Asymmetric Total Synthesis of (-)-Quinocarcin

(-)-Quinocarcin (I) has been synthesized in a longest linear sequence of 22 steps from 3-hydroxybenzaldehyde in 16% overall yield. The Pictet-Spengler reaction of L-tert-butyl-2-bromo-5-hydroxy phenylalanate, synthesized according to Corey-Lygo's enantioselective alkylation process, with benzoxyacetaldehyde under mild acidic conditions afforded 1,3-cis tetrahydroisoquinoline as an only isolable stereomer in 91% yield. The diazabicycle[3,2,1]-octane ring system of II was constructed by a silver tetrafluoroborate-promoted intramol. Mannich reaction using amino thioether as a latent N-acyliminium species and tethered silyl enol ether as a nucleophile. Using amino thioether instead of aminal as a precursor of N-acyliminium was of high importance to the success of this otherwise disfavored 5-endo-Trig cyclization. A Hf(OTf)4-catalyzed (0.1 equiv) transformation of aminal to amino thioether was uncovered in the course of this study, allowing the conversion of a tricyclic aminal to an amino thioether to be realized in high yield. From the bridged tetracyclic compd. III, a sequence of oxidn. of aldehyde to acid, global deprotection under hydrogenolysis conditions, and one-pot partial redn. of lactam to aminal/oxazolidine formation completed the total synthesis of the pentacyclic (-)-quinocarcin. [on SciFinder (R)]