Mbandakamine-Type Naphthylisoquinoline Dimers and Related

Article Cite This: J. Nat. Prod. 2018, 81, 918−933

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Mbandakamine-Type Naphthylisoquinoline Dimers and Related Alkaloids from the Central African Liana Ancistrocladus ealaensis with Antiparasitic and Antileukemic Activities Dieudonné Tshitenge Tshitenge,†,‡ Doris Feineis,† Virima Mudogo,§ Marcel Kaiser,⊥,∥ Reto Brun,⊥,∥ Ean-Jeong Seo,▽ Thomas Efferth,▽ and Gerhard Bringmann*,† †

Institute of Organic Chemistry, University of Würzburg, Am Hubland, D-97074 Würzburg, Germany Faculty of Pharmaceutical Sciences, University of Kinshasa, B.P. 212 Kinshasa XI, Democratic Republic of the Congo § Faculté des Sciences, Université de Kinshasa, B.P. 202, Kinshasa XI, Democratic Republic of the Congo ⊥ Swiss Tropical and Public Health Institute, Socinstrasse 57, CH-4002 Basel, Switzerland ∥ University of Basel, Petersplatz 1, CH-4003 Basel, Switzerland ▽ Institute of Pharmacy and Biochemistry, Department of Pharmaceutical Biology, University of Mainz, Staudinger Weg 5, D-55128 Mainz, Germany ‡

S Supporting Information *

ABSTRACT: Four new dimeric naphthylisoquinoline alkaloids, michellamine A5 (2) and mbandakamines C−E (4−6), were isolated from the Congolese plant Ancistrocladus ealaensis, along with the known dimer mbandakamine A (3). They represent constitutionally unsymmetric dimers, each consisting of two 5,8′-coupled naphthylisoquinoline monomers. While the molecular halves of michellamine A5 (2) are linked via C-6′ of both of the naphthalene moieties, i.e., via the least-hindered positions, so that the central biaryl axis is configurationally unstable and not an additional element of chirality, the mbandakamines 3−6 possess three consecutive stereogenic axes. Their monomeric units are linked through an unprecedented 6′,1″-coupling in the binaphthalene core, leading to a high steric load, since the central axis is located in one of the peri-positions, neighboring one of the outer axes. In addition, four new 5,8′-coupled monomeric naphthylisoquinolines, viz., ancistroealaines C−F (7−10), were identified, along with four “naphthalene-devoid” tetra- and dihydroisoquinolines, named ealaines A−D (11−14). The new mbandakamines C (4) and D (5) showed pronounced activities against the malaria parasite Plasmodium falciparum, and they were likewise found to display strong cytotoxic activities against human leukemia (CCRF-CEM) and multi-drug-resistant tumor cells (CEM/ADR5000). Ancistrocladus ealaensis J. Léonard1,2 has been recognized as its own species since its first botanical description in 1949. This woody liana is relatively widespread in lowland wet evergreen and swamp forests, on river banks, and along river islands in a quite large area in Central Africa, extending from the northwestern part of the Democratic Republic of the Congo (DR Congo) to the Central African Republic, and to the Republic of the Congo and Gabon in the West. Botanically, this palaeotropical plant belongs to the small monogeneric Ancistrocladaceae family,2 presently comprising 19 accepted species (13 from Africa and six from Asia).2,3 Phytochemically, Ancistrocladus lianas are characterized by the production of mono- and dimeric naphthylisoquinoline alkaloids,4,5 with their unique molecular architectures, which include stereogenic centers and chiral axes. Biosynthetically, they are the first polyketide-derived isoquinoline natural products, since not only the naphthalene parts but also the isoquinoline moieties originate from acetate−malonate units.6 Pharmacologically, © 2018 American Chemical Society and American Society of Pharmacognosy

several representatives have attracted attention due to their high activities against protozoan parasites that cause severe tropical diseases such as malaria, leishmaniasis, or trypanosomiasis.5,7−15 Some of the alkaloids have gained interest for their strong antiproliferative effects against human leukemia15−17 and multiple myeloma17 cell lines. Previous phytochemical investigations on the stems of A. ealaensis14 had revealed the presence of two 5,8′-linked naphthylisoquinoline alkaloids, along with three naphthoic acid derivatives related to the naphthalene portion of the alkaloids. More recent phytochemical studies on the leaves of A. ealaensis18 led to the discovery of unusual antiplasmodial heterodimers such as ealapasamine A (1) (Figure 1), consisting of two different biaryl moieties, viz., 5,8′- and 7,8′-coupled Received: December 11, 2017 Published: March 21, 2018 918

DOI: 10.1021/acs.jnatprod.7b01041 J. Nat. Prod. 2018, 81, 918−933

Journal of Natural Products

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Figure 1. Naphthylisoquinoline dimers of A. ealaensis: the recently identified ealapasamine A (1), a “hybrid” heterodimeric alkaloid composed of structurally divergent monomeric units with entirely different biaryl linkages, and five dimers comprising two 5,8′-coupled monomeric moieties. While the new michellamine A5 (2) possesses a freely rotating 6′,6″-linked central biaryl axis, mbandakamine A (3), already known from a related Congolese Ancistrocladus species, and the new mbandakamines C−E (4−6) show a highly unsymmetric 6′,1″-coupling in the central binaphthalene core.

Figure 2. Four new 5,8′-coupled monomeric naphthylisoquinoline alkaloids, ancistroealaines C−F (7−10), and four new related naphthalene-devoid tetra- and dihydroisoquinolines, ealaines A−D (11−14), discovered in A. ealaensis.

known parent compound mbandakamine A (3),21 are based on a highly unsymmetric 6′,1″-linkage in the binaphthalene core, leading to an extremely high steric load, so that, different from 1 and 2, the central biaryl axis of 3−6 constitutes an additional element of chirality. So far, a botanically yet undescribed Congolese Ancistrocladus species21,22 has been the only known source of such structurally and stereochemically fascinating mbandakamine-type naphthylisoquinolines.21,23 The new mbandakamines C (4) and D (5) showed remarkable activities against the malaria parasite Plasmodium falciparum. Mbandakamine A (3) and the related dimers 4 and 5 likewise displayed strong antiproliferative effects toward drug-sensitive acute

naphthylisoquinoline units, joined together via C-6′ of both of the naphthalene moieties. In this paper, we describe the isolation and structural elucidation of four new naphthylisoquinoline dimers (Figure 1), each consisting of two 5,8′-coupled monomeric units, from the leaves of A. ealaensis. Among these, michellamine A5 (2) has a freely rotating 6′,6″-linked central biaryl axis as in the case of 1. Michellamine-type quateraryls such as 2 have so far been found only in two Ancistrocladus species, namely, in the Cameroonian liana A. korupensis19 (six dimers) and in the Congolese plant A. congolensis20 (five dimers). The three further new dimeric naphthylisoquinoline alkaloids, mbandakamines C−E (4−6), which have now been isolated from A. ealaensis, along with the 919



