Blair D. Johnston

New glucosidase inhibitors from an ayurvedic herbal treatment for type 2 diabetes: structures and inhibition of human intestinal maltase-glucoamylase with compounds from Salacia reticulata.

An approach to controlling blood glucose levels in individuals with type 2 diabetes is to target alpha-amylases and intestinal glucosidases using alpha-glucosidase inhibitors acarbose and miglitol. One of the intestinal glucosidases targeted is the N-terminal catalytic domain of maltase-glucoamylase (ntMGAM), one of the four intestinal glycoside hydrolase 31 enzyme activities responsible for the hydrolysis of terminal starch products into glucose. Here we present the X-ray crystallographic studies of ntMGAM in complex with a new class of alpha-glucosidase inhibitors derived from natural extracts of Salacia reticulata, a plant used traditionally in Ayuverdic medicine for the treatment of type 2 diabetes. Included in these extracts are the active compounds salacinol, kotalanol, and de-O-sulfonated kotalanol. This study reveals that de-O-sulfonated kotalanol is the most potent ntMGAM inhibitor reported to date (K(i) = 0.03 microM), some 2000-fold better than the compounds currently used in the clinic, and highlights the potential of the salacinol class of inhibitors as future drug candidates.

Inhibition of recombinant human maltase glucoamylase by salacinol and derivatives

Inhibitors targeting pancreatic α‐amylase and intestinal α‐glucosidases delay glucose production following digestion and are currently used in the treatment of Type II diabetes. Maltase‐glucoamylase (MGA), a family 31 glycoside hydrolase, is an α‐glucosidase anchored in the membrane of small intestinal epithelial cells responsible for the final step of mammalian starch digestion leading to the release of glucose. This paper reports the production and purification of active human recombinant MGA amino terminal catalytic domain (MGAnt) from two different eukaryotic cell culture systems. MGAnt overexpressed in Drosophila cells was of quality and quantity suitable for kinetic and inhibition studies as well as future structural studies. Inhibition of MGAnt was tested with a group of prospective α‐glucosidase inhibitors modeled after salacinol, a naturally occurring α‐glucosidase inhibitor, and acarbose, a currently prescribed antidiabetic agent. Four synthetic inhibitors that bind and inhibit MGAnt activity better than acarbose, and at comparable levels to salacinol, were found. The inhibitors are derivatives of salacinol that contain either a selenium atom in place of sulfur in the five‐membered ring, or a longer polyhydroxylated, sulfated chain than salacinol. Six‐membered ring derivatives of salacinol and compounds modeled after miglitol were much less effective as MGAnt inhibitors. These results provide information on the inhibitory profile of MGAnt that will guide the development of new compounds having antidiabetic activity.

Facile syntheses of the enantiomers of sulcatol

The synthesis of R-(−)- and S-(+)-6-methyl-5-hepten-2-ol (sulcatol) from commercially available ethyl S-(−)-lactate via chiral methyloxiranes is reported. Both substances have crystalline intermedi...

Synthesis and conformational analysis of a pentasaccharide corresponding to the cell-wall polysaccharide of the Group A Streptococcus

The synthesis and conformational analysis of a pentasaccharide corresponding to a fragment of the cell-wall polysaccharide (CWPS) of the bacteria Streptococcus Group A are described. The polysaccharide consists of alternating alpha-(1 --> 2)- and alpha-(1 --> 3)-linked L-rhamnopyranose (Rhap) residues with branching 2-acetamido-2-deoxy-D-glucopyranose (GlcpNAc) residues linked beta-(1 --> 3) to alternate rhamnose rings. The pentasaccharide is of interest as a possible terminal unit on the CWPS, for use in a vaccine. The syntheses employed a trichloroacetimidate glycosyl donor. Molecular dynamics (MD) calculations of the pentasaccharide with the force fields CVFF and PARM22, both in gas phase and with explicit water present, gave different predictions for the flexibility and preferred conformational space. Metropolis Monte Carlo (MMC) calculations with the HSEA force field were also performed. Experimental data were obtained from 1D transient NOE measurements. Complete build-up curves were compared to those obtained by full relaxation matrix calculations in order to derive a model of the conformation. Overall, the best fit between experimental and calculated data was obtained with MMC simulations using the HSEA force field. Molecular dynamics and MMC simulations of a tetrasaccharide corresponding to the Group A-variant polysaccharide, which differs in structure from Group A in lacking the GlcpNAc residues, were also performed for purposes of comparison.

Conformational preferences in glycosylamines. implications for the exo-anomeric effect.

The conformational preferences about the C-N bond in N-(4-methoxyphenyl)-2,3,4,6-tetra-O-acetyl-alpha (1) and beta-D-glucopyranosylamine (2), in the solid state and in solution, have been investigated. The crystal structure of the axially substituted alpha anomer (1) indicates a conformational preference about the C-1-N bond in which nN-->sigma*C-O exo-anomeric interactions may be expressed, although this conformational preference is not displayed in solution. The solution conformation relieves steric interactions that result from expression of the exo-anomeric effect in the solid-state conformation. The conformational preference in the equatorially substituted beta anomer (2) both in solution and in the solid state is similar and permits expression of nN-->sigma*C-O exo-anomeric interactions. The structural data for 1 and 2 indicate significant differences in O-5-C-1-N-1 bond angles but insignificant differences in each of the O-5-C-1 or C-1-N-1 bond lengths. The J(C-1-H-1 coupling constants in 1 and 2 indicate a greater coupling constant for the alpha anomer that is consistent with a dominant nO-->sigma*C-H orbital interaction in the beta anomer that weakens the C-1-H-1 bond.

