Stereoelectronic Effects: The γ-Gauche Effect in Sulfoxides

The Conformational Behaviour of Fluorinated Tetrahydrothiopyran

We present a comprehensive study on the conformational behavior of diversely substituted 4-fluorotetrahydrothiopyran derivatives. Through quantum chemical simulations including DFT as well as NBO and NPA analysis, we elucidate the pivotal role of electrostatic interactions, occasionally complemented by hyperconjugative interactions, in stabilizing axial fluorine conformers. Less polar conformers were occasionally obtained, attributed to the interplay of electrostatic and hyperconjugative interactions. Experimental validation through NMR spectroscopy aligns with the computational analysis, thus providing a coherent understanding of the structural dynamics of these compounds.

Density functional theory-nuclear magnetic resonance-validated full structure elucidation of theionbrunonine C, an unstable N-oxide theionbrunonine from Mostuea brunonis

The structure elucidation of theionbrunonine C, a thioether-bridged dimeric monoterpene indole alkaloid (MIA), and more generally, one of the very few sulfur-containing MIA, is reported after its isolation from Mostuea brunonis (Gelsemiaceae). This unstable structure had already been targeted for isolation in our former, molecular network-guided, investigation of this plant, but this compound had degraded before sufficient spectroscopic data could have been acquired for a complete structure assignment. With this constraint in mind, the rapid acquisition of nuclear magnetic resonance (NMR) data enabled retrieving sufficient spectroscopic information for full structure elucidation, although from a partial set of spectroscopic information (¹ H and ¹³ C NMR; COSY, HSQC, and HMBC). In conjunction with biosynthetic considerations, the cursory examination of ¹³ C NMR data unambiguously defined the complete stereostructure of 1, as further supported by density functional theory (DFT)-NMR calculations and subsequent DP4 probability score.

Photoredox-Mediated Synthesis of Functionalized Sulfoxides from Terminal Alkynes

A photoredox-mediated protocol for the synthesis of α-alkoxy-β-ketosulfoxides and α,β-dialkoxysulfoxides using alkynes, thiol, and alcohols is reported. This work presents a rare single-step synthesis of α-substituted sulfoxides, involving tandem introduction of a thiol and alcohol as a key enabling advancement. Furthermore, the method can be easily employed to access vinyl sulfoxides and β-ketosulfoxides.

Questioning the γ-gauche effect: stereoassignment of 1,3-disubstituted-tetrahydro-β-carbolines using 1H-1H coupling constants

The Pictet-Spengler reaction of tryptophan esters and aldehydes has been widely applied in natural product synthesis and medicinal chemistry. To date, the trans- or cis-configuration of 1,3-disubstituted tetrahydro-β-carbolines (THβCs) formed in this reaction has most often been assigned based on the relative ¹³C chemical shifts of C1 and C3 in the diastereomers. Although the upfield shifts of C1 and C3 in trans-THβCs relative to cis-THβCs has been attributed to steric compression associated with the "γ-gauche" effect, we show that this effect is not borne out experimentally for other carbons that should suffer this same compression. Thus we developed a robust alternative method for stereochemical assignment based on ¹H NMR coupling constants (31 examples) and supported by extensive DFT-based conformational analysis and calculation of ¹H-¹H coupling constants. DFT calculations of ¹³C NMR chemical shifts also cast doubt upon the role of the "γ-gauche" effect on C1 and C3 chemical shifts in trans-THβCs.

Officinalins A and B, a pair of C23 terpenoid epimers with a tetracyclic 6/7/5/5 system from Salvia officinalis

Officinalins A (1) and B (2), a pair of 6/7/5/5 tetracyclic C₂₃ terpenoid epimers with a unique tetracycline-[9.6.0.0^3,8.0^12,16]-heptadecane core and a peroxide bridge, were isolated from the leaves of Salvia officinalis.