Organic chemists have always been fascinated by the cyclopropane subunit.1 The smallest cycloalkane is found as a basic structural element in a wide range of naturally occurring compounds.2 Moreover, many cyclopropane-containing unnatural products have been prepared to test the bonding features of this class of highly strained cycloalkanes3 and to study enzyme mechanism or inhibition.4 Cyclopropanes have also been used as versatile synthetic intermediates in the synthesis of more functionalized cycloalkanes5,6 and acyclic compounds.7 In recent years, most of the synthetic efforts have focused on the enantioselective synthesis of cyclopropanes.8 This has remained a challenge ever since it was found that the members of the pyrethroid class of compounds were effective insecticides.9 New and more efficient methods for the preparation of these entities in enantiomerically pure form are still evolving, and this review will focus mainly on the new methods that have appeared in the literature since 1989. It will elaborate on only three types of stereoselective cyclopropanation reactions from olefins: the halomethylmetal-mediated cyclopropanation reactions (eq 1), the transition metal-catalyzed decomposition of diazo compounds (eq 2), and the nucleophilic addition-ring closure sequence (eqs 3 and 4). These three processes will be examined in the context of diastereoand enantiocontrol. In the last section of the review, other methods commonly used to make chiral, nonracemic cyclopropanes will be briefly outlined.

Stereoselective Cyclopropanation Reactions

3-Acetoxyaminoquinazolinones (QNHOAc) as aziridinating agents: ring-opening of N-(Q)-substituted aziridines

The polyhydroxylated indolizidines castanospermine and swainsonine as well as some of their stereoisomers are powerful glycosidase inhibitors. An efficient and stereochemically flexible synthesis of racemic 1,7,8-trihydroxylated and 1,6,7,8-tetrahydroxylated indolizidines (castanospermine stereoisomers) from readily available N-substituted γ-oxygenated α,β-unsaturated sulfones 3 and 4 has been developed. The construction of the bicyclic skeleton of 1-hydroxyindolizidine has been accomplished by intramolecular conjugate addition of the nitrogen moiety of 3 and 4 to the α,β-unsaturated sulfone unit to give the pyrrolidine intermediates 5 and 6, followed by formation of the C(7)−C(8) bond by intramolecular acylation (or alkylation) of the α-sulfonyl carbanion. The stereoselectivity of the pyrrolidine synthesis was highly dependent on the bulkiness of the γ-oxygenated function; thus, the free alcohols gave predominantly cis-pyrrolidines while the OTIPS derivatives led to the trans isomers. After straightforwa...

Stereoselective Synthesis of Polyhydroxylated Indolizidines from γ-Hydroxy α,β-Unsaturated Sulfones

We report the syn-selective Michael addition of a series of substituted primary and secondary nitromethane derivatives 4a−g to chiral enoates (Z)-2a and (E)-2a in the presence of TBAF·3H2O or DBU. Regardless of the base employed, adducts syn-5a−g were obtained in good de (80−100%) from the reactions of 4a−g with (Z)-2a. However, in the addition to (E)-2a, the syn-diastereoselectivities depended on the structure of the nucleophile (80−90% de for nitromethane (4a), 80% de for phenylnitromethane (4g), 50 and 34% de for the primary nitromethane derivatives 4b and 4d, and 0 and 6% de for the secondary nitromethane derivatives 4c and 4f). A mixture of epimers (2:1/1:1) was obtained at the chiral center bearing the nitro group in 5b and 5d−g. The syn/anti ratio C-3,C-4 is kinetically controlled, while the epimeric ratio at the CNO2 chiral center (C-1‘) seems to be thermodynamically controlled. Adducts 5a,b,c,g were transformed into the respective cis-β,γ-disubstituted γ-butyrolactones 6a, 7, 9a, and 9c. A mechan...

Syn-Selective Michael Addition of Nitromethane Derivatives to Enoates Derived from (R)-(+)-Glyceraldehyde Acetonide

The conformational preferences of chiral vinylogous aminosulfonic acids (vs-amino acids) and of the corresponding oligomers (vs-peptides) were investigated by a combination of X-ray crystallography, variable-temperature (VT) 1H NMR spectroscopy, FT-IR spectroscopy, and NOE experiments. The major source of conformational freedom in the monomers is the rotation around the CC bond connecting the double bond with the allylic stereocenter (NC*CC). The allylic conformational preferences can be altered in the oligomers by the formation of secondary structures enforced by hydrogen bonding. Twelve-membered-ring hydrogen bonding is detected in the crystal structure of vs-dipeptide 9, while fourteen-membered-ring hydrogen bonding is the most common folding pattern for the oligomers in chloroform solution. The experimental results are complemented by computer modeling: suitable force-field (FF) parameters for the unsaturated sulfonamide group were developed from ab initio calculations. A Goodman–Still systematic pseudo-Monte-Carlo search was used for the conformational search. The conformers were minimized in chloroform with the GB/SA model. The calculations correctly predicted both the size of the hydrogen-bonded ring and its relative importance, in agreement with the experimental data in solution.

Conformational Studies of Chiral Vinylogous Sulfonamidopeptides

Steric and Electronic Effects in Conformational Preferences of C1-Oxygenated Chiral Alkenes

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Steric and Electronic Effects in Conformational Preferences of C1-Oxygenated Chiral Alkenes