Asymmetric thio-claisen rearrangement induced by an enantiopure alkylsulfinyl group. Unusual preference for a boat transition state in the acyclic series.

Ketene aminothioacetals 2a-c and 5a-b bearing an enantiopure vinylic alkylsulfinyl substituent were readily prepared from (R)-2-cyclohexylsulfinyl-N,N-dimethylethanethioamide 1 with full control of the geometry of their double bonds. They underwent a Claisen rearrangement upon heating at THF reflux to afford alpha-sulfinyl gamma-unsaturated thioamides 3a-c and 6a-b. With all substrates the asymmetric induction of the sulfinyl group was excellent. The determination of the absolute configurations of thioamides 3a-c and 6a-b was achieved either by X-ray crystallographic analysis or by chemical correlation. The stereochemical course of this [3,3] sigmatropic transposition was explained by an electronic model. Interestingly the Claisen rearrangement of the (ZE)-cinnamyl substrates 5b was shown to proceed through a boat transition state rather than a chair transition state; such a preference is quite unusual for acyclic systems.

First chiral selenium ylides used for asymmetric conversion of aldehydes into epoxides.

Enantioenriched selenonium ylides have been generated by addition of benzyl bromide to C2 symmetric (2R,5R)-2,5-dimethylselenolane in the presence of NaOH, and subsequently reacted with a variety of aldehydes to give oxiranes with excellent enantiomeric excesses (a catalytic version has been achieved); also, an aliphatic cyclic hypervalent dibromoselenurane structure has been demonstrated by X-ray analysis.

Proanthocyanidins from the bark of Hamamelis virginiana exhibit antimutagenic properties against nitroaromatic compounds.

The antimutagenic activity of Hamamelis virginiana bark was examined in the Ames assay. A commercial tincture and a methanolic extract showed dose-dependent inhibitory effects on mutagenicity induced by 2-nitrofluorene. Tannin-free samples did not display any inhibition. Bioassay-guided fractionation resulted in the isolation of two active fractions which were shown to contain oligomeric, proanthocyanidins. They were capable of inhibiting the mutagenicity of selected nitroaromatic compounds. The mechanism of antimutagenic action was also studied. The proanthocyanidins did not act as bioantimutagens, but rather as direct-acting desmutagens. The antimutagenic effect increased with an increasing degree of polymerisation in the proanthocyanidins. The most active fraction consisted of catechin and gallocatechin oligomers with an average polymerisation degree of 9.2.