Construction of the Fused Pentacycle of Talatisamine via a Combination of Radical and Cationic Cyclizations

A concise and stereoselective synthesis of the BCDF tetracyclic ring system of C19-diterpenoid alkaloids

A new synthetic route for the BCDF tetracyclic ring system of C19-diterpenoid alkaloids (C19-DTAs) has been developed. The key step is a Pd-catalyzed transannular alkenylation that installs a functionalized bridged F ring. The overall strategy is concise and stereoselective, and it provides a valuable new tool for the synthesis of C19-DTAs. The synthesis begins with a bridged [3.2.1] ring system, which is converted to a key intermediate through a series of highly regio- and stereoselective processes. The introduction of an allylic side chain with high precision is accomplished, culminating in a Pd-catalyzed transannular alkenylation that installs a functionalized bridged F ring to yield the BCDF tetracyclic analog of C19-DTAs.

Total Synthesis of Vilmoraconitine

Vilmoraconitine belongs to one of the most complex skeleton types in the C19-diterpenoid alkaloids, which architecturally features an unprecedented heptacyclic core possessing a rigid cyclopropane unit. Here, we report the first total synthesis of vilmoraconitine relying on strategic use of efficient ring-forming reactions. Key steps include an oxidative dearomatization-induced Diels-Alder cycloaddition, a hydrodealkenylative fragmentation/Mannich sequence, and an intramolecular Diels-Alder cycloaddition.

Total Synthesis of Puberuline C

Puberuline C (1) is an architecturally complex C₁₉-diterpenoid alkaloid with a unique ring fusion pattern. The 6/7/5/6/6/6-membered rings (ABCDEF-rings) contain one tertiary amine and six oxygen functionalities, and possess 12 contiguously aligned stereocenters, three of which are quaternary. These structural features of 1 make its chemical construction exceptionally challenging. Here, we disclose the first total synthesis of 1. The synthesis was accomplished from 2-cyclohexenone (9) by integrating radical cascade and Mukaiyama aldol reactions as the key transformations. A double Mannich reaction fused the A- and E-rings, and Sonogashira coupling attached the C-ring, efficiently leading to ACE-rings with the requisite 19 carbons of 1. The chemically stable tertiary chloride of the ACE-ring structure was then transformed to the corresponding bridgehead radical, which participated in the simultaneous cyclization of the B- and F-rings via a highly organized radical cascade process. This unusual step installed five contiguous stereocenters, including two quaternary carbons, without damaging the preexisting multiple polar functionalities. Subsequently, the intramolecular Mukaiyama aldol reaction between silyl enol ether and acetal was realized by applying a combination of SnCl₄ and ZnCl₂, forging the last remaining D-ring of the hexacycle. Finally, 3 was elaborated into 1 through regio- and stereoselective functionalizations of the BCD-rings. Our novel radical-based strategy achieved the total synthesis of 1 in 32 total steps from simple 9, demonstrating the power of the radical cascade reaction to streamline the assembly of highly complex molecules.

Stereocontrolled Synthesis of C20S-C26 and C20R-C26 Fragments of Amphidinolide L

Amphidinolide L is a cytotoxic macrolide isolated from marine symbiotic dinoflagellates of the genus Amphidinium. While its planar structure and the absolute stereochemistry of the C21-C26 part have been determined, six stereocenters have remained unassigned. Aiming at structure determination, we have developed a synthetic route to the C20S-C26 and C20R-C26 fragments via the Li-mediated stereocontrolled aldol reaction. Two aldehydes, 16 with the C22-hydroxy group and 19 with the C22-TES ether, were synthesized from lactone 4. The aldol reactions using the Li-enolate of 4-methyl-2-pentanone in THF provided the C20S-C26 fragment 20 from 16 and a 1:3.5 mixture of the C20-C26 fragment 22 favoring the C20R-isomer. Mechanistic studies based on an extensive search of transition states in explicit solvents indicated that the C20S-isomer would be generated via a tri-solvated transition state, while the C20R-isomer would be formed via a di-solvated transition state. The calculation emphasizes the importance of the coordination network as a higher-order complex composed of solvent molecules, aldehyde, enolate, and Li atoms in the reaction of 16 to minimize steric interactions but maximize the stabilizing effect by the coordination of solvents. The presence of the rotationally free aldehyde in the reaction of 19 results in moderate diastereoselectivity.

The diterpenoid alkaloids

The diterpenoid alkaloids are a family of extremely important natural products that have long been a research hotspot due to their myriad of intricate structures and diverse biological properties. This chapter systematically summarizes the past 11 years (2009-2019) of studies on the diterpenoid alkaloids, including the "so-called" atypical ones, covering the classification and biogenetic relationships, phytochemistry together with 444 new alkaloids covering 32 novel skeletons and the corrected structures, chemical reactions including conversion toward toxoids, synthetic studies, as well as biological activities. It should be noted that the synthetic studies, especially the total syntheses of various diterpenoid alkaloids, are for the first time reviewed in this treatise. This chapter, in combination with our four previous reviews in volumes 42, 59, 67, and 69, will present to the readers a more completed and updated profile of the diterpenoid alkaloids.

Total Syntheses of the C19 Diterpenoid Alkaloids (-)-Talatisamine, (-)-Liljestrandisine, and (-)-Liljestrandinine by a Fragment Coupling Approach

The C19 diterpenoid alkaloids (C19 DTAs) are a large family of natural products, many of which modulate the activity of ion channels in vivo and are therefore of interest for the study of neurological and cardiovascular diseases. The complex architectures of these molecules continue to challenge the state-of-the art in chemical synthesis, particularly with respect to efficient assembly of their polcyclic ring systems. Here, we report the total syntheses of (-)-talatisamine, (-)-liljestrandisine, and (-)-liljestrandinine, three aconitine-type C19 DTAs, using a fragment coupling strategy. Key to this approach is a 1,2-addition/semipinacol rearrangement sequence which efficiently joins two complex fragments and sets an all-carbon quaternary center.

