Asymmetric “Clip-Cycle” Synthesis of Pyrrolidines and Spiropyrrolidines

Organic Letters
2020.0

Abstract

The development of an asymmetric clip-cycle synthesis of 2,2- and 3,3-disubstituted pyrrolidines and spiropyrrolidines, which are increasingly important scaffolds in drug discovery programs, is reported. Cbz-protected bis-homoallylic amines were activated by clipping them to thioacrylate via an alkene metathesis reaction. Enantioselective intramolecular azaMichael cyclization onto the activated alkene, catalyzed by a chiral phosphoric acid, formed a pyrrolidine. The reaction accommodated a range of substitutions to form 2,2- and 3,3-disubstituted pyrrolidines and spiropyrrolidines with high enantioselectivities. The importance of the thioester activating group was demonstrated by comparison to ketone and oxoestercontaining substrates. DFT studies supported the aza-Michael cyclization as the rate- and stereochemistry-determining step and correctly predicted the formation of the major enantiomer. The catalytic asymmetric syntheses of N-methylpyrrolidine alkaloids (R)irnidine and (R)-bgugaine, which possess DNA binding and antibacterial properties, were achieved using the clip-cycle methodology.

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