New Recipe For Asymmetric Allenes

When it comes to making complex organic molecules in the drug discovery process, a catalytic route to the asymmetric allene class of compounds has been elusive. A research team led by Keiji Maruoka of Kyoto University, in Japan, now reports a strategy to resolve this shortcoming (Nat. Chem., DOI: 10.1038/nchem.1567). Allenes feature two consecutive double bonds in a carbon chain. This unusual pattern opens up the possibility of placing different substituent groups at each position on the terminal carbon atoms of the allene group, creating asymmetric molecules with useful biological activity. To make these allenes, Maruoka and colleagues used ammonium salts containing binaphthyl groups as chiral catalysts to generate enolate-type intermediates from alkylallene dicarboxylates. By treating the intermediates with imine electrophiles, the researchers prepared tetrasubstituted allenes containing a chiral amine group. When they used alkyl halides as the electrophile, they made tetra­sub­stituted alkylallenes (shown) or alkynes, depending on the starting allene and the choice of catalyst. The Kyoto researchers further showed that the new asymmetric allenes can easily undergo ring-closing reactions to form densely substituted pyrroles and furanones, which are desirable drug candidates.