The [4+2], not [2+2], Mechanism Occurs in the Gold-Catalyzed Intramolecular Oxygen Transfer Reaction of 2-Alkynyl-1,5-Diketones

Elucidation of the atroposelectivity in the synthesis of axially chiral thiohydantoin derivatives

Recently, Sarigul and Dogan have synthesized a number of enantiomerically enriched axially chiral atropoisomeric 2-thiohydantoins by the reaction of l-amino acid ester salts and o-aryl isothiocyanates in the presence of triethyl amine (TEA) in dichloromethane. The non-axially chiral derivative 5-methyl-3-phenyl-2-thiohydantoin gave a racemic product whereas the axially chiral 5-methyl-3-o-bromophenyl-2-thiohydantoin was less prone to racemize at C5 of the heterocyclic ring. In this study, we present a computational study (M06-2X/6-311+G(d,p) for C, H, O, N and S; M06-2X/6-311++G(3df,3pd) for Br) in order to propose plausible mechanisms for the racemization and cyclization steps for 2-thiohydantoin derivatives. The study includes rationalization based on steric as well as the electrostatic effects to elucidate the epimerization differences at C5.

(Radio)fluoroclick Reaction Enabled by a Hydrogen-Bonding Cluster

We have developed a widely applicable nucleophilic (radio)fluoroclick reaction of ynamides with readily available and easy-to-handle KF(18 F). The reactions exhibited high functional-group tolerance and needed only an ambient atmosphere. This 18 F addition to C-C unsaturated bonds proceeded with extraordinarily high radiochemical yields.

Nucleophile-dependent regio- and stereoselective ring opening of 1-azoniabicyclo[3.1.0]hexane tosylate

1-[(1R)-(1-Phenylethyl)]-1-azoniabicyclo[3.1.0]hexane tosylate was generated as a stable bicyclic aziridinium salt from the corresponding 2-(3-hydroxypropyl)aziridine upon reaction with p-toluenesulfonyl anhydride. This bicyclic aziridinium ion was then treated with various nucleophiles including halides, azide, acetate, and cyanide in CH3CN to afford either piperidines or pyrrolidines through regio- and stereoselective ring opening, mediated by the characteristics of the applied nucleophile. On the basis of DFT calculations, ring-opening reactions under thermodynamic control yield piperidines, whereas reactions under kinetic control can yield both piperidines and pyrrolidines depending on the activation energies for both pathways.

Solvent-catalyzed ring-chain-ring tautomerization in axially chiral compounds

The mechanism of ring-chain-ring tautomerization and the prominent effect of the solvent environment have been computationally investigated in an effort to explain the enantiomeric interconversion observed in 2-oxazolidinone derivatives, heterocyclic analogues of biphenyl atropisomers, which were isolated as single stable enantiomers and have the potential to be used as axially chiral catalysts. This study has shed light on the identity of the intermediate species involved in the ring-chain-ring tautomerization process as well as the catalytic effect of polar protic solvents. These mechanistic details will prove very useful in predicting and understanding ring-chain tautomeric equilibria in similar heterocyclic systems and will further enable experimentalists to devise appropriate experimental conditions in which axially chiral catalysts remain stable as single enantiomers.