Diastereoselective Ce(OiPr)(3)-Catalyzed Pinacol Couplings of Aldehydes

Aliphatic aldehydes as well as aromatic aldehydes can be converted by means of cerium-catalyzed pinacol coupling [Eq. (1)]. The coupling reaction products can be isolated with good to very good diastereoselectivities in favor of rac isomers (rac:meso>88:12). R=alkyl, aryl.     

Electronic determinants of photoacidity in cyanonaphthols

We present semiempirical AM1 calculations for the ground and excited state of 2-naphthol and some of its cyano derivatives in the gas phase. Following photoexcitation, the Mulliken electron density on the oxygen diminishes slightly for the acid and more conspicuously for the anionic conjugated base. This agrees with the measured solvatochromic parameters for 2-naphthol. In both electronic states, we find a nice correlation with the measured pK values in water. The electronic charge distribution on the distal ring of the anion agrees with the experimental acidity order in both S(0) and S(1). Upon excitation, it increases predominantly in positions 3, 5, and 8. The ring system of the anion assumes an alternate quinoidal structure in the ground state of the anion, which becomes more symmetric in the relaxed excited state. This suggests that the enhanced aromatic character of a 4n electron system in the excited state allows for better delocalization of the oxygen charge within the ring.     

PEG based resins for protease drug discovery synthesis, screening and analysis of combinatorial on-bead libraries

This review will cover the entire hit identification process performed with biocompatible, aqueous solvated, poly[ethylene glycol] (PEG) based resins - from synthesis, through screening, to analysis. The different types of resins (including their preparation) will be discussed and compared individually. Examples of one-bead-one-compound substrate libraries will be presented, as will one-bead-two-compounds libraries used for the discovery of enzyme inhibitors. The review includes a section covering organic and bio-organic reactions performed on all-PEG resins and discusses on-bead screening of the libraries with biomolecules. Finally, analysis of compounds on single beads, either via investigation by on-bead NMR or by ladder-coding of the combinatorial compound is covered. In general, the review will focus on chemistry, libraries, synthesis, screening, and analysis, using all-PEG based resins.     

Diffusion of reagents in macrobeads

A simple and effective method of measuring diffusion rates of various acylating reagents in macro beads (in this work diameters greater than 570 microm) of amino-functionalized resins is presented. The extent of diffusion at various points of time was determined by treating beads with a staining reagent ("chloranil", 2,3,5,6-tetrachloro-1,4-benzoquinone) that colored the regions of the beads that had not yet been permeated by reagent (the central part of the beads). The volume of unstained resin (permeated part) was compared with the volume of the stained part, and diffusion rate constants were calculated. Factors influencing diffusion such as reagent concentration, solvent, temperature, resin type, and mechanical effects were investigated. The results showed that diffusion was promoted by increased temperature, good swelling of the resin, small reagents, and high concentrations but not by sonication or mechanical agitation.     

SPOCC-194, a new high functional group density PEG-based resin for solid-phase organic synthesis

A novel polymer matrix for solid-phase synthesis, SPOCC(194) resin (1), was designed featuring a backbone of homogeneous tetraethylene glycol (TEG(194)) macromonomer linked by quaternary carbon junctions and terminating in primary alcohol functionality. Beaded SPOCC(194) resin was effectively prepared by suspension polymerization of oxetanylated TEG macromonomer 5 in stirred silicon oil. Mechanically stable and inert to a diverse range of reaction conditions, SPOCC(194) possessed a high hydroxyl group loading (0.9-1.2 mmol/g) for substrate attachment and swelled effectively ( approximately 2-4 mL/g) in a variety of organic and aqueous solvents. Developed for solid-phase synthesis at high reactant concentrations for driving organic and aqueous reactions to completion, SPOCC(194) exhibited high functional group density (mmol/mL) similar to that of low-loaded aminomethylated polystyrene-divinylbenzene copolymer (PS-1%DVB) yet significantly higher than that of PEGA(1900), SPOCC(1500), and TentaGel S. High-resolution MAS NMR spectra of Fmoc-derivatized SPOCC(194) indicate that monitoring of functional group transformation is possible. Moreover, by employment of a nonaromatic resin-linker combination, electrophilic chemistry, such as Lewis acid catalyzed glycosylation and Friedel-Crafts acylation, was selectively performed on substrate bound to SPOCC(194) resin. Such properties make SPOCC(194) resin a promising new polymer matrix for the support-bound construction of small organic molecules by parallel and combinatorial synthesis and the scavenging of solution-phase reactants or byproducts.     

Syntheses of (S)-(+)-trihexyphenidyl hydrochloride and (S)-(+)-procyclidine hydrochloride, two anticholinergics, using (S)-(-)-3-cyclohexyl-3-hydroxy-3-phenylpropanoic acid as chiral synthon

The absolute configuration of the more active (-)-enantiomer of the anticholinergic trihexyphenidyl hydrochloride has been established as (R) by syntheses of (S)-(+)-procyclidine hydrochloride, whose absolute configuration has been established previously, and (S)-(+)-trihexyphenidyl hydrochloride from the same chiral building block, viz. (S)-(-)-cyclohexyl-3-hydroxy-3-phenylpropanoic acid. Both enantiomers of this chiral synthon were prepared by optical resolution of the corresponding racemate, employing (R)- and (S)-1-phenylethylamine, respectively, as resolving agents.