Development of an efficient synthetic strategy for bioactive alkaloids possessing a spirocyclic ring system*

Efficient synthetic strategies for several bioactive alkaloids bearing a spirocyclic ring system have been developed. By application of an intramolecular aromatic oxidation of a phenolic intermediate, derived from L-tyrosine and glycine, with hypervalent iodine reagents, TAN1251A, isolated from Penicillium thomii RA-89, was synthesized in an optically pure form. Similarly, a novel synthetic path to TAN1251C and TAN1251D was established by employing an L-tyrosine dimer as a common starting material. Moreover, an enamide-phenol oxidative coupling reaction was applied to the synthesis of spirocyclic isoquinoline alkaloids, stepharine and annosqualine. Finally, stereoselective syntheses of securinine and viroallosecurinine in optically active forms were established by using a tandem ring-closing metathesis (RCM) of the enyne as a key reaction.