text: Compounds containing 2 geminal phosphonate groups are known as bisphosphonates. They were first synthesized in 1897 by Von Baeyer and Hofmann and now form the basis for an important class of drugs, used to treat osteoporosis and similar diseases. Examples include HEDP (etidronic acid or Didronel), which is prepared from phosphorous acid and acetic anhydride:
  :2 HPO + (CHCO)O → CHC(OH)(POH) + CHCOH