Cercosporin-Photocatalyzed [4+1]- and [4+2]-Annulations of Azoalkenes Under Mild Conditions

Philippe Icyishaka; Chenxing Li; Liushen Lu; Wenhao Bao; Jia Li; Yan Zhang; Yijian Rao

doi:10.3791/60786

JoVE Journal > Chemistry

Chemistry

Cercosporin-Photocatalyzed [4+1]- and [4+2]-Annulations of Azoalkenes Under Mild Conditions

Published: July 17, 2020

doi:

10.3791/60786

Philippe Icyishaka*¹, Chenxing Li*¹, Liushen Lu¹, Wenhao Bao¹, Jia Li¹, Yan Zhang², Yijian Rao¹

¹Key Laboratory of Carbohydrate Chemistry and Biotechnology, Ministry of Education, School of Biotechnology,Jiangnan University, ²School of Pharmaceutical Science,Jiangnan University

Summary

New routes for the synthesis of nitrogen-containing heterocycles utilizing cercosporin as a metal-free photocatalyst were developed.

Abstract

The interest on nitrogen-containing heterocycles has expanded rapidly in the synthetic community since they are important motifs for new drugs. Traditionally, they were synthesized through thermal cycloaddition reactions, whereas today, photocatalysis is preferred due to the mild and efficient conditions. With this focus, a new photocatalytic method for the synthesis of nitrogen-containing heterocycles is highly desired. Here, we report a protocol for the biosynthesis of cercosporin, which could function as a metal-free photocatalyst. We then illustrate cercosporin-photocatalyzed protocols for the synthesis of nitrogen-containing heterocycles 1,2,3-thiadiazoles through annulation of azoalkenes with KSCN, and synthesis of 1,4,5,6-tetrahydropyridazines [4+2] through cyclodimerization of azoalkenes under mild conditions, respectively. As a result, there is a new bridge between the microbial fermentation method and organic synthesis in a mild, cost-effective, environmentally friendly and sustainable manner.

Introduction

Nitrogen-containing heterocycles have drawn much attention since they are not only important skeletons for a wide range of natural products with bioactivities, but also the synthetic precursors for agrochemicals and drug molecules¹^,². Among the various N-heterocycles, 1,2,3-thiadiazoles³^,⁴ and 1,4,5,6-tetrahydropyridazines⁵^,⁶ are the most important molecules, which are utilized as versatile intermediates in the synthetic chemistry (Figure 1). Since modification of their functional groups always induces distinctive pharmacological activities, extensive efforts have been devoted to developing effective strategies for the synthesis of nitrogen-containing heterocycles and they were mostly synthesized through thermal cycloaddition reactions⁷^,⁸^,⁹^,¹⁰. Nowadays, to meet the requirements of sustainable development and green chemistry, photocatalysis has exerted great importance and advantages¹¹^,¹²^,¹³^,¹⁴, which includes effectiveness¹⁵^,¹⁶^,¹⁷^,¹⁸^,¹⁹ and avoidance of stoichiometric reagents for the activation²⁰^,²¹. The powerful and versatile four-unit intermediates, azoalkenes (1,2-diaza-1,3-dienes)²²^,²³^,²⁴^,²⁵^,²⁶^,²⁷^,²⁸^,²⁹, have been employed as precursors in metal-based Ru(bpy)₃Cl₂-photocatalyzed reactions with high efficiency for the annulation of halogeno hydrazine and ketocarbonyls³⁰. Furthermore, it was also used in the metal-free Eosin Y photocatalyzed system, but affording the desired product in only 7% yield. Since metal-free photocatalysts show great advantage over transition metal-based photocatalysts, regarding to the environmental factor as well as the cheaper prices¹⁸^,¹⁹, it is highly important to develop new metal-free photocatalytic systems for the synthesis of N-heterocycles.