Aggregation pheromone of square-necked grain beetle,Cathartus quadricollis (Guér.)

When feeding on rolled oats, male square-necked grain beetles,Cathartus quadricollis (Guér.), produced the aggregation pheromone (3R,6E)-7-methyl-6-nonen-3-yl acetate, for which the trival name “quadrilure” is proposed. The pheromone was highly attractive to both sexes in a two-choice, pitfall olfactometer modified to retain responding beetles by placing a food stimulus (an oat flake) in the glass vials containing the experimental and control stimuli. TheS enantiomer of the pheromone was inactive. Males also produced small amounts of (E)-7-methyl-6-nonen-3-one, (E)-7-methyl-6-nonen-3-ol, and (6E)-7-methyl-3-propyl-2,6-nonadienyl acetate, but these compounds were inactive in the laboratory bioassay. Segregated males and females both produced (R)-(−)-1-octen-3-ol, which by itself was repellent to both sexes but did not diminish beetle response to the aggregation pheromone.

A convenient synthesis of both enantiomers of seudenol and their conversion to 1-methyl-2-cyclohexen-1-ol (mcol)

Abstract PPL catalyzed transesterification of seudenol (2) in dry Et2O yielded both optically enriched enantiomers (≥ 95% ee). Epoxidation, mesylation and reductive ring opening furnished optically enriched MCOL (1)

Synthesis of oligosaccharide fragments of the glycosylinositolphospholipid of Trypanosoma cruzi: a new selenoglycoside glycosyl donor for the preparation of 4-thiogalactofuranosyl analogues†

Abstract A new selenoglycoside, phenyl 2,3,5,6-tetra-O-acetyl-4-thio-1-selenogalactofuranose, has been synthesized. This 4-thiogalactofuranosyl donor was used in the syntheses of heteroatom analogues of the di-, tri-, and tetrasaccharides corresponding to the oligosaccharide β- d -Galf-(1→3)-α- d -Manp-(1→2)-(β- d -Galf-(1→3))-α- d -Manp. These compounds represent fragments of the terminal end of the glycosylinositolphospholipid oligosaccharide found in the protozoan Trypanosoma cruzi, the causative agent of Chagas disease, and are intended for use as inhibitors of the enzymes that construct the native oligosaccharides. The syntheses employed the selective activation of a phenyl 4-thio-1-selenogalactofuranoside glycosyl donor over ethyl 1-thioglycoside glycosyl acceptors with NIS/TfOH.

Cucujolide biosynthesis in the merchant and rusty grain beetles

Most known aggregation pheromones of cucujid grain beetles are macrolides called “cucujolides”. It has recently been shown by us that cucujolide I[4(E),8(E)-4,8-dimethyldecadien-10-olide] is of terpenoid origin, and that cucujolide II[3(Z)-dodecen-11-olide] is of fatty acid. The objectives of this study were to determine if farnesol could serve as a precursor of cucujolide Iin vivo; to study the conversion of fatty acids to cucujolide II; and to study the stereochemistry of the lactonization reactions leading to cucujolides I and II. Experiments were performed through application of stable isotope-labeling techniques, using the merchant grain beetle, Oryzaephilus mercator (Fauvel), and/or the rusty grain beetle, Cryptolestes ferrugineus (Stephens), as study insects. Exogenous (E,E)-farnesol was converted to cucujolide I. Dual-labeling studies with D and 18O indicate that this conversion proceeded with retention of the hydroxyl oxygen. Lauric and 11-hydroxydodecanoic acids were not effective precursors of cucujolide II, whereas 3(Z)-dodecenoic acid and 11-hydroxy-3(Z)-dodecenoic acid were effective precursors of cucujolide II. These data support the hypothesis that the biosynthetic route from fatty acids to cucujolide II involves chain shortening through β-oxidation to a 3(Z)-dodecenoic acid intermediate and oxidation at carbon-11 to form a 11-hydroxy-3(Z)-dodecenoic acid intermediate, followed by cylization. Dual-labeling studies with D and 18O indicate that this cyclization proceeded with retention of the C-11 hydroxyl of the 11-hydroxy-3(Z)-dodecenoic acid intermediate.

Selenium coronands. A novel conformational pair

The synthesis and characterization of the novel selenium coronands, 1,3,7,9-tetraselenacyclododecane 1a, 1,3,7,9,13,15-hexaselenacyclooctadecane 2a, the corresponding β-gem-dimethyl derivatives 1b, 2b, and 1,5,9,13-tetraselenacyclohexadecane 3, and 1,5,9,13,17,21-hexaselenacyclotetracosane 4 are described. X-ray crystallographic analysis of 1a reveals three independent molecules that exist in two distinct conformations, one molecule having approximate two-fold symmetry together with two molecules (of similar conformation) each having crystallographic symmetry. The conformations are denoted as [3333] or [66]. Whereas one resembles that of cyclododecane and tetrathia-12-crown-4 with respect to torsion angles, the other resembles that of tetraoxa-12-crown-4. The solid state CP-MAS 77Se and 13C nmr spectra are interpreted in light of the crystallographic information. Crystal structure: formula Se4C8H16; fw = 428.05; monoclinic, P21/c; Z = 8; a = 15.823(2) A, b = 5.534(1) A, c = 27.962(5) A, β = 92.26(1)°; V =...