Cyclobutanes cleavage

It is well known that the [2+2] photodimerization of diolefinic compounds is allowed to occur photochemically but not thermally, whereas the cyclobutane cleavage reaction occurs both photochemically and thermally. The cleavage reaction occurs with irradiating light of shorter wavelength... 

Similarly, (ft)-( + )-6,6-dimcthylbicyclo[3.1, l]heptan-2-one [(R)-( + )-nopinone, 59] undergoes boron trifluoride promoted cyclobutane cleavage to give in 68% yield (4/ )-l-acetoxy-4-(2-acetoxy-2-propyl)cyclohex-l-ene (60).168,169... 

The ease of cyclobutane cleavage and the detailed mechanism can be affected by the nature of the substituents and the substitution pattern. While the above cyclobutanes are cleaved without a discernible intermediate, the n//-head-to-head dimer of dimethylindene shows a significantly different behavior. This substrate is cleaved in an apparent two-step process, involving a ring-closed radical cation (with spin and charge localized on one indan system) and a ring-opened 1,4-bifunctional radical cation. Apparently, the cleavage of the doubly benzylic cyclobutane bond is reversible. The involvement of more than one dimer radical cation is indicated by a unique polarization pattern (Fig. 13), which is incompatible with any one intermediate, but can be simulated on the basis of two successive radical cations (see Sect. 5.2) [256]. 

From a preparative point of view the heteroatom in P-position has an influence because important latent cyclobutane cleavage pathways exist in the product. The prototypical reaction of this type is the [2 + 2]-photocycloaddition/retro-aldol reaction sequence (de Mayo reaction) [55-57], the course of which is illustrated for substrate A3 (Q = O, PG = protecting group) in the reaction with ethylene as a generic olefin. 

The excitation energy of the olefinic groups in these photostable compounds is probably directed not toward cyclobutane cleavage but largely toward ds-trans isomerization. 

The PET-induced ring opening of oxygenated strained rings can be extended to cyclopropanes and cyclobutanes. One of the first examples of a PET-induced cyclobutane cleavage was reported in 1990 [67]. Irradiation of ketone 54 in the presence of triethylamine results in the formation of spiro compound 55 (Scheme 26) [68]. 

The biradical pathway for the cyclobutane cleavage and geometric isomerization was first pursued by Walters with cis- and ran -l,2-dimethylcyclobutane which interconvert roughly one-fifth as fast as they undergo cleavage to propylene at 400°C (Scheme 5.23). There is also 5-10% ethylene and 2-butene formed in the reaction, and, further, there is some preservation of stereochemistry at Cl and C2. The ratio of cis- to trans-2-buionQ formed from the trans compound is 0.125 while that from the cis compound is 2 at low conversions. ... 

A system analogous to the taxanes has been prepared by using the intramolecular de Mayo reaction of vinylogous esters. Irradiation of the mixture of diastereomers 223 results in cycloaddition of one diastereomer to give adduct 225 and recovery of the other isomer 224. The tetrahydrofuran is oxidized with RUO4 to the lactone, which is saponified with concomitant cyclobutane cleavage to the diketo acid 227. 

Perform an analysis of the thermochemistry for cyclobutane cleavage to two ethylenes. Does it predict that the tetramethyl-ene biradical (Section 11.12.2) is a viable reactive intermediate ... 

Oxidative cleavage of l,2-bis(trimethylsilyloxy)cyclobutene with Br2, to give cyclobutane-l,2-dione. 

In a recent total synthesis of the novel neurotrophic agent merrilactone A (22, Scheme 4) by Inoue and Hirama [24], key intermediate 21 with the cis-bicyclo[3.3.0] octane framework embedded within the caged pentacycle 22 was elaborated from cyclobutane 18 by a sequence of RCM and immediate cleavage of the resulting bicyclic vicinal diol 19 to raeso-diketone 20. Cyclooctenedione 20 then underwent regioselective transannular aldol reaction at low temperature (LHMDS, THF, -100 °C) to produce a 3 1 mixture of isomers in 85% combined yield. The major isomer 21 with the required stereochemistry was then converted into the racemic natural compound ( )-22 in 19 steps. 

Dihydropyran-4-ones are a source of phenols via an intramolecular [2+2] photocycloaddition reaction and a Lewis-acid catalysed cleavage of the cyclobutane moiety <96TL1663>. 

Carbocation-carbanion zwitterionic intermediates were proposed for the thermal cleavage of several cyclic compounds. In most of these reactions the ionically dissociating bond belongs to one of four strained ring systems, i.e. cyclopropane (13), cyclobutane (14), cyclobutene (15) or norbornadiene (16). The mechanism is distinguished from the formation of a diradical intermediate through homolysis in terms of solvent and substituent effects... 

Ejection of dinitrogen from the triazoline adducts to form the related aziridines was promoted by ultraviolet irradiation (300 nm, benzene) and usually proceeded in excellent yield. An exception was found in the irradiation of the triazoline substrate 59, where cleavage of the cyclobutane ring occurred as the dominant reaction pathway to form the pyridazino norbomadiene 61 (and secondary photoproducts derived therefrom), together with the triazole-4,5-diester 62. A role for the pyridazine ring and the 2-pyridyl substituents in stabilising the diradical intermediate 60 has been proposed for this abnormal outcome (Scheme 8). 

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