Cercosporin³¹^,³²^,³³^,³⁴^,³⁵, hypocrellin³⁶^,³⁷^,³⁸^,³⁹^,⁴⁰, elsinochrome⁴¹ and phleichrome⁴²^,⁴³ (Figure 2) belong to perylenequinonoid pigments (PQPs) in nature and are produced by endophytic fungi, which have been widely investigated regarding to their photophysical and photobiological properties, and applied in photodynamic therapy and photophysical diagnosis, due to their strong absorption in UV-vis region and unique properties of photosensitization³⁶^,⁴⁴^,⁴⁵^,⁴⁶^,⁴⁷. Upon irradiation, those PQPs can be prompted to excited state and then generate active species through energy transfer (EnT) and electron transfer (ET)³⁵^,³⁸^,⁴⁴^,⁴⁸^,⁴⁹^,⁵⁰^,⁵¹^,⁵²^,⁵³^,⁵⁴. Thus, we envisioned that these natural PQPs may be utilized as "metal-free" photocatalysts to drive organic reactions, which have rarely been investigated⁵⁵^,⁵⁶^,⁵⁷^,⁵⁸^,⁵⁹.

Herein, we report the protocol for the biosynthesis of cercosporin from liquid fermentation and then apply it as a metal-free photocatalyst for the [4+1] annulation reaction of azoalkenes and KSCN, as well as the [4+2] cyclodimerization of azoalkenes, which supply 1,2,3-thiadiazoles and 1,4,5,6- tetrahydropyridazines with high efficiency under mild conditions, respectively (Figure 3).

Protocol

NOTE: α-Halo-N-acyl-hydrazones were prepared according to a published procedure⁶⁰. All the solvents and other chemical reagents were obtained from commercial sources without further purification. We first described the synthesis of α-Halo-N-acyl-hydrazones and the biosynthesis of cercosporin as a metal-free photocatalyst. Next, we illustrated the protocols of the cercosporin-photocatalyzed reactions for the synthesis of 1,2,3-thiadiazoles and 1,4,5,6-tetrahydropyridazines.

CAUTION: All the manipulation should be conducted cautiously wearing gloves, lab-coat, and goggles. It is highly recommended to carefully read the MSDS for each chemical and solvent used in those reactions and purification process. Chemicals can be weighed out on a balance on the bench. All the organic reactions should be set up in the fume hood and the purification process should also be carried out in a fume hood.

1. Preparation of α-Halo-N-acyl-hydrazones

Weigh out 10 mmol of ketone and 10 mmol of benzoyl hydrazine into a flask.
Add 20 mL of CH₃OH to the flask.
Equip the flask with a rubber stopper and a stirring bar.
Inject 0.25 mL of HCl slowly into the mixture.
Incubate the flask in the air at room temperature for 4 h.
Collect the precipitate after reaction by filtration and wash with acetone.
Dry the product by vacuum and identify by NMR.

2. Preparation of cercosporin

Charge a 3 L shake flask with 1 L of S-7 medium.
Inoculate the cercosporin-producing strain⁵⁶ into the shake flask.
Culture the mixture under light conditions at 135 r/min, 25 °C for 2 weeks.
Subject the fermentation broth to vacuum filtration using a vacuum pump to obtain the supernatant and pellet.
Collect the pellet and dry it in a freeze dryer.
Extract the pellet and the supernatant separately with 3 x 50 mL of dichloromethane.
Combine the organic phases and wash with water 2-3 times.
Concentrate the organic phase under vacuum.
Re-dissolve the residue with analytical methanol, and filter through a 0.18 µm organic microfiltration membrane.
Purify the cercosporin with a Sephadex LH-20 column and identify by HPLC.

3. Preparation of 1,2,3-thiadiazoles

Weigh out the α-Halo-N-acyl-hydrazone (0.2 mmol, 1.0 eq), 1 mg of cercosporin (0.002 mmol, 0.01 equiv.), 27 mg of ^tBuOK (1.2 equiv) and 39 mg of KSCN (2 equiv) into a 10 mL Schlenk tub equipped with a rubber stopper and a stirring bar.
Purge the Schlenk tube with O₂ three times.
Inject dry CH₃CN (2 mL) to the Schlenk tube.
Subject the Schlenk tube to a 5 W blue LED from the bottom for 16 h.
Wash with 4 x 15 mL of saturated NaCl solution and combine the aqueous phase.
Re-extract the aqueous phase with 4 x 15 mL of ethyl acetate.
Combine organic phase and dry with anhydrous Na₂SO₄.
Remove the solvent with vacuum evaporator.
Purify the product 3 by silica gel column chromatography (eluent, petroleum: ethyl acetate = 10:1) and identify by NMR.

