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Six new cassaine diterpenoids (1, 3–7), along with three known ones (2, 8–9) were isolated from the seeds of Erythrophleum fordii. Their structures were elucidated by extensive spectroscopic methods and acid hydrolysis. Compound 2 was tested to be the most potent one and showed more sensitive activities on MCF-7 and A549 cancer cells with IC50 values of 3.66 ± 1.20 and 2.87 ± 0.46 μM, respectively. Furthermore, compound 2 reduced the number of cell colonies significantly in a dose-dependent manner in the colony formation assay and triggered apoptosis of MCF-7 cell.

The radical scavenging capacities of four new cassaine diterpenoid amides including 3β-hydroxydinorerythrosuamide (1), 3β-acetoxydinorerythrosuamide (2), 3β-tigloyloxydinorerythrosuamide (3), and 6α-hydroxydinorcassamide (4) present in leaf extract and four new cassaine diterpenoid amines namely erythroformine A (5), erythroformine B (6), 6α-hydroxy-nor-cassamine (7), and nor-erythrosuamine (8) recently identified in the extract of the bark of Erythrophleum fordii were elucidated using density functional theory (DFT) method. Different thermochemical properties characterizing antioxidant potential including bond dissociation enthalpy (BDE), proton affinity (PA), and adiabatic ionization potential (IP) were calculated at the B3LYP/6-311G(d,p) level of theory. Scavenging reaction mechanisms of cassaine diterpenes toward HOO

Two new cassaine-type diterpenoids, namely erythrofordins D (1) and E (2), sourced from a Cameroon collection of Erythrophleum suaveolens were isolated and assessed for anti-tumor activity. In the NCI-60 cancer cell assay, erythrofordins D (1) and E (2) were found to be cytotoxic in the low micro molar ranges with a mean GI50 value of 2.45 and 0.71 µM, mean TGI value of 9.77 and 2.29 µM, and a mean LC50 of 26.92 and 11.48 µM for 1 and 2 respectively. Using the COMPARE algorithm, the new compounds were found to have similar NCI-60 response profiles to the known cardiac glycosides hyrcanoside and strophanthin. In addition, in an assay examining the viability and contractile function in human cardiomyocytes derived from induced pluripotent stem-cells, erythrofordins showed cardiotoxicity effects at concentrations as low as 0.03 µg/mL.

Although the Na+-K+-ATPase inhibitor (+)-cassaine 4 was isolated from the bark of Erythrophleum guineense in 1935, the structure was not established until 1959. Intriguing features of 4 include the unsaturated amide and the axial secondary methyl group, both pendant to the C ring. Pierre Deslongchamps, now at Université Laval, envisioned (Org. Lett. 2013, 15, 6270) that the relative stereochemistry of the second­ary methyl could be established kinetically by intramolecular Michael addition of the enolate formed by the addition of the anion of 2 to the enone 1 to give 3. The sulfoxide 2 was readily prepared by the addition (Tetrahedron Lett. 1990, 31, 3969) of the anion derived from methyl phenyl sulfoxide to methyl crotonate. The enone 1 was prepared from commercial dihydrocarvone 5. Robinson annula­tion with ethyl vinyl ketone 6 (Tetrahedron 2000, 56, 3409) led to 7, that was reduc­tively methylated, reduced further, and protected to give 8. Oxidative cleavage of the pendant isopropenyl group followed by Baeyer–Villiger oxidation, hydrolysis, and further oxidation gave the ketone 9, that was methoxycarbonylated, then oxidized further to 1. The addition of the anion derived from 2 to 1 presumably gave initially the axial adduct. Subsequent intramolecular Michael addition then proceeded selectively to one face of the residual enone to give, after elimination of the sulfoxide, the enone 3. The anionic cascade annulation that formed the C ring having been accomplished, the ester of 3 was removed by exposure to ethoxide to give 10, having the alkene con­jugated with the B-ring ketone. Selective reduction followed by protection gave 11. In the course of the hydrogenolytic deprotection of the A-ring alcohol, selective hydrogenation of the tetrasubstituted alkene was also observed. Increasing the H2 pressure and extending the reaction time gave complete conversion to the desired 12, the rela­tive configuration of which was established by X-ray crystallography. A series of protection, reduction, and oxidation steps led to the C-ring ketone, that was methoxycarbonylated to give 14. Reduction followed by dehydration gave the unsaturated ester, that was reduced to the saturated ester with Mg in methanol. Reduction followed by oxidation then delivered the aldehyde 15. After some investi­gation, it was found that the aldehyde could be converted to the desired enol triflate by exposure to KHMDS and the Comins reagent.

