Synthesis of the Indolo[2,3-a]quinolizidine Ring through the Addition of 2-Siloxyfurans to Imines and Intrinsic Reaction Coordinate Calculations

Abstract

A concise asymmetric diastereoselective strategy for the synthesis of indolo[2,3-a]quinolizidine derivative 1 was developed using diastereoselective addition of 2-siloxyfurans 4 to imine 3 through chiral auxiliary induction. The addition of an ionic liquid as additive in the reaction favored the anti configuration in the major adduct. The stereochemical outcome of the anti/syn (threo/erythro) selectivity was rationalized based on transition state and IRC calculations at DFT (B3LYP) and MP2 theories. MP2 calculations was shown to be the method of choice in these systems, which orbital desymmetrizations were observed in the anti transition state of the addition of 4 to 3 and secondary orbital interactions allowed us to rationalize the production of the major anti-adduct 6. Furthermore, the work also suggested that 2-(triisopropylsiloxy)furan (4a) was the nucleophile of choice in this kind of Mannich reaction. Moreover, the strategy features the use of the Mitsunobu reaction to insert an amino group with the correct configuration into amine 2, key intermediate to achieve 1. The synthetic route can also be applied in the total synthesis of promising aza-β-carboline compounds.