Multicomponent reactions involving 2-methyleneaziridines: rapid synthesis of 1,3-disubstituted propanones

Ring opening of 1-alkyl-2-methyleneaziridines 1 or 2 is accomplished with organocopper reagents (R(2)CuLi or RMgX/CuI) in the presence of boron trifluoride diethyl etherate giving 1-substituted propan-2-ones 3-9 in 42-88% yield. Ring opening with RMgCl/CuI in the absence of the Lewis acid allows further alkylation of the metalloenamine (metalated imine) intermediate in a regiocontrolled manner. The sequential formation of two new intermolecular carbon-carbon bonds in this reaction provides a rapid entry into a variety of 1,3-disubstituted propan-2-ones, including 11 and 14-23. The scope and mechanism of this multicomponent reaction (MCR) has been assessed. It is established that this MCR tolerates alkyl, aryl, and benzylic Grignard reagents and a wide range of electrophiles, including alkyl iodides, bromides, and tosylates, as well as epoxides and aldehydes. In addition, gem-dimethyl substitution on the exocyclic double bond of the 2-methyleneaziridine is tolerated. This MCR has been applied to the one-pot synthesis of (Z)-6-heneicosen-11-one, 25, an important sex attractant of the Tussock moth. Using 3-deuterio-1-(1-phenylethyl)-2-methyleneaziridine, 26, we determined that this MCR occurs predominantly by direct ring opening at the sp(3)-hybridized aziridine carbon atom (C-3).