Lewis Acid‐Catalyzed Formal (4+2)‐ and (2+2+2)‐Cycloaddition Between 1‐Azadienes and Styrylmalonates as Analogues of Donor‐Acceptor Cyclopropanes

The reactions of donor‐acceptor cyclopropanes (DACs) and isomeric β‐styrylmalonates with conformationally non‐rigid α,β‐unsaturated imines in the presence of Lewis acids were studied. The effect of the structural features of starting compounds on the process course was determined. The main attention was focused to discover the reaction conditions for the formal (4+2)‐cycloaddition pathway with the possibility of variation of the α‐substituent in azadiene instead of known (3+2)‐cycloaddition synthetic methodology. Based on the results obtained, a method for the diastereoselective synthesis of substituted trans,trans‐tetrahydropyridines was developed. Three substrate‐sensitive diverse Lewis acid catalytic systems have been employed based on Sc(OTf)3, Sn(OTf)2, and GaCl3. The use of GaCl3 as a Lewis acid allows to obtain a minor product of the formal (2+2+2)‐cycloaddition, which represents a fully 1,2,3,4,5,6‐hexasubstituted cyclohexane derivative produced with diastereoselectivity >30:1. Appropriate mechanistic and stereochemical models have been suggested.