Single and double nucleophilic addition to methylated podocarpic acid coordinated to manganese tricarbonyl

Coordination of the Mn(CO)3+ moiety to the arene ring in dimethylated podocarpic acid (1) occurs with similar probability to both α and β faces (2). The nucleophile MeMgCl adds predominantly to the ortho C-13 site when the metal is situated β and to the meta C-14 site when the metal is α, to afford cyclohexadienyl complexes 3(β) and 4(α). The minor isomers 3(α) and 4(β) (Nu=Me) were isolated and characterized by X-ray diffraction, from which it is concluded that the diterpenoid Me-17 group interacts sterically with a carbonyl ligand when the metal is β and the nucleophile is at the meta C-14 position. Complexes 3(β) and 4(α) are readily converted into cationic Mn(CO)2NO+ salts 5(β) and 6(α), which add hydride ion to afford cyclohexadiene complexes.