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Androgen receptor (AR) is a viable therapeutic target for lethal castration-resistant prostate cancer (CRPC), because the continued progression of CRPC is mainly driven by the reactivation of AR transcriptional activity. The current FDA-approved AR antagonists binding to ligand binding domain (LBD) become ineffective in CRPC with AR gene amplification, LBD mutation, and the evolution of LBD-truncated AR splice variants. Encouraged by the fact that tricyclic aromatic diterpenoid QW07 has recently been established as a potential N -terminal AR antagonist, this study aims to explore the structure-activity relationship of tricyclic diterpenoids and their potential to suppress AR-positive cell proliferation. Dehydroabietylamine, abietic acid, dehydroabietic acid, and their derivatives were selected, since they have a similar core structure as QW07. Twenty diterpenoids were prepared for the evaluation of their antiproliferative potency on AR-positive prostate cancer cell models (LNCaP and 22Rv1) using AR-null cell models (PC-3 and DU145) as comparisons. Our data indicated that six tricyclic diterpenoids possess greater potency than enzalutamide (FDA-approved AR antagonist) towards LNCaP and 22Rv1 AR-positive cells, and four diterpenoids are more potent than enzalutamide against 22Rv1 AR-positive cells. The optimal derivative possesses greater potency (IC 50 = 0.27 µM) and selectivity than QW07 towards AR-positive 22Rv1 cells.

The cardiac effect of thirty-eight diterpenoid alkaloids was evaluated on the isolated bullfrog heart model. Among them, twelve compounds exhibited appreciable cardiac activity, with compounds 3 and 35 being more active than the reference drug lanatoside. The structure-cardiac activity relationships of the diterpenoid alkaloids were summarized based on our present and previous studies [2]: i) 1α-OMe or 1α-OH, 8-OH, 14-OH, and NH (or NMe) are key structural features important for the cardiac effect of the aconitine-type C19-diterpenoid alkaloids without any esters. C18-diterpenoid alkaloids, lycoctonine-type C19-diterpenoid alkaloids, and the veatchine- and denudatine-type C20-diterpenoid alkaloids did not show any cardiac activity; ii) the presence of 3α-OH is beneficial to the cardiac activity; iii) the effect on the cardiac action of 6α-OMe, 13-OH, 15α-OH, and 16-demethoxy or a double bond between C-15 and C-16 depends on the substituent pattern on the nitrogen atom.

A new and effective approach toward epoxide opening of a 7,17-seco-7,8-epoxy-C19-diterpenoid alkaloid is herein described. The starting epoxide was prepared from naturally occurring yunnaconitine via a nine-step transformation. Treatment of this epoxide with trifluoroacetic anhydride in dioxane at 110 degrees C followed by reduction with sodium boron hydride generated two epoxide opening compounds 7 and 8. Each of their structures is characteristic of a Δ8,15 bridgehead double bond and a 7β-oxygen-substituted group.

On the basis of intensive interpretation of the 1H NMR spectroscopic data, the ring A conformation of aconine (1) was speculated as twist boat in CDCl3, and as chair or twist boat in acetone-d6 and pyridine-d5. The ring A of pseudaconine (2) adopts the chair conformation in CDCl3, acetone-d6, and pyridine-ds. Accordingly, the boat conformation of ring A in these two diterpenoid alkaloids in CDCl3 reported in the literature [1] should be revised. The difference in 13C NMR data for the same compound (1 or 2) in two different solvents (CDCl3, pyridine-d5) can be attributed to solvent effects.

From the whole herb of Delphinium laxicymosum var pilostachyum, three new C20-diterpenoid alkaloids, laxipilostine, laxipilosdine, and laxipilosline (1-3), were isolated, together with fourteen known diterpenoid alkaloids. Their structures were determined by extensively interpretation of their spectroscopic data (1D- and 2D-NMR, MS, and IR).

Thirty three C19-diterpenoid alkaloids, twenty-two prepared from known C19-diterpenoid alkaloids and eleven isolated from Aconitum and Delphinium spp. were evaluated for their cardiac activity in the isolated bullfrog heart assay. Among them, eleven compounds exhibited cardiac activity, with average rate of amplitude increase in the range of 16-118%. Compound 7, mesaconine (17), hypaconine (25), and beiwutinine (26) exhibited strong cardiac activities relative to the reference drug. The structure-activity relationship data acquired indicated that an alpha-hydroxyl group at C-15, a hydroxyl group at C-8, an alpha-methoxyl or hydroxyl group at C-1, and a secondary amine or N-methyl group in ring A are important structure features necessary for the cardiac activities of the aconitine-type C19-diterpenoid alkaloids without any ester groups. In addition, an alpha-hydroxyl group at C-3 is also helpful for the cardiac activity of these alkaloids.