A full account of our anionic polycyclization approach to access highly functionalized tricycles related to quassinoids and terpenoids from several optically active bicyclic enone systems and Nazarov reagents is presented. (+)-Carvone is the only chiral source used to fix the entire stereochemistry of all of the tricycles, and the stereochemical outcome of this process was unambiguously determined by X-ray crystallographic analysis. The utility of this strategy was demonstrated by the stereocontrolled construction of advanced tricycles related to the highly potent anticancer natural product bruceantin, a member of quassinoid family, and the total synthesis of the cardioactive terpenoid (+)-cassaine, a nonsteroidal inhibitor of Na(+)-K(+)-ATPase.

A stereoselective total synthesis of (+)-cassaine (1) via an anionic polycyclization methodology is described. Commercially available (+)-carvone (5), the only chiral source, was used to fix the entire stereochemistry of the natural product. Anionic polycyclization of a new substituted 2-(methoxycarbonyl)cyclohex-2-en-1-one (4) with known 1-phenylysulfinyl-3-penten-2-one (3) provided the versatile tricycle (2) with requisite stereochemistry. A sequence of functional group manipulations of tricycle (2) furnished the natural product 1.

Activity-guided fractionation of Erythrophleum succirubrum for TRAIL resistance-overcoming activity led to the isolation of four new cassaine diterpenoid dimers, named erythrophlesins A–D ( 1 – 4 ). Their structures were elucidated by spectral data to show that they have an unsymmetrical dimeric structure through an ester bond between two cassaine diterpenoids. These new compounds were revealed to have a significant reversal effect on TRAIL resistance in human gastric adenocarcinoma (AGS) cells.

A stereoselective total synthesis of (+)-cassaine (1) via an anionic polycyclization methodology is described. Commercially available (+)-carvone (5), the only chiral source, was used to fix the entire stereochemistry of the natural product. Anionic polycyclization of a new substituted 2-(methoxycarbonyl)cyclohex-2-en-1-one (4) with known 1-phenylysulfinyl-3-penten-2-one (3) provided the versatile tricycle (2) with requisite stereochemistry. A sequence of functional group manipulations of tricycle (2) furnished the natural product 1.

Two new compounds, erythroformide (I) and 3β-acetoxynorcassamide (II), are isolated together with four known compounds and their effects on endothelial tube formation on matrigel are investigated.

The transannular Diels-Alder reaction of trans-trans-trans macrocyclic triene A, bearing two cis substiuents in C(12) and C(13) as well as a gem-dimethyl in C(4), was studied. Under thermal conditions, only the desired trans-anti-cis tricycle B was obtained. This tricycle represents an advanced intermediate toward the total synthesis of cassaine C.

