Organocatalytic, Enantioselective Synthesis of Cyclohexadienone Containing Hindered Spirocyclic Ethers through an Oxidative Dearomatization/Oxa-Michael Addition Sequence

An unprecedented enantioselective oxa‐Michael reaction of α‐tertiary alcohols using cinchona‐alkaloid‐based chiral bifunctional squaramide catalysts is reported. An oxidative dearomatization of phenol followed by an enantioselective oxa‐Michael addition sequence provided a broad array of chiral sterically hindered tetrahydrofurans and tetrahydropyrans attached to a cyclohexadienone moiety in spiro fashion. In general, good yields and excellent enantioselectivities (up to 99 %) were observed. The chiral oxo‐cycles obtained have easily been transformed into chromans without disturbing the enantioselectivity. Eine asymmetrische Oxa‐Michael‐Reaktion von α‐tertiären Alkoholen mit chiralen Squaramid‐Katalysatoren ergibt eine Reihe sterisch gehinderter spirocyclischer Tetrahydrofuran‐ und Tetrahydropyran‐Cyclohexadienone in guten Ausbeuten. Die Sauerstoffheterocyclen wurden ohne Verlust an Enantioselektivität in Chromane überführt. EWG=elektronenziehende Gruppe.