Construction of Pentacyclic Limonoid Skeletons via Radical Cascade Reactions

Limonoids 1 and 2 share a 6/6/6/5-membered ABCD-ring system and a six-membered oxacycle and differ in their C9-stereochemistries. A new radical-based strategy was devised to construct the pentacyclic skeletons of 1 and 2. An oxacycle-fused A-ring and enyne fragments were coupled to produce radical precursors 4a-4c with different C7-oxygen functionalities. The bridgehead tertiary bromide of 4a-4c participated in a radical cascade reaction with the three unsaturated bonds to cyclize the C9-diastereomeric BCD-rings.

Reactions of Allylmagnesium Reagents with Carbonyl Compounds and Compounds with C═N Double Bonds: Their Diastereoselectivities Generally Cannot Be Analyzed Using the Felkin-Anh and Chelation-Control Models

This review describes the additions of allylmagnesium reagents to carbonyl compounds and to imines, focusing on the differences in reactivity between allylmagnesium halides and other Grignard reagents. In many cases, allylmagnesium reagents either react with low stereoselectivity when other Grignard reagents react with high selectivity, or allylmagnesium reagents react with the opposite stereoselectivity. This review collects hundreds of examples, discusses the origins of stereoselectivities or the lack of stereoselectivity, and evaluates why selectivity may not occur and when it will likely occur.

The 1α-hydroxy-A-rings of norditerpenoid alkaloids are twisted-boat conformers

The skeletal conformations of naturally occurring norditerpenoid alkaloids fix their substituent functional groups in space, thereby directing their bioactivities.

Synthesis of tricyclo[7.2.1.09,10]dodecan-11-one core ring systems of norditerpenoid alkaloids and racemulosine

A synthetic strategy involving [2 + 2] enyne cycloaddition and pinacol rearrangement to construct common BCD ring systems of norditerpenoid alkaloids and racemulosine was developed.

Total Synthesis of Talatisamine

Talatisamine (1) is a member of the C₁₉ -diterpenoid alkaloid family, and exhibits K⁺ channel inhibitory and antiarrhythmic activities. The formidable synthetic challenge that 1 presents is due to its highly oxidized and intricately fused hexacyclic 6/7/5/6/6/5-membered-ring structure (ABCDEF-ring) with 12 contiguous stereocenters. Here we report an efficient synthetic route to 1 by the assembly of two structurally simple fragments, chiral 6/6-membered AE-ring 7 and aromatic 6-membered D-ring 6. AE-ring 7 was constructed from 2-cyclohexenone (8) through fusing an N-ethylpiperidine ring by a double Mannich reaction. After coupling 6 with 7, an oxidative dearomatization/Diels-Alder reaction sequence generated fused pentacycle 4 b. The newly formed 6/6-membered ring system was then stereospecifically reorganized into the 7/5-membered BC-ring of 3 via a Wagner-Meerwein rearrangement. Finally, Hg(OAc)₂ induced an oxidative aza-Prins cyclization of 2, thereby forging the remaining 5-membered F-ring. The total synthesis of 1 was thus accomplished by optimizing and orchestrating 33 transformations from 8.

Progress towards the synthesis of aconitine: construction of the AE fragment and attempts to access the pentacyclic core

We reported the first successful preparation of fully functionalized aconitine AE fragment and attempts to access the pentacyclic skeleton of aconitine via radical cascade.

Synthesis of Cardiopetaline via a Wagner-Meerwein Rearrangement without Preactivation of the Pivotal Hydroxy Group

A synthesis of cardiopetaline has been accomplished via a Wagner-Meerwein rearrangement of a diol having the denudatine skeleton. The Wagner-Meerwein rearrangement could be facilitated simply by heating the diol with p-toluenesulfonic acid in pivalic acid, without preactivating the pivotal hydroxy group. This strategy does not require differentiation of several hydroxy groups in the substrate for the Wagner-Meerwein rearrangement and could be applied to the synthesis of more highly oxygenated aconitine-type diterpenoid alkaloids.

Expedient Construction of the Hexacycle of Franchetine

Franchetine, a unique 7,17-seco type of norditerpenoid alkaloid, possesses a highly congested polycyclic architecture coupled with nine stereogenic centers. Here we present an efficient synthetic approach for the intact hexacyclic framework of franchetine from the known tricyle 16 in 20 steps. The synthesis features a diastereoselective 6-exo-tet radical cyclization for construction of ring A and a unique oxidative Wagner-Meerwein-type rearrangement to realize the functionalized [3.2.1] bridging ring CD.

Total synthesis of C19-diterpenoid alkaloid: construction of a functionalized ABCDE-ring system

An efficient synthetic approach for the ABCDE ring system of methyllycaconitine starting from a BCD tricycle precursor was developed.

Construction of the highly oxidized bicyclo[3.2.1]octane CD ring system of aconitine via a late stage enyne cycloisomerization

An enyne cycloisomerization enables assembly of the highly oxidized bicyclo[3.2.1]octane CD rings of the complex diterpenoid alkaloid aconitine.

Enabling syntheses of diterpenoid alkaloids and related diterpenes by an oxidative dearomatization/Diels–Alder cycloaddition strategy

Recent accomplishments in the total syntheses of diterpenoid alkaloids and their biosynthetically associated diterpenes have highly relied on an oxidative dearomatization/Diels–Alder cycloaddition strategy, and are highlighted in this article.