The Daphniphyllum alkaloids

Synthetic Approaches to Non-Tropane, Bridged, Azapolycyclic Ring Systems Containing Seven-Membered Carbocycles

This Short Review highlights various synthetic approaches to bridged azabicyclic ring systems containing seven-membered carbocyclic rings. Such ring systems are common to a number of biologically active natural products. The seven-membered ring in such systems is generally formed in one of four ways: 1) cyclization of an acyclic precursor; 2) ring expansion or rearrangement of a different ring size; 3) cycloaddition; and 4) use of a synthetic building block with the seven-membered ring already present. Representative examples of each approach from both total synthesis and methodological studies are discussed, with an emphasis on work published

in the last twenty years.

1 Introduction

2 Cyclization Reactions

3 Ring Expansions and Rearrangements

4 Cycloadditions

5 Strategies Involving Seven-Membered Ring Building Blocks

6 Conclusion

Computational studies of biologically active alkaloids of plant origin: an overview

Computational studies nowadays constitute a crucial source of information for drug development, because they provide information on many molecular properties and also enable predictions of the properties of not-yet-synthesized compounds. Alkaloids are a vast group of natural products exhibiting a variety of biological activities, many of which are interesting for drug development. On the other hand, computational studies of biologically active alkaloids have so far mostly focused on few particularly relevant or “popular” molecules, such as quinine, caffeine, or cocaine, with only few works on the other molecules. The present work offers an overview of existing computational studies on alkaloid molecules, from the earliest ones to the most recent, and considering all the theoretical approaches with which studies have been performed (both quantum mechanics and molecular dynamics). The considered studies are grouped according to their objectives and outcomes, such as conformational analysis of alkaloid molecules, effects of selected solvents on their properties, docking studies aimed at better understanding of the interactions between alkaloid molecules and biological targets, studies focusing on structure activity relationships, and computational studies performed to confirm experimental results. It is concluded that it would be important that computational studies on many other alkaloid molecules are performed and their results made available, covering their different classes as well as the variety of their biological activities, to attain better understanding of the properties not only of individual molecules, but also of groups of related molecules and of the overall alkaloids family.

Synthetic Entry to the 2-Azatricyclo[4.3.2.04,9]undecane Ring System via Tropone

A synthesis of the 2-azatricyclo[4.3.2.0^4,9]undecane ring system-a hitherto unreported bridged azatricyclic ring system-beginning from tricarbonyl(tropone)iron and allylamine was accomplished in three steps: (1) aza-Michael addition of allylamine to tricarbonyl(tropone)iron; (2) Boc-protection of the resulting secondary amine; and (3) oxidative demetallation leading to a spontaneous intramolecular Diels-Alder reaction. The effect of a variety of parameters on the intramolecular Diels-Alder reaction was investigated, including diene and dienophile substitution patterns and dienophile tether length.

Direct catalytic enantioselective amination of ketones for the formation of tri- and tetrasubstituted stereocenters

Herein, we report a Zn-ProPhenol catalyzed direct asymmetric amination reaction of unactivated aryl and vinyl ketones using di-tert-butyl azodicarboxylate as a cheap and practical electrophilic nitrogen source. Importantly, this methodology works with both α-branched and unbranched ketones for the construction of tri- and tetrasubstituted N-containing stereocenters. The reaction can be run at gram-scale with low catalyst loadings and features a recoverable and reusable ligand. Finally, the enantioenriched hydrazine products can be readily converted into versatile building blocks such as α-amino carbonyl compounds and β-amino alcohols.

Cycloaddition/annulation strategies for the construction of multisubstituted pyrrolidines and their applications in natural product synthesis

Pyrrolidines are privileged substructures of numerous bioactive natural products and drugs.

Recent Progress in the Chemistry of Daphniphyllum Alkaloids †

Daphniphyllum is an evergreen species known since 1826. After initial systematic investigations, more than 320 members of this family have been isolated, which comprise complex and fascinating structures. Unique azapolycyclic architectures containing one or more quaternary stereocenters render these alkaloids synthetically challenging. This review covers efforts toward the synthesis of Daphniphyllum alkaloids spanning the period from 2005 to the beginning of 2016, including reported biological activities as well as the isolation of new members of this genus.

