Cyclopropane ring homoconjugate

The direct observation of ion 46 is of particular interest in that it clearly does not involve a significantly opened cyclopropane ring, which could lead to the formation of an allylic cation. Thus, it must be considered as a bent cyclopropyl cation.164-170 It is, however, clear that the Ci-C6 ct-bond must to some degree interact with the empty p orbital at Cn and that homoconjugation between them becomes the important factor in stabilizing such a bent cyclopropyl species. 

Homoconjugative interactions between a cyclopropane ring and a double bond were studied for the three bicyclic compounds 7-9, each possessing a locked vinylcyclopropane fragment [84], The polarization patterns in the chloranil-sensitized photoreactions of these substrates were used to probe the spin density distribution in the radical cations. A strong dependence of electronic structure of the radical cations on steric effects was found (see Chart IV). 

The second of Corey s new approaches to PG synthesis is based on an examination of a process for selective cyclopropane ring cleavage by homoconjugate addition of an organocopper reagent [viz. (87) - (89) Scheme 18]. To this end Corey and Fuchs have shown that the tricyclic lactone ester (87), prepared from cw-cyclopent-2-ene-l, 4-diol (25), reacts with divinylcopper lithium in an expected homoconjugate additive fashion to produce (90),... 

In the 4q case, 6 is not an important contributor to the ground state description of the properties of either 4 or 5. However, with 4 there are alternative modes of homoconjugation possible that involve the external cyclopropane bonds30,32,37 38. This is shown in Scheme 2 for the bicyclo[3.1.0]hexenyl cation. This alternative mode of conjugation of a cyclopropane in a 4q situation, an option not available to the parent 4q antiaromatic unsaturated ring systems39, leads to a fundamentally different set of properties and reactions of these systems as compared to the potentially homoaromatic 4q + 2 cases. 

As part of a study of ring construction through transposition of activated cyclo-propanes, Danishefsky s group has scrutinized the feasibility of intramolecular homoconjugate nucleophilic additon.64 Two of the substrates examined were 34 and 35 and these were found to undergo rather efficient closure to bicyclo[3.3.0]octane derivatives when treated with strong base. The somewhat elevated temperatures allow both the cyclopropane fission and subsequent Dieckmann condensation to proceed at respectable rates. 

Danishefsky and co-workers have used activated cyclopropanes to promote intramolecular alkylation of an amino function, followed by lactam formation to provide a new entry to the pyrrolizidine ring system.20 This route is outlined in Scheme 1. Cyclopropanation of the phthalimido-olefin (19) was achieved with dimethyldiazomalonate in the presence of copper bronze. Treatment of the cyclopropane derivative (20) with hydrazine released the amine, which gave the lactam ester (21) in quantitative yield. This corresponds to intramolecular homoconjugate addition entirely in the spiro mode.21 It seems reasonable that the first step in this process is internal alkylation of the amine by the activated cyclopropane. The alternative... 

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