4. Preparation of 1,4,5,6-tetrahydropyridazine

Weigh out the α-Halo-N-acyl-hydrazone (0.5 mmol), 2.7 mg of cercosporin (0.01 equiv), and 195 mg of Cs₂CO₃ (1.2 equiv) into a 10 mL Schlenk tub equipped with a rubber stopper and a stirring bar.
Purge the Schlenk tube with N₂ three times.
Inject CH₃CN/H₂O (10:1, 2 mL) to the Schlenk tube.
Subject the Schlenk tube to a 5 W blue LED from the bottom for 16 h.
Wash with 4 x 15 mL of saturated NaCl solution and combine the aqueous phase.
Re-extract the aqueous phase with 4 x 15 mL of ethyl acetate.
Combine organic phase and dry with anhydrous Na₂SO₄.
Remove the solvent with vacuum evaporator.
Purify the product 4 by silica gel column chromatography (eluent, petroleum: ethyl acetate = 10:1) and identify by NMR.

Representative Results

Synthesis of α-Halo-N-acyl-hydrazones: They are synthesized according to Protocol 1.

Synthesis of cercosporin: It was synthesized and purified according to Protocol 2. ¹H NMR (400 MHz, CDCl₃): δ ppm 14.82 (s, 2H, ArH), 7.06 (s, 2H, ArH), 5.57 (s, 2H, CH₂), 4.20 (s, 6H, 2OCH₃), 3.62-3.57 (m, 2H, CH₂), 3.42-3.37 (m, 2H, CH₂), 2.93-2.88 (m, 2H, CH₂), 0.63 (d, 6H, J = 8 Hz, 2CH₃) (Figure 4). ¹³C NMR (101 MHz, CDCl₃): δ ppm 207.0, 181.8, 167.4, 163.4, 152.8, 135.4, 130.6, 127.9, 112.9, 109.3, 108.2, 92.6, 68.1, 61.2, 42.2, 19.3. HRMS (ESI-Q-TOF) exact mass calcd for C₂₉H₂₅O₁₀ [M-H]^– 533.1448, found 533.1468.

Synthesis of 4-Phenyl-1,2,3-thiadiazole (3a): It was synthesized and purified using Protocol 3 with 88% yield. ¹H NMR (400 MHz, CDCl₃): δ ppm 8.66 (s, 1H), 8.07-8.05 (m, 2H), 7.55-7.44 (m, 3H) (Figure 5). ¹³C NMR (100 MHz, CDCl₃): δ ppm 162.9, 130.8, 129.9, 129.4, 129.2, 127.4 (Figure 6). HRMS (ESI-Q-TOF) exact mass calcd for C₈H₇N₂S [M+H]⁺ 162.0330, found 163.0349.

Synthesis of 4-(4-Fluorophenyl)-1,2,3-thiadiazole (3b): It was synthesized and purified using Protocol 3 with 72% yield. ¹H NMR (400 MHz, CDCl₃): δ ppm 8.60 (s, 1H), 8.09-8.02 (m, 2H), 7.19-7.19 (m, 2H). ¹³C NMR (100 MHz, CDCl₃): δ ppm 164.3-161.9 (d, J_C-F = 240 Hz), 161.3, 133.6, 129.8 (d, J_C-F = 9.0 Hz), 127.8 (d, J_C-F = 3.0 Hz), 116.7 (d, J_C-F = 22.0 Hz). HRMS (ESI-Q-TOF) exact mass calcd for C₈H₆FN₂S [M+H]⁺ 181.0196, found 181.0191.

Synthesis of 4-(4-Chlorophenyl)-1,2,3-thiadiazole (3c): It was synthesized and purified using Protocol 3 with 87% yield. ¹H NMR (400 MHz, CDCl₃): δ ppm 8.65 (s, 1H), 8.00 (d, J = 8 Hz, 2H), 7.50 (d, J = 8 Hz, 2H). ¹³C NMR (100 MHz, CDCl₃): δ ppm 162.6, 135.5, 132.4, 129.4, 128.9, 128.7. HRMS (ESI-Q-TOF) exact mass calcd for C₈H₆ClN₂S [M+H]⁺ 196.9940, found 196.9940.

