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 isometric distribution of [( η 6-podo)Mn(CO) 3] BF 4 ([ 2]BF 4), in which the metal is situated on the α or β face. The nucleophiles PhLi, PhMgBr, MeLi, MeMgCl, LiCH 2C(O)CMe 3 and NaBH 4 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 α-isomer of 2 +, and at both ortho and meta sites in the case of the β-isomer. The X-ray structure is reported for a typical α- meta and a typical β- ortho cyclohexadienyl product, namely β-( η 5-podo · Ph)Mn(CO) 3 and α-( η 5-podo · CH 2C(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.