Stereocontrolled construction of conformationally constrained and rigid bis(a-amino acid) derivatives

Cystine is regarded as a four-atom bridged bis(α-amino acid). The bridge between the two glycine moieties in cystine has been replaced with all-carbon C4-bridges between the α-carbon of the glycines. The products are dicarba analogs of cystine. Conformational constraints have been conferred on these molecules by insertion of and double bonds in the bridge, by alkylidene substituents, by insertion of a triple bond, by insertion of aromatic or heteroaromatic rings, or otherwise by ring formation. Particularly rigidified dicarba analogs of cystine have been prepared where the α-amino acid carbon is quaternary and part of tricyclic ring structures. Ru(II)-catalyzed ring-closing metathesis (RCM) reactions have been widely used in the preparation of cyclic amino acids. Conformational constraints in acyclic structures result from substitution by simple alkyl, alkenyl, or alkynyl groups at the α-amino acid carbon in the formation of α-quaternary amino acid derivatives.