Redox Reorganization: Aluminium Promoted 1,5‐Hydride Shifts Allow the Controlled Synthesis of Multisubstituted Cyclohexenes

Abstract

AbstractAn efficient synthesis of cyclohexenes has been achieved from easily accessible tetrahydropyrans via a tandem 1,5‐hydride shift–aldol condensation. We discovered that readily available aluminium reagents, e.g. Al2O3 or Al(OtBu)3 are essential for this process, promoting the 1,5‐hydride shift with complete regio‐ and enantiospecificity (in stark contrast to results obtained under basic conditions). The mild conditions, coupled with multiple methods available to access the tetrahydropyran starting materials makes this a versatile method with exceptional functional group tolerance. A wide range of cyclohexenes (>40 examples) have been prepared, many in enantiopure form, showing our ability to selectively install a substituent at each position around the newly forged cyclohexene ring. Experimental and computational studies revealed that aluminium serves a dual role in facilitating the hydride shift, activating both the alkoxide nucleophile and the electrophilic carbonyl group.