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were subjected to preparative HPLC, permitting isolation of 18 metabolites, among them eight new naphthylisoquinoline alkaloids and four related compounds (Figures 1 and 2). Six of the compounds (Figure 3) were known from previous phytochemical investigations of other African and Asian Ancistrocladus species. Naphthylisoquinoline Alkaloids 15−19. One of the monomers was readily identified as the 5,8′-coupled alkaloid korupensamine A (15) (Figure 3), a known main constituent of the Cameroonian liana A. korupensis24 and the two Congolese species A. congolensis25 and A. likoko.26 It was isolated from the twig extracts of A. ealaensis, together with two further known 5,8′-coupled naphthylisoquinoline alkaloids (Figure 3), viz., ancistrolikokine B (16),26 which had previously been found in A. likoko, and ancistrotectoriline A (17), earlier detected in the Southeast Asian species A. tectorius27 and in the East African liana A. tanzaniensis.10 Korupensamine A (15)24−26 and ancistrolikokine B (16)26 are typical “mixed Ancistrocladaceae/Dioncophyllaceae-type” (i.e., so-called “hybrid-type”) naphthylisoquinolines with R-configuration at C-3 and an oxygen function at C-6. Ancistrotectoriline A (17),10,27 by contrast, is S-configured at C-3 and, in combination with its oxygen group at C-6, belongs to the subclass of Ancistrocladaceae-type alkaloids. Two further known Ancistrocladaceaetype metabolites were identified in the twigs of A. ealaensis, viz., the two N,C-coupled naphthyldihydroisoquinolinium salts ancistrocladinium A (18)12,13 and its bisphenolic analogue 1913 (Figure 3), earlier detected in a botanically undescribed Congolese Ancistrocladus species12 and in A. cochinchinensis13 from Vietnam. The isolation of hybrid-type alkaloids along with Ancistrocladaceae-type compounds from A. ealaensis is of special interest with respect to the chemotaxonomic relevance of naphthylisoquinoline alkaloids for the classification of the plants. Ancistrocladus species from Southeast Asia and East Africa typically produce Ancistrocladaceae-type alkaloids exclusively,4,10 whereas the metabolite patterns of most of the Central African species are dominated by the occurrence of hybrid-type alkaloids along with some Ancistrocladaceae-type compounds.4,8,14,18−21,23−26 The only other plant family that likewise produces naphthylisoquinolines, the West African Dioncophyllaceae, solely contains compounds with R-configuration at C-3, always lacking an oxygen function at C-6 (i.e., “Dioncophyllaceae-type” alkaloids). In view of the structures of the alkaloids found so far in A. ealaensis, this liana indeed represents a typical Central African Ancistrocladus species. Mbandakamine-Type Naphthylisoquinoline Dimers 3−6. LC-MS investigations on a crude leaf extract of A. ealaensis hinted at the presence of further constituents, with MS profiles typical of dimeric naphthylisoquinoline alkaloids. The chromatographic and spectroscopic data (HRESIMS, NMR, and electronic circular dichroism (ECD)) of one of these alkaloids isolated from the leaves of A. ealaensis were fully in accordance with those of the known dimer mbandakamine A (3)21 (Figure 1), which had recently been isolated from a yet undescribed Congolese Ancistrocladus species22 growing in the rain forests near the town of Mbandaka. Mbandakamine A (3) is the first naphthylisoquinoline dimer with a constitutionally highly unsymmetric 6′,1″-coupled central biaryl axis, which is, thus, located in the peri-position in close proximity to one of the outer axes. It displays a good antiplasmodial activity in vitro.21

lymphoblastic CCRF-CEM leukemia cells and their multi-drugresistant subline, CEM/ADR5000. A. ealaensis is the only Ancistrocladus species that has so far been found to produce ealapasamine- and michellamine-type dimers, i.e., quateraryls possessing two chiral outer biaryl axes and a configurationally unstable central biaryl axis, along with a series of mbandakamine-type compounds that possess three consecutive chiral biaryl axes. This finding clearly reveals the Central African plant to occupy a special phylogenetic position within the Ancistrocladaceae family. The geo- and chemotaxonomic relationships of A. ealaensis to other Ancistrocladus species from West and Central Africa are discussed in this paper. Furthermore, we report the isolation and structural elucidation of four new 5,8′-coupled monomeric naphthylisoquinolines from the leaves and twigs of A. ealaensis, viz., ancistroealaines C−F (7−10) (Figure 2), which were discovered together with five known10,12,13,24−27 alkaloids (Figure 3) earlier identified in related African and Asian

Figure 3. Known naphthylisoquinoline alkaloids from previous isolation work on related African and Asian Ancistrocladaceae plants,10,12,13,24−27 now identified for the first time in A. ealaensis.

Ancistrocladus species. Furthermore, four biosynthetically related, yet naphthalene-devoid tetra- and dihydroisoquinoline alkaloids, named ealaines A−D (11−14) (Figure 2), which had previously only been known as synthesized compounds,28 have now been identified for the first time as genuine natural products. Such simple isoquinoline alkaloids have so far been found in Ancistrocladaceae plants only rarely.24,29

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RESULTS AND DISCUSSION Isolation and Structural Elucidation of Naphthylisoquinoline Alkaloids. Leaves and twigs of A. ealaensis were collected in the Botanical Garden of Eala near the town of Mbandaka in Northwestern DR Congo. Air-dried material was ground and exhaustively extracted with MeOH. The crude extracts were macerated with water and then further purified by liquid−liquid partition using n-hexane and CH2Cl2. Fractionation of the CH2Cl2 layers by column chromatography (CC) and solid-phase extraction (SPE) using C18 reversed-phase silica gel provided a series of alkaloid-containing subfractions, which 920



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Table 1. 1H (600 MHz) and 13C (151 MHz) NMR Data of Mbandakamines C−E (4−6) and Michellamine A5 (2) in MeOH-d4 (δ in ppm) mbandakamine C (4) no. 1 3 4ax 4eq 5 6 7 8 9 10 1′ 2′ 3′ 4′ 5′ 6′ 7′ 8′ 9′ 10′ 1″ 2″ 3″ 4″ 5″ 6″ 7″ 8″ 9″ 10″ 1‴ 3‴ 4‴ax 4‴eq 5‴ 6‴ 7‴ 8‴ 9‴ 10‴ 1-CH3 3-CH3 8-OCH3 2′-CH3 4′-OCH3 2″-CH3 4″-OCH3 5″-OCH3 1‴-CH3 N‴-CH3 3‴-CH3 8‴-OCH3

δH (J in Hz) 4.65, 3.27, 2.55, 3.68,

q (6.7) m dd (16.9, 11.7) dd (17.1, 3.1)

6.48, s

6.74, d (1.0) 6.87, d (1.2)

6.44, s

6.79, s

6.99, d (8.0) 7.05, d (7.9)

4.64, 3.50, 2.42, 1.95,

q (6.7) m dd (17.4, 12.4) dd (17.5, 4.2)

δC, type 52.1, CH 51.7, CH 33.1, CH2 120.8, C 157.5, C 99.2, CH 158.1, C 113.5, C 136.0, C 118.9, CH 136.6, C 106.9, CH 157.6, C 151.2, C 124.2, C 134.9, CH 124.3, C 136.3, C 114.6, C 128.0, C 141.0, C 114.6, CH 155.4, C 158.5, C 104.7, CH 132.4, CH 126.5, C 138.3, C 116.3, C 50.0, CH 46.2, CH 33.2, CH2

d (6.6) d (6.5) s s s s

122.8, C 154.5, C 97.0, CH 156.7, C 113.5, C 133.7, C 20.6, CH3 18.9, CH3 56.0, CH3 22.3, CH3 56.9, CH3 21.5, CH3

4.15, s 1.53, d (6.7) 1.25, d (6.4) 3.05, s

5.32, s

1.76, 1.50, 3.83, 2.37, 4.09, 1.91,

mbandakamine D (5) δH (J in Hz) 4.68, 3.35, 2.71, 3.58,

q (6.4) m dd (17.9, 12.3) dd (16.7, 3.0)

6.48, s

6.77, d (1.1) 6.82, br s

6.51, s

6.83, s

7.02, d (8.1) 7.07, d (7.9)

3.52, m 2.85, m 2.00, dd (17.1, 4.9)

mbandakamine E (6)

δC, type 52,1, CH 51.7, CH 33.3, CH2 120.6, C 157.7, C 99.3, CH 158.3, C 113.5, C 135.4, C 119.3, CH 137.2, C 107.7, CH 157.5, C 152.4, C 123.6, C 135.4, CH 124.7, C 136.8, C 114.2, C 127.5, C 141.4, C 115.0, CH 155.6, C 159.1, C 104.7, CH 132.3, CH 124.8, C 138.0, C 116.3, C 175.8, C 50.2, CH 33.8, CH2

d (6.6) d (6.5) s s s s


57.0, CH3 19.0, CH3

4.16, s 2.65, d (1.8)