Synthetic Strategies toward Natural Products Containing Contiguous Stereogenic Quaternary Carbon Atoms

Strategies for the total synthesis of complex natural products that contain two or more contiguous stereogenic quaternary carbon atoms in their intricate structures are reviewed with 12 representative examples. Emphasis has been put on methods to create quaternary carbon stereocenters, including syntheses of the same natural product by different groups, thereby showcasing the diversity of thought and individual creativity. A compendium of selected natural products containing two or more contiguous stereogenic quaternary carbon atoms and key reactions in their total or partial syntheses is provided in the Supporting Information.

Alkaloids from the leaves of Daphniphyllum subverticillatum

Four new alkaloids, 17-hydroxydaphnigraciline (1), subdaphnidine A (2), daphnezomine L methyl ester (3), and 11-hydroxycodaphniphylline (4), along with 24 known analogues, were isolated from the leaves of Daphniphyllum subverticillatum. The structures of 1-4 were elucidated on the basis of spectroscopic methods and from chemical evidence. Daphnilongeridine (5) showed cytotoxicity against several tumor cell lines at IC(50) values in the range 2.4-9.7 microM and against the HMEC human microvascular endothelial cell line with an IC(50) of 2.7 microM.

Dapholdhamines A-D, alkaloids from Daphniphyllum oldhami

Four new Daphniphyllum alkaloids, dapholdhamines A-D (1-4), were isolated from the leaves of Daphniphyllum oldhami. The structures and relative configurations of 1-4 were elucidated on the basis of spectroscopic data.

Two new Daphniphyllum alkaloids from Daphniphyllum macropodum Miq

Two new Daphniphyllum alkaloids, 4,21-deacetyl-deoxyyuzurimine (1) and macropodumine L (2), together with the two known related alkaloids, have been isolated from the bark of Daphniphyllum macropodum Miq. The structures of the new compounds were elucidated on the basis of the detailed analysis of spectroscopic data, chemical method, and by comparison of the spectroscopic data with those of known compounds. Compounds 2 and 4 exhibited weak cytotoxicity against human carcinoma cell lines SMMC-7721 and HO-8910.

Alkaloids from the twigs of Daphniphyllum calycinum

Twelve new alkaloids, caldaphnidines G-R (1-12), along with 24 known ones, were isolated from the twigs of Daphniphyllum calycinum. Their structures were elucidated by spectroscopic methods, especially two-dimensional NMR techniques.

Daphniphyllum and diterpenoid alkaloids from Daphniphyllum longeracemosum

Four new Daphniphyllum alkaloids, daphlongamines A-D (1-4), were isolated from the fruits of Daphniphyllum longeracemosum. Their structures were characterized by spectroscopic methods, especially 2D NMR techniques. A single-crystal X-ray diffraction analysis was used to confirm the stereochemistry of 3. Remarkably, this is the first report of aconitine- and veatchine-type diterpenoid alkaloids from the genus Daphniphyllum.

Alkaloids from Daphniphyllum oldhami

Four new Daphniphyllum alkaloids, daphnioldhanines H-K ( 1- 4), along with 34 known alkaloids, were isolated from Daphniphyllum oldhami. The known alkaloid dehydrodaphnigraciline ( 5) is now reported as a natural product. Their structures were elucidated by spectroscopic methods, especially 2D NMR techniques. The effects against platelet aggregation of compounds 1, 3, and 5 were evaluated, and 3 showed stronger activity against platelet aggregation induced by PAF. This is the first report of quinolizidine alkaloids from the genus Daphniphyllum.

A Novel Alkaloid from the Seeds of Daphniphyllum calycinum

One novel alkaloid, named as caldaphnidine G (1), and two known alkaloid calycicine A (2), daphnezomine L (3) were isolated from the seeds of Daphniphyllum calycinum. The structure of 1 was established by spectral methods, especially 2D NMR techniques.

