Total Synthesis of Lycopodium Alkaloids Palhinine A and Palhinine D

Total Synthesis of Quebrachamine and Kopsiyunnanine D

The total syntheses of (±)-quebrachamine and (±)-kopsiyunnanine D are reported. Key transformations include an intermolecular Horner-Wadsworth-Emmons olefination to merge the two fragments convergently and an intramolecular Mitsunobu reaction to introduce the synthetically challenging nine-membered azonane ring efficiently.

Influence of Ring Strain on the Formation of Rearrangement vs Cyclization Isotwistane Products in the Acyl Radical Reaction of Bicyclo[2.2.2]octanone

An acyl radical reaction of bicyclo[2.2.2]octenone to yield either rearranged or cyclized isotwistane products is described. The influence of ring strain on the reaction was demonstrated by alternating the sizes of the fused ring in the starting material. DFT calculations showed that the reaction is under thermodynamic control and proceeds via a 5-exo-trig cyclization intermediate, which undergoes either hydrogen-atom transfer (HAT) to give a cyclized product or rearrangement via a twistane intermediate to give a rearranged product.

Total Synthesis of Huperserratines A and B

Huperserratines A (1) and B (2), two novel macrocyclic Lycopodium alkaloids, possess an aza-12-membered ring. Here, we describe the first total synthesis of these two natural products in 12 steps....

Total Synthesis of Lyconesidine B, a Lycopodium Alkaloid with an Oxygenated, Amine-Type Fawcettimine Core

This report describes the total synthesis of the complex, oxygenated tetracyclic alkaloid, lyconesidine B. The key synthetic challenge involves diastereoselective generation of a decahydroquinoline ring with a quaternary carbon at the angular position via domino cyclopropanation, ring-opening, and reduction. Another crucial step is the domino ene-yne metathesis involving a quaternary ammonium ion, leading to the construction of a decahydroazaazulen framework.

Hydroxymethylation of quinolines via iron promoted oxidative C-H functionalization: synthesis of arsindoline-A and its derivatives

Herein, we report a mild and efficient hydroxymethylation of quinolines via an iron promoted cross-dehydrogenative coupling reaction under external acid free conditions. Various hydroxyalkyl substituted quinolines were achieved in excellent yields with well tolerated functional groups. Importantly, a few of the hydroxylmethylated quinolines were further transformed into respective aldehydes, and were successfully utilized for the synthesis of alkaloid arsindoline-A and its derivatives.

O-Isocyanates as Uncharged 1,3-Dipole Equivalents in [3+2] Cycloadditions

1,3-Dipoles are commonly used in [3+2] cycloadditions, whereas isoelectronic uncharged dipole variants remain underdeveloped. In contrast to conventional 1,3-dipoles, uncharged dipole equivalents form zwitterionic cycloadducts, which can be exploited to build further molecular complexity. In this work, the first cycloadditions of oxygen-substituted isocyanates (O-isocyanates) were studied experimentally and by DFT calculations. This unique cycloaddition strategy provides access to a novel class of heterocycle aza-oxonium ylides through intramolecular and intermolecular cycloadditions with alkenes. This allowed a systematic study of the reactivity of the transient aza-oxonium ylide intermediate, which can undergo N-O bond cleavage followed by nitrene C-H insertion, and the formation of β-lactams or isoxazolidinones upon varying the structure of the alkene or O-isocyanate reagents.

Deciphering the Biosynthetic Mechanism of Pelletierine in Lycopodium Alkaloid Biosynthesis

Pelletierine, a proposed building block of Lycopodium alkaloids (LAs), was demonstrated to be synthesized via the non-enzymatic Mannich-like condensation of Δ¹-piperideine and 3-oxoglutaric acid produced by two new type III PKSs (HsPKS4 and PcPKS1) characterized from Huperzia serrata and Phlegmariurus cryptomerianus, respectively. The findings provide new insights for further understanding the biosynthesis of LAs such as huperzine A.

Strategies and Efforts towards the Total Synthesis of Palhinine Alkaloids

The palhinine family of Lycopodium alkaloids were first reported in 2010, which feature an intriguing isotwistane carbon cage and a nine-membered azonane ring. It is noteworthy that the tetracyclic 5/6/6/9 skeleton was unprecedented in Lycopodium alkaloids before their seminal discovery. Over the past decade, extensive synthetic efforts stemming from seven research groups have resulted in two racemic total syntheses to date. This review article takes the opportunity to survey these efforts and achievements so as to promote further research towards the asymmetric total synthesis of palhinine alkaloids.