57.1, CH3 24.8, CH3

18.9, CH3 55.4, CH3

1.28, d (6.7) 3.28, s

18.8, CH3 56.2, CH3

5.56, s

1.78, 1.54, 3.83, 2.38, 4.05, 1.90,

δH (J in Hz) 4.72, 3.66, 1.76, 2.89,

q (6.9) m dd (18.1, 11.5) dd (17.7, 4.9)

6.69, s

6.55, br s 6.82, br s

6.82, s

6.88, s

7.02, d (8.0) 6.98, d (8.0)

4.95, 3.96, 3.32, 2.08,

q (6.8) m dd (18.9, 11.7) dd (16.5, 5.4)

michellamine A5 (2)

δC, type 49.0, CH 44.6, CH 33.1CH2 119.7, C 156.9, C 97.9, CH 158.0, C 113.5, C 135.9, C 118.7, CH 137.2, C 107.6, CH 158.2, C 152.6, C 121.5, C 132.2, CH 124.3, C 136.3, C 116.3, C 128.3, C 137.2, C 114.6, CH 158.5, C 159.1, C 104.7, CH 132.7, CH 126.5, C 136.9, C 114.5, C 71.9, CH 62.3, CH 34.1, CH2

1.57, 1.16, 3.94, 2.29, 4.05, 2.07,

d (6.7) d (6.2) s s s s


4.16, 1.48, 3.01, 1.18, 3.43,

s d (6.6) s d (6.7) s

56.8, 16.5, 48.2, 13.6, 55.7,

5.61, s

CH3 CH3 CH3 CH3 CH3

δH (J in Hz) 4.59, 3.16, 1.86, 2.16,

q (6.6) m dd (17.4, 10.5) dd (17.8, 3.4)

6.75, s

6.67, br s 6.82, d (1.1)

6.57, s

6.85, s 7.01, s

7.01, s

3.45, m 2.75, dd (17.5, 11.7) 2.48, dd (17.8, 3.4)

6.28, s

1.68, 1.11, 3.90, 2.33, 4.15, 1.93, 4.16,

d (6.6) d (6.5) s s s s s

δC, type 52.0, CH 50.6, CH 33.1, CH2 119.7, C 156.9, C 99.3, CH 158.3, C 114.5, C 135.9, C 119.5, CH 137.2, C 107.6, CH 158.8, C 152.6, C 126.8, C 133.8, CH 125.1, C 136.9, C 114.6, C 114.8, CH 139.7, C 104.8, CH 158.4, C 155.7, C 123.3, C 132.1, CH 127.0, C 137.1, C 116.5, C 175.7, C 49.8, CH 33.8, CH2 125.8, C 166.8, C 98.7, CH 163.6, C 108.2, C 141.7, C 20.3, CH3 18.8, CH3 56.0, CH3 22.2, CH3 57.1, CH3 22.2, CH3 57.1, CH3

2.18, s

24.4, CH3

1.31, d (7.1) 3.67, s

18.4, CH3 56.0, CH3

unsymmetric dimer, which was confirmed by the presence of 48 signals in the 13C NMR spectrum (Table 1). The presence of 1 or related heterodimers was readily ruled out since the isolated metabolite showed substantially different 1D and 2D NMR spectroscopic data. The same number of signals as for

The second dimeric compound isolated from the leaf extract of A. ealaensis was obtained as a white powder. HRESIMS analysis gave a molecular formula of C48H52N2O8, identical to that of ealapasamine A (1)18 and mbandakamine A (3).21 The 1 H NMR data showed a full set of signals, indicative of an 921



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Figure 4. Selected 1H and 13C NMR shifts (in methanol-d4, δ in ppm) of the (A) “northwestern” and (B) “southeastern” naphthylisoquinoline portions of mbandakamine C (4) and key ROESY (double blue arrows) and HMBC (single red arrows) interactions indicative of the constitutions of the (C) “northwestern” and (D) “southeastern” halves of 4. (E) Selected ROESY correlations between the naphthalene part of the “northwestern” moiety and the isoquinoline portion of the “southeastern” half establishing the 6′,1″-coupling sites in the binaphthalene core (black arrows) and the relative configurations at the centers and the two outer axes (blue arrows) and at the central biaryl axis (green arrows) of 4.

signals it was evident that this second molecular portion was again a naphthyl-1,3-dimethyltetrahydroisoquinoline unit. The ROESY correlation between H-6″ and MeO-5″ and HMBC interactions from H-7″ to C-5″ (δC 158.5) and C-9″ (δC 138.3) and from H-6″ to C-8″ (δC 126.5) (Figure 4C) revealed C-8″ to be quaternary and hence an axis-bearing carbon atom. In the isoquinoline portion, the biaryl axis was found to be located at C-5‴ (δC 122.8), as shown by HMBC interactions of both H4‴ax (δH 2.42) and H-7″ with C-5‴, confirmed by the crosspeak of H-7‴ (δH 5.32) with C-5‴. The strongly upfield-shifted signal of Me-2″ (δH 1.91) hinted at the proximity of a second aryl substituent next to this methyl group, which was in agreement with the presence of the third, central biaryl axis connecting the two monomeric halves of the dimer by a 6′,1″coupling of the two naphthalene portions. The location of this central axis was evidenced by the 3J HMBC cross-peaks from H-3″, Me-2″, and H-7′ to C-1″ (Figure 4D) and from a series of unusal ROESY correlations between the naphthalene part of the “northwestern” half and the isoquinoline portion of the “southeastern” naphthylisoquinoline and between the two isoquinoline portions, e.g., from MeO-4′ (δH 4.09) to MeO8‴ (δH 3.05), MeO-4′ to H-1‴ (δH 4.64), and Me-3 (δH 1.50) to H-7‴ (Figure 4E). As in the case of mbandakamine A (3),21 parts of the two naphthylisoquinoline moieties were pressed into a dense spatial proximity due to the fact that the two naphthalene units were attached to the contiguous peripositions 1″ and 8″, thus leading to an extremely high steric load at the central biaryl axis. In conclusion, the new alkaloid was established to be an unsymmetric dimer with the same constitution as that of 3,21 consisting of two 5,8′-coupled naphthyltetrahydroisoquinolines linked via the extremely crowded 6′,1″-positions of the two naphthalene portions and

mbandakamine A (3), combined with the same spectroscopic features, suggested that the new dimer had the same constitution as 3, but was constructed from configurationally different monomeric halves. The 1H NMR spectrum of the first, “southeastern” molecular portion of the new alkaloid 4 (Table 1) showed the typical chemical shifts of a 5,8′-coupled naphthyltetrahydroisoquinoline bearing methyl groups at C-1 (δH 1.76), C-3 (δH 1.50), and C-2′ (δH 2.37), with four protons in the aromatic region, displaying two singlets, H-7 (δH 6.48) and H-7′ (δH 6.44), and a two-proton spin system, H-1′ (δH 6.74) and H-3′ (δH 6.87), with a meta-coupling pattern. This first naphthylisoquinoline moiety possessed two O-methyl groups, with NMR resonances at δH 3.83 and 4.09 (Figure 4B). Their positions were deduced at C-8 and C-4′, as evidenced from the ROESY correlation sequences {H-7 ↔ MeO-8 ↔ H-1} and {H-1′ ↔ Me-2′ ↔ H3′ ↔ MeO-4′} (Figure 4D). The 3J HMBC correlations from H-7, H-7′, and H-4eq (δH 3.68) to C-5 (δC 120.8) and between H-1′ and the quaternary carbon atom C-8′ (δC 124.3) showed that the two subunits of this naphthylisoquinoline moiety were 5,8′-coupled (Figure 4D). An HMBC cross-peak between H-7′ (δH 6.44) and a quaternary carbon atom belonging to a different naphthylisoquinoline part, viz., C-1″ (δC 128.0) in the second, “northwestern” molecular half, established C-6′ (δC 124.2) to be the coupling position of the central biaryl axis for this first, “southeastern” naphthylisoquinoline portion (Figure 4D). The “northwestern” half displayed signals for four aromatic protons in the 1H NMR spectrum (Figure 4A), viz., two singlets for H-3″ (δH 6.79) and H-7‴ (δH 5.32), and a twoproton spin system, H-6″ (δH 6.99) and H-7″ (δH 7.05), showing an ortho coupling pattern. From the 1H and 13C NMR 922



