Here we describe several useful strategies to a variety of unusual α-amino acid derivatives and peptides based on "building block" approach. These building blocks are suitable for modification at an amino acid as well as at a peptide level. Moreover, these methods have embedded several points for diversification and are capable of producing a library of modified amino acids and peptides. We have employed highly atom-economic processes such as the Diels-Alder reaction, [2 + 2 + 2] cycloaddition, Suzuki-Miyaura cross-coupling, and olefin metathesis as key steps to assemble various unnatural amino acid derivatives and peptides. In some instances, we have used rongalite to generate Diels-Alder precursors.

We describe diverse approaches to various dienes and their utilization in the Diels-Alder reaction to produce a variety of polycycles. The dienes covered here are prepared by simple alkylation reaction or via the Claisen rearrangement or by enyne metathesis of alkyne or enyne building blocks. Here, we have also included the Diels-Alder chemistry of dendralenes, a higher analog of cross-conjugated dienes. The present article is inclusive of o-xylylene derivatives that are generated in situ starting with benzosultine or benzosulfone derivatives. The Diels-Alder reaction of these dienes with various dienophiles gave diverse polycyclic systems and biologically important targets.

Benzosultine-sulfone 5 has been prepared as a o-xylylene or o-quinodimethane precursor by utilization of rongalite. Thermal activation of this hybrid molecule 5 has resulted a new sulfone-based building block 6. Building block 5 is a suitable precursor for the synthesis of unsymmetrically functionalized polycyclics through Diels-Alder (DA) chemistry. The dibromosulfone 24 and benzosultine-sulfone 5 has also been used for the synthesis of various sulfone based unnatural alpha-amino acid (AAA) derivatives.

Abstract:The direct nucleophilic addition of alkyl amines to the α(δ')-carbon atom of dimethyl (E)-hex-2-en-4-ynedioate to generate α,β-dehydroamino acid derivatives is reported. Herein, we have studied the reactivity of various primary and secondary alkyl amines in the α-selective nucleophilic conjugate addition to conjugated dimethyl (E)-hex-2-en-4-ynedioate. The reaction with primary alkyl amines gives only the (2E,4E)-stereoisomer, while that with secondary alkyl amines gives the (2E,4E) and (2Z,4E)-stereoisomers of dimethyl (2-alkylamino)-muconic ester.

Palladium-catalyzed Suzuki-Miyaura (SM) cross-coupling reaction with allylboronic acid pinacol ester and titanium assisted cross-metathesis (CM)/ring-closing metathesis (RCM) cascade has been used to synthesize macrocyclic cyclophane derivatives.