Synthesis of the AE bicyclic of Daphniphyllum alkaloid yuzurine

Recent Advances in the Synthesis of Propargyl Derivatives, and Their Application as Synthetic Intermediates and Building Blocks

The propargyl group is a highly versatile moiety whose introduction into small-molecule building blocks opens up new synthetic pathways for further elaboration. The last decade has witnessed remarkable progress in both the synthesis of propargylation agents and their application in the synthesis and functionalization of more elaborate/complex building blocks and intermediates. The goal of this review is to highlight these exciting advances and to underscore their impact.

Total Synthesis of Yuzurine-type Alkaloid Daphgraciline

The first total synthesis of daphgraciline has been achieved, which also represents the first example of the synthesis of Daphniphyllum yuzurine-type alkaloids (∼50 members). The unique bridged azabicyclo[4.3.1] ring system in the yuzurine-type subfamily was efficiently and diastereoselectively assembled via a mild type II [5+2] cycloaddition for the first time. The compact tetracyclic [6-7-5-5] skeleton was installed efficiently via an intramolecular Diels-Alder reaction, followed by a benzilic acid-type rearrangement. The synthetically challenging spiro tetrahydropyran moiety in the final product was installed diastereoselectively via a Ti^III-mediated reductive epoxide coupling reaction. Potential access to enantioenriched daphgraciline is presented.

A General Strategy for the Construction of Calyciphylline A-Type Alkaloids: Divergent Total Syntheses of (-)-Daphenylline and (-)-Himalensine A

An efficient general strategy for the synthesis of the Daphniphyllum alkaloids via the rapid construction of a common core intermediate has been established, based on which a divergent total synthesis of (-)-daphenylline and (-)-himalensine A has been accomplished in 16 and 19 steps, respectively. The present work features an enantioselective Mg(ClO₄ )₂ -catalyzed intramolecular amidocyclization to construct the aza-bridged core structure; a Cu-catalyzed intramolecular cyclopropanation and subsequent phosphine-catalyzed Cope-type rearrangement to furnish the himalensine A scaffold; and a one-pot Diels-Alder/aromatization method to assemble the aromatic skeleton of daphenylline.

The chemistry of Daphniphyllum alkaloids

The triterpenoids Daphniphyllum alkaloids share the unique fused hexacyclic ring framework are isolated from the genus Daphniphyllum. These natural products possess comprehensive biological activities and exhibit excellent potential medicinal appliment. This review covers the reported isolation studies and biological activities of Daphniphyllum alkaloids spanning the period from 1966 to the beginning of 2020, In the meantime, the total synthesis of Daphniphyllum alkaloids will be emphatically summarized for supplement over this review series.

Total Synthesis of (-)-Daphnezomines A and B

Daphnezomines A and B are structurally unusual Daphniphyllum alkaloids that contain a unique aza-adamantane core skeleton. Herein, a modular approach to these alkaloids is presented that exploits a diverse array of reaction strategies. Commencing from a chiral pool terpene-(S)-carvone, the azabicyclo[3.3.1]nonane backbone, which occurs widely in Daphniphyllum alkaloids, was easily accessed through a Sharpless allylic amination and a palladium-catalyzed oxidative cyclization. A protecting group enabled a stereoselective B-alkyl Suzuki-Miyaura coupling sequence and an Fe-mediated hydrogen atom transfer (HAT)-based radical cyclization were then applied to construct C6 and C8 stereocenters. A final epimer locking strategy enabled the assembly of the highly congested aza-adamantane core, thereby achieving the first total synthesis of (-)-daphnezomines A and B in 14 steps.

Enantioselective Total Synthesis of (-)-Caldaphnidine O via a Radical Cyclization Cascade

The synthetically challenging, diverse chemical skeletons and promising biological profiles of the Daphniphyllum alkaloids have generated intense interest from the synthetic chemistry community. Herein, the first and enantioselective total synthesis of (-)-caldaphnidine O, a complex bukittinggine-type Daphniphyllum alkaloid, is described. The key transformations in this concise approach included an intramolecular aza-Michael addition, a ring expansion reaction sequence, a Sm(II)/Fe(III)-mediated Kagan-Molander coupling, and the rapid formation of the entire hexacyclic ring skeleton of the target molecule via a radical cyclization cascade reaction, which was inspired by an unexpected radical detosylation observed in our recent dapholdhamine B synthesis.

Synthesis of the Core Structure of Daphnimacropodines

Daphniphyllum alkaloids daphnimacropodines A-C possess a highly congested ring system and share a common tetracyclic ring skeleton. To access the challenging chemical structure of daphnimacropodines, a divergent synthetic approach toward their total synthesis is described. A stereoselective synthesis of the core structure of daphnimacropodines has been achieved from a simple diketone building block. Our approach features an intramolecular carbamate aza-Michael addition and a hydropyrrole synthesis via a Au-catalyzed alkyne hydration followed by an aldol condensation, whereas all the other attempts failed.

Structural Revision of Macropodumine A and Structure of 2-Deoxymacropodumine A, Daphniphyllum Alkaloids with 11-Membered Macrolactone Rings

An unusual Daphniphyllum alkaloid, 2-deoxymacropodumine A (1), possessing an 11-membered macrolactone ring, was obtained from an extract of the stems of Daphniphyllum angustifolium. The structure of 1 was elucidated by 1D and 2D NMR spectroscopic methods and chemical calculations. Based on a comparison of the experimental and calculated NMR data, the structure of macropodumine A (2'), an analogue of 1, was also revised.