3-[[(Acetylamino)methyl]thio]-N-[(9H-fluoren-9-ylmethoxy)carbonyl]-L-valine

CAS 201531-76-2

A peptide with four thiols has three possible ways to pair them into two internal disulfide bridges. The bridge count can be correct while the connectivity is wrong. Fmoc-Pen(Acm)-OH helps address this problem through separate protection of amino nitrogen and penicillamine sulfur. Acm masks sulfur for a later stage, allowing it to participate in a differentiated protection strategy rather than becoming available automatically when Fmoc is removed. Penicillamine's two neighboring methyl groups are permanent structural features, not part of the temporary masks. Successful synthesis must distinguish intended intramolecular pairing from alternative connections, including bridges between different peptide molecules. This building block illustrates why programmed disulfide construction requires control over partners and timing, followed by evidence about the final topology.