José Barluenga of the Universidad de Oviedo described (Organic Lett. 2008, 10, 4469) a powerful route from lithiated arenes such as 1 to the benzocyclobutane 3, the immediate precursor to the powerful o-quinone methide Diels-Alder diene. Michael E. Jung of UCLA developed (Organic Lett. 2008, 10, 3647) a triflimide catalyst for the inverse electron demand coupling of the highly substituted diene 4 with the enol ether 5 to give 6 with high diastereocontrol. Joseph M. Fox of the University of Delaware showed (J. Org. Chem. 2008, 73, 4283) that the cyclopropene carboxylate 8 was a powerful and selective dienophile. Richard P. Hsung and Kevin P. Cole of the University of Wisconsin finally (Adv. Synth. Cat. 2008, 350, 2885) reduced to practice the long-sought enantioselective Diels-Alder cycloaddition of a trisubstituted aldehyde, 11. Li Deng of Brandeis University devised (J. Am. Chem. Soc. 2008, 130, 2422) a Cinchona -derived catalyst for Diels-Alder cycloaddition to the diene 13 with high ee. Miguel Á. Sierra of the Universidad Complutense, Madrid, and Alejandra G. Suárez of the Universidad Nacional de Rosario described (Organic Lett. 2008, 10, 3389) a clever switchable chiral auxiliary 16 that favored diastereomer S-18 on thermal addition, but R-18 with EtAlCl2. New approaches to the intramolecular Diels-Alder reaction continue to be introduced. Mathias Christmann, now at the TU Dortmund, showed (Angew. Chem. Int. Ed. 2008, 47, 1450) that a secondary amine organocatalyst converted the prochiral dialdehyde 19 into the bicyclic diene 20 with high de and ee. Martin G. Banwell of the Australian National University prepared (Organic Lett. 2008, 10, 4465) the triene 21 in high ee by microbiological oxidation of iodobenzene. On warming, 21 was converted smoothly into 22, which was carried on in a formal synthesis of platencin. Jun-ichi Matsuo of Kanazawa University was able (Organic Lett. 2008, 10, 4049) to induce (neat, 180 °C) the intermolecular Diels-Alder cycloaddition of 23 with 24, delivering the cycloadduct 25 with 11:1 diastereocontrol.

Phytochemical investigation of the EtOH extract of the leaves of Erythrophleum fordii led to the isolation of two oleanane-type triterpene saponins (1-2) and five cassaine-type diterpenoids (4-8) along with one known methyl 3 β-hydroxy-erythrosuamate (3). Their structures were established by extensive NMR, as well as ESI-MS analyses and acid hydrolysis. Biological evaluation of compounds 3- 8 against five human cancer cell lines revealed that compounds 5-7 exhibited potent cytotoxic activity with IC₅₀ values ranging from 1.51 to 8.68 µM.

A new three-dimensional model for the relative binding mode of cassaine 1 and digitoxigenin 2 at the digitalis receptor site is proposed on the basis of the structural and conformational similarities among 1, 2 and its 14,15-seco analogues 3 and 4. Accordingly, the speculation that also 17alpha-substituted derivatives of the digitalis 5beta,14beta-androstane skeleton could efficiently bind to the Na+,K+-ATPase receptor is put forward and verified through the synthesis of some related compounds. The binding affinity shown by 2-(N,N-dimethylamino)ethyl 3beta, 14-dihydroxy-5beta,14beta-androstane-17alpha-acrylate 6 (IC50 = 5.89 microM) and, much more significantly, by the corresponding 14, 15-seco-14-oxo derivative 9 (IC50 = 0.12 microM) substantiates the new hypothesis and opens new prospects to the design of novel inhibitors of Na+,K+-ATPase as potential positive inotropic compounds.

The transannular Diels−Alder reaction of trans-trans-trans macrocyclic triene A, bearing two cis substiuents in C12 and C13 as well as a gem-dimethyl in C4, was studied. Under thermal conditions, only the desired trans-anti-cis tricycle B was obtained. This tricycle represents an advanced intermediate toward the total synthesis of cassaine C.

Cassaidine has been converted into cassaine by way of bisdehydro-cassaidine. Both cassaine and bisdehydro-cassaidine are reduced to cassaidine by sodium borohydride. The ring A hydroxyl group of cassaic acid can therefore be assigned 3β equatorial configuration.