A new conversional synthesis of the ABC ring system of taxoids from the C19-diterpenoid alkaloid lycoctonine was developed in 6 steps with 2% overall yield. The distinctive features of the conversion include pinacol rearrangement, enlargement of ring B, and opening of a four-membered ring.

Treatment of a 7,17-seco-type C19-diterpenoid alkaloid (3), prepared from deltaline (8), with triethylamine in either DMF or TEG (triethylene glycol) at 120 degrees C provided two interesting compounds 6 and 7. The structures of compounds 3, 6, and 7 were established based on extensive interpretations of their 1D and 2D NMR data. Compound 6, a lycoctonine-type C19-diterpenoid alkaloid, can be transformed from alkaloid 3 via Grob fragmentation, Prins reaction, and intramolecular disproportionation. The mechanism of the formation of compound 6 was confirmed by deuteration experiments. Product 7 was formed through a pinacol-like rearrangement of alkaloid 3.

A convergent and efficient formal synthesis of (±)-atisine has been accomplished. The synthetic strategy is to efficiently construct the bicyclo[2.2.2]octane ring moiety by an oxidative dearomatization/intramolecular Diels-Alder cycloaddition cascade. The first total synthesis of another atisine-type C(20)-diterpenoid alkaloid, (±)-isoazitine, has also been achieved employing the same strategy.

Bioassay-guided fractionation of an n-BuOH extract of the lateral roots of Aconitum carmichaeli. led to the isolation of 5 cardioactive C(19)-diterpenoid alkaloids: N-deethylaconine (1), beiwutinine (2), hypaconine (3), mesaconine (4), and 15α-hydroxyneoline (5). N-Deethylaconine and beiwutinine are new aconitine-type C(19)-diterpenoid alkaloids. Hypaconine was isolated from this species for the first time. Among them, mesaconine, hypaconine, and beiwutinine showed the strongest cardiac actions on the isolated perfused bullfrog heart. Furthermore, mesaconine has protective effects, including improved inotropic effect and left ventricular diastolic function, on myocardial ischemia-reperfusion injury in rat at a dose of 10(-9) mol/L. However, mesaconine has almost no effect on heart rate.

The O-acyl group at C-1 of two C₁₉-diterpenoid alkaloids 2 and 5 was transferred to the secondary amine nitrogen to form amides 3 and 6 in the basic condition. This kind of O-to-N intramolecular acyl migration could be caused by the near distance between the nucleophilic nitrogen atom and the carbonyl group of the ester at C-1 in the C₁₉-diterpenoid alkaloids, which is consistent with the conformation of rings A and E in the C₁₉-diterpenoid alkaloids.

The first biomimetic conversion from the aconitine-type C(19)-diterpenoid alkaloids to the corresponding alkaloids of lactone-type C(19)-diterpenoid alkaloid has been achieved. Chasmanine was used as starting material with Baeyer-Villiger oxidation as a key reaction. It was also observed that the oxygenated group at C-16 did not change the relative migration tendencies of C-13 and C-9 during the oxidation. Meantime, a novel D-ring fragmented compound was obtained during the course of the present investigation. The plausible mechanism of the formation of this compound was also proposed.

Four novel taxoid analogs were conversionally synthesized from the C(19)-diterpenoid alkaloid deltaline, and their cytotoxic activities were evaluated against a small panel of cancer cell lines.

The conversional synthesis of taxoids by the BAC sequence from the C(19)-diterpenoid alkaloids, 14-acetyltalatisamine (1), yunaconitine (12), and 14-acetylchasmanine (19), was designed and explored. Two aconane-type diterpenes 17 and 28, the advanced intermediates for our conversional synthesis, were synthesized. The key steps include the rupture of the C(7)-C(17) bond, the formation of imine, and the denitrogenation.

Further phytochemical investigation of the whole herb of Delphinium anthriscifolium var. savatieri resulted in the isolation of two new C20-diterpenoid alkaloids, anthriscifolmines I (1) and J (2). The structures of the two new alkaloids were elucidated on the basis of spectral data, including 2D NMR and HRESIMS.

