Cyclobutane ring cleavage

Scheme 12 Ring enlargement reaction by cyclobutane ring cleavage.

Photochemical cyclobutane ring cleavage Tricyclo[3.3.2.04.6]deca-2,7,9-trienes... 

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). 

Cleavage of cyclobutane rings can occur easily. As indicated in the introductory section, the strain of the four-membered ring, the substituents on the ring, the nature of the reagents as well as the conditions of reaction are all responsible for the ease of cleavage of cyclobutanes 3). The substituents on the ring constitute one of the... 

The cleavage of the cyclobutane ring of (223) was achieved by reaction with KOH—KO Bu in Et20. The reaction proceeded with excellent stereoselectivity the -configuration of the product (224) was maintained (Table 13) 74). 

Cleavage of the cyclobutane ring of (277) by Lewis acid followed by cyclization of the allylsilane (278), furnished the tricyclic compound (278)94). 

Photolysis of the amide 31 in methylene chloride at room temperature results in the formation of the tricyclic lactam 32 in both syn and anti forms <99TL6001>. Treatment of 32 with BF3 etherate brings about cleavage of the cyclobutane ring in the syn isomer only, with the formation of 33. (Scheme 5). The corresponding esters undergo similar transformations, if somewhat less efficiently. 

Narasaka s chiral titanium catalyst, prepared from (Pr 0)2TiCl2 and a tartrate-derived (2R,3R)-l,l,4,4-tetraphenyl-2,3-0-(l-phenylethylidene)-l,2,3,4-butanetetrol, is utilized for the asymmetric [2+2] cycloaddition of A-acyl oxazolidinones to 1,2-propadienyl sulfides possessing a-substituents, which afford methylenecyclobutane derivatives with high enantiomeric purity. These chiral adducts are readily transformed to seven- and eight-membered carbocycles with chiral side chains by the ring-cleavage reaction and subsequent cationic cyclization of the chiral cyclobutane derivative [68] (Eq. 8A.44). 

Both of these reactions were expected to profit from relief of strain in the starting material and from a strain-free heterocycle in the product. Owing to the polar nature of the ylide it was desirable, however, to introduce at least 1 heteroatom into the cyclobutane ring in order to facilitate heterolytic cleavage of the system. The silacyclobutanes seemed to be an excellent choice, and, consequently, a project on ylide cleavage reaction of mono-and disilacyclobutanes was initiated. Although no stable cyclic pentaalkylphosphorane was obtained, it was possible to confirm the appearance of... 

---