Recent studies on chemical constituents of Ophiorrhiza plants

Ophiorrhiza plants (Family Rubiaceae) are known to produce diverse monoterpenoid indole alkaloids including camptothecin with potent antitumor activity. This review contains a summary of recent chemical studies reported over the past 10 years regarding alkaloids (monoterpenoid indole and tetrahydroisoquinoline alkaloids, and cyclopeptide) in Ophiorrhiza plants. In addition, the alkaloid biosynthetic pathways based on their reported structures were proposed.


Introduction
Ophiorrhiza plants belong to the Rubiaceae family and are widely distributed in tropical and subtropical Asia, Australia, New Guinea, and the Pacific Islands. Some of Ophiorrhiza plants have been used traditionally to treat snakebites, ulcers, skin disorders, etc. [1,2]. Ophiorrhiza species are known to produce diverse monoterpenoid indole alkaloids including camptothecin with potent antitumor activity [1][2][3][4][5]. Camptothecin biosynthesis has been investigated at the genetic level, and its biotechnological production continues to attract research interest [6][7][8][9][10]. This review contains a summary of recent chemical studies on alkaloids isolated from Ophiorrhiza plants over the past 10 years. In addition, the biosynthetic pathways for some isolated monoterpenoid indole and tetrahydroisoquinoline alkaloids based on their reported structures were proposed.
Ophiorrhisine A (11) is a cyclic tetrapeptide having a 14-membered paracyclophane ring. The characteristic functionalities of 11 include a carboxylate group at C-10 and an N,N,N-trimethylated tyrosine residue in the side chain. The structure and absolute configuration of 11 were determined by spectroscopic analyses and asymmetric total synthesis involving an intramolecular aromatic nucleophilic substitution reaction (S N Ar) of linear tripeptide 17 to construct a 14-membered paracyclophane ring (Fig. 3) [14]. No cytotoxic activity was observed for naturally occurring 11 with an ionic character against cancer cells. On the other hand, some 14-membered cyclophane derivatives without ionic character, such as primary amine derivative 18, N,N-dibenzyl derivative 19, N,N-dimethyl derivative 20, phenylalanine derivative 21, and tryptophan derivative 22, showed cytotoxicity against various types of human cancer cell lines (A549, HT29, HCT116) with IC 50 values in the range of 2.9-11.6 µM (Fig. 4).

Monoterpenoid indole alkaloid glycosides from Ophiorrhiza japonica
The isolation of several alkaloids from Ophiorrhiza japonica has been reported. In 2018, ophiorrhines A (23) and B (24), immunosuppressive monoterpenoid indole alkaloid glycosides, were isolated from O. japonica, a folk herbal medicine collected in China (Fig. 5) [15]. The structures of 23 and 24 were elucidated by spectroscopic analyses and single-crystal X-ray diffraction. Both 23 and 24 possess a novel spirocyclic ring system and bridged carbon ring system. The major alkaloid isolated from the plant is 5-oxodolichantoside (25). The same group isolated ophiorrhines F (26) and G (27) from O. japonica in 2022, and proposed a biosynthetic pathway for ophiorrhines A (23) and B (24) via 26 and 27 as described (vide infra) [16]. The structures of 26 and 27 were elucidated by spectroscopic methods, ECD, and calculated NMR with DP4 + analysis. Then, the inhibitory activity against

Alkaloids from Ophiorrhiza cantoniensis
In 2021, ophiorrhines C-E (28-30) were isolated from Ophiorrhiza cantoniensis collected in China, together with one known alkaloid Δ 1',2' -deoxytubulosine (31) (Fig. 6) [17]. The structures of 28-30 and their absolute configurations were elucidated by spectroscopic methods, ECD, and calculated NMR with DP4 + analysis. The relative configuration of 28 at C-16 was assigned by gauge-independent atomic orbital (GIAO) 13 C NMR calculations and DP4 + analysis. Immunosuppressive activity assays demonstrated the inhibitory activity of compounds 28 and 29 against Con A-induced T cell proliferation with IC 50 values of 23.6 and 17.9 μM, respectively, and the inhibitory activity of 28 against LPSinduced B cell proliferation with an IC 50 value of 8.7 μM. Vincoside lactam (32) was also isolated from O. cantoniensis [18].
The proposed biosynthetic pathways for monoterpenoid tetrahydroisoquinoline alkaloids in Ophiorrhiza plants are shown in Fig. 9. The condensation of dopamine with secologanin is considered to produce deacetylisoipecoside (48) with H-1α and deacetylipecoside (49) with H-1β, although neither 48 nor 49 has been isolated from Ophiorrhiza plants to date. Isoalangiside (15) having H-1α and demethylalangiside (13) and alangiside (14) having H-1β would be formed from 48 and 49, respectively, via lactam formation between N-2 and the methyl ester group. On the other hand, the hydrolysis of the glucose unit in 48 and piperidine ring formation in the resulting aldehyde intermediate 50, followed by a sequence of reactions, would give 10-O-demethylprotoemetine (16). The reaction of 16 or its analogs with a second dopamine would yield 7',10-dide-O-methylcephaeline (12), whereas the reaction of 16 with tryptamine would lead to the formation of ophiorrhine E (30) via Δ 1',2'deoxytubulosine (31).

Conclusion
This review contains a summary of chemical studies reported over the past 10 years regarding the alkaloidal constituents of Ophiorrhiza plants. A number of alkaloids having unique chemical structures have been isolated, including monoterpenoid indole alkaloid glycosides, monoterpenoid tetrahydroisoquinoline alkaloids, and a cyclopeptide. Among them, some (including synthetic analogs) have demonstrated useful biological activities. The second half of this review discussed the plausible biosynthetic pathways for the isolated monoterpenoid indole and tetrahydroisoquinoline alkaloids based on their reported structures. Their diverse chemical structures would be derived from common intermediates obtained by the condensation of secologanin with tryptamine (tryptophan) or dopamine. It is highly anticipated that the candidate biosynthetic intermediates of the related alkaloids and novel alkaloids having unique skeletons and biological activities would be discovered from Ophiorrhiza plants in the future.
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.