Conformations of diterpenoids: an X-ray determination of the molecular structure of 6α-bromo-13-hydroxy-14-isopropylpodocarpa-8,11,13-trien-7-one and of the structure and absolute configuration of methyl 6α-bromo-13-isopropyl-7-oxopodocarpa-8,11,13-trien-15-oate
The crystal and molecular structures of the title compounds have been determined by X-ray diffraction methods from photographic data by the heavy-atom method; the absolute configuration was established for the second named. Crystals of the former (II) are tetragonal, a= 12·54, c= 23·97, Å, space group P41212; the structure was refined by least squares to R 0·088. The latter (III) is triclinic, a= 6·20, b= 7·73, c= 10·79 Å, α= 97·0, β= 79·3, γ= 104·6°, space group P1; refined to R 0·064. In (II) ring A has a chair conformation with a trans A/B ring fusion; ring B has an approximate boat conformation and the bromine atom has the α-configuration. The bromine is in the same configuration in compound (III) and ring A also adopts the chair conformation. There is trans A/B ring fusion with ring B somewhat distorted but describable as a ‘half-boat’. The stereochemistries at asymmetric centres confirm earlier evidence. There is considerable variation in detailed conformation, however, and a comparison between these compounds and with the molecular structure of methyl 6α-bromo-12-methoxy-7-oxopodocarpa-8,11,13-trien-16-oate leads to the view that steric rather than electronic factors are largely responsible for the path of bromine attack.