Barium manganate in microwave-assisted oxidation reactions: synthesis of solvatochromic 2,4,6-triarylpyrimidines

Synthesis of 2,4,6-trisubstituted pyrimidines by tandem oxidation/heterocyclocondensation of propargylic alcohols and amidines is effected rapidly and efficiently under microwave dielectric heating using barium manganate as oxidant. Irradiation at 150 °C in ethanol–acetic acid for 45 min results in dramatic improvements in yield over the corresponding manganese dioxide-mediated method and establishes a rapid route to triarylpyrimidines in order to investigate their photophysical properties.     

Design of Multichannel Optical Splitter without Bends

The theory of a planar geometry splitter involving only parallel waveguide channels of rectangular cross-section is presented. The strictly parallel geometry allows for a semi-analytical analysis that makes possible the design of a splitter, which can divide an optical signal into uncoupled outputs carrying designated fractions of the input power. The theoretical intrinsic losses in the proposed splitter is shown to be minimal, and device design parameters are provided.     

Rheology of precursors for sol-gel derived YBa2Cu3O7−x superconductors

Aqueous-based precursors for high T_c superconductors are prepared by mixing solutions of copper acetate, barium acetate and yttrium hydroxide. The time-dependent dynamic moduli show the kinetics and extent of gelation of the precursor solution o be directly related to the age of yttrium hydroxide component; “age” being the length of time between the preparation of the yttrium component and its use in the precursor solution. The viscosity of the colloidal suspension of the yttrium hydroxide at low shear rates increases with the age of the material by an order of magnitude in two months. Dynamic oscillatory and controlled-stress experiments on the yttrium hydroxide suspension also reveal increases in moduli, and formation of network structure as this material ages. The particle size of the yttrium hydoxide, as measured by dynamic light scattering, almost doubles in two months of aging. Superconducting films made from different batches of precursors containing fresh and old yttrium components show differences in their morphology and electrical behavior. Fired films obtained from precursors containing aged yttrium hydroxide had a single phase perovskite and showed superconducting behavior whereas films made from fresh yttrium had an additional structural phase and never achieved zero resistance. A possible mechanism relating the changes in the yttrium hydroxide properties as it ages to the sol-gel kinetics of the precursor solution and the characteristics of the fired film is presented.     

Stereoselective synthesis of the gamma-lactam hydrolysate of the thiopeptide cyclothiazomycin

[reaction: see text] Bohlmann-Rahtz pyridine synthesis of a chiral nonracemic enamine and thiazolylpropynone gives a terminal-protected pyridine-containing gamma-amino acid in high optical purity in a sequential one-pot multicomponent reaction that proceeds with total control of regiochemistry and with minimal racemization. Further elaboration has established the synthesis of the gamma-lactam acidic hydrolysate of the macrocyclic thiopeptide antibiotic cyclothiazomycin, a selective renin inhibitor, in only four steps and 30% overall yield and has confirmed its structure.     

Synthesis of dimethyl sulfomycinamate

[reaction: see text] Dimethyl sulfomycinamate, the oxazole-thiazole-pyridine product generated in the methanolysis of the thiopeptide antibiotic sulfomycin I, is prepared in 13 steps and 8% overall yield by the Bohlmann-Rahtz heteroannulation of 1-(oxazol-4-yl)enamines and methyl 4-(trimethylsilyl)-2-oxobut-3-ynoate.