Constructing complex tetrahydropyrans by stereoselective multicatalytic reactions
Résumé
Selectively functionalizing alcohols commonly requires adjusting the oxidation state through several waste-generating oxidation and reduction steps, necessary in order to obtain the desired reactivity at the carbonyl level. In order to bypass these redox steps, several catalytic transformations were invented based on an initial metal-catalyzed dehydrogenation of the alcohol generating the reactive carbonyl compound and a metal-hydride. Upon functionalization of the carbonyl compound, hydrogen backtransfer in a borrowing hydrogen-type catalytic cycle generates the final alcohol. Alternatively, the metalhydride can insert in a pro-nucleophile such as allylic acetates to generate a reactive organometallic specie condensing with the carbonyl in a Krische-type reductive coupling catalytic cycle.
In our group, we recently took advantage of those two strategies to prepare functionalized chiral tetrahydropyrans through their combination with oxa-Michael additions. 1 The challenge at developing these transformations lied in the selective dehydrogenation of the allylic alcohol and the control of the stereoselectivity over the newly formed stereocenters during the highly reversible oxa-Michael addition. Key to success was the implementation of a multicatalytic strategy merging the metal complexes catalytic cycles (iridium or iron complex), with a secondary amine organocatalysis.
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