Catalytic enantioselective construction of vicinal quaternary carbon stereocenters

This review summarizes the advances in the catalytic enantioselective construction of vicinal quaternary carbon stereocenters, introduces major synthetic strategies and discusses their advantages and limitations, highlights the application of known protocols in the total synthesis of natural products, and outlines the synthetic opportunities.

This review summarizes the advances in catalytic enantioselective construction of vicinal quaternary carbon stereocenters, introduces major synthetic strategies and discusses their advantages and limitations, and outlines the synthetic opportunities.

Total Synthesis of Fawcettimine-Type Alkaloid, Lycojaponicumin A

The efficient total synthesis of lycojaponicumin A (1) has been accomplished for the first time. The remarkable features of this novel strategy include the following: (1) rapid construction of tricyclic intermediate 4 through a regio- and stereoselective semipinacol ring expansion, which simplified the construction of rings A and B of 1; (2) the subsequent regio- and stereoselective formation of the highly strained rings C-E of 1 through a tandem oxa-hetero [3 + 2] cycloaddition/N-cycloalkylation.

Recent applications of the Wittig reaction in alkaloid synthesis

The Wittig reaction is the chemical reaction of an aldehyde or ketone with a triphenyl phosphonium ylide (the Wittig reagent) to afford an alkene and triphenylphosphine oxide. Noteworthy, this reaction results in the synthesis of alkenes in a selective and predictable fashion. Thus, it became as one of the keystone of synthetic organic chemistry, especially in the total synthesis of natural products, where the selectivity of a reaction is paramount of importance. A literature survey disclosed the existence of vast numbers of related reports and comprehensive reviews on the applications of this important name reaction in the total synthesis of natural products. However, the aim of this chapter is to underscore, the applications of the Wittig reaction in the total synthesis of one the most important and prevalent classes of natural products, the alkaloids, especially those showing important and diverse biological activities.

Arylboronic Acid-Catalyzed C-Allylation of Unprotected Oximes: Total Synthesis of N-Me-Euphococcine

O-Unprotected keto- and aldoximes are readily C-allylated with allyl diisopropyl boronate in the presence of arylboronic acid catalysts to yield highly substituted N-α-secondary and tertiary homoallylic hydroxylamines. The method was used in the total synthesis of the trace alkaloid N-Me-Euphococcine.

Synthesis and Formation Mechanism of a Compound with an Unprecedented Skeleton: Dodecahydro-4,10:5,9-diepoxydipyrrolo[3,4-b:3',4'-f][1,5]diazocine

The reaction of N-(2-{[(tert-butyldimethylsilyl)oxy]imino}ethyl)-4-methyl-N-(3-phenylprop-2-yn-1-yl)benzenesulfonamide (6b) with BF3·OEt2 afforded a compound with an unprecedented dodecahydro-4,10 : 5,9-diepoxydipyrrolo[3,4-b:3',4'-f][1,5]diazocine skeleton (7) after aqueous work-up. The formation mechanism of meso-7 appears to involve dimerization of the hydrated forms (6aS)-C and (6aR)-C of the initial racemic product 9 via cation B generated by facile protonation at the C3a position of 9. Extensive computational studies revealed that the driving force of the facile hydration of 9 is probably release of the ring strain of 9, which arises in part from the bent sp2-hybridized C3a carbon.

An approach towards the construction of the tetracyclic skeleton of palhinine alkaloids

A bifunctional Lewis acid catalyzed Diels–Alder/carbocyclization cascade process was developed for the rapid construction of the tetracyclic core of palhinine alkaloids.

Total Synthesis of (-)-Himeradine A

(-)-Himeradine A is a complex lycopodium alkaloid with seven rings and ten stereogenic centers that shows anticancer activity against lymphoma L1210 cells. A total synthesis has been developed that builds off prior work on (+)-fastigiatine. A 2,4,6-trisubstitited piperidine ring forms the core of the quinolizidine segment, and was prepared by diastereoselective reduction of a pyridine and classic resolution of an intermediate. The remaining secondary amine was introduced with a catalyst-controlled Overman rearrangement. The piperidine segment was coupled in a B-alkyl Suzuki reaction with a bicyclic bromoenone, which was a key intermediate for the synthesis of (+)-fastigiatine. The final transformation featured a transannular Mannich reaction and cyclization to complete the quinolizidine. Five bonds and four new rings were generated in this one-pot procedure. (-)-Himeradine A was prepared in 17 steps in the longest linear sequence.

