Convergent Synthesis of trans-2,6-Disubstituted Piperidine Alkaloid, (?)-iso-6-Spectaline by Palladium-Catalyzed Cyclization

摘要

The plant alkaloids, iso -6-spectaline and spectaline, isolated from the Cassia or Senna genera contain a characteristic 2,6-disubstituted piperidin-3-ol scaffold. Although both natural products are reported to exhibit a variety of interesting biological activities, few stereo-selective schemes for the construction of the 2,6-disubstituted scaffold have been reported. Following our previous studies regarding the synthesis of (+)-spectaline, herein we report the first convergent synthesis of (?)- iso -6-spectaline using a cross-metathesis under thermal conditions where the cis -2,6-disubstituted piperidin-3-ol scaffold is condensed with a long alkyl chain containing a terminal olefin. The cis -2,6-disubstituted piperidin-3-ol used in the synthesis was prepared simply via Pd(II)-catalyzed diastereoselective cyclization. It was confirmed that (+)-spectaline, an epimer of (?)- iso -6-spectaline, was selectively synthesized by the cross-metathesis reaction under less intense thermal conditions starting from the same cis -2,6-disubstituted piperidin-3-ol derivative.