Vinylcyclopropanes cyclopentene ring

The thermal expansion of a vinylcyclopropane to a cyclopentene ring is a special case of a [l,3]-sigmatropic migration of carbon, although it can also be considered an internal [ 2 + 2]-cycloaddition reaction (see 15-63). It is known as a vinylcyclopropane rearrangementThe reaction has been carried out on many vinylcyclopropanes bearing various substituents in the ring or... 

Cyclopentene ring from vinylcyclopropanes Stereospecific cyclopentene annelation... 

The synthesis of useful organic motifs via molecular rearrangement of strained molecules continues to be an area of profound interest to synthetic chemists. The strain energy of a variety of three-membered carbocycles has been utilized to develop new synthetic transformations. One of the important reactions that has been ejq)lored within this class of molecules is the vinylcyclopropane (VCP)-cyclopentene (CP) rearrangement, which involves the formation of a cyclopentene ring from the ring expansion of the VCP via C—C bond cleavage fScheme 11.11. 

Thermal Reactions.—Vinylcyclopropanes and Related Compounds. The vinyl-cyclopropane rearrangement " provides a convenient method for cyclopentene ring formation even in highly constrained ring-fused systems. A trans-... 

Adducts derived from cyclopropyl-TMM reactions are versatile synthetic intermediates. Alkylidenecyclopropanes have been proven useful in further Pd-cata-lyzed transformations [4], On the other hand, vinylcyclopropanes can undergo smooth thermal ring-expansion to cyclopentenes. Thus, a total synthesis of 11-hy-droxyjasionone (27) was achieved with the cyclopropyl-TMM cycloaddition as the crucial step, and the thermal rearrangement of the initial adduct (28) as an entry to the bicyclo[6.3.0]undecyl compound (29), a key intermediate in the synthetic sequence (Scheme 2.9) [19]. 

The direction of ring opening by homolytic cleavage of a cyclopropane bond is controlled by the stability of the diradical species formed. Upon heating of the mono-deuterated vinylcyclopropane 3, a mixture of the two isomeric mono-deuterated cyclopentenes 4 and 5 is formed ... 

One other 100% atom economic rearrangement is worth mentioning briefly. When vinylcyclopropanes are heated they readily undergo ring expansion to cyclopentenes. The temperature required varies significantly, depending on the molecule for example l-phenyl-2-vinylcyclopropane may be converted to phenylcyclopentene in reasonable yield at 200 °C. 

The photoindueed 1,7-cycloaddition of carbon monoxide across the divinyl-cyclopropane derivative 32 yields the two cyclic dienyl ketones 34, via the ferracyclononadiene intermediate 33 [18]. (Scheme 11) cyclopentene rearrangement. The dienylcyclopropane 35 is capable of forming the complex 36, followed by ring enlargement to 37 [19]. 1,1-Dicyclopropylethylene 29 is also converted to the 1-cyclopropyl-1-cyclopentene 38. The additional functionality of vinylcyclopropanes is necessary to serve as a 7t-donor... 

Vinylcyclopropanes are now widely used as precursors to five-membered rings. Various methods have been designed to construct vinylcyclopropanes and bring about their rearrangement — most frequently thermolytically — to cyclopentenes. 

The extrapolation of the vinylcyclopropane-cyclopentene rearrangement to a vinyl-cyclobutaiie-cyclohexene synthesis begins to create new insights into the synthesis of six membered ring natural products. The eudesmane sesquiterpene (—)-P-selinene, 217 illustrates such a strategy as summarized in Scheme 14 80). A suitable cyclohexene... 

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... 

The vinylcyclopropane radical cation, generated at 77 K by X-irradiation of (139) in a Freon-113 matrix, was shown to rearrange at 105-110 K to afford two ring-opened distonic radical cationic species.300 The rearrangement reactions of the radical cations of 1,3- and 1,4-pentadiene and cyclopentene and the formation of spin adducts with 2.4.6-tri-/-butylnitrosobenzene (BNB) are discussed. The pulse radiolysis of 1,1 -binaphthyl-2,2,-diyl hydrogenphosphate (BiNPCUH) (140) in deaerated f-butanol at... 

Donor-acceptor-substituted vinylcyclopropanes such as (203) have been shown to undergo thermal ring enlargement to functionalized cyclopentene derivatives (204) at... 

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