Synthesis of 4-(4-Bromophenyl)-1,2,3-thiadiazole(3d): It was synthesized and purified using Protocol 3 with 78% yield. ¹H NMR (400 MHz, CDCl₃): δ ppm 8.66 (s, 1H), 7.94 (d, J = 8 Hz, 2H), 7.65 (d, J = 8 Hz, 2H). ¹³C NMR (100 MHz, CDCl₃): δ ppm 161.2, 134.3, 132.7, 130.4, 129.6, 119.1. HRMS (ESI-Q-TOF) exact mass calcd for C₈H₆BrN₂S [M+H]⁺ 240.9435, found 240.9429.

Synthesis of (3,6-Diphenyl-5,6-dihydropyridazin-1(4H)-yl)(phenyl)methanone (4a): It was synthesized and purified using Protocol 4 with 80% yield. ¹H NMR (400 MHz, CDCl₃): δ ppm 7.84-7.82 (m, 2H), 7.60-7.58 (m, 2H), 7.49-7.44 (m, 3H), 7.33-7.30 (m, 5H), 7.26-7.24 (m, 1H), 7.18 (d, J = 8 Hz, 2H), 6.09 (s, 1H), 2.71-2.67 (m, 1H), 2.43-2.16 (m, 3H) (Figure 7).

Synthesis of (3,6-Bis(4-fluorophenyl)-5,6-dihydropyridazin-1(4H)-yl)(phenyl)methanone (4b): It was synthesized and purified using Protocol 4 with 72% yield. ¹H NMR (400 MHz, CDCl₃): δ ppm 7.80-7.78 (m, 2H), 7.57-7.55 (m, 2H), 7.52-7.43 (m, 3H), 7.16-7.12 (m, 2H), 7.03-6.97 (m, 4H), 6.05 (s, 1H), 2.69-2.65 (m, 1H), 2.40-2.25 (m, 2H), 2.18-2.13 (m, 1H). ¹³C NMR (100 MHz, CDCl₃): δ ppm 170.2, 163.4 (d, ¹J_C-F = 248.1 Hz), 162.0 (d, ¹J_C-F = 244.1 Hz), 146.0, 135.5 (d, ⁴J_C-F = 3.1 Hz), 135.1, 133.2 (d, ⁴J_C-F = 3.2 Hz), 130.4, 129.9, 127.5, 127.2 (d, ³J_C-F = 8.2 Hz), 127.1 (d, ³J_C-F = 8.0 Hz), 115.7 (d, ²J_C-F = 21.5 Hz), 115.4 (d, ²J_C-F = 21.6 Hz), 50.9, 24.0, 18.7. ¹⁹F NMR (376 MHz, CDCl₃) (ppm) -111.7, -115.5. HRMS (ESI-Q-TOF) exact mass calcd for C₂₃H₁₉F₂N₂O [M+H]⁺ 377.1465, found 377.1482.

Synthesis of (3,6-Bis(4-chlorophenyl)-5,6-dihydropyridazin-1(4H)-yl)(phenyl)methanone (4c): It was synthesized and purified using Protocol 4 with 70% yield. ¹H NMR (400 MHz, CDCl₃): δ ppm 7.78 (d, J = 4 Hz, 2H), 7.50-7.43 (m, 5H), 7.30-7.26 (m, 5H), 7.10 (d, J = 8 Hz, 2H), 6.03 (s, 1H), 2.68-2.63 (m, 1H), 2.39-2.26 (m, 2H), 2.20-2.11 (m, 1H). ¹³C NMR (100 MHz, CDCl₃): δ ppm 170.2, 145.8, 138.3, 135.4, 135.3, 134.9, 133.2, 130.5, 129.9, 129.0, 128.6, 127.5, 126.9, 126.6, 51.2, 29.7, 19.8, 18.6. HRMS (ESI-Q-TOF) exact mass calcd for C₂₃H₁₉Cl₂N₂O [M+H]⁺ 409.0874, found 409.0864.