The lasting attention that researchers have devoted to diterpenoid alkaloids is due to their various bioactivities and toxicities, structural complexity, and intriguing chemistry. From 1998 to the end of 2008, more than 300 new diterpenoid alkaloids were isolated from Nature. This review focuses on their structural relationships, and investigations into their chemical reactions, synthesis, and biological activities. A table that lists the names, plant sources, and structural types is given along with 363 references.

The diterpenoid alkaloids, with an intriguing chemistry and numerous varied bioactivities, constitute the largest and most complicated group of terpenoid alkaloids. Among them, the C19-diterpenoid alkaloids have attracted extensive and lasting attention from researchers. This chapter with 1075 references systematically summarizes the past 30 years of studies on the C19-diterpenoid alkaloids, covering the classification and distribution, biogenesis and biosynthesis, applications in the phytochemotaxonomy of Chinese Aconitum L., spectral properties (1H and 13C NMR, MS), X-ray crystallographic analysis, chemical reactions and chemical conversions (as a continuation of Volume 42 of this treatise), and pharmacological activity of this important group of alkaloids. This review on the C19-diterpenoid alkaloids, in combination with two previous reviews in this treatise (C20-diterpenoid alkaloids, Volume 59, and C18-diterpenoid alkaloids, Volume 67), will present readers with a complete and updated profile of the diterpenoid alkaloids. The only exception is the description of the total syntheses of the diterpenoid alkaloids, which will be summarized in a subsequent individual chapter.

For evaluation of C(18)- and C(19)-diterpenoid alkaloids as analgesics, three C(19)-diterpenoid alkaloids were isolated from the roots of Aconitum hemsleyanum var. circinatum and A. transsecutum; and twenty-five semisynthetic C(18)- or C(19)-diterpenoid alkaloids were prepared from lappaconitine, crassicauline A or yunaconitine. In a mice acetic acid-induced abdominal constriction assay, four crassicauline A analogs and three yunaconitine analogs exhibited good analgesic activities with 77.8-94.1% inhibition range in 0.1-10 mg/kg subcutaneous (s.c.) dose range at the point of 20 min after drug administration. Among them, 8-O-deacetyl-8-O-ethylcrassicauline A (ED(50)=0.0972 mg/kg) and 8-O-ethylyunaconitine (ED(50)=0.0591 mg/kg) were the most potent analgesics relative to the reference drugs lappaconitine (ED(50)=3.50 mg/kg) and crassicauline A (ED(50)=0.0480 mg/kg). Analgesic activity data of these C(18)- and C(19)-diterpenoid alkaloids indicate that a tertiary amine in ring A, an acetoxyl or an ethoxyl group at C-8, an aromatic ester at C-14, and the saturation state of the ring D are important structural features necessary to the analgesic activity of the C(19)-diterpenoid alkaloids.

Two new, rare heteratisine-hetidine-type bisditerpenoid alkaloids designated as trichocarpines A 1 and B 2, together with twelve known compounds have been isolated from the whole plants of Aconitum tanguticum var. trichocarpum. Their structures were elucidated by spectroscopic data interpretation and chemical transformation.

A new and efficient approach toward the conversion of C(19)-diterpenoid alkaloids into diterpenes with [6+8+5+6] ring system is reported. Treatment of imines 5, 14, and 24 derived from the C(19)-diterpenoid alkaloids with NaNO(2)-NaOAc-HOAc afforded a series of novel rearrangement diterpenes 6-8, 15-19, and 25-27, respectively. The lactone 11 was obtained in 41% yield by treating 5 with NaNO(2)-HBr-Br(2). The formation of diazonium intermediate is postulated, which was subsequently subjected to skeletal rearrangement, leading to the enlargement of B ring. All the new compounds were isolated and fully characterized.

Three new C(19)-diterpenoid alkaloids, anthriscifoldines A-C (1-3), and two new C(18)-diterpenoid alkaloids, anthriscifolcines F and G (5, 6), were obtained from the whole herbs of Delphinium anthriscifolium var. savatieri during our further investigation on a larger scale of recollected plants. Their structures were elucidated based on the interpretation of NMR and high-resolution ESI-MS data, and chemical transformation. In addition, five known C(19)-diterpenoid alkaloids were also isolated and identified as nudicaulamine (4), delbruninol (7), blacknine (8), winkleriline (9), and deltaline (10).

