Migration cyclopropane rings

The key step in this sequence, achieved by exposure of 46 lo a mixture of sulfuric acid and acetic anhydride, involves opening of the cyclopropane ring by migration of a sigma bond from the quaternary center to one terminus of the former cyclo-l>ropane. This complex rearrangement, rather reminiscent of the i enone-phenol reaction, serves to both build the proper carbon. keleton and to provide ring C in the proper oxidation state. 

Quinone methides 156 and 157 were generated on irradiation of the cis and trans forms of 155, respectively (Eqs. 1.42 and 1.43).99 100 The mechanism is believed to involve a radical ring opening of the cyclopropane ring, followed by hydrogen or methyl migration. 

As mentioned above, the conversion of cyclopropane to propene radical cation has been investigated by ab initio calculations. The general course of this reaction was confirmed, or anticipated, by product studies in the electron transfer-sensitized conversion of 1,1,2,2-tetraphenylcyclopropane (37) to 1,1,3,3-tetra-phenylpropene (38). The sequence of the key steps, migration versus ring opening cannot be derived from the results. In the case of 37, the four phenyl substituents may actually favor a ring-opened bifunctional radical cation. 

Artemisyl, Santolinyl, Lavandulyl, and Chrysanthemyl Derivatives.— The presence of (41) in lavender oil has been reported earlier. Poulter has published the full details of his work (Vol. 5, p. 14) on synthetic and stereochemical aspects of chrysanthemyl ester and alkoxypyridinium salt solvolyses (Vol. 3, pp. 20—22) and discussed its biosynthetic implications. Over 98% of the solvolysis products are now reported to be artemisyl derivatives which are formed from the primary cyclopropylcarbinyl ion (93) which results from predominant (86%) ionization of the antiperiplanar conformation of (21)-)V-methyl-4-pyridinium iodide the tail-to-tail product (96 0.01%) may then result from the suprafacial migration of the cyclopropane ring bond as shown stereochemically in Scheme 3. This is consistent with earlier work (Vol. 7, p. 20, ref, 214) reporting the efficient rearrangement of the cyclobutyl cation (94) to (96) and its allylic isomer, via the tertiary cyclopropylcarbinyl cation (95). ... 

The mechanism proposed for the isomerization of perfluoro(alkylcyclopropanes)24 involves abstraction of fluoride ion from the a-position to form a cation that is partially stabilized by delocalization into the cyclopropane ring. Ring opening, addition of fluoride, and bond migration give the observed products, as shown for pcrfluoro(methylcyclopropane) (27a). 

Eormation of a Cyclic Transition State Structure Erontier Orbital Approach Some Examples of Hydrogen Shifts Migrations in Cyclopropane rings Migrations of Atoms or Groups other than Hydrogen Selection Rules... 

Rupture of a carbon-carbon bond in the cyclopropane ring to yield the trimethylene biradical, followed in a distinctly separate stage by hydrogen migration, viz. 

C]-mevalonic acid (56a) in Pinus palustris or P. sylvestris showed that the Relabeling is predominantly at (4) . Namely, the C(4) of the monoterpene is derived from C(2) of mevalonic acid (56) rather than C(4) as had been assumed and therefore, the cyclopropane ring formation must be accompanied by a double bond migration if a-car-3-ene (61) is indeed dervied from a-terpinyl ion (62) or its biosynthetic equivalents. 

That is in fact the case 3S>36) (Scheme 94), not only for the 1-seleno-l-vinylcyclopropa-nes belonging to the methylseleno series but also for their phenylseleno analogs, which both lead to cyclobutanones in the presence of p-toluenesulfonic acid on the condition 35,36) t jat the sp2-carbon linked to the cyclopropane is identically or more highly substituted than the other sp2-carbon. If that is not the case, migration of the carbon-carbon double bond from the vinylic to the allylic position (relative to the cyclopropane ring) mainly takes place and only a trace amount of cyclobutanone (< 5 %) and presumably cyclopentanone is formed (Scheme 94e). 

In the dehydrobromination of 9-bromobicyclo[6.1.0]nonane (37), which has no alkyl group geminal to the bromine, the initial product is a cyclopropene intermediate in which the double bond undergoes rapid base-catalyzed migration to a position exo to the cyclopropane ring. An 80% yield of product was thereby obtained in which the major component (95%) was bicyclo[6.1.0]non-l -ene (38). ... 

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