We report 8‐step syntheses of (−)‐minovincine and (−)‐aspidofractinine using easily available and inexpensive reagents and catalyst. A key element of the strategy was the utilization of a sequence of cascade reactions to rapidly construct the penta‐ and hexacyclic frameworks. These cascade transformations included organocatalytic Michael‐aldol condensation, a multistep anionic Michael‐S
N
2 cascade reaction, and Mannich reaction interrupted Fischer indolization. To streamline the synthetic routes, we also investigated the deliberate use of steric effect to secure various chemo‐ and regioselective transformations.
We report a novel
reductive interrupted Fischer indolization process
for the concise assembly of the 20-oxoaspidospermidine framework.
This rapid complexity generating route paves the way toward various
dihydroindole
Aspidosperma
alkaloids with different
C-5 side chain redox patterns. The end-game redox modulations were
accomplished by modified Wolff–Kishner reaction and photo-Wolff
rearrangement, enabling the total synthesis of (−)-aspidospermidine,
(−)-limaspermidine, and (+)-17-demethoxy-
N
-acetylcylindrocarine and the formal total synthesis of (−)-1-acetylaspidoalbidine.
A total synthesis of the monocyclic sesquiterpene elemol has been accomplished in seven steps and 16.7% overall yield. The key new carbon-carbon bonds were formed by the [2 + 2] photochemical cycloaddition reaction between 2,2,6-trimethyl-4íí-l,3-dioxin-4-one and -terpineol, a reaction that occurred with essentially complete regioselectivity (head/head) and good alkene face selectivity (3.35/1). The 35-38% enantiomeric purity of the starting (-)-a-terpineol was translated into optically active elemol predominating in the unnatural (+) enantiomer (ee = 31-40%).
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