Synthesis and Conformation of Chiral Biheteroaryls

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.

Design and synthesis of new alkyl-based chiral phosphoric acid catalysts

Using chiral BINOL-derived phosphoric acids (PA's) to activate substrates for enhanced reactivity is now regarded as a powerful strategy to control enantioselectivity in asymmetric synthesis. Generally, most substituents at the 3,3'-positions of BINOL PA's are aryl derivatives. These derivatives are pivotal in attaining high selectivity. PA's with alkyl substituents in these positions have rarely been reported. Herein, we introduced alkyl-based substituents at the 3,3'-position of PA's. These new potential catalysts, if applied in reactions, may allow altered noncovalent interactions (as opposed to the typical aryl substituents in these positions) with substrates used in chiral PA-catalyzed chemistry in the future.

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.