Enantioselective Synthesis of Tropane Scaffolds by an Organocatalyzed 1,3-Dipolar Cycloaddition of 3-Oxidopyridinium Betaines and Dienamines.

Autor: Lamhauge JN; Department of Chemistry, Aarhus University, Langelandsgade 140, 8000, Aarhus C, Denmark., McLeod DA; Department of Chemistry, Aarhus University, Langelandsgade 140, 8000, Aarhus C, Denmark., Barløse CL; Department of Chemistry, Aarhus University, Langelandsgade 140, 8000, Aarhus C, Denmark., Oliver GA; Department of Chemistry, Aarhus University, Langelandsgade 140, 8000, Aarhus C, Denmark., Viborg L; Department of Chemistry, Aarhus University, Langelandsgade 140, 8000, Aarhus C, Denmark., Warburg T; Department of Chemistry, Aarhus University, Langelandsgade 140, 8000, Aarhus C, Denmark., Anker Jørgensen K; Department of Chemistry, Aarhus University, Langelandsgade 140, 8000, Aarhus C, Denmark.
Jazyk: angličtina
Zdroj: Chemistry (Weinheim an der Bergstrasse, Germany) [Chemistry] 2023 Sep 01; Vol. 29 (49), pp. e202301830. Date of Electronic Publication: 2023 Aug 02.
DOI: 10.1002/chem.202301830
Abstrakt: Tropane alkaloids constitute a compound-class which is structurally defined by a central 8-azabicyclo[3.2.1]octane core. A diverse bioactivity profile combined with an unusual aza-bridged bicyclic framework has made tropanes molecules-of-interest within organic chemistry. Enantioselective examples of (5+2) cycloadditions between 3-oxidopyridinium betaines and olefins remain unexplored, despite 3-oxidopyridinium betaines being useful reagents in organic synthesis. The first asymmetric (5+2) cycloaddition of 3-oxidopyridinium betaines is reported, affording tropane derivatives in up to quantitative yield and with excellent control of peri-, regio-, diastereo-, and enantioselectivity. The reactivity is enabled by dienamine-activation of α,β-unsaturated aldehydes combined with in situ formation of the pyridinium reaction-partner. A simple N-deprotection protocol allows for liberation of the tropane alkaloid motif, and synthetic elaborations of the cycloadducts demonstrate their synthetic utility to achieve highly diastereoselective modification around the bicyclic framework. DFT computations suggest a stepwise mechanism where regio- and stereoselectivity are defined during the first bond-forming step in which the pyridinium dipole exerts critical conformational control over its dienamine partner. In the second bond-forming step, a kinetic preference toward an initial (5+4) cycloadduct was identified; however, a lack of catalyst turn-over, reversibility, and thermodynamic bias favoring a (5+2) cycloadduct rendered the reaction fully periselective.
(© 2023 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)
Databáze: MEDLINE
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