Synthesis of (3,6-Bis(4-bromophenyl)-5,6-dihydropyridazin-1(4H)-yl)(phenyl)methanone (4d): It was synthesized and purified using Protocol 4 with 82% yield. ¹H NMR (400 MHz, CDCl₃): δ ppm 7.78 (d, J = 8 Hz, 2H), 7.52-7.40 (m, 9H), 7.04 (d, J = 8 Hz, 2H), 6.01 (s, 1H), 2.67-2.62 (m, 1H), 2.39-2.25 (m, 2H), 2.20-2.11 (m, 1H). ¹³C NMR (100 MHz, CDCl₃): δ ppm 170.2, 145.9, 138.9, 135.8, 134.8, 132.0, 131.6, 130.5, 129.9, 127.5, 127.2, 126.9, 119.6, 121.2, 51.3, 29.7, 19.8, 18.5. HRMS (ESI-Q-TOF) exact mass calcd for C₂₃H₁₉Br₂N₂O [M+H]⁺ 498.9845, found 498.9799.

These representative results demonstrate how 4-aryl-1,2,3-thiadiazoles and 1,4,5,6-tetrahydropyridazines can be conveniently synthesized by cercosporin-catalyzed photocatalytic reactions from α-Halo-N-acyl-hydrazone (Figure 8).

4-aryl-1,2,3-thiadiazoles were obtained with those conditions: 1 (0.2 mmol), KSCN (0.4 mmol), ^tBuOK (0.24 mmol), CH₃CN (2.0 mL), cercosporin (1 mol%), 5 W blue LED, 16 h, at room temperature under O₂ atmosphere (Figure 3 and Figure 8). The procedure was suitable for substrates bearing both electron-donating and electron-accepting groups on the phenyl ring, providing the desired products with moderate to good yields.

1,4,5,6-tetrahydropyridazines were obtained with those conditions: 1 (0.5 mmol), Cs₂CO₃ (1.2 equiv) and cercosporin (1 mol%) in the mixture of MeCN and H₂O (10:1) under N₂ atmosphere (Figure 3 and Figure 8). The desired products were obtained in good to excellent yields.

Figure 1: Bioactive molecules with N-heterocycles motifs. Adapted with permission from Zhang Y., Cao Y., Lu L. S., Zhang S. W., Bao W. H., Huang S. P., Rao Y. J. Perylenequinonoid-Catalyzed [4+1]-and [4+2]-Annulations of Azoalkenes: Photocatalytic Access to 1, 2, 3-Thiadiazole/1, 4, 5, 6-Tetrahydropyridazine Derivatives, Journal of Organic Chemistry. 84 (12), 7711-7721, (2019). Copyright (2019) American Chemical Society. Please click here to view a larger version of this figure.

Figure 2: Representative perylenequinonoid pigments in nature. Adapted with permission from Zhang Y., Cao Y., Lu L. S., Zhang S. W., Bao W. H., Huang S. P., Rao Y. J. Perylenequinonoid-Catalyzed [4+1]-and [4+2]-Annulations of Azoalkenes: Photocatalytic Access to 1, 2, 3-Thiadiazole/1, 4, 5, 6-Tetrahydropyridazine Derivatives, Journal of Organic Chemistry. 84 (12), 7711-7721, (2019). Copyright (2019) American Chemical Society. Please click here to view a larger version of this figure.

Figure 3: Cercosporin-Catalyzed Synthesis of 1,2,3-Thiadiazoles and 1,4,5,6- Tetrahydropyridazines. Adapted with permission from Zhang Y., Cao Y., Lu L. S., Zhang S. W., Bao W. H., Huang S. P., Rao Y. J. Perylenequinonoid-Catalyzed [4+1]-and [4+2]-Annulations of Azoalkenes: Photocatalytic Access to 1, 2, 3-Thiadiazole/1, 4, 5, 6-Tetrahydropyridazine Derivatives, Journal of Organic Chemistry. 84 (12), 7711-7721, (2019). Copyright (2019) American Chemical Society. Please click here to view a larger version of this figure.