For the purpose of comparison, 3β- and 7β-hydroxycassan-19-oic acids (V a and VIa, respectively) have been prepared. The two triols XI and XII, derived from cassaine, are also described.

Cassaidin-7α-T und Cassaidinsaure-7α- T (spezifische Aktivitat jeweils 20 mC/mMol) werden dargestellt durch Reduktion der Ketogruppe an C-7 von Cassain mit Natriumborhydrid-T (spezifische Aktivitat 80 mC/mMol), gegebenenfalls gefolgt von saurer Hydrolyse des Alkaloids. Die Reduktion der teilweise gehinderten Ketogruppe liefert ausschlieslich den aquatorialen Alkohol. Wie gezeigt wird, geht die unerwartete Abwesenheit des axialen Alkohols auf den sterischen Ausschlus seiner Bildung durch die benachbarte cis-standige Methylgruppe an C-14 zuruck. Die markierten Verbindungen sind geeignet zur Erforschung des Vorkommens von primaren Redoxreaktionen an C-7 im tierischen Organismus und, wenn dies ausgeschlossen ist, zur Abklarung des allgemeinen Stoffwechsel-Schicksals der Erythrophleum-Alkaloide. Cassaidine-7α-T and cassaidic acid-7α-T (specific activities 20 mC/mMole) are prepared by reduction of the C-7 keto group of cassaine with sodium borohydride-T (specific activity 80 mC/ mMole) eventually followed by acid hydrolysis of the alkaloid. The reduction of the partially hindered keto group yields exclusively the equatorial alcohol. The unexpected absence of the axial alcohol is shown to be due to steric exclusion of its formation by the neighboured cis-positioned methyl group at C-14. The labelled compounds are suited for the exploration of the occurrence of primary oxidation-reduction reactions at C-7 in the animal metabolism and, if this is excluded, for the clarification of the general metabolic fate of of Erythrophleum alkaloids.

The structures of four synthetic isomeric analogues of cassaine were confirmed by detailed interpretation of their NMR. spectra.

SummaryThe erythrophleum alkaloids, cassaine and coumingine, affect active transport of potassium by human erythrocytes as do the cardiac glycosides. At low concentrations both groups of compounds slightly increase K transfer from plasma to cells: higher concentrations inhibit the process. These effects probably reflect activation and inhibition of a membrane ATPase which others have implicated in the active movements of cations in erythrocytes. Erythrophleum alkaloids and cardiac glycosides also share similar effects on mammalian hearts. In view of the structural differences between the 2 classes of compounds, and their biological similarities in 2 kinds of tissue, it is suggested that previous views on minimal structural requirements for digitalis-like activity in hearts and erythrocytes be revised.Cassaine sulfate was supplied by Dr. Robert Dowben from stock originally obtained from Prof. Ruzicka. Coumingine was donated by Dr. A. C. Keyl. We would like to thank both for their generosity. It is a pleasu...

Publisher Summary This chapter discusses the chemical studies of Erythrophleum alkaloids. Research on the isolation and characterization of new alkaloids from the various Erythrophleum species has been hampered by the shortage of plant material, in part because of the political unrest in the regions where many of the species are found. The alkaloids are in general N-alkylaminoetyhl esters of diterpene acids containing a perhydrophenanthrene skeleton. It is remarkable that compounds of this general structure should have the same type of pharmacological activity as the non-nitrogenous digitalis group of steroid glycosides. The diterpene portion of the Erythrophleum alkaloids is devoid of this characteristic biological activity. Like digitalis compounds, the Erythrophleum alkaloids show a high degree of toxicity. Cassaine, the alkaloid available in greatest quantity, has been the one most studied chemically.

In addition to cassaine, two new alkaloids have been isolated from the bark ofErythrophleum guineenseDon, in crystalline form. Their empirical formulae have been established by analysis of the free bases and of some of their crystalline salts.