Cyclobutane ring opening photochemical

It has been found that certain 2 + 2 cycloadditions that do not occur thermally can be made to take place without photochemical initiation by the use of certain catalysts, usually transition metal compounds. Among the catalysts used are Lewis acids and phosphine-nickel complexes.Certain of the reverse cyclobutane ring openings can also be catalytically induced (18-38). The role of the catalyst is not certain and may be different in each case. One possibility is that the presence of the catalyst causes a forbidden reaction to become allowed, through coordination of the catalyst to the n or s bonds of the substrate. In such a case, the... 

Irradiation of isophorone and a 15-fold excess of enol ether 177 results in the formation of a diastereomeric mixture of adducts 178 in quantitative yield. On brief treatment with tetrabutylammonium fluoride, the mixture of photoadducts 178 undergo cyclobutane ring-opening to give two epimeric diketo esters 179 in 70% yield. The Claisen condensation of these diketo esters proceeds on treatment with sodium hydride in refluxing THF, and a single enolized trione 180 is isolated in good yield. This photochemical approach provides easy access to a series of bicyclic 1,3-cyclohexanediones. 

The route employed to prepare indanone 51 involved the cycloaddition-hydrolysis-aldol sequence shown in Scheme 3.9. Accordingly, condensation of cyclopentenone 52 with ynamine 53 (84) afforded the bicyclic enamine 54 which was converted to indanones 51 and 55 by hydrolytic cyclobutane ring opening followed by intramolecular aldol condensation. Interestingly, treatment of 54 with aqueous formic acid yielded indanone 51 which has stereochemistry complementary to that at C(15) and C(20) in reserpine. In contrast, hydrolysis of this substance with aqueous hydrochloric acid afforded the trans-fused indanone 55. Subsequent to this work, the Ficini group found that esterification of 51 followed by photochemically induced addition of methanol afforded adduct 56 which has four of the reserpine stereocenters in place (23). While no further work on this problem has been reported, these preliminary investigations demonstrate a novel use of [2 -h 2] photocycloaddition chemistry in potential approaches to yohimbane alkaloid synthesis. 

Photochemical [2 + 2] cycloaddition is a powerful way to produce cyclobutanes, which, in turn, are reactive synthesis intermediates. N-Methylpyrrole adds aldehydes via [2 -I- 2] photocycloaddition to give transient oxetanes with high regioselectivity Ring-opening produces 3-(oi-hydroxyalkyl)pyrroles which are oxidized easily to 3-arylpyrroles, such as 3-BUTYROYL-l-METHYL-PYRROLE. With a special apparatus, ethylene is conveniently added to 3-methyl-... 

During the synthesis of the Woodward reserpine precursor (17 Scheme 4), Peailman used the protected acylacetal (13) to control the stereochemistry of an intramolecular photochemical cycloaddition to (14). The strategy for opening the cyclobutane ring employed the Baeyer-Villiger reaction to convert the 7-keto ester (15) to a 3-hydroxy ester (16), which underwent retroaldolization to (17). 

Photochemical cyclobutane annealings are much more promising and find their way into synthesis to a greater extent. Photochemical cycloaddition reactions to enolized 1,3-diketones and consecutive ring openings via retroaldol reactions have been applied in ring expansion reactions numerous times. 

Cyclobutane formation, occasionally in fairly low yield due to ring-opening reactions, also occurred by photochemical [2 - - 2] cycloaddition of vinylcyclopropane derivatives to an a, -unsaturated ketone moiety, " a cyclobuta-1,3-diene, a 1,4-benzoquinone, 2-acylthiophenes, and on irradiation of e t/o,enr/o-2,4-bis[( )-2-(methoxycarbonyl)vinyl]bi-cyclo[1.1.0]butane. A formal [2-1-2] cycloaddition also occurred on reaction of tricyclo[3.1.0.0 ]hex-3-ene with methyl 6-oxo-5-phenyl-l,3,4-oxadiazine-2-carboxylate to give methyl 9-oxo-10-phenyl-8-oxapentacyclo[4.4.0.0 ". 0 .0 °]decane-7-carboxylate, albeit in very low yield, after nitrogen extrusion. ... 

The photochemical [2+2]cycloaddition of alkenes to coumarins provides a mixture of the exo- and e <7o-cyclobuta[c]coumarins. Treatment with CH2=SOMe2 opens both the lactone and cyclobutane rings and both diastereomers are considered to give the same quinone methide from which a single dibenzofuran-4-ol stereoisomer results <04SL1897>. 

A survey of the literature reveals some characteristic photochemical features of homoquinones (1) [2+2]-photocycloaddition with alkenes and alkynes, (2) hydrogen abstraction and/or ring opening of incorporated cyclopropane, and (3) photoinduced electron transfer reactions, as described below. The photocycloaddition with alkenes and alkynes took place exclusively at the ethylene linkage of homobenzoquinones to afford cyclobutanes (or cyclobutene derivatives) regio- and exo/endo stereoselectively, depending on the identity of both the homobenzoquinones as well as the alkenes. 

Ring and open-chain derivatives of L-erythritol 1,4-dicinnamate have been irradiated yielding products of photochemical asymmetric synthesis, in which intramolecular 2-1-2 cycloaddition occurs to give L-erythritol esters of cyclobutane-1,2-dicarboxylic acid the 2,3-di-O-methyl derivative of l-erythritol gave a high degree of asymmetric induction, whereas the 2,3-0-isopropylidene derivative was much less stereospecific, and gave the opposite cyclobutane enantiomer. ... 

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