Fmoc-Cys(tBu)-OPfp

Fmoc-Cys(tBu)-OPfp is an Fmoc-protected cysteine derivative bearing a tert-butyl-protected side-chain thiol and an OPfp ester leaving group on the carboxylate, giving a chiral amino acid building block with a defined α-amino stereocenter and a reactive activated ester handle. The molecule combines an Fmoc carbamate for orthogonal amine protection, a tBu thioether/thiol-protecting strategy that supports chemoselective deprotection to regenerate a nucleophilic cysteine side chain, and a pentafluorophenyl (OPfp) ester that can participate in acyl transfer under peptide coupling conditions. The presence of the electron-withdrawing OPfp group increases acyl electrophilicity relative to unactivated esters, while the Fmoc group enables controlled N-terminal exposure for stepwise peptide assembly. The resulting reactivity profile makes Fmoc-Cys(tBu)-OPfp suitable as a peptide coupling intermediate and as a cysteine-functional scaffold for downstream thio chemistry, including disulfide formation or thiol-directed conjugation after orthogonal deprotection.

1. Peptide Coupling

Fmoc-Cys(tBu)-OPfp is applied in peptide synthesis workflows where OPfp-activated carboxylates enable efficient amide bond formation to incorporate cysteine residues with defined stereochemistry. The Fmoc-protected α-amine supports standard N-deprotection and re-coupling logic, while the tBu-protected side-chain thiol preserves sulfur functionality from side reactions during chain elongation. The OPfp ester can be used as an acyl donor for coupling to amines, supporting Cys-containing peptide construction that later permits thiol unmasking for disulfide engineering or native cysteine chemistry. Downstream peptide analogs prepared from this building block can be used for structure-activity relationship studies, peptide library generation, and cysteine-dependent biochemical investigations, aligning amino acid derivatization with peptide science requirements.

2. Protected Amino Acid Chemistry

Fmoc-Cys(tBu)-OPfp is used as a protected amino acid derivative for orthogonal protection strategy development, combining Fmoc N-protection with a thiol-protecting tert-butyl group and an activated carboxylate for controlled acylation. The Fmoc group allows selective deprotection to reveal the free amine under base conditions, while the tBu-thiol protection can be removed to regenerate a reactive cysteine side chain for subsequent functional group transformations. The OPfp ester functionality serves as a chemically addressable handle for conversion into amides without requiring carboxylate activation infrastructure at the point of coupling. This configuration supports preparation of peptide building block preparations, intermediate generation for Cys-modified scaffolds, and controlled sequencing of protection/deprotection steps in synthetic organic chemistry.

3. Bioconjugation And Labeling

Fmoc-Cys(tBu)-OPfp is suitable for chemical biology and bioconjugation chemistry where cysteine side-chain thiols are used as nucleophilic attachment points after orthogonal deprotection. The tBu-protected sulfur can be unmasked to generate a thiol that can undergo thiol-directed coupling reactions, including disulfide exchange strategies or selective thioether formation depending on the chosen electrophile. The OPfp-activated carboxylate and Fmoc-protected backbone can be leveraged to install cysteine-containing motifs onto amine-bearing biomolecules or onto peptide linkers used in labeling workflows. Downstream conjugates derived from this amino acid intermediate can serve in biomolecule labeling, probe construction, and molecular recognition studies that rely on cysteine-specific chemoselectivity.

4. Peptidomimetics And SAR Studies

Fmoc-Cys(tBu)-OPfp is applied in peptidomimetic construction and structure-activity relationship studies where cysteine-bearing fragments are incorporated into constrained or functionalized peptide-like scaffolds. The amino acid backbone protected as an Fmoc derivative supports iterative assembly of cysteine-containing sequences, while the protected thiol enables introduction of sulfur-dependent functional motifs without premature oxidation or side reactions. The OPfp ester handle supports conversion into amide-linked fragments that can be further elaborated into thioether, disulfide, or thiol-reactive analogs used to probe binding-site interactions. Resulting cysteine-modified peptidomimetics can be used as SAR tools to map how side-chain sulfur chemistry influences molecular recognition and stability in applied biochemical research.

5. Pharmaceutical Intermediate Preparation

Fmoc-Cys(tBu)-OPfp is relevant to pharmaceutical manufacturing and fine chemical synthesis as a process-compatible intermediate for producing cysteine-containing peptide intermediates and activated acyl building blocks. The combination of Fmoc N-protection and thiol protection provides a controlled functional-group profile that can be carried through coupling steps and then deprotected to yield defined reactive handles for further derivatization. The OPfp activated ester format can be integrated into manufacturing routes that require predictable acyl transfer behavior for assembling amide-linked intermediates under peptide chemistry conditions. Downstream outputs include cysteine-containing protected fragments used for drug discovery chemistry, process development for peptide-derived candidates, and scalable preparation of functional intermediates for specialty chemical production.

6. Industrial Process Chemistry

Fmoc-Cys(tBu)-OPfp is utilized in process chemistry contexts where activated amino acid esters facilitate reproducible coupling steps and streamlined intermediate handling. The OPfp ester provides an electrophilic carboxylate that can participate in acylation chemistry with amine partners, while the Fmoc carbamate and tBu thiol protection reduce undesired side reactions during multi-step synthesis. The chiral cysteine center and protected thiol allow controlled downstream conversion to sulfur-functional derivatives, including disulfide-containing materials or thiol-functional intermediates for further manufacturing steps. Industrially, cysteine-functional peptide building blocks derived from this compound can support specialty chemical production and applied synthetic methodology development in amino acid derivative manufacturing.

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