Multiplicities and continuum γ-rays following heavy-ion reactions

The continuous γ-ray spectrum following (HI, xn) reactions is analyzed in terms of the competition between collective (rotational) and non-collective (statistical) modes of de-excitation at high spin. The dependence of the γ-ray intensity and of the multiplicity upon the parameters that define the rotational and the statistical modes is studied in an application of the model to the reaction ¹⁷⁶Yb(¹²C, 8n)¹⁸⁰Os at 120 MeV. It is found that that competition may be relevant throughout a rather large energy interval.     

Probing nuclear surfaces by heavy ion reactions

An analysis of the recently measured ¹⁶O + ²⁶Mg one- and two-proton transfer reaction is given. It is shown that these reactions probe progressively deeper regions of the surface of the two colliding nuclei.     

A flexible stereospecific synthesis of polyhydroxylated pyrrolizidines from commercially available pyranosides

Nitrogen-containing sugar analogues, known as azasugars or iminosugars, such as polyhydroxylated piperdines, pyrrolidines, pyrrolizidines, and indolizidines, have the potential to become important therapeutic agents due to their ability to inhibit glycosidases. Synthetic pathways that are able to systematically produce a variety of these azasugars are eagerly sought after, since even minute structural or stereochemical changes often significantly alter the degree of inhibition. The synthesis of tetrahydroxylated pyrrolizidines 40 and 41 starting from methyl alpha-d-glucopyranoside is described and will be used as a template to develop syntheses of all the stereoisomers of polyhydroxylated pyrrolizidine 9 as well as other analogous bicyclic polyhydroxylated iminosugars. The key steps in this synthesis involve a one-pot conversion of a halopyranoside to a divinylamine by employing a simultaneous Zn reduction and reductive amination of the resulting aldehyde. After protection of the amine, a ring-closing metathesis results in a multifunctional eight-membered ring that then undergoes an internal S(N)2 cyclization to form an alkene-containing pyrrolizidine 33. Dihydroxylation of the alkene followed by hydrogenolysis of the benzyl protecting groups results in tetrahydroxylated pyrrolizidines 40 and 41.     

Upward straight-line embeddings of directed graphs into point sets

In this paper we study the problem of computing an upward straight-line embedding of a planar DAG (directed acyclic graph) G into a point set S, i.e. a planar drawing of G such that each vertex is mapped to a point of S, each edge is drawn as a straight-line segment, and all the edges are oriented according to a common direction. In particular, we show that no biconnected DAG admits an upward straight-line embedding into every point set in convex position. We provide a characterization of the family of DAGs that admit an upward straight-line embedding into every convex point set such that the points with the largest and the smallest y-coordinate are consecutive in the convex hull of the point set. We characterize the family of DAGs that contain a Hamiltonian directed path and that admit an upward straight-line embedding into every point set in general position. We also prove that a DAG whose underlying graph is a tree does not always have an upward straight-line embedding into a point set in convex position and we describe how to construct such an embedding for a DAG whose underlying graph is a path. Finally, we give results about the embeddability of some sub-classes of DAGs whose underlying graphs are trees on point set in convex and in general position.     

Diastereoselective addition of chlorotitanium enolate of N-acyl thiazolidinethione to O-methyl oximes: a novel,stereoselective synthesis of alpha,beta-disubstituted beta-amino carbonyl compounds via chiral auxiliary mediated azetine formation

We have discovered a novel and highly diastereoselective synthesis of azetinyl thiazolidine-2-thiones that utilizes additions of the chlorotitanium enolates of N-acyl thiazolidin-2-thiones to O-methyl aldoximes. The "anti" azetines can be subsequently converted to the corresponding alpha,beta-disubstituted beta-amino carbonyl compound with retention of stereochemistry. The formation of azetine and the product of its "hydrolytic" opening has been confirmed by X-ray crystallographic analyses.