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displaying a total of seven stereogenic elements (four centers and three axes). From a ROESY correlation between H-1 (δH 4.65) and H-3 (δH 3.27), the 1,3-cis relative configuration in the “southeastern” half (Figure 4E) was deduced, while a ROESY interaction between Me-1‴ (δH 1.53) and H-3‴ (δH 3.50) revealed a 1‴,3‴-trans relative configuration in the isoquinoline portion of the “northwestern” half (Figure 4E). The absolute configurations at C-3 and C-3‴ in the two tetrahydroisoquinoline portions were determined by ruthenium-mediated oxidative degradation as described earlier,30 by stereochemical analysis of the resulting amino acids using gas chromatography with mass-selective detection (GC-MSD) after derivatization with the R-enantiomer of Mosher’s acid chloride. The exclusive formation of the R-enantiomer of 3-aminobutyric acid established the new metabolite to be R-configured at both C3 and C-3‴, which implied the absolute configurations of the molecular halves to be 1‴R,3‴R in the trans-configured “northwestern” part and 1S,3R in the cis-configured “southeastern” half (Figure 4E). Long-range ROESY correlations between H-1′ (δH 6.74) and H-4ax (δH 2.55) and between H-7′ (δH 6.44) and H-4eq (δH 3.68) in the “southeastern” portion and between H-7″ (δH 7.05) and H-4eq‴ (δH 1.95) in the “northwestern” half (Figure 4E) assigned the axial configurations in the two monomeric portions relative to the stereocenters, which, together with the absolute configurations at C-1 and C-3 and at C-1‴ and C-3‴, evidenced the two outer axes to be P-configured. ROESY correlations from H-3 (δH 3.27), Me-3, H-4ax, and H-1′ to the upfield shifted proton at C7‴ (δH 5.32), those from H-3′ to MeO-8‴, and between H-1‴ (δH 4.64) and MeO-4′ (δH 4.09) (Figure 4E) established the central biaryl axis to be P-configured, too. This stereochemical assignment was confirmed by the fact that the ECD spectrum of the new dimeric alkaloid was virtually identical to that of mbandakamine A (3), which is P-configured at the central biaryl axis.21 The new dimer isolated from A. ealaensis was, thus, (1S,3R,5P,6′P,5‴P,3‴R,1‴R)-configured and, consequently, possessed the full absolute stereostructure 4 as shown in Figure 1. It was, hence, the 1-epi-analogue of the co-occurring mbandakamine A (3); in continuation of the series of mbandakamine-type dimers, the new alkaloid 4 was named mbandakamine C. From another dimer-enriched subfraction of the leaves of A. ealaensis, a second new alkaloid 5 was obtained as a yellowish powder, corresponding to a molecular formula of C48H50N2O8 according to HRESIMS. The NMR data (for 1H and 13C NMR shifts, see Table 1; for ROESY and HMBC correlations, see Supporting Information) and the MS data hinted at a further unsymmetric 6′,1″-coupled dimer consisting of two 5,8′-linked monomers, with a molecular architecture similar to those of the mbandakamines A (3) and C (4), but with two protons fewer than 3 and 4. The downfield-shifted signal of one of the Cmethyl groups (δH 2.65) and the lack of one of the quartets representing a proton at C-1 or C-1‴ were typical of the presence of a 1,3-dimethyldihydroisoquinoline moiety (Figure 5A). This was further evidenced by HMBC interactions from H-7‴ (δH 5.56) and Me-1‴ (δH 2.65) to the unshielded C-1‴ (δC 175.8) and by ROESY correlations from H-7‴ and Me-1‴ to MeO-8‴ (δH 3.28) (Figure 5B). The NMR data of the “southeastern” naphthylisoquinoline portion of the new dimer revealed the same constitution and the same relative 1,3-cisconfiguration in the tetrahydroisoquinoline subunit as in the respective “southeastern” molecular half of mbandakamine C

Figure 5. Selected NMR data (methanol-d4, δ values in ppm) of the “northwestern” naphthyldihydroisoquinoline half of mbandakamine D (5): (A) 1H and 13C NMR shifts (δ in ppm) and HMBC interactions (single red arrows), indicative of the constitution of 5; (B) key ROESY cross-peaks (double blue arrows) for the elucidation of the relative configuration.

(4). Oxidative degradation30 established the R absolute configuration at both C-3 and C-3‴, which, in combination with the relative cis-configuration in the “southeastern” portion, determined the stereocenter at C-1 to be S-configured. ROESY correlations from H-4eq to H-7′ and from H-4ax to H-1′, like in the cis-configured tetrahydroisoquinoline part of 4 (Figure 4E), and between H-7″ and H-4‴eq in the “northwestern” half (Figure 5B) assigned the two outer biaryl axes to be again Pconfigured. ROESY correlations between the naphthalene unit of the “southeastern” naphthylisoquinoline monomer and the dihydroisoquinoline part of the “northwestern” molecular half similar to those observed for mbandakamine C (4), e.g., between H-7‴ and H-4eq and between MeO-8‴ and H-3′ (Supporting Information), established the central axis to be Pconfigured, too. This stereochemical assignment was further supported by the ECD spectrum of the new dimer, which was similar to those of the likewise P-configured mbandakamines A (3) and C (4) (see Supporting Information). Hence, this new dimer possessed the full stereostructure 5, with (1S,3R,5P,6′P,5‴P,3‴R)-configuration, as displayed in Figure 1. It was named mbandakamine D. The third new compound, 6, isolated as a trace metabolite from the leaves of A. ealaensis, was obtained as a white powder. Its monoprotonated ion [M + H] + (m/z 799.39125) corresponded to a molecular formula of C49H55N2O8, as revealed by HRESIMS, in agreement with the number of signals in the 13C NMR spectrum. The NMR data of the new dimer again showed features resembling those of an unsymmetric 6′,1″-coupled dimer consisting of two 5,8′-linked monomeric naphthyltetrahydroisoquinolines, but, different from the mbandakamines A (3) and C (4), equipped with an additional O- or N-methyl group. A three-proton singlet resonating at δH 3.01 (Table 1), typical of an N-Me group, suggested this methyl function to be attached to the nitrogen atom in one of the isoquinoline subunits. 923



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Figure 6. Selected NMR data (methanol-d4, δ values in ppm) and HMBC (single red arrows) interactions evidencing the constitution of (A) the “northwestern” and (B) the “southeastern” molecular portion of mbandakamine E (6), and ROESY (double blue arrows) correlations indicative of the relative configuration at the biaryl axis and the stereogenic centers (C) in the “northwestern” and (D) for the “southeastern” half of 6.

