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.

Structure proof and synthesis of kotalanol and de-O-sulfonated kotalanol, glycosidase inhibitors isolated from an herbal remedy for the treatment of type-2 diabetes.

Kotalanol and de-O-sulfonated-kotalanol are the most active principles in the aqueous extracts of Salacia reticulata which are traditionally used in India, Sri Lanka, and Thailand for the treatment of diabetes. We report here the exact stereochemical structures of these two compounds by synthesis and comparison of their physical data to those of the corresponding natural compounds. The candidate structures were based on our recent report on the synthesis of analogues and also the structure-activity relationship studies of lower homologues. The initial synthetic strategy relied on the selective nucleophilic attack of p-methoxybenzyl (PMB)-protected 4-thio-D-arabinitol at the least hindered carbon atom of two different, selectively protected 1,3-cyclic sulfates to afford the sulfonium sulfates. The protecting groups consisted of a methylene acetal, in the form of a seven-membered ring, and benzyl ethers. Deprotection of the adducts yielded the sulfonium ions but also resulted in de-O-sulfonation. Comparison of the physical data of the two adducts to those reported for de-O-sulfonated natural kotalanol yielded the elusive structure of kotalanol by inference. The side chain of this compound was determined to be another naturally occurring heptitol, d-perseitol (d-glycero-d-galacto-heptitol) with a sulfonyloxy group at the C-5 position. The synthesis of kotalanol itself was then achieved by coupling PMB-protected 4-thio-d-arabinitol with a cyclic sulfate that was synthesized from the naturally occurring d-perseitol. The work establishes unambiguously the structures of two natural products, namely, kotalanol and de-O-sulfonated kotalanol.

Stereoselective synthesis of 4'-selenonucleosides using the Pummerer glycosylation reaction.

The syntheses of four selenonucleosides, namely 4'-beta-selenoadenosine, -cytidine, -thymidine, and -uridine are described. Commercially available D-ribonolactone was converted to the key intermediate 1,4-anhydro-4-seleno-D-ribitol in seven steps in overall excellent yield. Oxidation of the seleno-d-ribitol with MCPBA gave a single diastereomeric selenoxide in excellent yield, which upon Pummerer reaction in the presence of silylated purine or pyrimidine bases gave stereoselectively the corresponding 4'-beta-selenonucleosides. The stereochemistry at the anomeric center was determined by means of 1D-NOE experiments.

New synthetic routes to chain-extended selenium, sulfur, and nitrogen analogues of the naturally occurring glucosidase inhibitor salacinol and their inhibitory activities against recombinant human maltase glucoamylase.

Six heteroanalogues (X = S, Se, NH) of the naturally occurring glucosidase inhibitor salacinol, containing polyhydroxylated, acyclic chains of 6-carbons, were synthesized for structure-activity studies with different glycosidase enzymes. The target zwitterionic compounds were synthesized by means of nucleophilic attack of the PMB-protected 1,4-anhydro-4-seleno-, 1,4-anhydro-4-thio-, and 1,4-anhydro-4-imino-D-arabinitols at the least hindered carbon atom of 1,3-cyclic sulfates. These 1,3-cyclic sulfates were derived from D-glucose and D-galactose, and significantly, they utilized butane diacetal as the protecting groups for the trans 2,3-diequatorial positions. Deprotection of the coupled products proceeded smoothly, unlike in previous attempts with different protecting groups, and afforded the target selenonium, sulfonium, and ammonium sulfates with different stereochemistry at the stereogenic centers. The four new heterosubstituted compounds (X = Se, NH) inhibited recombinant human maltase glucoamylase (MGA), one of the key intestinal enzymes involved in the breakdown of glucose oligosaccharides in the small intestine. The two selenium derivatives each had Ki values of 0.10 microM, giving the most active compounds to date in this general series of zwitterionic glycosidase inhibitors. The two nitrogen compounds also inhibited MGA but were less active, with Ki values of 0.8 and 35 microM. The compounds in which X = S showed Ki values of 0.25 and 0.17 microM. Comparison of these data with those reported previously for related compounds reinforces the requirements for an effective inhibitor of MGA. With respect to chain extension, the configurations at C-2' and C-4' are critical for activity, the configuration at C-3', bearing the sulfate moiety, being unimportant. It would also appear that the configuration at C-5' is important but the relationship is dependent on the heteroatom.