Total synthesis of dysiherbaine

A convergent total synthesis of the marine natural product dysiherbaine was accomplished. The key steps of the synthesis are an alkylation at the gamma-carbon of a protected glutamate with a highly substituted pyran derived from mannose, which was followed by a ring-contraction cascade reaction, which simultaneously gave the tetrasubstituted carbon and the hexahydrofuro[3,2-b]pyran ring system of the natural product.     

Excitation dependent quenching of luminescence in LED phosphors

Droop, the decrease of efficiency with increased power density, became a major topic with InGaN LEDs, after its introduction in 2007. This paper provides insight into droop in localized center luminescence phosphors, exemplified here by Eu²⁺ doped materials. This topic is of increasing importance, as high brightness blue LEDs have reached outputs >1 W/mm². The nonlinearities in phosphor quantum efficiency result in drive‐dependent color point shift and reduction of overall efficiency of phosphor converted white LEDs which utilize Eu²⁺ activated phosphors. The efficiency quenching can be traced back to two processes, well‐known in laser physics, excited state absorption or/and cross relaxation by Foerster/Dexter transfer. Both processes lead to reduction in phosphor efficiency, but they can be differentiated. Understanding the root cause of efficiency quenching opens ways to minimize the practical consequences. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Synthesis of Octasubstituted Porphyrins

The synthesis of porphyrins having high solubility in organic solvents requires the presence of long chain aliphatic substituents. Such substituted porphyrins are readily prepared in good yields by extension of our general procedure¹ which involves the synthesis of α,β-unsaturated ketones, conversion of these to 3,4-disubstituted pyrroles followed by acid catalyzed condensations of these pyrroles with formaldehyde in the presence of air.     

Theoretical study of nucleophilic substitution at the disulfide bridge of cyclo-l-cystine

The reaction of cyclo-l-cystine with thiolate is examined at the B3LYP/6-31+G level. The two isomers of cyclo-l-cystine differ in their dihedral angle about the disulfide bond; the M isomer (with dihedral angle of -90.1 degrees) is found to be slightly lower in energy. The nucleophilic substitution reaction at sulfur follows the addition-elimination mechanism, exemplified by the hypercoordinate sulfur intermediate on the reaction surface. The reaction is exergonic (DeltaG = -6.16 kcal mol(-1)), and both the entrance and exit transition state lie below the reactant energies.