Facile and scalable synthesis of baphicacanthin A by a two-pot procedure

Abstract Facile two-pot total synthesis of baphicacanthin A, a natural phenoxazinone alkaloid isolated from the roots of Baphicacanthus cusia which has been utilized as a traditional chinese medicine to effectively treat disease caused by coronavirus, has been developed from simple and commercially available starting materials. Catalytic aerobic oxidative cross-cyclocondensation of equimolar 2-aminophenol and 3-methoxy-2-hydroxylphenol in water was used to construct the key molecular skeleton 2-hydroxy-3H-phenoxazin-3-one. Gram scale synthesis was realized in 80% overall yield with practical convenience. Graphical Abstract


Intruduction
Contemporary natural product synthesis is pursuing more and more synthetic efficiency (Ball 2015;Newhouse et al. 2009). Synthetic endeavors are paid to not only invent innovative strategies and methods to generate efficiently the key skeletal bonds of a target molecule (Gao and Ma 2021), but also concern applicability and efficiency of synthetic process for the purpose of providing synthetic sample rapidly. Considering increasingly cognizance of sustainable chemistry (Isidro-Llobet et al. 2019), several metrics are proposed and well-accepted as rules for synthetic efficiency (Reisman and Maimone 2021), including atom economy (Trost 1991), redox economy (Newhouse et al. 2009), time economy and pot economy (Hayashi 2021), as well as scalability (Kuttruff 2014) and ideality (Peters et al. 2021).
In the past 20 years, the coronaviruses, COVID-19, SARS and Middle East respiratory syndrome have caused three global health crises. For treatment of these diseases, traditional chinese medicines have proven its effectiveness Zhao et al. 2021). Therefore, a traditional Chinese anti-viral medicine Nan-Ban-Lan-Gen, the root of Baphicacanthus cusia, has been listed as one of the 8 major anti-SARS medicines of China during the outbreak of SARS in 2003 (Feng et al. 2016), and arose a new wave of research (Li et al. 2018;Zhu et al. 2020). In 2016, Jiang and his co-authors isolated an anti-viral ingredient 2,4-dimethoxyl-3H-phenoxazin-3-one named as baphicacanthin A (1) from Nan-Ban-Lan-Gen (Feng et al. 2016;Wang et al. 2017). In 2019, its first and so far, the only total synthesis was reported (Ahn et al. 2019), which employed 2,6dimethoxy-1,4-benzoquinone as the starting material and gave a 22% total yield through six steps and five pots reactions.
Many endeavours have been paid to construct the dimeric natural products , involving the employment of oxidative cyclization (Lu et al. 2015;Liu et al. 2018;Jiang et al. 2021). Recently we described a novel and sustainable methodology for the preparation of the phenoxazin-3-one scaffold Li et al. 2021), which enables to produce 4-methoxyl-2-hydroxy-3H-phenoxazin-3-one (2) efficiently, the key molecular skeleton of 1. Considering its relevant anti-viral activity, we herein report a novel, efficient, and scalable two-pot synthesis of 1, involving catalytic aerobic oxidative cross-cyclocondensation in water and methylation (Scheme 1).

Results and discussion
Compound 2, as the core skeleton of 1, were synthesized through a catalytic aerobic oxidative cross-cyclocondensation with simple and commercially available 2-aminophenol (3) and 3-methoxy-2-hydroxylphenol (4) as the starting materials by following our protocol reported previously ) (Scheme 1). The synthesis was carried out on gram scale by using equimolar 3 and 4 with the combination of natural renewable gallic acid and Mn(OAc) 2 as a catalytic system (Song et al. 2019) under 0.3 MPa of dioxygen and pH 10. As we established before, this cross-cyclocondensation occurred in a one-pot tandem-reactions mode, involving aerobic oxidation of 4 to the corresponding o-benzoquinone intermediate, two consecutive 1,4-additions on 3 (an intermolecular aza-Michael addition and an intramolecular oxa-Michael addition) to form phenoxazine-diol skeleton, and two oxidations to afford 2. Remarkably, the reaction proceeded with a low catalyst loading [0.5 mol % gallic acid and 0.5 mol% Mn(OAc) 2] under mild conditions, and accomplished in a short course (1 h), showing excellent synthetic efficiency. In consideration of the low yield of the by-product, it was expected to be removed in the next reaction. So, the gram-scale test was conducted, followed by a simply work-up of acidification, extraction and concentration, which gave a 98% yield of the crude product with 99% HPLC purity. The crude product of first step without further purification was directly used in the methylation to prepare 1. The reaction also took place at room temperature. By a simply work-up operation of dilution, acidification, extraction, concentration and recrystallization, the gram-scale preparation gave an 80% yield of pure 1. Its 1 H NMR and 13 C NMR spectral data of synthetic 1 were in good accord with those of the natural product (Feng et al. 2016, Ahn et al. 2019).

General information
All starting materials and catalysts were purchased from commercial sources and used without further treatment unless noted. 1 H NMR and 13 C NMR spectra were recorded on 600 MHz or 400 MHz BRUKER spectrometers. High resolution mass spectra (HRMS) data were measured on an AB SCIEX TripleTOF 6600 Mass spectrometer by means of the positive ESI modes. The melting points were determined by an X-4 micro-melting point apparatus (Beijing, China). The pH values were determined by a REX PHS-3C pH meter of Shanghai Rex Instrument Factory. A KQ3200E ultrasonic cleaner of Kunshan Ultrasonic Instrument Co., Ltd. was used in all ultrasonication.

4-Methoxyl-2-hydroxy-3H-phenoxazin-3-one (2)
2-aminophenol (0.55 g, 5 mmol), 3-methoxy-2-hydroxylphenol (0.70 g, 5 mmol) and H 2 O (200 mL) were added into a 500-mL beaker. The mixture was treated by ultrasound sonication in an ultrasonic cleaner. After sonication, the resulting clear solution, gallic acid (0.5 mol%) and Mn(OAc) 2 (0.5 mol%) were transferred into the autoclave, and the pH value was adjusted to 10 by NaOH solution (1 M) under stirring. After the reactor was closed, the atmosphere over the mixture was exchanged with O 2 for three times. The reaction was stirred at 25 C under 0.3 MPa until no observation of the decrease of the oxygen pressure. Notably, when the pressure dropped down below 0.2 MPa, O 2 was recharged up to 0.3 MPa. The final reaction mixture was quenched by adding 10 mL water, acidized with HCl solution (1 M) to pH 1-2, and then centrifugated. The solid cake was washed with water for three times to afford the crude product of 2 (1.19 g), which was pure enough and used directly in the next step without further purification.

Conclusions
In summary, we have accomplished a novel, highly efficient and scalable total synthesis of baphicacanthin A that was found in traditional chinese anti-viral herbal Baphicacanthus cusia. The key reaction is an aerobic oxidative cross-cyclocondensation of equimolar 2-aminophenol and 3-methoxy-2-hydroxylphenol, which constructs the near-complete molecular structure of baphicacanthin A in a convergent way by onepot process in water. Gram scale synthesis with practical convenience further illustrates the practicability of our route.

Disclosure statement
No potential conflict of interest was reported by the authors.

Funding
The author(s) reported there is no funding associated with the work featured in this article.