The NMR data set of the “southeastern” naphthyltetrahydroisoquinoline portion (Figure 6B) revealed the same constitution and the same relative configuration as in the respective “southeastern” half of mbandakamine A (3). The relative configuration at C-1 versus C-3 in the new dimer was assigned to be trans from ROESY correlations between Me-1 (δH 4.72) and H-3 (δH 3.66) (Figure 6D). The ROESY series {H-1‴ ↔ N‴-Me ↔ Me-3‴} assigned the location of the Nmethylation to be on the “northwestern” molecular part, likewise indicating a 1,3-diaxial relationship of H-1‴ (δH 4.95) and H-3‴ (δH 3.96) and thus a relative cis-configuration at C1‴ versus C-3‴ (Figure 6A and C). The oxidative degradation procedure30 afforded the R-enantiomers of 3-aminobutyric acid and its N-methyl derivative. Consequently, given the different relative trans- and cis-configurations established above, the new dimer was deduced to be R-configured at both C-3 and C-3‴, and also at C-1, whereas the absolute configuration at C-1‴ was determined to be S (Figure 6C and D). From specific ROESY interactions across the biaryl axis between H-7″ and H-4‴eq and from the results of the oxidative degradation, the absolute axial configuration in the “northwestern” half was deduced to be P (Figure 6C). In the “southeastern” portion, by contrast, the biaryl axis was established to be M-configured, as concluded from ROESY interactions between H-7′ and H-4ax and between H-1′ and H4eq (Figure 6D). As for the mbandakamines described above, the absolute configuration of the new dimer at the central biaryl linkage was determined to be P, as evident from ROESY and ECD measurements (Supporting Information). Thus, the new compound had the full stereostructure 6 as shown in Figure 1. It was hence the first mbandakamine-type dimer consisting of two biaryl halves with different axial configurations that has been discovered in nature. In continuation of the series of 6′,1″coupled dimeric naphthylisoquinoline alkaloids, compound 6 was named mbandakamine E. Michellamine A5 (2). The fourth new alkaloid, 2, was isolated as a trace constituent from the leaves of A. ealaensis. HRESIMS analysis gave a molecular formula of C48H50N2O8, i.e., the same as that of mbandakamine C (5). 1H and 13C NMR data (Table 1) suggested the presence of a further unsymmetric naphthylisoquinoline dimer consisting of two 5,8′-linked monomers, with structural features similar to those of

mbandakamine C (5) (Figure 1). Indeed, the molecular framework of the “southeastern” half of the molecule was found to be fully in accordance with that of 5. The “northwestern” part differed from 5 only by the coupling position of the central biaryl axis. HMBC interactions of H-7″ (δH 7.01) with C-5‴ (δC 125.8) and with a quaternary carbon atom (δC 126.8) belonging to the naphthalene system (C-6′) of the “southeastern” portion of the dimer clearly established the central biaryl linkage to be located at C-6″ (δC 123.3) in the “northwestern” naphthylisoquinoline half (Figure 7). Hence, in contrast to 5, dimer 2 possessed a 6′,6″-coupled central binaphthalene core; thus, the new metabolite was not a mbandakamine-type dimer, but belonged to the subclass of michellamine-type naphthylisoquinoline alkaloids with a central

Figure 7. Selected 1H and 13C NMR shifts (methanol-d4, δ values in ppm), key HMBC (single red arrows), and ROESY (double black arrows) interactions indicative of the constitution of michellamine A5 (2), and decisive ROESY (double blue arrows) evidencing the relative configurations at the two “outer” biaryl axes of 2 versus the stereogenic centers. 924



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Table 2. 1H (400 MHz) and 13C (101 MHz) NMR of Ancistroealaines C−F (7−10) in MeOH-d4 (δ in ppm, J in Hz) ancistroealaine C (7) no. 1 3 4ax 4eq 5 6 7 8 9 10 1′ 2′ 3′ 4′ 5′ 6′ 7′ 8′ 9′ 10′ 1-CH3 3-CH3 6-OCH3 8-OCH3 2′-CH3 4′-OCH3 5′-OCH3

δH (J in Hz) 4.63, 3.26, 2.19, 2.42,

q (6.7) m dd (17.8, 11.9) dd (17.4, 3.4)

δC, type 52.3, CH 50.8, CH 33.2, CH2

1.80, d (6.5) 1.21, d (6.5)

119.4, C 156.7, C 102.8, CH 156.3, C 113.0, C 135.0, C 119.1, CH 137.5, C 107.5, CH 158.0, C 155.6, C 110.4, CH 131.4, CH 124.8, C 137.6, C 115.1, C 19.8, CH3 18.7, CH3

2.34, s 4.09, s

22.2, CH3 56.8, CH3

6.47, s

6.79, s 6.81, s

6.80, d (7.7) 7.08, d (7.9)

ancistroealaine D (8) δH (J in Hz) 4.90, 3.50, 2.23, 2.62,

q (6.2) m dd (18.4, 12.0) dd (18.2, 4.6)

6.55, d (1.4)

6.65, d (1.3) 6.61, s

6.87, d (7.8) 7.07, d (7.8)

1.68, 1.28, 3.67, 3.96, 2.33, 4.08,

d (6.7) d (6.4) s s d (0.5) s

ancistroealaine E (9)

δC, type 48.2, CH 44.7, CH 31.9, CH2 121.0, C 158.5, C 94.2, CH 156.1, C 113.4, C 131.8, C 118.0, CH 136.7, C 106.7, CH 156.1, C 154.3, C 109.4, CH 129.4, CH 123.3, C 135.5, C 113.6, C 18.6, CH3 18.9, CH3 56.1, CH3 55.5, CH3 21.9, CH3 56.2, CH3

δH (J in Hz) 3.81, m 2.25, dd (16.8, 9.5) 2.62, dd (16.6, 5.4)

6.55, d (1.4)

6.55, s 6.68, d (1.4)

6.90, d (7.8) 7.07, d (7.9)

2.89, 1.28, 3.82, 4.10, 2.36, 4.11,

s d (6.9) s s s s

δC, type 173.4, C 47.7, CH 31.6, CH2 122.5, C 166.9, C 93.9, CH 163.7, C 108.0, C 132.7, C 117.4, CH 136.6, C 106.7, CH 156.6, C 154.9, C 109.5, CH 129.9, CH 121.4, C 135.2, C 113.7, C 24.4, CH3 17.2, CH3 56.2, CH3 56.0, CH3 22.2, CH3 56.2, CH3

ancistroealaine F (10) δH (J in Hz) 3.80, m 2.28, dd (17.0, 10.0) 2.71, dd (17.0, 5.4)

6.84, s

6.46, br s 6.67, d (1.4)

6.94, d (8.0) 7.10, d (7.9)

2.82, 1.20, 3.85, 4.15, 2.24,

d (1.1) d (6.7) s s s

4.11, s

δC, type 176.0, C 49.2, CH 32.7, CH2 123.6, C 168.6, C 95.9, CH 166.4, C 109.0, C 141.6, C 116.4, CH 139.6, C 113.4, CH 156.3, C 157.9, C 104.4, CH 129.5, CH 126.3, C 137.1, C 114.8. C 25.1, CH3 17.9, CH3 57.1, CH3 57.2, CH3 22.0, CH3 56.9, CH3

them (δH 6.81 and 6.79) belonged to the naphthalene part, located meta to each other. The coupling site was deduced from 3 J HMBC interactions between H-1′ and the quaternary carbon atom C-8′, thus revealing that the naphthalene portion was coupled via C-8′. In the tetrahydroisoquinoline portion, HMBC interactions from H-7′ and H-7 to C-5 established the axis to be located at C-5. From a NOESY interaction between H-1 (δH 4.63) and H-3 (δH 3.26) (Figure 8A), a relative cis-configuration of the two stereocenters at C-1 and C-3 was determined. Rutheniummediated oxidative degradation30 and specific NOESY interactions across the biaryl axis proved the new metabolite to be (1S,3R)-configured in the tetrahydroisoquinoline part and evidenced the biaryl axis to be P. This assignment was further corroborated by the resemblance of the ECD spectrum of the compound with that of the P-configured korupensamine A (15)24−26 (Figure 8A). The isolated alkaloid thus had the full absolute stereostructure 7 as shown in Figure 2 and was, hence, the 1-epi-analogue of the co-occurring 15 (Figure 3). In continuation of the series of monomeric 5,8′-coupled alkaloids isolated from A. ealaensis, compound 7 was named ancistroealaine C. 1 H and 13C NMR data (Table 2) as well as specific NOE correlations (Figure 8B) of the second pure monomer 8 isolated from the twig extract hinted at the presence of yet another 5,8′-coupled naphthyltetrahydroisoquinoline. The NMR data revealed a strong structural similarity to 7, but differed in the OH/OMe substitution pattern and the relative trans-configuration of the two methyl groups at C-1 and C-3, as deduced from a NOESY correlation between Me-1 (δH 1.68) and H-3 (δH 3.50). This was confirmed by oxidative

