Cycloreversion, thermal cyclobutanes

A great deal is already known about the pyrolysis of pinenes," which constitutes a perfect case for the study of cyclobutane cycloreversion reactions. In practice, this avenue was first explored with the hope of obtaining products with commercial value.99 Unfortunately, the application of these reactions to organic synthesis is somewhat restricted, because complex product mixtures cause complications. For the sake of clarity Table 6100 110 outlines only the cycloreversion products and their straightforward secondary derivatives nevertheless, it demonstrates some of the synthetic uses of these thermal cleavage reactions. 

The mechanistic and synthetic groundwork has been unequivocally established for the consecutive cycloreversion transannular ene reaction sequence, from which /ra/w-decalin derivatives with a hydroxyl group at the ring junction are produced.146,147 As an example, the thermally induced cycloreversion of the ester 43 at 200°C affords 45 in an astonishing 96% yield.146 Presumably the initial cycloreversion product 44 is converted by a transannular ene reaction to generate the decalin 45.146 However, not all the cycloreversion reactions proceed to give a single product as predicted, as can be shown by the examples collected in Table 7. In fact, closer inspection of work already reported has shown that complex product mixtures are usually obtained from cyclobutane cycloreversion reactions.143,148 152... 

Cyclobutanes may be converted to alkenes thermally, the reverse of the [2 + 2] cycloaddition reaction. These retroaddition or cycloreversion reactions have important synthetic applications and offer further insights into the chemical behavior of the 1,4-diradical intermediates involved they may proceed to product alkenes or collapse to starting material with loss of stereochemistry. Both observations are readily accommodated by the diradical mechanism. Generation of 1,4-tetramethylene diradicals in other ways, such as from cyclic diazo precursors, results in formation of both alkenes and cyclobutanes, with stereochemical details consistent with kinetically competitive bond rotations before the diradical gives cyclobutanes or alkenes. From the tetraalkyl-substituted systems (5) and (6), cyclobutane products are formed with very high retention stereospecificity,while the diradicals generated from the azo precursors (7) and (8) lead to alkene and cyclobutane products with some loss of stereochemical definition. ... 

Eight-membered rings can be obtained by [4+4]-cycloadditions of 1,3-dienes [1] via diradicals or other intermediates. Synthesis of such compounds has been achieved by thermal, [2] photochemical, [3] and by metal-catalyzed [4] processes these reactions have been the subject of extensive mechanistic [5] and theoretical [5c] studies. Their strategic applications in natural product synthesis have been reviewed. [5d] The thermal version has generated little interest, except in orthoquino-dimethane dimerizations and in cycloreversions the Cope rearrangement of 1,2-divinyl-cyclobutanes [3] is more commonly used. [4+4]-Cycloadditions are also used with 1,3-dipoles or mesoionic heterocycles for the synthesis of six- and seven-membered rings. Sometimes also [6+4]-cycloadditions are... 

Certain transition metals catalyse the thermally forbidden [2 + 2] cycloaddition reactions to form a cyclobutane ring or the corresponding cycloreversion reaction. One of the proposed mechanisms for this reaction (Scheme 3) involves the metal acting as a template which provides /-orbitals of the... 

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