Fmoc-D-Cys(Bzl)-OH

Fmoc-D-Cys(Bzl)-OH is a chiral cysteine derivative bearing the D-configuration at the α-carbon, with an N-(9H-fluoren-9-ylmethoxycarbonyl) (Fmoc) protecting group and a benzyl (Bzl) thioether protecting group on the side-chain sulfur. The molecule contains an Fmoc carbamate, a carboxylic acid for C-terminal activation, and a protected thiol equivalent (thioether) that can be selectively transformed during peptide assembly or subsequent functionalization. The benzyl thioether stabilizes the sulfur functionality under standard coupling and base conditions used in solid-phase peptide synthesis, while the D-stereocenter supports stereochemically defined incorporation into peptide sequences. The overall structure is suited to chiral amino acid intermediate workflows, where orthogonal protection and controlled deprotection enable downstream thiol-based derivatization, disulfide formation, and thioether-to-thiol conversion strategies.

1. Protected Amino Acid Synthesis

Fmoc-D-Cys(Bzl)-OH is applied in protected amino acid synthesis workflows where orthogonally protected sulfur and an Fmoc carbamate are required for controlled handling. The Fmoc group provides base-labile N-protection for stepwise peptide coupling, while the Bzl thioether masks the cysteine side-chain to suppress undesired oxidation or side reactions during intermediate preparation. The free carboxylic acid enables conversion to activated esters or coupling-ready derivatives compatible with peptide coupling chemistry. Downstream processing can include thiol-unmasking and conversion to cysteine-like functionality for further derivatization, supporting the preparation of stereodefined amino acid building blocks and peptide-ready intermediates for fine chemical synthesis.

2. Peptide Synthesis

Fmoc-D-Cys(Bzl)-OH is used as a peptide building block in solid-phase and solution-phase peptide synthesis targeting sequences that require D-cysteine incorporation. The Fmoc-protected amine supports standard Fmoc deprotection and subsequent amide bond formation, while the carboxylic acid participates as the C-terminal coupling handle. The benzyl-protected sulfur maintains thioether stability during coupling cycles, enabling reliable assembly of peptide chains that later undergo sulfur-specific transformations such as thiol generation for disulfide engineering or native-like cysteine chemistry. The defined D-configuration supports stereochemical control in peptide analog construction, including studies where inversion of stereochemistry is used to tune folding, stability, or receptor recognition patterns in peptide science.

3. Bioconjugation Chemistry

Fmoc-D-Cys(Bzl)-OH is suitable for bioconjugation chemistry routes that rely on cysteine-derived functional handles after orthogonal deprotection. The protected side-chain sulfur can be converted into a thiol-bearing intermediate under sulfur-selective conditions, enabling conjugation strategies such as maleimide coupling, thiol-reactive electrophile attachment, or disulfide exchange to generate stable thioether-linked bioconjugates. The amino acid scaffold and protected termini allow incorporation into peptide linkers, affinity tags, or targeting motifs prior to conjugation, supporting workflow designs that separate peptide assembly from labeling steps. Stereodefined D-cysteine incorporation can also be leveraged to modulate conjugate stability and protease resistance in chemical biology and applied biomolecule modification programs.

4. Peptidomimetics And SAR

Fmoc-D-Cys(Bzl)-OH supports peptidomimetic construction and structure-activity relationship studies where controlled cysteine stereochemistry and sulfur functionality are required for scaffold diversification. The D-configuration provides a stereochemical variable for mapping how side-chain orientation affects binding interactions, while the protected sulfur enables installation of thiol-reactive motifs at defined positions after peptide or fragment assembly. The Fmoc-protected amine and carboxylic acid support systematic variation of neighboring residues through peptide coupling chemistry, facilitating generation of analog libraries for SAR workflows. The sulfur handle can be further transformed into disulfide, thioether, or other sulfur-containing motifs, enabling downstream derivatization that expands chemical space while preserving the stereochemical identity of the cysteine element.

5. Pharmaceutical Intermediate Preparation

Fmoc-D-Cys(Bzl)-OH is employed in pharmaceutical intermediate preparation where protected amino acid building blocks are manufactured for downstream peptide or peptide-like ingredient synthesis. The orthogonal protection pattern, with Fmoc on nitrogen and benzyl masking on sulfur, supports process-compatible handling that reduces side reactions during activation, coupling, and intermediate storage. The carboxylic acid functionality enables formation of activated intermediates used in controlled assembly of larger molecules, including peptide fragments and constrained cysteine-containing motifs. The resulting derivatives can serve as controlled-entry materials for later sulfur deprotection and functional group interconversions, aligning amino acid chemistry with process chemistry intermediate design and specialty chemical production needs.

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