While our mechanistic understanding of naturally occurring Diels-Alder is still in its infancy, more information on protein-catalysed [4+2] additions has been obtained with catalytic antibodies. Several Diels-Alderase antibodies have been generated from synthetic haptens mimicking more often the highly ordered transition state of Diels-Alder reactions. Surprisingly, only one example of antibody-mediated hetero Diels-Alder (HDA) reaction of carbonyl compounds has been reported despite the enormous synthetic potential of this particular transformation. The proposed work describes the development of a novel highly enantio-selective antibody-route to access hetero Diels-Alder adducts of carbonyl compounds that we expect might be amenable to the preparation of various synthetic intermediates for natural product synthesis. Two mechanistic pathways can operate for Lewis acid-mediated hetero Diels-Alder reactions of carbodienes with carbonyl compounds.
These two pathways are formulated as a pericyclic cycloaddition by analogy with the catalysed all-carbon Diels-Alder reaction or the formation of a Mukaiyama-aldol intermediate followed by an intra-molecular Michael-type addition. To a great extent, these are a function of the catalyst and solvent system along with the structural features in the reactants. Specifically, we hypothesized that the commercially available aldolase class I antibody 84G3 is a potential catalyst for the preparation of HDA adducts of carbonyl compounds according to a stepwise pathway where an aldol condensation might serve to join the reactants and a ring closure on the ß-carbon of the hydroxyen one would complete the cyclocondensation. The originality of our approach relies on the combined use of biocatalysis and metal (palladium) catalysis to address respectively the aldol step and the cyclisation.
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