Interphylal product splicing:: The first total syntheses of cephalostatin 1, the north hemisphere of ritterazine G, and the highly active hybrid analogue, ritterostatin GN1N

Convergent total syntheses of the extremely potent cell growth inhibitor cephalostatin 1 and two hybrid analogues, ritterostatins G(N)1(N) and G(N)1(S), have been achieved. Ritterostatin G(N)1(N) displays sub-nanomolar activity in the 60 cell line human tumor panel of the National Cancer Institute. The North hemisphere of ritterazine G was efficiently constructed from hecogenin acetate in 15% yield over 13 steps. Extension of a key photolysis/Prins sequence to intermediates 19 and 32 proceeded in excellent yield, leading to installation of the Delta(14) moiety in the-North G-and South I steroidal subunits. Application of a method for directed unsymmetrical coupling furnished the natural and analogue pyrazines in good yield from the cephalostatin and ritterazine components.