Vinylcyclobutane-cyclohexene rearrangements

Cycloaddition of OTr/n-bomyltriazolinedione 706 to 9-thia-l-azabicyclo[4.2.1]nona-2,4-diene 9,9-dioxide 705 gave compound 707 as a diastereoisomeric mixture (Scheme 112). The formation is attributed to the sequential operation of a regioselective [2+2] cycloaddition and a heteroatomic variant of the vinylcyclobutane-cyclohexene rearrangement <2006JOC6573>. 

Useful examples of the cyclohexene to vinylcyclobutane rearrangement for preparative purposes are given in Section 4.2.3. 

Photochemical [2 + 2] cycloadditions followed by vinylcyclobutane to cyclohexene rearrangements often yield the same products as the thermal Diels-Alder reactions. Strongly directing substituents on unsymmetrical dienes, however, may lead to constitutionally different isomers depending on the reaction path.19 21... 

Acceleration of the vinylcyclobutane to cyclohexene rearrangement of vinylcyclobutanols by conversion to the potassium alkoxide was first reported by Wilson and Mao.50 Typically, the [1,3] shift occurs at a satisfactory rate in tetrahydrofuran between room and reflux temperature. Sodium51 or lithium alkoxides52 also rearrange in this temperature range. The reaction rate can be further increased by addition of crown ethers or hexamethylphosphoric triamide.50,53... 

Styrene type [59a], As will be noted further on, the tendency of these initially formed vinylcyclobutane adducts to quickly rearrange via a cation radical mechanism to the cyclohexene (i.e., Diels-Alder) adducts is also more pronounced. In the case of a very highly electron rich substrate such as A-methyl-A-vinylacetamide, even the reaction with the rigidly s-cw-l,3-cyclohexadiene is CB periselective (Scheme 28) [61]. 

The conclusion would appear to be that the vinylcyclobutane rearrangement involves biradicals, possibly formed by conrotatory-level motions (see Chapter 7, Section 3.14), Scheme 7.103, which are incompletely equilibrated before cyclization to cyclohexene because there is always a preference for utilization of the allylic moiety in a suprafacial way. 

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