Disulfide-Based Protecting Groups for the Cysteine Side Chain.

Two new disulfide-based protecting groups (SIT and MOT) are proposed for Cys thiol in the substitution of StBu, which is often difficult to remove. Both groups are based on a secondary thiol with a branched point in the β-position for an efficient modulation of its lability and/or stability. This unique structure allows them to be fully compatible with Fmoc/tBu SPPS. At the end of the synthesis, these groups are removed in a straightforward manner with dithiothreitol with some H2O.