Furanones photocycloaddition

A very remote secondary H/D isotope effect has been measured for the 2 + 2-cycloaddition of TCNE to 2,7-dimethylocta-2,fran -4,6-triene. The reaction of nitric oxide with iV-benzylidene-4-methoxyaniline to produce 4-methoxybenzenediazonium nitrate and benzaldehyde is thought to proceed via a 2 + 2-cycloaddition between nitric oxide and the imine double bond. A novel mechanism for the stepwise dimerization of the parent silaethylene to 1,3-disilacyclobutane involves a low-barrier [1,2]-sigmatropic shift. Density functional, correlated ab initio calculations, and frontier MO analysis support a concerted 2 + 2-pathway for the addition of SO3 to alkenes. " The enone cycloaddition reactions of dienones and quinones have been reviewed. The 2 + 2-photocycloadditions of homochiral 2(5H)-furanones to vinylene carbonate are highly diastereoisomeric. ... 

H)-Furanones (4-oxa-2-cyclopentenones) have been extensively explored as [2 + 2]-photocycloaddition substrates [59, 60]. A recent intermolecular example (Scheme 6.19) illustrates the application of their photocycloaddition chemistry to... 

The [2 + 2]-photocycloaddition chemistry of a,(3-unsaturated lactones has been widely explored. The factors governing regio- and simple diastereoselectivity are similar to what has been discussed in enone photochemistry (substrate class Al, Section 6.2). The HT product is the predominant product in the reaction with electron-rich alkenes [84]. A stereogenic center in the y-position of ot,P-unsaturated y-lactones (butenolides) can serve as a valuable control element to achieve facial diastereoselectivity [85, 86]. The selectivity is most pronounced if the lactone is substituted in the a- and/or P-position. The readily available chiral 2(5H)-furanones 79 and 82 have been successfully employed in natural product total syntheses (Scheme 6.30). In both cases, the intermediate photocycloaddition product with 1,2-dichloroethylene was reductively converted into a cyclobutene. In the first reaction sequence, the two-step procedure resulted diastereoselectively (d.r. = 88/12) in product 80, which was separated from the minor diastereoisomer (9%). Direct excitation (Hg lamp, quartz) in acetonitrile solution was superior to sensitized irradiation (Hg lamp, Pyrex) in acetone, the former providing the photocycloaddition products in 89% yield, the latter in only 45%. Cyclobutene 80 was further converted into the monoterpenoid pheromone (+)-lineatin (81) [87]. In the second reaction... 

Paddlanes such as 23 can be readily obtained in good yields from di-(2n + 2n) photocycloaddition of the linked 1,3-divinylbenzenes 24 (Inokuma et al.), and in the crystalline state the enamides 25 give head-to-tail dimers 26 in yields exceeding 87% (Song et al). The diastereoselectivity of the (2n + 2n) photocycloaddition of vinylene carbonate to homochiral furanones 27 is observed to be very dependent on the nature of the R group (Gregori et al.) and these adducts have been used in a synthetic route to carbohydrate derivatives. 

Mechanistic details for the attack of olefins on CS-substituted cyclopentenones are discussed at length by Scharf et al. [114]. All steps of the photocycloaddition are reversible except for the final cyclization, and diastereoselectivity is dominated by only one mechanistic step, that being the approach of the olefin to the vibrationally relaxed furanone (56, see Fig. 7). The P carbon structure... 

Face-selective [2 + 2] photocycloaddition reactions for a series of racemic 2,2-disubstituted furanones 25 were examined83. Irradiation of 25 in the presence of excess cyclopentene led to the formation of the diastereomeric adducts 26 and 27. The ratio of photoproducts (26/27) ranged from modest (42 58) to significant (4 96) as the steric bulk of the substituent R was increased from ethyl to rm-butyl. 

Intermolecular de Mayo reactions are efficient for cyclic 1,3-diketones such as dimedone (5,5-dimethyl-l,3-cyclohexanedione)96,103,104 and acyclic systems such as acetylacetone93-95. Unsymmetrical acyclic /l-diketones, such as 1-phenyl-1,3-butanedione98 can enolize in two directions, however, reaction normally occurs preferentially from a single enol form. Examples of alkene photocycloaddition to trapped ends of /(-dicarbonyl compounds (e.g., 2,2-dimethyl-3(2/f)-furanone and 2,2.6-trimethyl-4/f-l,3-dioxin-4-one) are given in Table 1 (entries 26, 27) and Table 2 (entry 35) 10°. If the enol is stabilized by derivatization (e.g., acetylated dimedone 3-acetoxy-5,5-dimethyl-2-cyclohexenone), the primary cyclobutane photoproducts can be isolated96. 

The enhanced regioselectivity observed in photocycloadditions of both 3(2//)-thiophenones and 3(2 0-furanones as compared to the corresponding carbocyclic enones, e.g. cyclopent-2-enones has been ascribed to the possibility of CT-interaction in the excited heteracyclopentenones thus increasing the charge separation on the olefinic carbon atoms [49] (Scheme 13). 

Similarly photocycloaddition of ethylene with chiral 5-alkoxyfuranones 181 proceeds with facial diastereoselection [153], Although selectivity remains low, the reaction has been used for the enantiospecihc synthesis of grandisol because it is easy to separate the initial diastereoisomers [154]. Selectivity is greatly influenced by temperature and by substituents on the ethylenic bond of the furanone ring. It was concluded that steric hindrance of the alkoxy group and stereoelectronic effects of substituents on the conjugated system are responsible for the selectivity observed. It was also proposed that the diastereoselection of the photocycloaddition of ethylene with 181 was dominated by the approach of ethylene to the vibrationally relaxed (71,71 ) excited furanone rather than by the selectivity of the reactions of biradicals. 

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... 

A paper disclosing details of the stereoselective [2+2]photocycloaddition of chiral 2(5H)furanones to alkenes has appeared (Scheme 23). ... 

Related Photocycloadditions Furanones, Maleic Anhydride, and Maleimides 71-10... 

Furanones, maleic anhydride, maleimides, and cyclopentenones behave similarly when irradiated in the presence of alkenes. [2-1-2]-Photocycloaddition reactions of furanones, especially y-substituted furanones with alkenes, can be highly diastereoselective. A detailed analysis of the cycloaddition of ethylene with substituted furanones gave a deep insight into the origin of the diastereoselectivity. Based on product ratio dependence on temperature and the substituents, it was proposed that a pyramidalization of the P-carbon of furanones in their relaxed excited state and a homo-anomeric effect determined the... 

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