X-Ray investigation of several phorbol derivatives. The crystal and molecular structure of phorbol bromofuroate–chloroform solvate at –160°

The structure and stereochemistry of phorbol has been determined by a three-dimensional X-ray analysis of phorbol 5-bromofuroate–chloroform solvate. The crystals are monoclinic, space group P21, with two molecules of C25H29O8Br,(CHCl3)0·8 in a unit cell of dimensions a= 10·54, b= 10·35, c= 13·80 Å, β= 107·6°. The structure was solved by the heavy-atom method and refined by isotropic block-diagonal least-squares calculations from photographic data collected at –160°. The final value of R is 0·151. The molecules are held in the crystal by an intricate network of intermolecular hydrogen bonds involving C–H ⋯ O (C ⋯ O = 3·17 Å), O–H ⋯ Br (O ⋯ Br = 3·17 Å) and O–H ⋯ O bonds.Details are given of an unsuccessful attempt to solve the phorbol structure by multiple isomorphous replacement methods by use of iodobenzene–, 1,3-dibromobenzene–, and bromothiophen–phorbol triacetate solvate crystals.