Lewis-Base Dependent (3+3) Annulations of Acetoxy Allenoates with Iminoindolines: α-Carboline Scaffolds with Varied Substituents

Lewis base dependent (3+3) annulations of beta & PRIME;/delta-acetoxy allenoates with iminoindolines offer alpha-carbolines with varying substituents depending on the base used as well as subtle changes in the reaction conditions. The phosphine-catalyzed annulation of delta-acetoxy allenoates with iminoindolines involves 6-exo-trig cyclization, tosyl anion elimination/trapping, and ethyl acetate elimination as key steps in delivering beta-H and gamma-tosyl containing alpha-carbolines. An unobvious elimination (by C-alpha-C-beta bond cleavage) of the -CH2CO2Et moiety is observed here. The same reactants under DBU catalysis offer alpha-carbolines that retain -CH2CO2Et moiety but are devoid of -Ts group via 6-exo-dig cyclization. The reaction of beta & PRIME;-acetoxy allenoate with iminoindolines is completely tertiary amine dependent; the use of DABCO affords tetrahydro-alpha-carbolines exclusively with excellent stereoselectivity while DBU offers substituted alpha-carbolines that are distinct from those using DABCO. Several control experiments and HRMS studies have been done in support of a plausible reaction mechanism.