Intramolecular (4+3) Cycloadditions of Oxidopyridinium Ions: Towards Daphnicyclidin A

Reductive samarium (electro)catalysis enabled by SmIII-alkoxide protonolysis

Samarium diiodide (SmI2) is a privileged, single-electron reductant deployed in diverse synthetic settings. However, generalizable methods for catalytic turnover remain elusive because of the well-known challenge associated with cleaving strong SmIII-O bonds. Prior efforts have focused on the use of highly reactive oxophiles to enable catalyst turnover. However, such approaches give rise to complex catalyst speciation and intrinsically limit the synthetic scope. Herein, we leveraged a mild and selective protonolysis strategy to achieve samarium-catalyzed, intermolecular reductive cross-coupling of ketones and acrylates with broad scope. The modularity of our approach allows rational control of selectivity based on solvent, pKa (where Ka is the acid dissociation constant), and the samarium coordination sphere and provides a basis for future developments in catalytic and electrocatalytic lanthanide chemistry.

Synthesis of the ABC Core of Daphniphyllum Alkaloids with a [5-6-7] Azatricyclic Scaffold via Ring Expansion of Azabicyclic and Azatricyclic Building Blocks

The [5-6-7] azatricyclic ABC core, found in several Daphniphyllum alkaloids, has been synthesized through a novel route involving ring expansion of a perhydroindolone to afford the AC ring system and a radical B ring closure as key steps. The level of functionalization of the reported octahydro-1,7-ethanocyclohepta[b]pyrroles suggests that they can serve as valuable building blocks in this alkaloid field. Also reported is the first synthesis of homomorphans by the ring enlargement of 2-azabicyclo[3.3.1]nonanes.

Recent Strategies in the Nucleophilic Dearomatization of Pyridines, Quinolines, and Isoquinolines

Dearomatization reactions have become fundamental chemical transformations in organic synthesis since they allow for the generation of three-dimensional complexity from two-dimensional precursors, bridging arene feedstocks with alicyclic structures. When those processes are applied to pyridines, quinolines, and isoquinolines, partially or fully saturated nitrogen heterocycles are formed, which are among the most significant structural components of pharmaceuticals and natural products. The inherent challenge of those transformations lies in the low reactivity of heteroaromatic substrates, which makes the dearomatization process thermodynamically unfavorable. Usually, connecting the dearomatization event to the irreversible formation of a strong C-C, C-H, or C-heteroatom bond compensates the energy required to disrupt the aromaticity. This aromaticity breakup normally results in a 1,2- or 1,4-functionalization of the heterocycle. Moreover, the combination of these dearomatization processes with subsequent transformations in tandem or stepwise protocols allows for multiple heterocycle functionalizations, giving access to complex molecular skeletons. The aim of this review, which covers the period from 2016 to 2022, is to update the state of the art of nucleophilic dearomatizations of pyridines, quinolines, and isoquinolines, showing the extraordinary ability of the dearomative methodology in organic synthesis and indicating their limitations and future trends.

Total Synthesis of Four Classes of Daphniphyllum Alkaloids

The macrodaphniphyllamine-type, calyciphylline A-type, daphnilongeranin A-type, and daphnicyclidin D-type alkaloids are four structurally related classes of Daphniphyllum alkaloids. On the basis of a systematic analysis of the biogenetic network of these classes, we developed synthetic strategies centered on the C4-N and C1-C8 bonds of calyciphylline A, which took full advantage of the suitable substrates, reactions, and pathways that are altered from their counterparts in the postulated biogenetic network. Through this generalized biomimetic approach, we achieved the first synthesis of 14 Daphniphyllum alkaloids from the four subfamilies.

Total Synthesis of (±)- and (-)-Daphnillonin B

The first total synthesis of (±)- and (-)-daphnillonin B, a daphnicyclidin-type alkaloid with a new [7-6-5-7-5-5] A/B/C/D/E/F hexacyclic core, has been achieved. The [6-5-7] B/C/D ring system was efficiently and diastereoselectively constructed via a mild type I intramolecular [5+2] cycloaddition, followed by a Grubbs II catalyst-catalyzed radical cyclization. The [5-5] fused E/F ring system was synthesized via a diastereoselective intramolecular Pauson-Khand reaction. Notably, the synthetically challenging [7-6-5-7-5-5] hexacyclic core was reassembled by a unique Wagner-Meerwein-type rearrangement from the [6-6-5-7-5-5] hexacyclic framework found in calyciphylline A-type Daphniphyllum alkaloids.

First synthesis of an ABCE ring substructure of daphnicyclidin A

The ABCE tetracyclic ring system of daphnicyclidin A was prepared using an intramolecular (4 + 3) cycloaddition of an oxidopyridinium ion as the key step.