Total Synthesis of Unsymmetrically Oxidized Nuphar Thioalkaloids via Copper-Catalyzed Thiolane Assembly

An asymmetric total synthesis of (+)-6-hydroxythiobinupharidine (1b) and (-)-6-hydroxythionuphlutine (2b), a set of hemiaminal containing dimeric sesquiterpenes isolated from yellow water lilies of the Nuphar genus, is described. The central bis-spirocyclic tetrahydrothiophene ring was forged through the Stevens rearrangement of a sulfonium ylide, generated in situ from the coupling of a copper-carbene with a spirocyclic thietane. This strategy diverges both from the proposed biosynthesis1 and previous syntheses of this family of alkaloids,2,3 all of which employ dimerization of symmetric monomers to form the aforementioned thiaspirane. The coupling of unsymmetrical monomers allowed access to the unsymmetrically oxidized product 2b for the first time.