Synthesis of (±)-Serralongamine A and the Revised Structure of Huperzine N

Huperzine alkaloids: forty years of total syntheses

Covering: up to 2023Huperzine alkaloids are a group of natural products belonging to the Lycopodium alkaloids family. The representative member huperzine A has a unique structure and exhibits potent inhibitory activity against acetylcholine esterase (AChE). This subfamily of alkaloids provides a great opportunity for developing synthetic methodologies and asymmetric synthesis. The efforts towards the synthesis of huperzine A have cultivated dozens of total syntheses and a rich body of new chemistry. Impressive progress has also been made in the synthesis of other huperzine alkaloids. The total syntheses of huperzines B, U, O, Q and R, structure reassignment and total syntheses of huperzines K, M and N have been reported in the past decade. This review focuses on the synthetic organic chemistry and the biosynthesis and medicinal chemistry of huperzines are also covered briefly.

Decahydroquinoline Ring 13C NMR Spectroscopic Patterns for the Stereochemical Elucidation of Phlegmarine-Type Lycopodium Alkaloids: Synthesis of (-)-Serralongamine A and Structural Reassignment and Synthesis of (-)-Huperzine K and (-)-Huperzine M (Lycoposerramine Y)

Analysis of ¹³C NMR spectroscopic data of the phlegmarine subset of Lycopodium alkaloids revealed spectral patterns that allowed the stereochemical arrangement of the four stereogenic carbons in the decahydroquinoline core to be established. A relatively simple predictive set of chemical shift combinations is reported, providing a tool for the challenging stereochemical assignment of phlegmarine-type alkaloids. Based on the chemical shifts in their NMR spectroscopic profiles, the alkaloids huperzine K and huperzine M, formally reported as cis derivatives, were structurally reassigned as trans-decahydroquinolines. The NMR spectroscopic data for huperzine M were identical to those reported for lycoposerramine Y and, hence, also implied the configurational reassignment of the latter. The revised structures of the above alkaloids were confirmed by enantioselective total synthesis. Additionally, the synthesis of (-)-serralongamine A via a common intermediate precursor is reported.

Enantioselective Construction of Octahydroquinolines via Trienamine-Mediated Diels-Alder Reactions

A trienamine-mediated asymmetric Diels-Alder reaction using a 5-nitro-2,3-dihydro-4-pyridone derivative as a dienophile in the presence of a secondary amine organocatalyst derived from cis-hydroxyproline was discovered. The reaction provides optically active octahydroquinolines through an endo-selective [4 + 2] cyclization pathway. The following stereoselective denitration, isomerization, and/or hydrogenation generated divergent stereoisomers of decahydroquinolines, which are useful synthons for the total synthesis of Lycopodium alkaloids.

Synthesis of cis-hydrindan-2,4-diones bearing an all-carbon quaternary center by a Danheiser annulation

A straightforward synthetic entry to functionalized hydrindane compounds based on a rapid assembly of the core nucleus by a Danheiser cycloaddition is reported. Valuable bicyclic building blocks containing the fused five and six-membered carbocyclic ring system can be achieved in only four steps from a simple acyclic β-keto ester.

Identification of Lycopodium Alkaloids Produced by an Ultraviolet-Irradiated Strain of Paraboeremia, an Endophytic Fungus from Lycopodium serratum var. longipetiolatum

12- epi-Lycopodine (1), a Lycopodium alkaloid, along with lycopodine (2) and huperzine A (3), were discovered in the mycelium of Paraboeremia sp. Lsl3KI076, a UV-irradiated strain of Paraboeremia sp. Lsl3, an endophytic fungus from Lycopodium serratum Thunb. var. longipetiolatum Spring. Additionally, a trace of 1 was isolated from Phlegmariurus nummulariifolius (Blume) Ching, and the structure was elucidated on the basis of spectroscopic data. This is the first report proving that a new naturally occurring Lycopodium alkaloid can be obtained from an endophytic fungus.

Total Synthesis of (±)-Cermizine B

A practical synthesis of (±)-cermizine B was achieved. The nine-step synthesis mainly comprised two uninterrupted Michael additions including a highly diastereoselective 1,4-addition of 2-picoline to methyl 4-methyl-6-oxocyclohex-1-ene-1-carboxylate, Krapcho decarboxylation, a double reductive amination that resulted in ring closure and dearomatization of pyridine in 24% overall yield.