Vinylcyclopropane- cyclopentene rearrangement photochemical

Photochemical vinylcyclopropane-cyclopentene rearrangements open an alternative to prepare racemic grandisol from (optically active) A -carene (equation 184). On the other hand, irradiation of tricyclic enones has resulted in two different product types as shown in equation 185, depending on the nature of substituent R both classes of compounds should be of synthetic value . ... 

Vinylcyclopropane-cyclopentene rearrangement Thermal or photochemical rearrangement of substituted vinylcyclopropanes to substituted cyclopentenes. 470... 

Several photochemically induced vinylcyclopropane to cyclopentene rearrangements of nor-carene derivatives to form bicyclo[3.2.0]heptenes can be understood as ring contractions of cyclohexenes to cyclobutanes. Upon direct irradiation of norcar-2-ene (bicyclo[4.1.0]hept-2-ene) at 214 nm (pentane solution), however, complex product mixtures were obtained containing only small amounts of bicyclo[3.2.0]hept-2-ene, while toluene sensitized photolysis in 50 millimolar solution in degassed pentane at 254 nm gave mainly the cyclobutane derivative 13 in addition to EjZ-isomeric hepla-l,3,6-trienes.72... 

Cooke, R.S. (1970) A photochemical vinylcyclopropane to cyclopentene rearrangement. Journal of the Chemical Society D Chemical Communications, (7), 454—455. 

The photochemical transformations of vinylcyclopropanes are of mechanistic interest, but because of the high energy of the reaction intermediates, especially in the triplet manifold, practical applications are scarce. The regio- and stereo-selective aspects are, however, similar to those of the thermal processes. Additional methods that provide for the cyclopentene rearrangement involve the degenerate photochemical rearrangements observed by Wender during meta-photocycloaddition of arenes and alkenes and utilized extensively in the synthesis of triquinane sesquiterpenes. -" -" (See Section 8.1.9 or ref. 41 for a recent summary.)... 

Cooke, R. S. Photochemical vinylcyclopropane to cyclopentene rearrangement. J. Chem. Soc., Chem. Common. 1970, 454-455. 

Occasionally, photolysis of vinylcyclopropanes has been used for preparative purposes to effect the vinylcyclopropane to cyclopentene rearrangement in good yield. Bicyclic ketones and alkenes are available by the photolysis of endocyclic vinylcyclopropanes of type 1, see also Table 3. For a more detailed listing of photochemical transformations consult the recent reviews. ... 

When carbene 21 is generated from the diazo precursor photochemically in solution, it reacts with added alcohols by 0,H insertion (equation 55), in contrast to the gas-phase copyrolysis where the silene intermediate is trapped . Similarly, photolysis of 20 in the presence of 2,3-dimethylbutadiene gives mainly vinylcyclopropane 184, while after copyrolysis of 20 and the same diene one finds that most of the vinylcyclopropane rearranged to the cyclopentene, together with the [4-1-2] cycloaddition product of diene and silene 22 °. Furthermore carbene 21, generated photochemically or thermally (at 117-148 °C) in solution, undergoes [1 4-2] cycloaddition to alkynes to give cyclopropenes 185. 

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