biaryl axis that is configurationally unstable. The formation of (R)-3-aminobutyric acid in the oxidative degradation30 proved the new dimer to be R-configured at both C-3 and C-3‴ and, given the relative cis-configuration of the two methyl groups in the tetrahydroisoquinoline moiety in the “southeastern” naphthylisoquinoline half, established C-1 to be S-configured. Specific ROESY interactions between H-4eq and H-7′, and between H-4ax and H-1′, in conjunction with the Rconfiguration at C-3 in both of the naphthylisoquinoline halves (Figure 7), confirmed the two outer biaryl axes to be Pconfigured. This stereochemical assignment was further supported by the ECD spectrum of the new compound, which was highly similar to that of the known, structurally closely related michellamine A.19 Consequently, the new dimer had the full stereostructure 2 as presented in Figure 1. In continuation of the series of michellamine-A-related dimers previously isolated from A. korupensis19 and A. congolensis,20 compound 2 was named michellamine A5. Monomeric Alkaloids (7−10). Besides the aforementioned dimers, four new monomers were discovered in the extracts of the twigs and the leaves of A. ealaensis. The molecular formula of the first new alkaloid, 7, obtained as a white, amorphous solid, was C23H25NO4, as deduced from HRESIMS. The compound displayed NMR signals (Table 2) typical of a naphthyltetrahydroisoquinoline with one methoxy function (δH 4.09), which was located at C-4′ of the naphthalene unit by its HMBC interaction to C-4′ and its ROESY correlation to H-3′. As typical of 6-oxygenated naphthylisoquinoline alkaloids with a 5,8′-linkage, the 1H NMR spectrum showed three singlets and two doublets in the aromatic region, each corresponding to one proton. Two of 925



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Figure 8. Decisive NOESY interactions relevant for the constitution and the relative configurations at the stereogenic centers and axes of (A) ancistroealaine C (7) and (B) ancistroealaine D (8) and confirmation of the absolute axial configuration of 7 and 8 by comparison of their ECD spectra with that of the known24−26 related 5,8′-coupled alkaloid korupensamine A (15).

Figure 9. NOESY interactions indicative of the constitution and the configuration at the biaryl axes of (A) ancistrealaine E (9) and (B) ancistroealaine F (10) relative to the stereogenic center and confirmation of the absolute axial configuration of 9 and 10 by comparison of their ECD spectra with that of the known15 5,8′coupled alkaloid ancistectorine D (20).

degradation,30 establishing a (1S,3S) absolute configuration in the tetrahydroisoquinoline part. In contrast to 7, the new metabolite possessed three methoxy groups, with NMR resonances at δH 3.67, 3.96, and 4.08. Their positions were deduced from HMBC correlations to C-6, C-8, and C-4′, respectively, in accordance with the NOESY series {Me-1 ↔ MeO-8 ↔ H-7 ↔ MeO-6} and the NOESY interaction between H-3′ and MeO-4′. Specific NOE effects between H-7′ (δH 7.07) and H-4eq (δH 2.62) and between H-1′ (δH 6.65) and H-4ax (δH 2.23) revealed the biaryl axis to be M-configured, as depicted in Figure 8B. This stereochemical assignment was confirmed by the almost mirror-image-like ECD spectrum of the new alkaloid compared to that of ancistroealaine C (7). The metabolite, thus, had the stereostructure 8 as displayed in Figure 2. It is the atropodiastereomer of the co-occurring ancistroealaine B14 and was named ancistroealaine D. The molecular formula (C25H27NO4) of the third new monomeric compound 9, isolated as a yellow, amorphous solid, was deduced from HRESIMS and 13C NMR data. With respect to 1H NMR data (Table 2), the only structural differences between 8 and the new alkaloid were the lack of the H-1 quartet typical of naphthytetrahydroisoquinolines (usually at ca. δH 4.50−4.80), the downfield-shifted signal of Me-1 (δH 2.89) and its multiplicity (singlet), and the deshielded signal of C-1 (δC 173.4) in the 13C NMR spectrum, which indicated the presence of a naphthyl-1,3-dimethyldihydroisoquinoline alkaloid. The oxidative degradation30 established the (3S) absolute configuration. This, in combination with NOE interactions of H-1′ and H-7′ with the two diastereotopic protons at C-4 (Figure 9A), similar to those monitored for 8 (Figure 8B), established the new alkaloid to be M-configured at the biaryl axis, too. This was confirmed by the ECD spectrum of the new compound 9 (Figure 9A), which was highly similar to that of the related M-configured ancistectorine D (20),15 a 5,8′coupled naphthyldihydroisoquinoline from the extracts of the twigs and stems of the Chinese liana A. tectorius. The new

alkaloid thus possessed the structure 9 (Figure 2) and was named ancistroealaine E. The molecular formula of the fourth new monomeric alkaloid 10, obtained as a yellow solid from the extract of the twigs of A. ealaensis, was again C25H27NO4, as deduced from HRESIMS. The metabolite showed 1H and 13C NMR data suggesting the presence of a further 5,8′-linked naphthyldihydroisoquinoline alkaloid, strongly resembling the constitution of ancistroealaine E (9), yet displaying a different OMe/OH substitution pattern in the naphthalene portion. Based on an NOE correlation between MeO-5′ and H-6′, the methoxy group resonating at δH 4.11 was assigned to C-5′, which was in agreement with the HMBC interactions from both MeO-5′ and H-7′ to C-5′. Oxidative degradation30 of the new alkaloid gave R-configured aminobutyric acid, thus establishing the (3R) absolute configuration. This, in combination with the relative configuration at the axis, as evident from NOESY interactions of H-4ax with H-1′ and H-4eq with H-7′ (Figure 9B), determined the biaryl axis to be Pconfigured. The absolute axial configuration was confirmed by the fact that the ECD spectrum of the new alkaloid was virtually opposite that of the M-configured ancistroealaine E (9) (Figure 9B). The new monomer thus had the structure 10 as outlined in Figure 2 and was named ancistroealaine F. Ealaines A−D (11−14). The isolation work revealed the presence of four further alkaloids, which, although showing a close structural similarity to the 6,8-dioxygenated 1,3dimethyltetra- and dihydroisoquinoline parts of some of the aforementioned monomeric naphthylisoquinoline alkaloids, differed by the complete absence of NMR signals corresponding to the usual 2-methyl-4,5-dioxynaphthalene portions of these biaryl natural products. The first such noncoupled, hence “naphthalene-devoid”, compound, 11, was isolated as a colorless, amorphous powder. This minor alkaloid possessed a molecular formula of C12H17NO2, as deduced by HRESIMS. 1D and 2D NMR investigations (see Table 3 and Supporting Information) 926



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Table 3. 1H (400 MHz) and 13C (101 MHz) NMR of Ealaines A−D (11−14) in MeOH-d4 (δ in ppm, J in Hz) ealaine A (11) no. 1 3 4ax 4eq 5 6 7 8 9 10 1-CH3 3-CH3 6-OCH3 8-OCH3

δH (J in Hz) 4.58, 3.39, 2.86, 2.86, 6.28,

q (6.7) m m m d (2.5)

6.31, d (2.5)

