Self-Regeneration of Chirality with l-Cysteine through 1,3-Dipolar Cycloadditions between Diazoalkanes and Enantiomerically Pure Thiazolines: Experimental and Computational Studies

Stereoselective Synthesis of β-Thiolated Aryl Amino Acids

A variety of β-thiolated aryl amino acids were readily synthesized in moderate to excellent yields via 1,4-addition from a readily-accessible thiazoline precursor, which can be prepared from cysteine methyl ester hydrochloride in just three steps. The thiolated products can be subsequently deprotected within one single step in high yields and used directly for subsequent solid-phase peptide synthesis.

General Solution to the Preparation of β-Thiolated/Selenolated Amino Acids Via Visible Light Catalyzed Asymmetric Giese Reaction

Thiolated/selenolated amino acids are irreplaceable despite their rare abundance in proteins. They play critical roles in regulating the conformation and function of proteins and peptide design as well as bioconjugation. Furthermore, β-thiolated/selenolated amino acids are important motifs in native chemical ligation-dechalcogenation strategy for protein synthesis. However, a universal method to access enantiopure β-thiolated/selenolated amino acids has not been reported. Herein, we developed a practical strategy for the preparation of a variety of enantiopure β-thiolated/selenolated amino acids via photoredox-catalyzed Giese reaction.

Origins of stereoselectivity in uncatalyzed and ZnBr2-catalyzed Diels-Alder reactions of a chiral sulfinylquinone

Density functional theory calculations are reported which explore how a chiral sulfinyl substituent controls the stereoselectivities of the Diels-Alder reactions of a 2-p-tolylsulfinylbenzoquinone. The π-facial stereoselectivities vary depending on the diene (cyclopentadiene or trans-piperylene) and on the presence or absence of a ZnBr₂ catalyst. The stereoselectivities are shown to be controlled by steric effects and non-covalent (CH-π) interactions. The calculations reveal that the active dienophile in the ZnBr₂-catalyzed reactions is likely to be a 1 : 2 complex of the dienophile and catalyst, not a 1 : 1 complex as commonly assumed.

Synthesis of Enantiopure Constrained α,β-Cycloaliphatic Cystines via Diels-Alder Reaction with Homochiral Thiazolines

The behavior of homochiral 2,3-dihydrothiazoles, easily available from l-cysteine in Diels-Alder reaction with different dienes, "en route" to sterically constrained modified cystines, has been studied. The oxidation level of the sulfur atom of the heterocyclic ring was crucial for the course of the reaction. Whereas 2,3-dihydrothiazoles did not lead to Diels-Alder adducts, 1-oxide and 1,1-dioxide derivatives afforded the exo adduct enantiopurely in high yields and diastereoselectivities. Further elaboration of the resulting adducts provided conformationally restricted quaternary cystines. DFT calculations correctly predict both the reactivity and stereoselectivity observed experimentally.