Natural Product Research Formerly Natural Product Letters
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Optimized protocol for multigram preparation of emodin anthrone, a precursor in the hypericin synthesis Renato Sonchini Gonçalves, Gabriel Batista César, Patrícia Magalhães Barbosa, Noboru Hioka, Celso Vataru Nakamura, Marcos Luciano Bruschi & Wilker Caetano To cite this article: Renato Sonchini Gonçalves, Gabriel Batista César, Patrícia Magalhães Barbosa, Noboru Hioka, Celso Vataru Nakamura, Marcos Luciano Bruschi & Wilker Caetano (2018): Optimized protocol for multigram preparation of emodin anthrone, a precursor in the hypericin synthesis, Natural Product Research, DOI: 10.1080/14786419.2018.1457661 To link to this article: https://doi.org/10.1080/14786419.2018.1457661
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Natural Product Research, 2018 https://doi.org/10.1080/14786419.2018.1457661
SHORT COMMUNICATION
Optimized protocol for multigram preparation of emodin anthrone, a precursor in the hypericin synthesis Renato Sonchini Gonçalvesa , Gabriel Batista Césara, Patrícia Magalhães Barbosaa, Noboru Hiokaa, Celso Vataru Nakamurab, Marcos Luciano Bruschib and Wilker Caetanoa a
Department of Chemistry, State University of Maringá, Maringá, Brazil; bDepartment of Pharmacy, State University of Maringá, Maringá, Brazil
ABSTRACT
Emodin reduction to emodin anthrone comprise one of three process steps involved in the hypericin synthesis, a powerful natural photosensitiser found in plants of the genus Hypericum. In this communication, an optimized protocol was established for emodin reduction enabling an efficient multigram preparation of emodin anthrone. A screening of reducing agent (SnCl2·2H2O and HClconc) under different reaction times was employed in micro-scale and monitored by electronic absorption spectroscopy technique. Data showed lower yields of emodin anthrone when some experimental conditions previously described in the literature were reproduce. However, using the optimized protocol for the emodin reduction these yields were overcoming, and a gram-scale supply experiment was reproducible for the preparation of 10 grams of emodin anthrone with excellent yield.
ARTICLE HISTORY
Received 25 October 2017 Accepted 18 March 2018 KEYWORDS
Hypericin; emodin; emodin anthrone; tin(II) chloride dihydrate; reduction reaction; electronic absorption spectroscopy; optimized protocol
CONTACT Renato Sonchini Gonçalves
[email protected] Supplemental data for this article can be accessed at https://doi.org/10.1080/14786419.2018.1457661. © 2018 Informa UK Limited, trading as Taylor & Francis Group
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1. Introduction Emodin is a naturally occurring anthraquinone derivative found in a variety of medicinal plant species. Although emodin has been widely studied due its broad spectrum of pharmacological activities, it serves as a precursor to access the emodin anthrone in the preparation of naphtodianthrones, comprising mainly the chemical synthesis and biosynthesis of hypericin and its derivative fagopyrin (Falk 1999; Abd-Alla et al. 2009; Zdunić et al. 2011; Sytar et al. 2013; Huang et al. 2014; Dong et al. 2016; Braumann et al. 2017). During last years, a number of antimicrobial and anticancer effects of hypericin and fagopyrine have been discovered, justifying the importance of developing a methodology for the preparation of its precursor (Wada et al. 2002; Karioti and Bilia 2010; Sytar et al. 2016; Montanha et al. 2017). Therefore, in order to overcome the main difficulty concerning the hypericin isolation in a pure state direct from plant material, in the last decade a number of synthetic routes have been proposed, standing out one simplified three process steps starting from emodin reduction reaction (Rodewald et al. 1977; Mazur et al. 1989). However, a large number of divergence in the experimental conditions, mainly related to initial emodin reduction step can be found in the previous works (Falk et al. 1993, 1995, 1998; Altmann et al. 1998; Motoyoshiya et al. 2007; Tobia et al. 2012). Moreover, some experimental conditions reproduced by us led to very lower yields of emodin anthrone, and considering the expensive commercial cost of emodin (30 mg ~ USD 270) an increase in the final production cost of hypericin is unavoidable hindering its production on large scale. In this present communication, we show that the yield of the emodin anthrone and consequently the overall yield of hypericin synthesis provided by previous works can be significantly improved if the emodin reduction step is performed under optimal experimental condition set here (Scheme 1).
