been a popular target molecule for photochemical syntheses. Another approach to this molecule using (-)-quinic acid as the starting material has been described. The cycloaddition provided a route to the key intermediate (42). Further chemical transformation of this afforded the final product. 

3 Additions to Cyclohexenones and Related Systems - Irradiation of 3-methylcyclohexenone in methanol solution in the presence of the ester (42) results in the synthesis of adduct (43) in moderate yields. This product was used as the starting material for an approach to the synthesis of trichodiene. The enone (44) undergoes photochemical reaction with 2,3-dimethylbut-2-ene in benzene or acetonitrile solution and using 350 nm light.The four products were identified as (45), (46), (47) and (48). The formation of the oxetane (45) follows the conventional route and the cycloadduct (48) arises via the biradical 

The chromone (52) undergoes photochemical addition of ethene. The primary product from this cycloaddition, presumed to be (53), is photo-chemically reactive and is converted into (54) and (55). The former of these is a key intermediate in a synthetic strategy to tricothecene analogues. Both (54) and (55) arise via the Norrish Type II reactivity of (53). Thus hydrogen abstraction from the methoxy substituent by the excited carbonyl group results in a 1,4-biradical that either ring closes to (54) or fragments with the loss of methanal to yield the enol of (55). 

Acetone-sensitized addition of 2,3-dimethylbut-2-ene to the enone (56) affords both the (2 + 2)-cycloadduct (57) and the cyclopentanobenzofuran (58). The latter product arises by a stepwise addition of the alkene to the enone ethene bond and the attached cyano group. The authors suggest that this mode of addition arises from an upper excited triplet state and similar behaviour is observed with (59) to give the adduct (60). 

Photodimerisation of (61) affords the cw,a t/,cw-head-to-tail product (62). The position of the fluoro substituent appears to play some part in the outcome of the reaction since irradiation of the isomer (63) affords the cis,syn,cis head-to-head dimer. Further studies of the influence of fluoro substituents on the dimerisation of the styrylcoumarins (64) have also been reported.  

An examination of the fluorescent behaviour of 7-hydroxycoumarin (umbelli-ferone) has been reported. The authors suggest that dimerization may result on irradiation of solutions of high concentration. A conformational analysis has been carried out on the C4 photocycloadducts formed from 4,5, 8-trimethyIpsor-alen.  

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Additions to Cyclohexenones and Related Systems - Caldwell and his coworkers have studied the photochemical addition of 1,1-diphenylethene to 4,4-dimethylcyclohex-2-enone. The products from this reaction, carried out in cyclohexane, are shown in Scheme 1. Although other evidence (see reference 9b above and references cited therein) has suggested that an exciplex is not a key interaction in such (2+2)-photocycloaddition reactions Caldwell et al. conclude from their detailed study of this system that a triplet exciplex is involved. Schuster and his coworkers have reported that a variety of cyclic enones (17) -(19) add photochemically to fullerene. The yields of the adducts vary but with some of the less heavily substituted enones the yields can be reasonable as shown by the data under the appropriate structure. Suginome et aO report the synthesis of the cycloadducts (20) by the photochemical addition of various ethenes to the enone (21). The adduct (22) was also synthesized by photochemical addition of methoxycyclohexene to the enone (20). 

Additions to Cyclohexenones and Related Systems. Singh and Samanta have described the photochemical addition of substituted cyclohexa-2,4-dienones to cycloheptadiene as a route to 10,10-dimethyltricyclo[7.2.2.0 ]-... 

Additions to Cyclohexenones and Related Systems. Irradiation of the diox-enone derivative (29) in acetonitrile-acetone at 0°C affords the adduct (30) in 60% yield. This compound was a key intermediate in the first total synthesis of racemic ingenol. A further study has shown that the enone (31) also undergoes cycloaddition on irradiation under the same conditions. This yields the adduct... 

Additions to Cyclohexenones and Related Systems - A re-investigation of the photodimerization of isophorone (37) has been reported. The study examined the influence of solvent and of the concentration of the enone. Some of the results and the yields of dimers obtained are shown in Scheme 4. From this detailed study the authors suggest that supramolecular structures are involved in the dimerization. These apparently take part even at low concentrations of enone. The photocycloaddition of enones such as (39) to buckminsterfullerene (Ceo) has been studied. The outcome of the addition is the formation of low yields of furanylfullerenes. This addition occurs to the exclusion of de Mayo type of addition. Photocycloaddition of the cyanocyclohexenone derivative (40) to alkenes has been reported. ... 

Inter molecular Additions to Cyclohexenones and Related Systems. Calculations have been reported that deal with a transition state analysis of the regioselectivity encountered in triplet-state (2 + 2)-cycloaddition reactions of cyclohex-2-enone. Diastereoselective (2 + 2)-cycloaddition of ethene to cyclohexenone carboxylates in the presence of chiral auxiliaries has been described. Yields of bicyclo[4.2.0]octanone derivatives can be obtained with de as high as 81%. The enone (16) can be tethered to a polymer substrate via the tether group R. Irradiation of this material in toluene with ethene at —78°C gives a 68% yield of the adduct (17). The de of the product is reasonable at 72%. Addition of 1,1-diethoxyethene to the cyclohexenone (18, R = Ph) results in the formation of the two stereoisomeric head-to-tail regioisomers (19) and (20). The outcome of the reaction is dependent on the rate of formation of 1,4-biradical intermediates. This can be seen in the dependence of the cisjtrans ratio on the solvent and on the temperature at which the reaction is carried out. Thus with enone (18, R = Ph) in acetone at 3°C, a cjt ratio of 4.4 is obtained, and this changes to 2.1 at —40°C. In acetonitrile the cjt ratio is only 1.9 at the same temperature. With the other derivative of (18), the cjt ratio is 1.3 in acetone and 0.8 in acetonitrile. Photoaddition of ethene to the enone carboxylate [21, R = (-)-8-(2-naphthyl)menthyl] results in the formation of the diastereoisomers (22) and (23) with a de of 56% at —78°C. The diastereo-selectivity can be enhanced by the addition of naphthalene derivatives to the solution. Thus with naphthalene, a de of 71% is obtained, and this can be increased to 83% with 1-phenylnaphthalene. ... 

Intramolecular Additions to Cyclohexenones and Related Systems. 3-Styrylcyclohex-2-enone undergoes photochemical cyclization to afford 2,3-dihydro-[177]-phenanthrene-4-one. Work carried out on the photostability of curcumin has also examined the photolability of the chalcone (38). This undergoes cyclization to (39) on irradiation. ... 

Additions to Cyclopentenones and Related Systems. (2 + 2)-Cycloadditions are reported following the irradiation of mixtures of alkyl and aryl 2-thioxo-3/f-benzoxazole-3-carboxylates with alkenes. Cycloaddition also occurs to the CS double bond. The photochemical additions of arylalkenes to 3-phenylcyclo-pentenone and 3-phenyl cyclohexenone have been studied. The regio- and stereochemistry observed in the additions has been rationalized in terms of the stability of the intermediate biradicals. Photocycloaddition of allene to the cyclopentenone derivative (6) in methylene chloride solution at — 78°C affords... 

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