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An original tandem reaction consisting of a Wittig reaction-ring contraction process between α-hydroxycyclobutanone and phosphonium ylides has been developed. Highly functionalized cyclopropanecarbaldehydes are obtained in good to high yield.

A general strategy for the synthesis of indolyl cyclopropanecarbaldehydes and ketones via a Brønsted acid-catalyzed indole nucleophilic addition/ring-contraction reaction sequence has been exploited. The procedure leads to a wide panel of cyclopropyl carbonyl compounds in generally high yields with a broad substrate scope.

A unified protocol for the construction of 3-(2-bromoethyl)benzofurans and 2-(benzofuran-3-yl)ethylamines from bis[(trimethylsilyl)oxy]cyclobutene has been developed. This mild and facile strategy was applied for the synthesis of a series of 5-HT serotonin receptor agonists, underlining its potential for the syntheses of bioactive compounds and natural products.

The first examples of the asymmetric synthesis of P-stereogenic vinylic phospholene boranes are described. The synthetic approach is concise and flexible. The route involves (i) asymmetric deprotonation-allylation of a dimethyl phosphine borane; (ii) telescoped regioselective deprotonation, paraformaldehyde trapping, and hydroxyl group elimination to give a diene; and (iii) ring-closing metathesis.

[reaction: see text] Acid-catalyzed ring expansion of chiral cyclopropyl and cyclobutyl derivatives for the synthesis of carbo- and heterocyclic compounds is reported. The starting materials have been successfully prepared by l-proline-catalyzed direct asymmetric aldol reactions of 1-phenylthiocycloalkyl carboxaldehydes with ketones.

[reactions: see text] Regioselective synthesis of 2,4,5- or 3,4,5-trisubstituted 2,3-dihydrofurans has been realized by using donor-acceptor cyclopropanes or by a Corey ylide reaction with alpha-sulfenyl-, alpha-sulfinyl-, or alpha-sulfonylenones. The method allowed a straightforward synthesis of the natural product calyxolane B.

[reaction: see text] The stereochemistry of the cyclobutanones 3, obtained by lithium salt catalyzed ring expansion of the optically pure oxaspiropentanes 2, depends not only on the lithium salt but also on the stereochemistry of 2. They constitute the starting material for the syntheses of the acetogenin (-)-(4R,5R)-muricatacin and the pheromone (R)-japonilure.

[reaction: see text] The thionium ion, generated through a cyclopropylcarbinyl-cyclobutyl ring expansion, is, for the first time, intramolecularly intercepted by activated aromatic rings to generate new versatile 2a-methyl-8b-(phenylsulfanyl-1,2a,3,8b-tetrahydro-2H-cyclobuta[c]chromenes.