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

Abstract Lewis base dependent (3+3) annulations of β′ / δ ‐acetoxy allenoates with iminoindolines offer α ‐carbolines with varying substituents depending on the base used as well as subtle changes in the reaction conditions. The phosphine‐catalyzed annulation of δ‐ acetoxy allenoates with iminoindolines involves 6‐ exo‐trig cyclization, tosyl anion elimination/trapping, and ethyl acetate elimination as key steps in delivering β ‐H and γ ‐tosyl containing α ‐carbolines. An unobvious elimination (by C α ‐C β bond cleavage) of the −CH 2 CO 2 Et moiety is observed here. The same reactants under DBU catalysis offer α ‐carbolines that retain −CH 2 CO 2 Et moiety but are devoid of −Ts group via 6‐exo‐dig cyclization. The reaction of β′‐ acetoxy allenoate with iminoindolines is completely tertiary amine dependent; the use of DABCO affords tetrahydro‐ α ‐carbolines exclusively with excellent stereoselectivity while DBU offers substituted α ‐carbolines that are distinct from those using DABCO. Several control experiments and HRMS studies have been done in support of a plausible reaction mechanism. magnified image