2. Results and discussion Initially a significant amount of emodin was accessed with high purity degree (Figure S1) by reproduce of the optimized protocol for the isolation of anthraquinones from Rhamnus Frangula L. reported by us in a previously communication (Gonçalves et al. 2017). To an initial emodin solution (5.0 × 10−2 mol·L−1) in acetic acid, tin(II) chloride dihydrate (SnCl2·2H2O) and concentrated hydrogen chloride were employed as reducing agents in agreement with the reported works (Mazur et al. 1989; Falk et al. 1993). Thus, we performed a screening of the molar amounts of reducing agents and of reaction time (Table S1), with monitoring of the reaction progress by electronic absorption technique. In the first screening (entries 1–3),
Scheme 1. Optimized experimental condition for the emodin reduction reaction.
NATURAL PRODUCT RESEARCH
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no significant change was observed in the intensity of emodin electronic absorption band at λmax = 450 nm, even to a prolonged reaction time of 6 h. Next, we set the same molar amount of the reducing agents (entry 4), described by Falk et al. (1993), however the emodin anthrone yield was much lower than that of reported reaction. For an added 50-fold molar excess of hydrogen chloride, an improvement of 61% in the emodin reduction was achieved for a reaction time of 3 h (entry 5). At this stage, it becomes evident that a large excess of HClconc should be used to promote an effective reduction of emodin with a significant reducing of the reaction time. For our delight, the optimal experimental condition was set for the combination of 3.2 and 100.0 equivalents of SnCl2·2H2O and HClconc, respectively resulting in a very efficient conversion of emodin to emodin anthrone with 87% yield (Figures S2 and S3), and with an expressive reduction of the reaction time for 30 min (entry 6). The UV-vis monitoring show that in the first five minutes of reaction, the emodin absorption band (λmax = 450 nm) decrease rapidly from 0.4 to approximately 0.1, accompanied of an increase of 0.2 in the emodin anthrone absorption band at λmax = 356 nm, which reached a maximum value of 0.7 for a reaction time of 30 min (Figure S4(A)). In this way, the next experiments were performed out in order to demonstrate the effect of some extreme experimental conditions, reported previously. For instance, Motoyoshiya et al. described the use of a large excess of reducing agents (10.0 and 2.505 M equivalent of SnCl2·2H2O and HClconc, respectively) and a prolonged reaction time of 24 h for the reduction of an 0.048 mol·L−1 initial emodin solution (Motoyoshiya et al. 2007). For comparison purposes, the optimal molar amounts of reducing agents was maintained, and the emodin reducing was evaluated with reaction times of 3, 5 and 6 h (entries 7–9). However, for 3 h of reaction, the maximum intensity of emodin anthrone absorption band show an expressive decrease of 25% (0.7 to 0.5) (Figure S4(B)). Moreover, the decrease of absorption intensity become even more pronounced for reaction times of 5 and 6 h, leading to a dramatic decrease of 42 and 58%, respectively. Interestingly, at this reaction stage (6 h), the reaction proceeded with a yellow (optimal condition) to green color change (Figure S5). Analyzing the 1H NMR spectrum of crude product, the presence of additional aromatic and aliphatic hydrogen signals were observed (Figure S6), probably, due the correspondent formation of over-reduced products, justifying this abrupt color change. Finally, a gram-scale supply experiment was performed in order to evaluate the reproducibility of the optimal experimental condition, initially accessed in micro-scale (entry 6). However, 10.0 g of emodin (0.1 mol·L−1) was reduced to emodin anthrone with excellent yield after a reaction time of ca. 90 min, proving the efficiency of the set optimized parameters. Furthermore, the optimal protocol provided here can be extended to other anthraquinones derivatives.
Disclosure statement No potential conflict of interest was reported by the authors.
Funding This work was supported by Conselho Nacional de Desenvolvimento Científico e Tecnológico [grant number 308142/2014‐4].
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ORCID Renato Sonchini Gonçalves
http://orcid.org/0000-0003-2701-6101
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