Publication : Enantioselective Access to Robinson Annulation Products and Michael Adducts as Precursors

The Robinson annulation is a reaction that has been useful for numerous syntheses since its discovery in 1935, especially in the field of steroid synthesis. The products are usually obtained after three consecutive steps: the formation of an enolate (or derivative), a conjugate addition, and an aldol reaction. Over the years, several methodological improvements have been made for each individual step or alternative routes have been devised to access the Robinson annulation products. The first part of this Review outlines the most relevant developments towards the formation of monocarbonyl-derived Robinson annulation products (MRA products, MRAPs) and activated monocarbonyl-derived Robinson annulation products (AMRA products, AMRAPs). The following sections are then devoted to the diastereoselective and enantioselective synthesis of these products, while the last section describes the enantiomeric resolution of racemic mixtures.

Gallier, F.; Martel, A.; Dujardin, G., Angew. Chem. Int. Ed. 2017, 56, 12424-12458. 

DOI 10.1002/anie.201701401

 

 

Publication : Enantioselective 1,3‐DC reactions of C‐Carboxy Ketonitrones and Enals with MacMillan catalyst: Evidence of a non‐concerted mechanism

Highly diastereo- and enantioselective 1,3-dipolar cycloadditions between functional ketonitrones and β-substituted enals are promoted by organocatalysis with the imidazolidinium catalyst of MacMillan. Study of the scope of the reaction shows that high selectivities are conserved by varying the N-protecting group or the ester function. However it is sensitive to sterical interaction with the C-substituent of the nitrone. Reaction proceeds in all cases with a high exo selectivity. In most cases, a third diastereomer, not compatible with a concerted mechanism, was observed, although in minute amount. DFT calculations evidence that the cycloaddition proceeds in a non-concerted fashion by a first oxa Michael-type addition of the nitrone to the double bond followed by a cyclization. This mechanism explains the formation of the observed minor diastereomers. In addition, the diastereo- and enantioselectivities of the reaction were shown to be intermediately thermodynamically controlled and the diastereomeric ratio is modulated by the kinetics of iminium hydrolysis.

 

Ayed, K. B.; Laurent, M. Y.; Martel, A.; Selim, K. B.; Abid, S.; Dujardin, G., Eur. J. Org. Chem. 2017, accepted

http://doi.org/10.1002/ejoc.201701307

 

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