Butenolides photocycloadditions

Scheme 40 Synthesis of furanose-fused y-butyrolactones by photocycloaddition of vinylene carbonate to substituted butenolides...

A novel entry to decahydrocyclopentacyclooctene derivatives via the intramolecular photocycloaddition of fused a,/3-unsaturated y-lactones has been developed (80CC1011). Irradiation of the butenolide (153) in acetone solution gave both the fused and bridged photoadducts (154) and (155) (2-3 1). The major adduct was hydrolyzed, oxidized and esterified to afford (156). Reductive cleavage of the unsaturated keto ester (156) with lithium in ammonia afforded a five-component mixture of a,/3- and /3,y-unsaturated esters. Equilibration with 0.1M sodium methoxide in methanol converted the mixture into a single a,j8-unsaturated ester (157 Scheme 34). This annelative two-carbon ring expansion method may find application in the synthesis of ophiobolin and ceroplastol sesterterpenes. 

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... 

The crossed intramolecular [2 + 2]-photocycloaddition of allenes to a, 3-unsat-urated y-lactones has been extensively studied by Hiemstra et al. in an approach to racemic solanoedepin A (87). The sensitized irradiation of butenolide 85 in a 9 1 mixture of benzene and acetone, for example, led selectively to the strained photocycloadduct 86 (Scheme 6.31) [89]. The facial diastereoselectivity is determined by the stereogenic center, to which the allene is attached. The carbon atom in exposition to the carbonyl carbon atom is attacked from its re face, forming a bond to the tertiary allene carbon atom, while the P-carbon atom is being connected to the internal allene carbon atom by a si face attack. The method allows facial diaster-eocontrol over three contiguous stereogenic centers in the bicyclo[2.1.1]heptane part of the natural product. 

Photocycloaddition of allene to the cyclopentenone derivative (6) in methylene chloride solution at — 78°C afforded a cycloadduct (7) in 84% yield, which was a key intermediate for the construction of the AB ring core of Taxol (Shimada et al, Chapter 2). Similarly, photocycloadditions of ethene to enan-tiopure butenolides (8) at — 78°C have been studied by de March et al. (Chapter 2). The product distribution was found to depend on the protecting group R, but with R = TMS, the product stereoisomer (9) was obtained in 83% yield, and an efficient synthesis of (-I- )-grandisol was developed. 

Intramolecular cycloaddition can often provide an easy synthetic entry to novel strained compounds. Thus the photocycloaddition of the butenolides (3) yields the adducts shown in the Scheme 22 Irradiation of the enone (4) at 330 nm affords the... 

An intramolecular [2-1-2] photocycloaddition reaction was reported as the key step in constructing the tricyclic core 218 of solanoeclepin A, which includes an intricate bicycle[ 2.1.1 jhexanone moiety. Allene butenolide 217 as... 

The intramolecular photocycloadditions of butenolides with a 2,3-butadienyl substituent at the 5-position (16) produced tricyclic cycloadducts (17) with a methylenecyclobutane moiety as the core unit. The key reaction is the regioselective crossed 2- -2-cycloaddition of the internal allene double bond (Scheme 5). The ruthenium-catalysed 2- -2-intramolecular cycloaddition of allenes and alkynes (18) yielded bicyclo[3.2.0]heptane cycloadducts (19) under mild conditions and with a high diastereo-selectivity (Scheme 6). ... 

Intramolecular 2 -f 2-photocycloadditions of butenolides with an allenylmethyl substituent at the 5-position have been reported (Scheme 67). ... 

Tanaka, M., Tomioka, K., and Koga, K. (1994) Enantioselective total synthesis of (+)-stoechospermol via stereoselective intramolecular (2+2) photocycloaddition of the chiral butenolide. Tetrahedron, 50, 12829-12842. 

The high facial stereoselectivity observed for the [2-t-2]-photocycloadditions of alkenes with chiral butenolides and the polyfimctional nature of the cycloadducts have led to interesting new apphcations in the field of natural products. The total synthesis of (-t-)-stoechospermoP and (-l-)-spatol (Scheme 23), as well as new syntheses of (-i-)-grandisol, > including photochemical cycloaddition as a key step, have been recently reported (Scheme 23). 

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