An Asymmetric Organocatalyst Having an Isothiourea Moiety

HBTM-2.1 (1) is an isothiourea derivative having two asymmetric carbons and is used as an asymmetric organocatalyst.^1-4) 1 can be used as a catalyst for condensations of two different carbonyl components and consequently affording cyclic compounds by intramolecular cyclizations. For example, an acyl iminium cation intermediate 2 is generated by the reaction of 1 with an anhydride prepared from a carboxylic acid and pivaloyl chloride, followed by the asymmetric 1,4-addition with an unsaturated carbonyl compound giving an enolate intermediate 3. The formed intermediate 3, having an active amide moiety, is cyclized by the intramolecular cyclization from the enolate moiety to afford the desired 6-membered lactone derivative.¹⁾ The condensation reactions using 1 have wide availabilities because various cyclic compounds can be synthesized by replacing the unsaturated compounds with other ones. Especially, the synthesis of β-lactam derivatives using imines is considered as one of the efficient conversions.²⁾