Studies toward the Synthesis of Palhinine Lycopodium Alkaloids: A Morita–Baylis–Hillman/Intramolecular Diels–Alder Approach

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.

Synthesis of Functionalized Bicyclo[2.2.2]octan-2-ones Skeleton via Tandem Process of Amino Acid Involved Formal [4 + 2] and Decarboxylative-Mannich Sequence

An efficient approach to a functionalized bicyclo[2.2.2]octan-2-one scaffold has been developed through a one-pot cascade process including amino acid involved successive Michael addition and decarboxylative-Mannich sequence. Starting from α,β-unsaturated ketones and amino acids, a series of desired products 7a-7m and 8a-8o were obtained with moderate yields. In addition, the tandem process was reasonably explained by the results of DFT calculations.

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.

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.

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.

Morita-Baylis-Hillman reaction of a chiral aziridine aldehyde

The Morita-Baylis-Hillman reaction of (R)-1-((R)-1-phenylethyl)aziridine-2-carbaldehyde with alkyl acrylate was carried out under various conditions by changing solvents, bases, and alcohol additives. The reaction at room temperature under neat conditions (no solvent) with quinuclidine as an amine nucleophile, in the presence of benzyl alcohol as an additive, afforded a product, γ-(aziridin-2-yl)-β-hydroxy-α-methylene butanoate, in 97% yield with a diastereomeric ratio of anti and syn as 86 : 14.

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.

Organocatalyzed Synthesis of [3.2.1] Bicyclooctanes

Organocatalysis constitutes one of the main research areas in organic chemistry from the last two decades. This chemistry has been applied to the synthesis of many natural products and structures in a manner that reduces the residues and so the ecological impact. In this review, we consider the work that has been done for the synthesis of bicyclo[3.2.1]octane framework. This structure is present in many natural products with very important biological activities.

Chemical Constituents of Plants from the Genus Phlegmariurus

Phlegmariurus is a genus of ca. 200 species in the family Huperziaceae. Up to now, six species of the Phlegmariurus genus have been chemically investigated, and 89 compounds, including Lycopodium alkaloids possessing diverse structures and serratane-type triterpenes, have been isolated. These compounds show potent bioactivities, such as acetylcholinesterase inhibitory and cytotoxic activities.

Cooperative Trifunctional Organocatalysts for Proficient Proton Transfer Reactions

Cooperativity is essential to proficient catalysis, and designing biomimetic, cooperative catalysis is a major avenue to finding new and efficient chemical reactions with practical applications. One challenge in designed cooperative catalysis is to access high catalytic proficiency (large enhancement in both rate and enantioselectivity) as seen in biocatalysis. Here described is an approach of developing and investigating trifunctional organocatalysts with three distinct catalytic functionalities, in order to understand how cooperativity could be organized for enantioselective activation that confers the observed proficiency in a tandem Michael-aldol-proton transfer elimination model reaction. This in future may assist in finding not just cooperative but also regulated catalysis to expand the level of catalytic complexity and efficiency in biomimetic systems.

Construction of Optically Active Isotwistanes and Aminocyclitols Using Chiral Cyclohexadiene as a Common Intermediate

We have developed a new method for synthesizing chiral isotwistane and homoisotwistane skeletons as well as aminocyclitols in a highly stereoselective manner. These results were achieved through the use of a common intermediate, which was derived from the ytterbium-catalyzed asymmetric Diels-Alder reaction of Danishefsky diene.

Efficient construction of the A/C/D tricyclic skeleton of palhinine A

An efficient approach for the synthesis of the 9/6/6 tricyclic structure ofLycopodiumalkaloid palhinine A has been accomplished. This protocol can undergo ring constriction to rapidly construct the highly strained nine-membered azonane ring of palhinine A.

Recent applications of intramolecular Diels–Alder reaction in total synthesis of natural products

Diels–Alder (D–A) reaction is undoubtedly the most powerful [4 + 2] cycloaddition reaction in organic synthesis.

Lycopodine-Type Alkaloids from Lycopodium japonicum

Three new lycopodine-type alkaloids, 4α-hydroxyanhydrolycodoline (1), 4α,6α-dihydroxyanhydrolycodoline (2), and 6-epi-8β-acetoxylycoclavine (3), and an artifact, lycoposerramine G nitrate (4), along with seventeen related known compounds, were isolated from the club moss Lycopodium japonicum Thunb. ex Murray (Lycopodiaceae). Their structures were elucidated by extensive spectroscopic methods as well as X-ray analysis. Compounds 1-4 were evaluated for their acetylcholine esterase inhibitory activity.