Far Away from Flatland. Synthesis and Molecular Structure of Dihetera[3.3.n]propellanes and Trihetera[3.3.n]propellanes: Advanced Analogues of Morpholine/Piperazine

An approach to di- and trihetera[3.3.n]propellanes (n = 2-4 ), advanced morpholine and piperazine analogues, is developed. The key step of the reaction sequence included a [3 + 2] cycloaddition reaction of unsaturated vicinal dicarboxylic acid derivatives and in situ generated azomethine ylide resulting in the formation of the pyrrolidine ring. One more heteroaliphatic ring (i.e., pyrrolidine or tetrahydrofuran) was annelated by nucleophilic cyclization of the appropriate 1,4-dielectrophilic intermediates. There were 11 examples of the title products obtained in 3-5 steps on a multigram scale with 10-72% overall yields. Additionally, molecular structures of homologous dihetera[3.3.n]propellanes, analogues of morpholine, were obtained from X-ray diffraction studies and analyzed using exit vector plots (EVPs). It was shown that the scaffolds obtained are somewhat larger as compared to the parent morpholine and bicyclic 3-oxa-7-azabicyclo[3.3.0]octane. Moreover, despite very similar chemical structures, they provide a very distinct spatial position of heteroatoms, which is clearly seen from the conformation adopted by a formal eight-membered ring including both N and O atoms (i.e., crown, boat-chair, twist chair-chair, and boat-boat for the oxaza[3.3.2]-, -[3.3.3]-, -[4.3.3]propellanes, and 3-oxa-7-azabicyclo[3.3.0]octane, respectively).

Synthesis of the Tricyclic Caged Core of Palhinine Alkaloids Based on a Non-Diels-Alder-Type Strategy

A concise synthesis of the tricyclo[4.3.1.0^3,7]decane caged core of palhinine alkaloids was developed with SmI₂-mediated cyclization and light-initiated radical addition-fragmentation as key steps. Compared with the reported racemic routes which are all based on Diels-Alder-type key reactions, our strategy would be more readily accessible to the asymmetric total syntheses of the palhinine alkaloids.

Asymmetric Total Synthesis of Fawcettimine-Type Lycopodium Alkaloid, Lycopoclavamine-A

An asymmetric total synthesis of lycopoclavamine-A (1), a structurally unique fawcettimine-type Lycopodium alkaloid, was achieved via a stereoselective Pauson-Khand reaction and a stereoselective conjugate addition to construct a quaternary carbon center at C-12.

Visible light-induced direct α C-H functionalization of alcohols

Considering the synthetic value of introducing active alcoholic hydroxyl group, developing C-H functionalization of alcohols is of significance. Herein, we present a photochemical method that under visible light irradiation, selectfluor can effectively promote the oxidative cross-coupling between alcohols and heteroarenes without the external photocatalysis, achieving the selective α sp³ C-H arylation of alcohol, even in the presence of ether. The N-F activation of selectfluor under blue LEDs irradiation is evidenced by electron paramagnetic resonance (EPR) study, which is the key process for the oxidative activation of α sp³ C-H alcohols. The observed reactivity may have significant implications for chemical transformations.

Biomimetic Syntheses of (±)-Isopalhinine A, (±)-Palhinine A, and (±)-Palhinine D

The first total synthesis of isopalhinine A, as well as unified syntheses of palhinine A and palhinine D, were successfully accomplished by means of a biomimetic strategy that proceeds through a bioinspired 5/6/6/9 tetracyclic intermediate, which mimics the amino ketone form of palhinine D. An early-stage direct S_N 2 cyclization to construct the nine-membered azonane ring minimized the transannular strain that would otherwise be increased by the twisted nature of the isotwistane skeleton. Then, a diastereoselective Diels-Alder reaction of a masked ortho-benzoquinone using the nine-membered ring as a steric shielding group furnished a functionalized 6/6/9 tricyclic skeleton and established the desired stereochemistry at the C3, C7, C12, and C15 positions in one step. A thiol-mediated acyl radical cyclization gave the bioinspired intermediate bearing three differentiated oxygen-containing functional groups, from which all three total syntheses could be completed in either two or three additional steps.

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 the reported structure of 13a-hydroxytylophorine

The first total synthesis of the reported structure of 13a-hydroxytylophorine was accomplished. The key step was an unprecedented NaBH₄-promoted one-pot reductive cyclization cascade that efficiently yielded a hydroxyl azonane intermediate. The indolizidine framework was obtained by means of oxidation and a subsequent unexpected protecting-group migration. This total synthesis revealed that the reported structure of the naturally isolated compound is incorrect.

Cyclols Revisited: Facile Synthesis of Medium-Sized Cyclic Peptides

Medium-sized rings, particularly the corresponding cyclic peptides, are challenging synthetic targets. In the present study, we report an approach to medium-sized cyclic peptides through targeted formation and collapse of cyclol intermediates. This methodology operates on β-amino imides derived from 2,5-diketopiperazines and offers a straightforward transition from frequently examined scaffolds in drug discovery to a rarely visited class of medium-sized rings.