Figure 4: ¹H-NMR spectrum of cercosporin (400 MHz, CDCl₃). Reprinted with permission from Zhang Y., Cao Y., Lu L. S., Zhang S. W., Bao W. H., Huang S. P., Rao Y. J. Perylenequinonoid-Catalyzed [4+1]-and [4+2]-Annulations of Azoalkenes: Photocatalytic Access to 1, 2, 3-Thiadiazole/1, 4, 5, 6-Tetrahydropyridazine Derivatives, Journal of Organic Chemistry. 84 (12), 7711-7721, (2019). Copyright (2019) American Chemical Society. Please click here to view a larger version of this figure.

Figure 5: Representative ¹H-NMR spectrum of 3a (400 MHz, CDCl₃). Reprinted with permission from Zhang Y., Cao Y., Lu L. S., Zhang S. W., Bao W. H., Huang S. P., Rao Y. J. Perylenequinonoid-Catalyzed [4+1]-and [4+2]-Annulations of Azoalkenes: Photocatalytic Access to 1, 2, 3-Thiadiazole/1, 4, 5, 6-Tetrahydropyridazine Derivatives, Journal of Organic Chemistry. 84 (12), 7711-7721, (2019). Copyright (2019) American Chemical Society. Please click here to view a larger version of this figure.

Figure 6: Representative ¹³C-NMR spectrum of 3a (400 MHz, CDCl₃). Reprinted with permission from Zhang Y., Cao Y., Lu L. S., Zhang S. W., Bao W. H., Huang S. P., Rao Y. J. Perylenequinonoid-Catalyzed [4+1]-and [4+2]-Annulations of Azoalkenes: Photocatalytic Access to 1, 2, 3-Thiadiazole/1, 4, 5, 6-Tetrahydropyridazine Derivatives, Journal of Organic Chemistry. 84 (12), 7711-7721, (2019). Copyright (2019) American Chemical Society. Please click here to view a larger version of this figure.

Figure 7: Representative ¹H-NMR spectrum of 4a (400 MHz, CDCl₃). Reprinted with permission from Zhang Y., Cao Y., Lu L. S., Zhang S. W., Bao W. H., Huang S. P., Rao Y. J. Perylenequinonoid-Catalyzed [4+1]-and [4+2]-Annulations of Azoalkenes: Photocatalytic Access to 1, 2, 3-Thiadiazole/1, 4, 5, 6-Tetrahydropyridazine Derivatives, Journal of Organic Chemistry. 84 (12), 7711-7721, (2019). Copyright (2019) American Chemical Society. Please click here to view a larger version of this figure.

Figure 8: Cercosporin-Catalyzed Synthesis of 4-aryl-1,2,3-thiadiazoles and 1,4,5,6-tetrahydropyridazines. Adapted with permission from Zhang Y., Cao Y., Lu L. S., Zhang S. W., Bao W. H., Huang S. P., Rao Y. J. Perylenequinonoid-Catalyzed [4+1]-and [4+2]-Annulations of Azoalkenes: Photocatalytic Access to 1, 2, 3-Thiadiazole/1, 4, 5, 6-Tetrahydropyridazine Derivatives, Journal of Organic Chemistry. 84 (12), 7711-7721, (2019). Copyright (2019) American Chemical Society. Please click here to view a larger version of this figure.

Discussion

Nitrogen-containing heterocycles are important motifs for many new drugs and were traditionally synthesized through thermal cycloaddition reactions. Due to great interest, a new photocatalytic method for the synthesis of these compounds is highly desired. To take advantage of the excellent photosensitization properties of cercosporin, we applied cercosporin as a metal-free photocatalyst in two categories of annulation reactions to synthesize nitrogen-containing heterocycles.

First, we reported the protocol of cercosporin-photocatalyzed [4+1] annulation of azoalkenes with KSCN under standard conditions: α-halo-N-acyl-hydrazone 1 (0.2 mmol), ^tBuOK (1.2 equiv), KSCN 2 (2 equiv), cercosporin (0.01 equiv), dry CH₃CN (2 mL), and the resulting mixtures were subjected to 5 W blue LED for 16 h under an O₂ atmosphere. KSCN functionalized as an ambident nucleophilic unit here. Cercosporin, ^tBuOK, blue light and O₂ were all prerequisites for this reaction. CH₃ CN supplied the best yield of product and 0.01 equiv. of cercosporin was the optimized ratio.

