A facile [4+1] type synthetic route to thiophenes from dienol silyl ethers and elemental sulfur

A facile method for the synthesis of thiophene derivatives via the reaction of readily accessible dienol silyl ethers with elemental sulfur is described. Dienol silyl ethers and elemental sulfur, when heated at 180 °C in the presence of MS4A, provided 3-siloxythiophene derivatives in yields up to 98%. In this reaction, thiophene derivatives might be formed through 1,2-dithiines and eight-membered cyclic tetrasulfides.     

Two new steroidal saponins from "Gualou-xiebai-baijiu-tang" consisting of Fructus trichosanthis and Bulbus allii macrostemi

Two new steroidal saponins were isolated from the Chinese folk medicine called Gualou-xiebai-baijiu-tang. The structures were determined to be spirost 25(27)-ene-2beta,3beta-diol-3-O-beta-D-glucopyranosyl(1-->2)-beta-D-galactopyranoside and 26-O-beta-D-glucopyranosyl-22alpha-hydroxy-5beta-furost-25(27)-ene-1beta,3beta,6beta,26-tetraol-3-O-beta-D-galactopyranoside, respectively, based on chemical evidences and spectral analysis.     

MNDO MO theoretical study of electronic structure and homolytic dissociation of perfluoroalkanoyl peroxides

The MNDO molecular orbital method was applied to alkanoyl peroxides and the effect of fluorination on the electronic structure and the rate of the homolytic 0-0 cleavage of alkanoyl peroxides were analyzed. The fluorine atom introduced at the carbon atom makes the peroxy 0-0 bond long and the dihedral angle between two COO planes large. The analysis of two-atom energy and its components energies has shown that the rate of the 0-0 cleavage correlates well with the destabilization of the resonance energy of the 0-0 bond and not with the electrostatic energy. The effect of elongation of the perfluoroalkyl group is also well elucidated by the two-atom energy. The perfluoroalkyl group lowers the σ*(0-0) anti-bonding orbital considerably, and this fully explains the observed rate of induced decomposition of perfluoroalkanoyl peroxides with benzene.     

Rotational isomerism involving an acetylenic carbon IV: synthesis and structure of bis(1,1';3',1"-terphenyl-2'-yl)ethynes: molecular design of sterically congested alkynes toward restricted rotation about acetylenic axis

The title diphenylethyne derivative with 4-methylphenyl (tolyl) groups at all the ortho positions was synthesized by the Stille or Sonogashira coupling from the corresponding iodide. The X-ray structure revealed that the two terminal phenyl groups at the sp carbons are twisted by 63 degrees out of the coplanar conformation to avoid steric interactions between the tolyl groups. The relative stabilities of possible conformers were analyzed by the PM3 calculations. The axially chiral derivative with two methoxymethyl groups showed no evidence of restricted rotation about the acetylenic axis by VT NMR measurements, its barrier being less than 35 kJ mol(-1). The spectroscopic features and reactivities of this sterically congested alkyne are also described.     

The design of cone-fixed calix[4]arene analogs by takingsyn-[2.n]metacyclophanes as a building block

Rigidified calix[4]arene analogs were synthesized fromsyn-[2.n]metacyclophanes as a building block. Their structure was firmly locked in the cone conformation. An enlarged calix[4]arene analog was obtained after the cyclobutane ring cleavage of the parent analog by Birch reduction. Several ionophores have been derived from the analogs and been found to select the larger ions during the extraction of alkali metals, transition metals, and lanthanoids. The ionophore having oligoethylene glycol units showed an effective catalytic activity for S_N2 reactions such as ester synthesis, Williamson ether synthesis, and Finkelstein reaction in several media.This paper is dedicated to the commemorative issue on the 50th anniversary of calixarenes.     

Self-assembled metal colloid films: two approaches for preparing new SERS active substrates

