Cyclobutanes ring opening

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... 

A cyclobutane ring-opening of the photoadduct (426) by the reaction of BF3 Etfi in refluxing benzene gave isopropenylcyclohexenone (427). The reaction could be applied to more complex compounds. Thus, (428), (430), (432) and (434) were converted accordingly to (429), (431), (433) as well as (435) 145). 

If hydrogen occupies all sites, the dual-site mechanism may operate over two adjacent /2g sites 42). The importance of active site periodicity and the screening of the adjacent C—C bond is valid in this case, too. This (assumedly adsorbed) hydrogen does not participate in C5 cyclic reactions. There is some indication, however, that it might be mobilized for cyclobutane ring opening 97, 97a). 

Considering the actual cyclobutane ring opening step Pyr +OPyr Pyr+-Pyr (Scheme 4.5.4), however, where the C(6)-C(6 ) a bond is broken and a N(1)=C(6) n bond is simultaneously formed, this cleavage should be slower the more the involved orbitals, e.g. the SOMO and the c orbitals of the N(l)-C(6) and C(6)-C(6 ) bonds, deviate from coplanarity (Scheme 4.5.8). 

Compound VIII thus formed would then undergo ring contraction as postulated before, followed by a now logical cyclobutane ring opening that would occur under the auspices of acetate and the C-C bond weakening effect of the sulfonium unit. 

Orbital symmetry restraints exist in both directions of a transformation, so this treatment will apply to cyclobutane ring opening, as well as to olefin fusion. Further, these arguments will not be restricted to the fusion of 77 bonds, but may apply generally to the concerted fusion of any two bonds in the following manner ... 

Ammonium ceric nitrate / , -Ethyleneoxo compds. by cyclobutane ring opening... 

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. 

Durst T, Breau L (1991) Cyclobutane ring opening. In Trost BM, Fleming I (eds) Comprehensive organic synthesis. Pergamon, Oxford (Chapter 6)... 

Ammonium ceric nitrate Oxidatiye cyclobutane ring opening... 

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. 

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