Benzotriazole-1-yl-oxy-tris-(dimethylamino)-phosphonium hexafluorophosphate; Castro′s Reagent; BOP

BOP, also known with the name of Castro’s reagent, is a phosphonium coupling reagent used for peptide synthesis. BOP can be employed in presence of a base such as triethylamine or Hünig’s base and forms HOBt in situ, which then generates an activated ester. BOP is a very effective coupling reagent, achieving high yields, low epimerisation and high conversion rates. However, it forms the extremely toxic and carcinogenic HMPA in situ, therefore the equally efficient pyrrolidino derivative PyBOP was introduced, which leads to a less toxic by-product.