Additions to Cyclopentenones and Related Systems

Photocycloaddition has often been used as the key step towards an important 

2 Additions to Cyclopentenones and Related Systems - The photophysics of a series of cyclopentenones (12) has been studied. Irradiation in methylene chloride of a solution of the enone (13) and the alkene (14) results in the formation of the adduct (15) in 47% yield. The reaction is best carried out at temperatures around 0°C. The adduct (15) has been converted into the 

The photochemical cycloaddition of ethene to the bis-butenolides (20) has been examined in an attempt to establish the influence of the ether-protecting groups of the diol system. Generally only two adducts are formed as can be seen from the results shown for the appropriate structures. The most effective ether protecting group is the trimethylsilyl function and here the facial selectivity yields predominantly the anti,anti adduct (21). With the unprotected systems (20, R = H), there is virtually no selectivity and in this case the three adducts (21), (22) and (23) are formed. Irradiation of the butenolides (20a) and (20b) in the absence of ethene leads to intramolecular hydrogen abstraction (a Norrish Type II process) with the formation of the products (24a) and (24b) in 79% and 76%, respectively. 

A further example of photochemically induced addition to the enone double bond in (25) has been reported. In this example irradiation using benzophe-none as the radical producing agent in methanol results in a 51% yield of the adduct (26). Pete and his co-workers have reported the sensitized addition of tertiary amines such as (27) to the furanone double bond in (28). The reaction involves electron transfer from the amine to the sensitiser which ultimately 

Further reports on the addition of 2,3-dimethylbut-2-ene to enones such as (41) have been made.  

A series of (2+2) photocycloaddition reactions have been carried out using (5R)-5-menthyloxy-2(5H)-furanone (32) as the substrate. Photoaddition of cyclopentenone to this substrate gives the four products (33)-(36) with some level of regioselectivity but no facial selectivity. Interestingly, cyclohexenone, cyclohepten-one and cyclooctenone fail to undergo the mixed addition. High facial selectivity is observed when more complex enones such as the 3,5,5-trimethylcyclohexenone and isophorone, (37) are used. The reaction affords adducts of the type illustrated 

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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... 

Additions to Cyclopentenones and Related Systems. Bach et al have examined the (2 -H 2)-intramolecular addition within some furanones. Irradiation at... 

Intermolecular Additions to Cyclopentenones and Related Systems. Mehta and co-workers have described the photochemical addition of 1,2-dichloroethene to the enone (12). This affords the adduct (13), which can be transformed into the protoilludane-type molecule shown in Scheme 4. A further study related to the (2 + 2)-cycloaddition reactions of compounds such as (14) and (15) has determined the crystal structures of these compounds. The... 

The [2+2+1] cycloaddition of an alkene, an alkyne and carbon monoxide is commonly known as the Pauson-Khand reaction. This transformation has been adopted many times in the synthesis of complex natural products and related compounds, which contain a cyclopentenone moiety, for example, prostaglandins. Two independent reports of this reaction appeared almost simultaneously in late 2002 by Iqbal and co-workers25 and Fisher and co-workers26, respectively. They not only used very similar substrate systems in their studies, but they also reached very similar conclusions Toluene was found to be the preferred solvent in this reaction, even though it is a very poor microwave absorber. A reaction time between 5 and 10 min, using dicob alto ctacar-bonyl or dicobalthexacarbonyl as the carbon monoxide source, and a temperature of 100-120°C resulted in high yields of the products. Fisher and co-workers used 20 mol% Co2(CO)8 and cyclohexylamine as an additive (Scheme 5.12), since this system had been used previously in order to allow a catalytic reaction. Iqbal and co-workers did not use cyclohexylamine, but instead used 1 equiv. of the carbon monoxide (Co2(CO)6) source. In both reports, the products were formed in 40-70% yield. 

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