THE SYNTHESIS AND ELECTROPHILIC REACTIVITY OF MANGANESE TRICARBONYL COMPLEXES OF THE C-RING AROMATIC DITERPENOID METHYL O-METHYLPODOCARPATE

Abstract

The aromatic ring in the diterpenoid methyl O-methylpodocarpate (podo) coordinates readily to the Mn(CO)(3)(+) moiety to afford in high yield a nearly 1:1 isomeric distribution of [(eta(6)-podo)Mn(CO)(3)]BF4 ([2]BF4), in which the metal is situated on the alpha or beta face. The nucleophiles PhLi, PhMgBr, MeLi, MeMgCl, LiCH2C(O)CMe(3) and NaBH4 were found to add cleanly to the activated aromatic ring in 2(+) to give neutral cyclohexadienyl complexes. Nucleophilic addition occurs predominantly meta to the OMe substituent in the case of the alpha-isomer of 2(+), and at both ortho and meta sites in the case of the beta-isomer. The X-ray structure is reported for a typical alpha-meta and a typical beta-ortho cyclohexadienyl product, namely beta-(eta(5)-podo.Ph)Mn(CO)(3) and alpha-(eta(5)-podo.CH2C(O)CMe(3))Mn(CO)(3) The high yield and regioselectivity of the nucleophilic additions suggests that the manganese-mediated functionalization of aromatic diterpenes and steroids will prove to provide a useful synthetic methodology.