ASYMMETRIC SYNTHESIS OF THE ABCD RING SYSTEM OF DAPHNILACTONE B VIA A TANDEM, DOUBLE INTRAMOLECULAR, [4+2]/[3+2] CYCLOADDITION STRATEGY

An asymmetric synthesis of the ABCD ring system of daphnilactone B is described. The synthesis features a tandem, double intramolecular, [4+2]/[3+2] cycloaddition of a highly functionalized, enantiomerically enriched nitroalkene to generate a pentacyclic nitroso acetal. The cycloaddition establishes six contiguous stereogenic centers including the critical CD ring junction that bears two quaternary stereogenic centers. Hydrogenolysis of the nitroso acetal followed by amide reduction and cyclization provided the AB rings. The methyl substituent on the A ring was installed in the correct configuration via hydrogenation of an exocyclic olefin in the final step.

Bisphosphine-catalyzed mixed double-Michael reactions: asymmetric synthesis of oxazolidines, thiazolidines, and pyrrolidines

Bisphosphine-catalyzed mixed double-Michael reactions have been developed to afford β-amino carbonyl derivatives of oxazolidines, thiozolidines, and pyrrolidines in excellent yields and with high diastereoselectivities. Efficient reactions between amino acid-derived pronucleophiles, e.g., β-amino alcohols,β-amino thiols, and γ-amino diesters, as Michael donors and electron-deficient acetylenes, e.g., propiolates, acetylacetylene, and tosylacetylene, as Michael acceptors provided access to azolidines containing both diversity of substituents and asymmetry. This methodology—the first examples of mixed double-Michael reactions of acetylenes—is operationally simple and involves mild conditions. Mechanistically, it constitutes a rare example of the anchimeric assistance of bisphosphines in organocatalysis.

Daphnipaxianines A-D, alkaloids from Daphniphyllum paxianum

Four new Daphniphyllum alkaloids, daphnipaxianines A-D ( 1- 4), along with six known ones, have been isolated from the leaves and fruits of Daphniphyllum paxianum. Daphnipaxianines A and B ( 1, 2), a pair of epimers differing at C-10, are the first caliciphylline A type Daphniphyllum alkaloids with a Delta (9(15))-unsaturated cyclic ketone unit, and daphnipaxianine D ( 4) is the first yuzurine-type Daphniphyllum alkaloid containing a hexacyclic ring system. The structures of these alkaloids were characterized by spectroscopic methods, especially 2D NMR techniques. A single-crystal X-ray diffraction analysis was used to confirm the structure of 1.

Pordamacrines A and B, alkaloids from Daphniphyllum macropodum

The new Daphniphyllum alkaloids, pordamacrines A (1) and B (2), have been isolated from the leaves of Daphniphyllum macropodum, and their structures were elucidated on the basis of interpretation of spectroscopic data and by the single-crystal X-ray diffraction analysis of 2. Pordamacrines A (1) and B (2) exhibited moderate vasorelaxant effects on the rat aorta.

Daphlongeranines A and B, two novel alkaloids possessing unprecedented skeletons from Daphniphyllum longeracemosum

Two novel alkaloids, daphlongeranines A (1) and B (2), with an unprecedented ring system, were isolated from the fruits of Daphniphyllum longeracemosum. Their unique structures and relative stereochemistries were established on the basis of spectroscopic and single-crystal diffraction analysis. The proposed biosynthetic pathway was also discussed. Compounds 1 and 2 showed weak inhibition on platelet aggregation.

Isolation, X-ray Crystallography, and Computational Studies of Calydaphninone, a New Alkaloid from Daphniphyllum calycillum

[structure: see text]. A new hexacyclic alkaloid, calydaphninone (1), containing a 4-azatricyclo[5.2.2.0(1,4)]undecan ring system, has been isolated from the leaves and twigs of Daphniphyllum calycillum (Daphniphyllaceae). Its structure was elucidated by spectral and crystallographic means. Furthermore, conformations of calydaphninone (1) in solution are discussed on the basis of NMR spectral analysis and computational (B3LYP/6-31G*) results.