From the whole herbs of Delphinium majus, three new C(19)-diterpenoid alkaloids, majusines A-C (1-3), and six new C(20)-diterpenoid alkaloids, majusimines A-D (4-7) and majusidine A and B (8 and 9), have been isolated, together with 15 known compounds. The structures of compounds 1-9 were elucidated by spectroscopic data interpretation.

In the title study, the synthesis of the vital intermediate C-nor-aconanone (3) from 4 was completed through 11 steps, mainly including semipinacol rearrangement, formation of the imines by the treatment of 10 or 20 with NBS, the cleavage of N-C19 bonds in 11 or 21 by treatment with m-CPBA and subsequently with LTA, as well as the rupture of the N-C17 bonds in 22 by a modified Nef reaction (NaH/t-BuOH --> KMnO4/H2O). One-pot procedure was successfully developed starting from 11 or 21 to afford the N,19-seco-C-nor product 15 or 22, respectively, in reasonably good yields.

Treatment of compounds 2 and 5, both from the C19-diterpenoid alkaloid deltaline 1, with 5% NaOH methanol (room temperature, 16 h) and NaOH-N,N-dimethylformamide (reflux, 10 h) led to the two novel skeletal rearrangement products 4 (90%) and 6 (62%), respectively, involving Wagner-Meerwein rearrangement and Grob fragmentation. Their structures were fully characteristized based on 2D NMR and HR-MS data.

Treatment of some C(19)-diterpenoid alkaloids (3, 6, 10 and 12) with anhydrous DMSO at 100-170 degrees C for 3-7 h led to the formation of the corresponding imines (4, 7/8, 11, 13/14) in 65-83% yield, respectively. This is a new simpler formation of the imines of the C(19)-diterpenoid alkaloids.

Seven new C19-diterpenoid alkaloids, circinasines A-G (1-7), together with six known compounds, talatisamine, yunaconitine, senbusine A, sachaconitine, hemsleyanisine, and isohemsleyanisine, were isolated from the roots of Aconitum hemsleyanum var. circinatum. The structures of 1-7 were determined by the interpretation of spectroscopic data and by the single-crystal X-ray crystallographic analysis of 6 and the acetonide derivative of 1. In addition, the structures of hemsleyanisine and isohemsleyanisine were revised from 8 and 9 to 10 and 11, respectively.

Three new C20-diterpenoid alkaloids, trifoliolasines D-F (1-3), were isolated from the aerial parts of Delphinium trifoliolatum, and their structures were determined by the interpretation of spectroscopic data and by the single-crystal X-ray crystallographic analysis of 1.

Two new C19-diterpenoid alkaloids, kongboentine A (1) and kongboentine B (3), were isolated from the roots of Aconitum kongboense Lauener and their structures were elucidated by spectral data.

Further investigation of the roots of Delphinium giraldii DIELS led to the isolation of three new C(19)-diterpenoid alkaloids, giraldines G (1), H (2), and I (3). The structures of 1-3 were established based on spectroscopic evidence.

Three new norditerpenoid alkaloids, trifoliolasines A (1), B (3), and C (5), were isolated from the whole plant of Delphinium trifoliolatum FINET et GAGNEP, and their structures were established based on the spectral data.

A new franchetine-type (leueandine 1) and two new lycoctonine-type [potanisines F (3) and G (5)] C19-diterpenoid alkaloids have been isolated from the roots of Aconitum hemsleyanum var. leueanthus and Delphinium potaninii, respectively, and their structures were established on the basis of spectral data.

The new compound hemsleyatine (1) was isolated along with four known C(19)-diterpenoid alkaloids: indaconitine (4), yunaconitine (5), chasmanine (6), and talatisamine (7) from the roots of Aconitum hemsleyanum PRITZ. Structures were established by spectral analysis, including tow dimeusional (2D) NMR spectroscopy and a chemical method. Hemsleyatine (1) is the first C(19)-diterpenoid alkaloid bearing the 8-amino group. In addition, the assignments of some (13)C signals for pseudaconine (3) were revised by comparison with those of hemsleyatine (1).

Four new norditerpenoid alkaloids, leueantines A (1), B (2), C (3), and D (4), were isolated from the roots of Aconitum hemsleyanum var. leueanthus. The structures of 1-4 were established by spectroscopic evidence.

From the roots of Delphinium potaninii var. jiufengshanense, two new lycoctonine-type C(19)-diterpenoid alkaloids called jiufengdine (1) and jiufengtine (4) have been isolated. The structures of the new alkaloids (1, 4) were established by 1D and 2D NMR spectra.