The quest for selenocycles: From an ESR spectrum to a commercial product

Selenium compounds have a checkered history. Originally considered to be highly toxic, the tide turned in the 20th century when selenium was discovered to be an essential trace element; indeed, selenium is the least abundant element on Earth to have a well-defined biological role. Despite this new-found importance, organoselenium compounds were largely curiosities because methods for their synthesis were cumbersome and unpleasant and often involved toxic reagents and/or hazardous procedures. This paper describes how work carried out in collaboration with Alwyn Davies in the late 1980s, aimed at acquiring Electron Spin Responance (ESR) spectra of selenophene radical anions and cations, led to the development of free-radical methods for the synthesis of numerous selenium-containing heterocycles, many of which showed interesting and useful biological properties. This journey ends with the development of selenium-containing carbohydrates (selenosugars) that exhibit unique skin-repair properties and the establishment of Seleno Therapeutics as a vehicle for the commercialization of these selenosugars.

Sweet Selenium: Synthesis and Properties of Selenium-Containing Sugars and Derivatives

In the last decades, organoselenium compounds gained interest due to their important biological features. However, the lack of solubility, which characterizes most of them, makes their actual clinical exploitability a hard to reach goal. Selenosugars, with their intrinsic polarity, do not suffer from this issue and as a result, they can be conceived as a useful alternative. The aim of this review is to provide basic knowledge of the synthetic aspects of selenosugars, selenonium salts, selenoglycosides, and selenonucleotides. Their biological properties will be briefly detailed. Of course, it will not be a comprehensive dissertation but an analysis of what the authors think is the cream of the crop of this interesting research topic.

1,4-Anhydro-4-seleno-d-talitol (SeTal): a remarkable selenium-containing therapeutic molecule

1,4-Anhydro-4-seleno-d-talitol is an exceptional selenium-containing small molecule with significant therapeutic potential; its beneficial actions firmly establish a new therapeutic paradigm in which selenium plays a central role.

Preventing protein oxidation with sugars: scavenging of hypohalous acids by 5-selenopyranose and 4-selenofuranose derivatives

Heme peroxidases including myeloperoxidase (MPO) are released at sites of inflammation by activated leukocytes. MPO generates hypohalous acids (HOX, X = Cl, Br, SCN) from H(2)O(2); these oxidants are bactericidal and are key components of the inflammatory response. However, excessive, misplaced or mistimed production can result in host tissue damage, with this implicated in multiple inflammatory diseases. We report here methods for the conversion of simple monosaccharide sugars into selenium- and sulfur-containing species that may act as potent water-soluble scavengers of HOX. Competition kinetic studies show that the seleno species react with HOCl with rate constants in the range 0.8-1.0 × 10(8) M(-1) s(-1), only marginally slower than those for the most susceptible biological targets including the endogenous antioxidant, glutathione. The rate constants for the corresponding sulfur-sugars are considerably slower (1.4-1.9 × 10(6) M(-1) s(-1)). Rate constants for reaction of the seleno-sugars with HOBr are ~8 times lower than those for HOCl (1.0-1.5 × 10(7) M(-1) s(-1)). These values show little variation with differing sugar structures. Reaction with HOSCN is slower (~10(2) M(-1) s(-1)). The seleno-sugars decreased the extent of HOCl-mediated oxidation of Met, His, Trp, Lys, and Tyr residues, and 3-chlorotyrosine formation, on both isolated bovine serum albumin and human plasma proteins, at concentrations as low as 50 μM. These studies demonstrate that novel selenium (and to a lesser extent, sulfur) derivatives of monosaccharides could be potent modulators of peroxidase-mediated damage at sites of acute and chronic inflammation, and in multiple human pathologies.

Synthesis of phosphate derivatives related to the glycosidase inhibitor salacinol

The syntheses of polyhydroxylated imino- and anhydro thio-alditol compounds related to the naturally occurring glycosidase inhibitor, salacinol, containing a phosphate group in the side chain are described. The compounds lack hydroxyl groups on the acyclic side chain and are prototypes of the exact salacinol analogue. The synthetic strategy relies on the Mitsunobu reaction of N- and S-hydroxyalkyl derivatives of 2,3,5-tri-O-benzyl-1,4-dideoxy-1,4-imino-D-arabinitol and 1,4-anhydro-2,3,5-tri-O-benzyl-1-thio-D-arabinitol with dibenzyl phosphate to yield the corresponding protected heteroalditol phosphates. Screening of these compounds against recombinant human maltase glucoamylase (MGA), a critical intestinal glucosidase involved in the processing of oligosaccharides of glucose into glucose itself, shows that they are not effective inhibitors of MGA and demonstrates the importance of the hydroxyl and/or sulfate substituents present on the side chain for effective inhibition. The attempted synthesis of the exact analogue of salacinol by opening of cyclic phosphates is also described.

Zwitterionic glycosidase inhibitors: salacinol and related analogues

Natural products with interesting biological properties and structural diversity have often served as valuable lead drug candidates for the treatment of human diseases. Salacinol, a naturally occurring alpha-glucosidase inhibitor, was shown to be one of the active principles of the aqueous extract of a medicinal plant that has been prescribed traditionally as an Ayurvedic treatment for type II diabetes. Salacinol contains an intriguing zwitterionic sulfonium-sulfate structure that comprises a 1,4-anhydro-4-thio-D-arabinitol core and a polyhydroxylated acyclic chain. Due to the unique structural features and its potential to become a lead drug candidate in the treatment of type II diabetes, a great deal of attention has been focused on salacinol and its analogues. Since the isolation of salacinol, several papers describing various synthetic routes to salacinol and its analogues have appeared in the literature. This review is aimed at highlighting the synthetic aspects of salacinol and related compounds as well as their structure-activity relationship studies.