Chemo-, regio-, and stereoselectivity of F-ring opening reactions in the cephalostatin series

In an effort to prepare unsymmetrical cephalostatin analogues with multi-functionality, we tried the route of selective opening of the spiroketal joining rings E and F. In this study, we have tested several borane complexes (like borane-9-BBN, borane-(N-tosyl)-d-valine, and borane-catechol) with some bis-steroidal pyrazine derivatives like 3, 4, and 16 aiming at opening ring-F at only one spiro-system of the dimer. Upon testing these borane reagents, satisfying results were obtained in the case of the keto-methylene 4 using the catechol-borane complex. The structures of the resulting mono-opened and also some double-opened spiro dimers have been completely confirmed. Some of the prepared compounds were tested against three cancer cell lines: HM02 (stomach cancer), HEP G2 (hepatocellular cancer), and MCF 7 (breast cancer).