Ring currents in the porphyrins: pi shielding, delocalisation pathways and the central cation

It is shown that the ipsocentric orbital-based model explains how the charge of the central cation drives the delocalisation pathway in metalloporphyrins. A positive charge +Ze at the centre of the porphin ring gives rise to a two-way radial transfer of charge within the pi structure of the porphin macrocycle. This manifests itself in a change of pathway of the global pi current, as Z increases from Z = 0, from an inner- through a bifurcated- to an outer-pathway. Changes of pathway can be interpreted in terms of a specific pi shielding effect whereby electrons in high-lying pi orbitals are screened from the central charge by the electrons in lower-lying orbitals of the same symmetry. These changes in pi structure are essentially independent of accompanying changes in the sigma structure.     

Synthesis of C-linked glycopyranosyl serines via a chiral glycine enolate equivalent

[formula: see text] The stereoselective preparation of C-linked D-gluco- and D-galactopyranosyl L-serines in their alpha and beta forms is herein reported. The syntheses require the conversion of the allyl C-glycopyranosides into their iodoethyl derivatives, which then undergo substitution with the Williams' chiral glycine enolate equivalent. Deprotection and acetylation affords Boc-protected amino acids for peptide synthesis.     

Pentaheptite modifications of the graphite sheet

Pentaheptites (three-coordinate tilings of the plane by pentagons and heptagons only) are classified under the chemically motivated restriction that all pentagons occur in isolated pairs and all heptagons have three heptagonal neighbors. They span a continuum between the two lattices exemplified by the boron nets in ThMoB4 (cmm) and YCrB4 (pgg), in analogy with the crossover from cubic-close-packed to hexagonal-close-packed packings in 3D. Symmetries realizable for these pentaheptite layers are three strip groups (periodic in one dimension), p1a1, p112, and p111, and five Fedorov groups (periodic in two dimensions), cmm, pgg, pg, p2, and p1. All can be constructed by simultaneous rotation of the central bonds of pyrene tilings of the graphite sheet. The unique lattice of cmm symmetry corresponds to the previously proposed pentaheptite carbon metal. Analogous pentagon-heptagon tilings on other surfaces including the torus, Klein bottle, and cylinder, face-regular tilings of pentagons and b-gons, and a full characterization of tilings involving isolated pairs and/or triples of pentagons are presented. The Kelvin paradigm of a continuum of structures arising from propagation of two original motifs has many potential applications in 2D and 3D.     

6-Endo Heck Cyclization of 3-(2-Iodophenoxy)methylbenzofurans: A Useful Approach to Pterocarpenes

Mitsunobu coupling of 3-hydroxymethylbenzofurans with o-iodophenols affords 3-(2-iodophenoxy)methylbenzofurans which under go a useful 6-endo Heck cyclization under Jeffery conditions to afford pterocarpene-type heterocycles. The unsubstituted parent pterocarpene thus prepared was readily converted to coumestan via subsequent PCC oxidation.

Supplemental materials are available for this article. Go to the publisher's online edition of Synthetic Communications® to view the free supplemental file.     

Optimal synthesis of heat exchanger network for thermochemical S-I cycle

In this paper, a brief survey of hydrogen production methods is presented with a focus on S-I cycle. Based on heat duty data of sulfuric acid decomposition in S-I cycle, optimization models are developed to explore the minimum utility consumption and the minimum number of heat exchangers. Finally an optimal heat exchanger network for S-I thermochemical cycle is defined by a mixed integer optimization model.     

The MacLane class and complex differential equations in the unit disk

The interaction between the coefficients and solutions for linear differential equations and for Riccati differential equations is investigated in the unit disk in terms of their asymptotic values as described by the MacLane class. This interaction is further explored within the context of a meromorphic version of the MacLane class.