1.72, d (6.6) 1.46, d (6.5) 3.73, s

δC, type 52.4, CH 51.3, CH 35.5, CH2 105.7, CH 161.6, C 101.7, CH 157.3, C 113.7, C 135.8, C 19.7, CH3 18.9, CH3 55.8, CH3

ealaine B (12) δH (J in Hz) 4.67, 3.79, 2.79, 3.07, 6.27,

q (6.7) m dd (18.0, 10.6) dd (17.6, 4.6) d (2.4)

6.30, d (2.3)

1.61, d (6.8) 1.46, d (6.4) 3.73, s

ealaine C (13) δC, type 51.1, CH 45.1, CH 35.5, CH2 105.7, CH 161.6, C 101.7, CH 157.2, C 113.5, C 135.8, C 19.7, CH3 18.9, CH3 55.8, CH3

δH (J in Hz) 3.92, 2.82, 3.06, 6.49,

m dd (16.6, 11.8) dd (16.4, 5.2) dt (2.3, 1.1)

6.42, d (2.4)

2.78, d (1.5) 1.43, d (6.7) 3.88, s

ealaine D (14) δC, type 176.0, C 49.7, CH 35.3, CH2 108.9, CH 169.7, C 101.2, CH 165.9, C 108.1, C 142.7, C 24.5, CH3 18.3, CH3 56.7, CH3

δH (J in Hz) 3.86, 2.78, 3.01, 6.39,

m dd (16.4, 11.9) dd (16.4, 5.3) d (1.0)

δC, type 174.4, C 49.6, CH 35.6, CH2

2.04, s 1.42, d (6.7)

111.1, CH 163.1, C 100.0, CH 166.8, C 107.5, C 143.8, C 23.7, CH3 18.3, CH3

3.93, s

56.6, CH3

6.41, d (1.9)

was named ealaine B. Hence, from its constitution and relative configuration, 12 is the 8-O-demethyl analogue of the heterocyclic molecular half of ancistroealaine B14 and its atropodiastereomer ancistroealaine D (8) (Figure 2), and also of the known10,27 ancistrotectoriline A (17), identified in A. ealaensis (see Figure 3). Two further “naphthalene-devoid” compounds, 13 and 14, were obtained that were structurally related to ealaine A (11) according to 1H and 13C NMR data (Table 3). With a molecular formula of C12H15NO2 determined by HRESIMS, their 1H NMR data differed from those of 11 essentially by the lack of the H-1 quartet and the downfield-shifted signals of Me1 (δH 2.04 and 2.78), thus hinting at the presence of a 1,3dimethyldihydroisoquinoline-type structure. This assumption was corroborated by the chemical shifts of the C-1 peaks of the two compounds (δC 174.4 and 176.0) in the 13C NMR spectra. By oxidative degradation, 30 the two metabolites were established to be S-configured at C-3. The oxygenation pattern of one of these compounds was identical to that of the ealaines A (11) and B (12), with a methoxy function at C-6 and a free hydroxy group at C-8, thus possessing the stereostructure 13 as shown in Figure 2. This new alkaloid was named ealaine C. It is the 8-O-demethylated heterocyclic moiety of the two cooccurring naphthylisoquinoline alkaloids ancistroealaine A14 (not shown) and E (9) (Figure 2). The second of these two alkaloids, compound 14, was a regioisomer, differing from 13 by the methoxy group now located at C-8, and with a free phenolic group at C-6. It was named ealaine D and is the 6-Odemethyl analogue of the heterocyclic half of the ancistroealaines A14 and E (9) (Figure 2). All of the ealaines A−D (11−14) are typical Ancistrocladaceae-type compounds, i.e., with a (3S)-configuration and an oxygen function at C-6. As useful chiral building blocks in the stereoselective total synthesis of naphthylisoquinoline alkaloids,4,31 they had been prepared by regio- and stereoselective synthesis,28 but were now identified in A. ealaensis as new natural products. The discovery of the ealaines A−D in Ancistrocladus plants is of special relevance with respect to the proposed biosynthetic route to naphthylisoquinoline alkaloids. The co-occurrence of tetra- and dihydroisoquinolines in A. ealaensis and of naphthoic acid derivatives14 related to the naphthalene portions of naphthylisoquinoline alkaloids is in agreement with the postulated separate formation of the two molecular halves of naphthylisoquinolines from joint polyketide

established the constitution of a 1,3-dimethyl-6-methoxy-8hydroxy-1,2,3,4-tetrahydroisoquinoline, as evident, for example, from the presence of two aromatic protons with a metacoupling pattern (δH 6.28 and 6.31), two C-bonded methyl groups (δH 1.46 and 1.72), and two diastereotopic protons (δH 2.86, m). A singlet (δH 3.73) corresponding to three protons indicated the presence of a methoxy group, which, by its NOESY correlations to H-7 (δH 6.31) and H-5 (δH 6.28) and by its HMBC interaction with C-6, was located at C-6 (δC 161.6). From an NOE correlation between H-1 (δH 4.58) and H-3 (δH 3.39) the relative configuration at C-1 versus C-3 was deduced to be cis (Figure 10). Ruthenium-mediated oxidative

Figure 10. Constitution of ealaine A (11) and its relative cisconfiguration at C-1 versus C-3, as deduced from characteristic chemical shifts and NOESY interactions.

degradation30 gave (S)-3-aminobutyric acid, and, thus, together with the relative cis-configuration of the two methyl groups at C-1 and C-3, the new alkaloid had to possess the full stereostructure 11, with (1R,3S)-configuration. According to its isolation from A. ealaensis and being related to the molecular halves of the ancistroealaines, compound 11 was simply named ealaine A. A second “naphthalene-free” tetrahydroisoquinoline, 12, with a chromatographical behavior similar to that of 11, was obtained from the extract of the leaves of A. ealaensis, showing a close structural relationship to 11 with respect to its 1H and 13C NMR data (Table 3). It had the same molecular formula and the same constitution as ealaine A (11), differing only by the relative configuration of the two stereocenters C-1 and C-3, which was deduced from a significant NOESY interaction of H3 (δH 3.79) with the pseudoaxial methyl group at C-1 (δH 1.61). By oxidative degradation,30 the new metabolite 12 was determined to be (1S,3S)-configured as shown in Figure 2. It 927



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Table 4. Antiparasitic Activities of the Mbandakamine Dimers 4 and 5 and of the Monomeric Naphthylisoquinoline Alkaloids 7−9 from A. ealaensis against Plasmodium falciparum (NF54 and K1 Strains), Trypanosoma cruzi, Trypanosoma brucei rhodesiense, and Leishmania donovani and Cytotoxicities against Rat Skeletal Myoblast (L6) Cells IC50 [μM]a compound standard 4 5 7 8 9

P. falciparum 0.008 (NF54) 0.291 (K1)c 0.047 (NF54) 0.064 (K1) 0.337 (NF54) 0.126 (K1) 9.94 (NF54) 12.9 (K1) 1.74 (NF54) 0.76 (K1) 2.69 (NF54) 1.06 (K1)

T. cruzi c

3.56 41.8

d

T. brucei rhodesiense 0.0075

e

L. donovani 0.432

f

L6 cells (cytotoxicity) 0.041

1.8

>100

15.9

4.1

>100

8.87

237

15.3

26.4

79.1

32.5

3.85

37.9

77.8

43.4

1.46

13.4

101

g

selectivity indexb −b −b 339 (NF54) 249 (K1) 26.3 (NF54) 70.4 (K1) 7.95 (NF54) 6.13 (K1) 44.7 (NF54) 102 (K1) 37.5 (NF54) 95.3 (K1)

a

The IC50 values are the means of two independent assays; the individual values vary by a factor of less than 2. bThe selectivity index (SI) is calculated as the ratio of the IC50 values for the L6 cells to the IC50 data relative to P. falciparum. Within this test series, the IC50 values for chloroquine against L6 cells and P. falciparum were not determined. cChloroquine. dBenznidazole. eMelarsoprol. fMiltefosine. gPodophyllotoxin.

