6-Endo-dig versus 5-exo-dig: Exploring Radical Cyclization Preference with First-, Second-, and Third-row Linkers using High-level Quantum Chemical Methods

As an expansion upon Baldwin rules, the cyclization reactions of hex-5-yn-1-yl radical systems with different first-, second-, and third-row linkers are explored at the CCSD(T) level via means of the SMD(benzene)-G4(MP2) thermochemical protocol. Unlike C, O, and N linkers, systems with B, Si, P, S, Ge, As, and Se linkers are shown to favor 6-endo-dig cyclization. This offers fundamental insights into the rational synthetic design of cyclic compounds. A thorough analysis of stereoelectronic effects, cyclization barriers, and intrinsic barriers illustrates that structural changes alter the cyclization preference by mainly impacting 5-exo-dig reaction barriers. Based on the high-level computational modeling, we proceed to develop a new tool for cyclization preference prediction from the correlation between cyclization barriers and radical structural parameters (e. g., linker bond length and bond angle). A strong correlation is found between the radical attack trajectory angle and the reaction barrier heights, i. e., cyclization preference. Finally, the influence of stereoelectronic effects on the two radical cyclization pathways is further investigated in stereoisomers of hypervalent silicon system, which provides novel insight into cyclization control.

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.

Alkyl selenol reactivity with common solvents and ligands: influences on phase control in nanocrystal synthesis

This study develops mechanistic understanding of the factors which control the phase in syntheses of copper selenide nanocrystals by investigating how the chemistry of the dodecylselenol reactant is altered by the ligand and solvent environment. ¹H NMR and ⁷⁷Se NMR were used to study how commonly used solvents (octadecene and dioctylether) and ligands (oleylamine, oleic acid, stearylamine, stearic acid and trioctyl phosphine) change the nature of the dodecylselenol reactant at 25 °C, 155 °C and 220 °C. Unsaturations were prone to selenol additons, carboxylates underwent selenoesterification, amines caused the release of H₂Se gas, and the phosphine formed phosphine selenide. Adventitious water caused oxidation to didodecyldiselenide. NMR studies were correlated with the phases that resulted in syntheses of nanocrystalline copper selenides, in which berzalianite, umangite or a metastable hexagonal phase were produced as identified by X-ray diffraction, depending on the ligand and solvent environemnts. Formation of the rare hexagonal Cu_2-xSe phase could be assigned to cases that included DD₂Se₂ as a reactive intermediate, or strong L-type ligation of amines which was dependant on alkyl chain length.

Synthesis and Th1-immunostimulatory activity of α-galactosylceramide analogues bearing a halogen-containing or selenium-containing acyl chain

A novel series of CD1d ligand α-galactosylceramides (α-GalCers) were synthesized by incorporation of the heavy atoms Br and Se in the acyl chain backbone of α-galactosyl-N-cerotoylphytosphingosine. The synthetic analogues are potent CD1d ligands and stimulate mouse invariant natural killer T (iNKT) cells to selectively enhance Th1 cytokine production. These synthetic analogues would be efficient X-ray crystallographic probes to disclose precise atomic positions of alkyl carbons and lipid-protein interactions in KRN7000/CD1d complexes.