Michael in a comment here on the mechanism of the Masamune-Bergman reaction notes that when it occurs as part of the Calicheamicin (an antibody-drug conjugate or ADC) version of this mechanism, a pre-step is first necessary. As discussed in this review article,[1] the trisulfide linkage is reduced and the resulting thiolate undergoes a facile 1,4-addition to the adjacent enone.
References
- V. Kostova, P. Désos, J. Starck, and A. Kotschy, "The Chemistry Behind ADCs", Pharmaceuticals, vol. 14, pp. 442, 2021. https://doi.org/10.3390/ph14050442