μM). Ancistroealaine B14 (IC50 = 1.27 μM) and its 5-epimer 8 (IC50 = 1.74 μM), by contrast, exhibited comparable antiplasmodial activities. The three monomeric alkaloids were only weakly active against Trypanosoma and Leishmania parasites. The “naphthalene-devoid” ealaines A−D (11−14) showed no antiparasitic activity at all, thus indicating that the pronounced antimalarial,5,7,8 antileishmanial,5,11−15 or antitrypanosomal5,9,10,15 activities reported for some of the naphthylisoquinoline alkaloids seem to originate from the specific combination of their tetra- or dihydroisoquinoline and naphthalene portions. Antileukemic Activities of the Mbandakamine-Type Dimers 3−5 and the Monomer Ancistroealaine F (10). The development of multi-drug resistance is a principal mechanism by which many malignancies become tolerant to conventional agents used in chemotherapeutic regimens. It is thus a major factor in the failure of cancer treatment.32,33 Therefore, novel anticancer drugs for clinical pharmacotherapy are urgently needed to overcome this phenomenon. Resistance to therapy is frequently conferred by the ATP-dependent efflux pump P-glycoprotein, which actively expels pharmaceutical agents from the interior of tumor cells, so that anticancer drugs can no longer exert their antiproliferative and cytotoxic properties.33,34 More recently, several monomeric naphthylisoquinolines15−17 from Asian and Central African Ancistrocladus species, among them also 5,8′-linked alkaloids such as ancistectorine D (20),15 were found to strongly inhibit the viability of parental drug-sensitive CCRF-CEM leukemic cells and their multi-drug-resistant P-glycoprotein-overexpressing subline, CEM/ADR5000, which indicates that these alkaloids may bear therapeutic potential. The compounds showed a strong growth-retarding activity against these two lymphoblastic leukemia cell lines, similar to the one induced by the standard anticancer drug doxorubicin, with half-maximum inhibitory concentrations in the low micromolar range and very low degrees of cross-resistance toward CEM/ADR5000 cells.15−17 These findings prompted us to also evaluate the cytotoxic properties of two of the new dimers, mbandakamines C (4) and

precursors, followed by phenol-oxidative coupling to give the corresponding alkaloids.4,6 Antiprotozoal Activities of Mbandakamines and Related Monomers. The two new mbandakamines C (4) and D (5) exhibited good in vitro antiplasmodial activities against chloroquine-sensitive (NF54) and chloroquine-resistant (K1) strains of the malaria parasite Plasmodium falciparum, but exerted only weak cytotoxicities against rat skeletal myoblast (L6) cells, thus showing a structure-dependent specificity of their antiplasmodial activities (Table 4). The strongest effects were found for mbandakamine C (4), with a half-maximum inhibitory concentration (IC50) of 0.047 μM against the NF54 strain, thus being 2 to 3 times more effective than its 1-epimer mbandakamine A (3) (IC50 = 0.13 μM).21 Noteworthy was also the excellent antiplasmodial activity of 4 (IC50 = 0.064 μM) against the chloroquine-resistant strain K1, with an inhibitory potential significantly better than that of the standard chloroquine by a factor of 4.5. Likewise remarkable was the comparatively low cytotoxicity (15.9 μM) of 4 toward L6 cells, which led to good selectivity indices of nearly 250 (K1) and 340 (NF54). Similar to 3,21 the mbandakamines C (4) and D (5) exhibited only moderate to low antitrypanosomal activities (Table 4) against the pathogens of African sleeping sickness (Trypanosoma brucei rhodesiense) and Chagas’ disease (T. cruzi) and displayed virtually no activities against Leishmania donovani, the causative agent of visceral leishmaniasis. For lack of sufficient material, mbandakamine E (6) and michellamine A5 (2) were not tested. The strong antiprotozoal activities displayed by some 5,8′coupled monomeric naphthylisoquinolines7−10,14,15,17,24−26 prompted us to screen the new alkaloids ancistroealaines C− E (7−9), too. Ancistroealaine F (10) could not be investigated for lack of material. Compounds 7−9 showed only moderate to low antiplasmodial activities (Table 4). The effects of 8 and 9 were more pronounced against the chloroquine-resistant K1 strain than against the chloroquine-sensitive NF54 strain, whereas ancistroealaine C (7) exhibited nearly similar activities toward these two Plasmodium strains. Interestingly, the inhibitory potential of korupensamine A (15)24 (IC50 = 0.82 μM) against the NF54 strain was significantly better (by a factor of 12) than that of the related 1-epimer, 7 (IC50 = 9.94 928



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D (5), and of the known, but previously not yet tested mbandakamine A (3). Two 5,8′-coupled monomeric naphthylisoquinoline alkaloids, the ancistroealaines E (9) and F (10), were also investigated for their antiproliferative effects on human leukemia CCRF-CEM and CEM/ADR5000 cells. For this purpose, the tumor cells were treated with different concentrations of the respective compounds in a range from 0.001 to 100 μM or with doxorubicin as the reference drug. Cell viability was assessed by the resazurin assay. Within the series of three mbandakamine-type dimers, mbandakamine C (4) showed the highest growth-retarding activity against the drug-sensitive CCRF-CEM tumor cell line (Figure 11A) with a very good half-maximum inhibitory concentration in the low micromolar range (IC50 = 1.49 μM). Considerable antiproliferative activities against CCRF-CEM leukemia cells were also observed for the mbandakamines A (3) (Figure 11B) and D (5) (Figure 11C). The growth-inhibitory potential of 3 and 5 was distinctly lower than that of 4 (Table 5), with IC50 values of 7.40 μM for 3 and 2.95 μM for 5, thus showing that, compared to mbandakamine C (4), the cytotoxicities of the related mbandakamines A (3) and D (5) were reduced by a factor of ca. 5 for 3 and ca. 2 for 5. Furthermore, the cytotoxic potential of the compounds 3−5 against the CEM/ADR5000 P-glycoprotein overexpressing leukemia subline was investigated, to study more closely the cross-resistance profiles of the mbandakamines A (3), C (4), and D (5) in comparison to the parental cell line, CCRF-CEM. As outlined in Table 5, multi-drug-resistant CEM/ADR5000 cells exhibited cross-resistance only to a low degree to all of the dimers tested in this study, ranging from 3.2- to 18.5-fold as compared to that of the reference drug doxorubicin (ca. 1770fold). The highest value of cross-resistance (18.5-fold) was determined for mbandakamine C (4), which displayed the strongest growth-inhibiting effects against the CCRF-CEM cells, whereas the degrees of resistance to the mbandakamines A (4) and D (5) were distinctly lower (3.2-fold and 6.4-fold, respectively). The lymphoblastic leukemia cells were treated with different concentrations of the respective naphthylisoquinoline or with doxorubicin. Cell viability was assessed by the resazurin assay. Mean values and standard deviation of three independent experiments each with six parallel measurements are shown. The degrees of resistance were calculated by division of the IC50 values of the compounds for CEM/ADR5000 by the corresponding IC50 values for CCRF/CEM cells. Among the monomeric naphthyldihydroisoquinoline alkaloids, ancistroealaine F (10) significantly inhibited the viability of the CCRF-CEM tumor cells (IC50 = 11.7 μM) and their multi-drug-resistant CEM/ADR5000 subline (IC50 = 19.9 μM) at a micromolar range, at quite similar concentrations (Table 5), thus inhibiting the cell viability of drug-sensitive and multidrug-resistant leukemia cells with comparable efficacies (Figure 11D). Ancistroealaine E (9) (Figure 2), by contrast, differing from 10 by its O-methylation pattern in the naphthalene portion and its R-configuration at C-3, displayed only weak activities on the viability of CCRF/CEM and CEM/ADR5000 cells, inducing a reduction of the growth of the tumor cells by

Mbandakamine-Type Naphthylisoquinoline Dimers and Related

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