A facile efficient approach to the 2-styrylmalonates via the Lewis acid-catalyzed isomerization of 2-arylcyclopropane-1,1-dicarboxylates has been developed. The efficiency of this method was demonstrated for a representative series of such cyclopropanes. The isomerization proceeds chemo-, regio-and stereoselectively to afford E-styrylmalonates in good yields.
(3 + 2)-Annulation of donor-acceptor cyclopropanes to alkynes induced by both Lewis and Brønsted acids has been developed. The reaction provides a rapid approach to functionalized indenes displaying intense visible emission (λmax = 430 nm, Φ = 0.28-0.34).
The ability of donor-acceptor cyclopropanes to (3 + 3)-cyclodimerize is disclosed. It has been found that Lewis acid-induced transformations of 2-(hetero)arylcyclopropane-1,1-dicarboxylates containing electron-abundant aromatic substituents led to the construction of six-membered cyclic systems. Depending on the substrate properties and the Lewis acid applied, three types of products can be obtained: (1) 1,4-diarylcyclohexanes, (2) 1-aryl-1,2,3,4-tetrahydronaphthalenes, and (3) 9,10-dihydroanthracenes.
A novel Lewis acid-catalyzed [3 + 4] annulation of 2-(heteroaryl)cyclopropane-1,1-dicarboxylates with cyclopentadiene is reported. This reaction proceeds via an electrophilic attack of the Lewis acid-activated donor-acceptor cyclopropane onto cyclopentadiene followed by Friedel-Crafts intramolecular alkylation of the heteroarene substituent. This is the first general example of reactions of donor-acceptor cyclopropanes wherein the donor substituent serves as a nucleophile. The described annulation represents a convenient approach to bicyclo-A C H T U N G T R E N N U N G [3.2.1]octa-2,6-dienes with heteroarenes annulated to C(2)-C(3) bond. Its efficiency was demonstrated for a series of furyl, thienyl, pyrrolyl, benzofuryl, benzothienyl, and indolyl substituted cyclopropanes. Additionally, in the case of 2-(5-methyl-2-furyl)cyclopropane-1,1-diester we observed the predominant formation of product of the [3 + 4] annulation or the tetracyclic 5,8-methanocyclopenta[a]azulene derivative, depending on the reaction conditions.
Quo vadis? The Lewis acid catalyzed reaction of (hetero)aryl-derived donor-acceptor cyclopropanes with alkenes can be selectively directed along a [3+2] annulation pathway (see scheme). This new process provides convenient and efficient access to indanes and other cyclopentannulated (hetero)arenes, among which polyoxygenated 1-arylindanes exhibit significant cytotoxicity against several cancer cell lines with an IC50 of 10(-6)-10(-5) M.
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