Total Syntheses of the Securinega Alkaloids (+)-14,15-Dihydronorsecurinine, (−)-Norsecurinine, and Phyllanthine^†

A new strategy for enantiospecific construction of the Securinega alkaloids has been developed and applied in total syntheses of (+)-14,15-dihydronorsecurinine (8), (−)-norsecurinine (6), and phyllanthine (2). The B-ring and C7 absolute stereochemistry of these biologically active alkaloids originated from trans-4-hydroxy-l-proline (10), which was converted to ketonitrile 13 via a high-yielding eight-step sequence. Treatment of this ketonitrile with SmI₂ afforded the 6-azabicyclo[3.2.1]octane B/C-ring system 14, which is a key advanced intermediate for all three synthetic targets. Annulation of the A-ring of (−)-norsecurinine (6) with the required C2 configuration via an N-acyliminium ion alkylation was accomplished using radical-based amide oxidation methodology developed in these laboratories as a key step, providing tricycle 33. Annulation of the D-ring onto α-hydroxyketone 33 with the Bestmann ylide 45 at 12 kbar gave (+)-14,15-dihydronorsecurinine (8). In the securinine series, the D-ring was incorporated using an intramolecular Wadsworth−Horner−Emmons olefination of phenylselenylated α-hydroxyketone 47. The C14,15 unsaturation was installed late in the synthesis by an oxidative elimination of the selenoxide derived from tetracyclic butenolide 50 to give (−)-norsecurinine (6). The A-ring of phyllanthine (2) was formed from hydroxyketone 14 using a stereoselective Yb(OTf)₃-promoted hetero Diels−Alder reaction of the derived imine 34 with Danishefsky's diene, affording adduct 35. Conjugate reduction and stereoselective equatorial ketone reduction of vinylogous amide 35 provided tricyclic intermediate 36, which could then be elaborated in a few steps to stable hydroxyenone 53 via α-selenophenylenone intermediate 52. The D-ring was then constructed, again using an intramolecular Wadsworth−Horner−Emmons olefination reaction to give phyllanthine (2).