A short route toward chiral, polyhydroxylated indolizidines and quinolizidines

In this paper, a rapid route toward functionalized bicyclic alkaloids is presented. In only three steps, an easily accessible carbohydrate derivative was converted into iodomethyl indolizidine 13, which can equilibrate to the corresponding iodoquinolizidine 15. We provide strong evidence that this equilibration proceeds via an aziridinium ion intermediate. Furthermore, nucleophilic substitution of the iodomethyl indolizidine as well as the aziridinium intermediate gives access to highly functionalized indolizidine and quinolizidine alkaloids.     

Pathological tremors: Deterministic chaos or nonlinear stochastic oscillators?

Pathological tremors exhibit a nonlinear oscillation that is not strictly periodic. We investigate whether the deviation from periodicity is due to nonlinear deterministic chaotic dynamics or due to nonlinear stochastic dynamics. To do so, we apply various methods from linear and nonlinear time series analysis to tremor time series. The results of the different methods suggest that the considered types of pathological tremors represent nonlinear stochastic second order processes. Finally, we evaluate whether two earlier proposed features capturing nonlinear effects in the time series allow for a discrimination between two pathological forms of tremor for a much larger sample of time series than previously investigated. (c) 2000 American Institute of Physics.     

De novo synthesis of uronic acid building blocks for assembly of heparin oligosaccharides

An efficient de novo synthesis of uronic acid building blocks is described. The synthetic strategy relies on the stereoselective elongation of thioacetal protected dialdehydes 12 a and 17. The dialdehydes are prepared from D-xylose, a cheap and commercially available source. A highly stereoselective MgBr(2)OEt(2)-mediated Mukaiyama aldol addition to C4-aldehyde 12 a is performed to obtain D-glucuronic acid building block 16, whereas L-iduronic acid building block 22 is prepared by MgBr(2)OEt(2)-mediated cyanation of C5-aldehyde 17. Synthesis of a heparin disaccharide demonstrates the utility of the de novo strategy for the assembly of glycosaminoglycan oligosaccharides.     

Selective Cross‐Metathesis of C‐Allyl‐Glycosides

The efficient and selective cross‐metathesis (CM) of both the α‐ and β‐anomers of C‐allyl‐glucose and N‐acetyl‐C‐allyl‐glucosamine with electron‐deficient olefins is reported. The applicability of our CM approach in the synthesis of glycoside‐conjugates is illustrated by the CM of α‐C‐allyl‐glycosamine 2 with uridinyl vinylphosphonate 22, to produce UDP‐GlcNAc analog 23.     

Stereocontrolled synthesis of fully functionalized D-glucosamine monosaccharides via a domino nitro-Michael/Henry reaction

A diastereoselective domino nitro-Michael/Henry reaction involving a beta-hydroxyaldehyde and a nitroalkene provides direct access to fully functionalized D-glucosamine monosaccharides.     

Preorganized frustrated Lewis pairs

Geminal frustrated Lewis pairs (FLPs) are expected to exhibit increased reactivity when the donor and acceptor sites are perfectly aligned. This is shown for reactions of the nonfluorinated FLP tBu(2)PCH(2)BPh(2) with H(2), CO(2), and isocyanates and supported computationally.