In this paper, we propose two new approaches for preparing active substrates for surface-enhanced Raman scattering (SERS). In the first approach (method 1), one transfers AgI nanoparticles capped by negatively charged mercaptoacetic acid from a AgI colloid solution onto a quartz slide and then deoxidizes AgI to Ag nanoparticles on the substrate. The second approach (method 2) deoxidizes AgI to Ag nanoparticles in a colloid solution and then transfers the Ag nanoparticles capped by negatively charged mercaptoacetic acid onto a quartz slide. By transfer of the AgI/Ag nanoparticles from the colloid solutions to the solid substrates, the problem of instability of the colloid solutions can largely be overcome. The films thus prepared by both approaches retain the merits of metal colloid solutions while they discharge their shortcomings. Accordingly, the obtained Ag particle films are very suitable as SERS active substrates. SERS active substrates with different coverages can be formed in a layer-by-layer electrostatic assembly by exposing positively charged surfaces to the colloid solutions containing oppositely charged AgI/Ag nanoparticles. The SERS active substrates fabricated by the two novel methods have been characterized by means of atomic force microscopy (AFM) and ultraviolet-visible (UV-vis) spectroscopy. The results of AFM and UV-vis spectroscopy show that the Ag nanoparticles grow with the increase in the number of coverage and that most of them remain isolated even at high coverages. Consequently, the surface optical properties are dominated by the absorption due to the isolated Ag nanoparticles. The relationship between SERS intensity and surface morphology of the new active substrates has been investigated for Rhodamine 6G (R6G) adsorbed on them. It has been found that the SERS enhancement depends on the size and aggregation of the Ag particles on the substrates. Especially, we can obtain a stronger SERS signal from the substrate prepared by method 1, implying that for the metal nanoparticles capped with stabilizer molecules such as mercaptoacetic acid, the in situ deoxidization in the film is of great use in preparing SERS active substrates. Furthermore, we have found that the addition of Cl- into the AgI colloid solution changes the surface morphology of the SERS active substrates and favors stronger SERS enhancement.     

Novel asymmetric michael addition of alpha-cyanopropionates to acrolein by the use of a bis(oxazolinyl)phenylstannane-derived rhodium(III) complex as a chiral Lewis acid catalyst

The rhodium complex prepared in situ by simply mixing [[RhCl(c-octene)2]2] and [(Phebox)SnMe3] (1) (Phebox = 2,6-bis(oxazolinyl)phenyl) was found to serve as an efficient catalyst for the asymmetric Michael addition of alpha-cyanopropionates (4) to acrolein under mild and neutral conditions. In the present catalytic system, both the temperature of catalyst preparation and the order of the addition of the substrates were very important for the catalytic efficiency and enantioselectivity. Detailed mechanistic studies of this catalytic system revealed that the [(Phebox)RhIII(SnMe3)Cl] complex (9), generated by oxidative addition of [[RhCl(c-octene)2]2] to 1, is an active catalyst and the turnover number (TON) of the present actual catalyst existing in a reaction mixture is greater than 10,000. The obtained (R) stereochemistry of the Michael adducts 5 can be explained by N-bonded enol intermediates C', which are formed by enolization of 4 bound to the Lewis acidic rhodium complex 9. We also found that the active catalyst 9 gradually decomposed in the presence of the remaining [[RhCl(c-octene)2]2] in the reaction mixture to form the catalytically nonactive [(Phebox)RhCl2] fragment A, whose structure was characterized by an X-ray crystallographic study after converting to the tBuNC complex 10.     

Palladium-catalyzed [3 + 2] cycloaddition reaction of vinylcyclopropanes with α,β-unsaturated esters or ketones

Reaction of vinylcyclopropanes having two electron-withdrawing groups with α,β-unsaturated esters or ketones in the presence of Pd₂(dba)₃.CHC1₃-1,2- bis(diphenylphosphino)ethane (dppe) or tributylphosphine catalyst gave vinyl- cyclopentanes in good yields.     

Confirmation of the crystal structure of poly(p‐phenylene benzobisoxazole) by the X‐ray structure analysis of model compounds and the energy calculation

To check the previously proposed crystal structure of poly(p‐phenylene benzobisoxazole) [PBO], we performed an X‐ray structure analysis for single crystals of low molecular weight model compounds with the following chemical formulas: Both of these two model compounds show essentially the same molecular and subcell structures as those of PBO: the molecular chains take an almost perfect planar conformation and are packed together with a relative height between the adjacent chains of about 3 Å along the chain axis, although for the polymer the chains are shifted by the same value but in a disordered mode with respect to the direction of the shift (upward or downward), different from the regular packing in model compounds. These structural features are reproduced well with energy calculations. Structural ordering in PBO fibers caused by heat treatment at high temperatures, as clarified by X‐ray diffraction measurement, are interpreted on the basis of the energy calculations. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1296–1311, 2001     

High-efficiency and minimum-waste continuous kinetic resolution of racemic alcohols by using lipase in supercritical carbon dioxide

A novel continuous-flow scCO(2) process for kinetic resolution of racemic alcohols can be performed with an immobilized lipase to lead to a quantitative mixture of the corresponding optically active acetates with up to 99% ee and unreacted alcohols with up to 99% ee, in which the productivity of the optically active compounds was improved by over 400 times compared to the corresponding batch reaction using scCO(2).