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Abstract: An AE azabicyclic fragment, with a β-hydroxyl group at C-5 and a substituent at C-1, of the C19-diterpenoid alkaloids, was synthesized. The key reactions include an intramolecular Claisen-type condensation, double Mannich reaction, and stereoselective nucleophilic addition of carbonyl group with the assistance of steric effect of 1β-substituent. [Copyright &y& Elsevier]

Treatment of 3-acetylyunaconitine 2 from yunaconitine 1 with NBS at 40°C for 12 h afforded the 3-acetylyunaconitine azomethine 3 (48%). Reaction of 3 with CH3I in MeOH at room temperature overnight gave the iminium salt 4 (100%). Unexpectedly, treatment of 4 with 5% NaOH in methanol at room temperature for 20 min produced the rearrangement alkaloid 5 (38%) besides other unidentified compounds. Acetylation of 5 with Ac2O/pyridine containing a small amount of TsOH gave its two derivatives 6 (28%) and 7 (56%). The structure of 5 was confirmed using 2D NMR spectra and single crystal X-ray analysis of its acetyl derivative 7. [Copyright &y& Elsevier]

Abstract: A novel diterpene with an unprecedented pentacyclic skeleton, designated as atropurpuran (1), featuring a unique motif to fuse two bicyclo[2.2.2]octane units, was isolated from the roots of Aconitum hemsleyanum var. atropurpureum. Its structure and relative configuration were established by NMR techniques and single-crystal X-ray diffraction. The biosynthetic origin of 1 was proposed to be Hetidane-type diterpenoid. [Copyright &y& Elsevier]

Abstract: An tricyclic AEF fragment of C19-diterpenoid alkaloid bearing an angular methyl group and the oxygen function groups at C-1 and C-6 was successfully synthesized. The synthesis features a stereoselective intramolecular Diels–Alder reaction and a highly efficient intramolecular Mannich reaction as well as efficient functional groups transformations. [Copyright &y& Elsevier]

Abstract: Treatment of imines 5 and 7, both prepared from the C19-diterpenoid alkaloid deltaline (1), with NaNO2–HCl resulted in the formation of diterpene 6 with an eight-membered ring through a sequence of denitrogenation and enlargement of ring B. In addition, two unusual skeletal rearrangements of diterpene 6 were also observed, leading to two structurally novel products 8 and 10. The structures of compounds 8 and 10 were confirmed by their X-ray crystallographic analyses and 2D NMR data. [Copyright &y& Elsevier]

Abstract: A concise strategy for construction of B/C/D ring system of C19-diterpenoid alkaloids was described, which features a highly diastereoselective intramolecular Diels–Alder reaction of masked o-benzoquinone and an efficient Wagner–Meerwein 1,2-shift rearrangement. The functional group transformations of rearrangement product 19 demonstrate a potential for stereoselective installation of oxygen functions on D-ring of many type of C19-diterpenoid alkaloids. [Copyright &y& Elsevier]