Cyclopropane skeletal structure

The skeletal isomerization of C4 and C5 n-olefins is an acid-catalyzed reaction requiring relatively strong acid sites that proceeds via carbenium ion intermediates formed upon protonation of the double bond (17). Double bond cis-trans isomerization usually occurs on the acid sites before skeletal isomerization. The general reaction mechanism for branching isomerization is depicted in Fig. 2 2. Protonation of the double bond leads to a secondary carbenium ion, which then rearranges into a protonated cyclopropane (PCP) structure. In the case of n-butenes,... 

Ring-protonated alkylcyclopropanes, well-known in solution , were suggested to explain the most unusual behaviour of heptyl ions. Extensive C- and D-labelling as well as a computer simulation and the analysis of CA data indicate that the major fragmentation reactions of CvHj s ions, generated via loss of I from 1-heptyl iodide, are preceded by extensive skeletal reorganization which via cyclopropane-like structures (140, 142, 144) eventually isomerize to tertiary carbocations (145 and 146, Scheme 23 where only the carbon skeleton is shown). The latter serve as actual precursors for the loss of CsH. ... 

The above mentioned isomerization of CCCLI to CCCLII constitutes a route from the 9/3,19-cyclosteroids to a new skeletal type which is characteristic of a small group of Buxus alkaloids and will be dealt with in the subsequent text. Another reaction applied to an analogous structural type and leading to cleavage of the cyclopropane ring was reported by Kupchan and Abushanab 219) who had subjected 9, 19-cyclo-5a-pregnane-3,l 1,20-trione-3,2()-diethylene ketal to Wolff-Kishner reduction and obtained a mixture of 10-epimeric ketals CCCLX and CCCLXI. 

The name of a cycloalkane reveals the number of carbon atoms in the ring. For example, cyclopropane (CjHg) is the smallest cycloalkane, with three carbon atoms in the ring. Cyclobutane (C4Hg) has a ring with four carbon atoms. A cyclic hydrocarbon can be represented by condensed, skeletal, and line structures. These structures are illustrated below for cyclobutane. Chemists usually use line structures to depict cycloalkanes. 

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