The asymmetric total synthesis of farnesin, a rearranged ent-kaurenoid, was achieved through a convergent approach involving photo-Nazarov and intramolecular aldol cyclizations to build the syn-syn-syn hydrofluorenol ABC ring system and bicyclo[3.2.1]octane CD ring system in the first application of a UV-light-induced excited-state Nazarov cyclization of a non-aromatic dicyclic divinyl ketone in a total synthesis. Unlike the conventional acid-promoted ground-state Nazarov reaction, the excited-state Nazarov reaction enables stereospecific formation of the highly strained syn-syn-synfused hydrofluorenone scaffold through a disrotatory cyclization.
The asymmetric total synthesis of farnesin, a rearranged ent‐kaurenoid, was achieved through a convergent approach involving photo‐Nazarov and intramolecular aldol cyclizations to build the syn‐syn‐syn hydrofluorenol ABC ring system and bicyclo[3.2.1]octane CD ring system in the first application of a UV‐light‐induced excited‐state Nazarov cyclization of a non‐aromatic dicyclic divinyl ketone in a total synthesis. Unlike the conventional acid‐promoted ground‐state Nazarov reaction, the excited‐state Nazarov reaction enables stereospecific formation of the highly strained syn‐syn‐syn‐fused hydrofluorenone scaffold through a disrotatory cyclization.
The reaction conditions and scope of the excitedstate Nazarov reaction of dicyclicvinyl ketones were studied. The stereochemistry of this electrocyclization is consistent with the mechanism of the pericyclic reaction and Woodward−Hoffmann rule. UV-light-promoted excited-state Nazarov reactions gave hydrofluorenones bearing a syn−cis configuration via a disrotatory cyclization. The core tricyclic hydrofluorenones of pyrrocidines and wortmannines were constructed via the excited-state Nazarov reactions, which demonstrated their synthetic potential in complex natural product total synthesis.
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