Yuzurimine-type alkaloids from Daphniphyllum yunnanense

Five new yuzurimine-type alkaloids, yunnandaphnines A-E (2-6), together with two known analogues, macrodaphniphyllamine (1) and calycinine A (7), have been isolated from the leaves and twigs of Daphniphyllum yunnanense. The structures of the new alkaloids were elucidated by spectroscopic methods. Yunnandaphnine E (6) is a novel heptacyclic yuzurimine-type alkaloid with an oxazine ring.

Alkaloids from the leaves of Daphniphyllum longeracemosum

Two new C-21 Daphniphyllum alkaloids, longeracinphyllins A (1) and B (2), have been isolated from the leaves of Daphniphyllum longeracemosum. Longeracinphyllin A (1) is a new daphnilongeranin B type alkaloid with a rearranged alpha,beta-unsaturated ketone group, and the structure is supported by X-ray crystal data. Longeracinphyllin B (2) is a new seco-10,17-daphnilongeranin B type alkaloid.

Macropodumines A–C: Novel Pentacyclic Alkaloids with an Unusual Skeleton or Zwitterion Moiety fromDaphniphyllum macropodum Miq

Three novel alkaloids, macropodumines A-C (1-3), were isolated from the stem of Daphniphyllum macropodum Miq. Interestingly, the structure of macropodumine A (1) was characterized as having a fused pentacyclic system including an unusual eleven-membered macrolactone ring, whereas macropodumine B (2) contains a rare cyclopentadienyl carbanion, which is stabilized as a zwitterion by an internal iminium cation. The structures of these new metabolites were established on the basis of their detailed spectroscopic analysis. In particular, the unique structure of zwitterion 2 was further confirmed by using single-crystal X-ray diffraction analysis.

Daphmanidins E and F, alkaloids from Daphniphyllum teijsmannii

Two new Daphniphyllum alkaloids, daphmanidins E (1) and F (2), have been isolated from the leaves of Daphniphyllum teijsmannii, and the structures were elucidated on the basis of spectroscopic data. Daphmanidins E and F showed a moderate vasorelaxant effect on rat aorta.

Alkaloids from Daphniphyllum longeracemosum

Four new Daphniphyllum alkaloids, daphnilongeranins A-D (1-4), along with four known ones, daphniphylline, longistylumphylline A, deoxyisocalyciphylline B, and daphnicyclidin D, were isolated from the leaves and stems of Daphniphyllum longeracemosum. Daphnilongeranin A (1) is the first seco-10,17-longistylumphylline A type Daphniphyllum alkaloid, and daphnilongeranin B (2) is a new C-22 nor-Daphniphyllum alkaloid based on a new C21 skeleton. Their structures and stereochemistry were determined using spectroscopic data and chemical methods.

Daphmanidins C and D, Novel Pentacyclic Alkaloids fromDaphniphyllumteijsmanii

[structure: see text] Two novel alkaloids with an unprecedented fused-pentacyclic skeleton, daphmanidins C (1) and D (2), have been isolated from the leaves of Daphniphyllum teijsmanii, and the structures were elucidated on the basis of spectroscopic data. The relative stereochemistry of 1 and 2 was assigned by combination of NOESY correlations and a simulation analysis. Daphmanidin C (1) elevated activity of NGF biosynthesis.

New alkaloids from Daphniphyllum calycinum

Four new alkaloids, 17-hydroxyhomodaphniphyllic acid (1), daphcalycinosidine C (2, a new iridoid alkaloid), yuzurimine E (3), and yuzurimic acid B (4), were isolated from the seeds of Daphniphyllum calycinum. The structures of these Daphniphyllum alkaloids were determined by spectroscopic analysis including mass spectrometry and 2D NMR.