Second, we reported the protocol of cercosporin-photocatalyzed [4+2] annulation of azoalkenes under standard conditions: α-halo-N-acyl-hydrazone 1 (0.5 mmol), Cs₂CO₃ (1.2 equiv), cercosporin (0.01 equiv) (CH₃CN/H₂O = 10:1) 2 mL, and the resulting mixtures were subjected to a 5 W blue LED for 16 h under a N₂ atmosphere. The control experiments have been done for the [4+2] reaction as it was for the [4+1] reaction. In this protocol, the addition of water and Cs₂CO₃ was critical for the self-condensation of α-halo-N-acyl-hydrazone. The ratios of water and Cs₂CO₃ were also critical to provide best yield for the product.

In summary, we have reported the biosynthesis protocol for cercosporin and then applied it as a metal-free photocatalyst for the synthesis of N-heterocycles 4-aryl-1,2,3-thiadiazoles and 1,4,5,6-tetrahydropyridazines under mild conditions, through [4+1] annulation of azoalkenes with KSCN and [4+2] annulation of azoalkenes, respectively. Those reactions made use of cost-effective 5 W LED and could be processed easily, which supplied a new application in synthesis. Most importantly, we built a bridge between biosynthesis and organic synthesis for the design of N-heterocycles in a mild, cost-effective, environmentally friendly and sustainable manner.

Disclosures

The authors have nothing to disclose.

Acknowledgements

We thank for the National Key R&D Program of China (2018YFA0901700), Natural Science Foundation of Jiangsu Province (Grants No. BK20160167), the Thousand Talents Plan (Young Professionals), the Fundamental Research Funds for the Central Universities (JUSRP51712B), the National First-class Discipline Program of Light Industry Technology and Engineering (LITE2018-14) and Postdoctoral Foundation in Jiangsu Province (2018K153C) for the funding support.

Materials

2,4'-Dibromoacetophenone	ENERGY	D0500850050
2'-bromo-4-chloroacetophenone	ENERGY	A0500400050
2-Bromo-4'-fluoroacetophenone	ENERGY	A050037-5g
2-Bromoacetophenone	ENERGY	A0500870050
4-Bromobenzhydrazide	ENERGY	B0103390010
4-Chlorobenzhydrazide	ENERGY	D0511130050
4-Fluorobenzhydrazide	ENERGY	B010461-5g
5 W blue LED	PHILIPS	29237328756
Benzoyl hydrazine	ENERGY	D0500610250
CH₂Cl₂	SINOPHARM	80047360
CH₃CN	SINOPHARM	S3485101
CH₃OH	SINOPHARM	100141190
Cs₂CO₃	ENERGY	E060058-25g
Ethyl acetate	SINOPHARM	40065986
freeze dryer	LABCONCO	7934074
HPLC	Agilent	1260 Infinity II
KSCN	ENERGY	E0104021000
Na₂SO₄	SINOPHARM	51024461
organic microfiltration membrane	SINOPHARM	92412511
S-7 medium			Gluose 1g; Fructose 3g; Sucrose 6g; Sodium acetate 1g; Soytone 1g; Phenylalanine 5mg; Sodium benzoate 100mg; 1M KH₂P0₄ buffer ph6.8; Biotin 1mg; Ca(NO₃)₂ 6.5mg; Pyridoxal 1mg; Calcium pantothenate 1mg; Thiamine 1mg; MnCl₂ 5mg; FeCl₃ 2mg; Cu(NO₃)₂ 1mg; MgSO₄ 3.6mg; ZnSO₄ 2.5mg
Schlenk tub	Synthware	F891910
sephadex LH-20 column	GE	17009001
shaker	Lab Tools	BSH00847
silica gel	ENERGY	E011242-1kg
^tBuOK	ENERGY	E0610551000
vacuum bump	Greatwall	SHB-III
vacuum evaporator

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Icyishaka, P., Li, C., Lu, L., Bao, W., Li, J., Zhang, Y., Rao, Y. Cercosporin-Photocatalyzed [4+1]- and [4+2]-Annulations of Azoalkenes Under Mild Conditions. J. Vis. Exp. (161), e60786, doi:10.3791/60786 (2020).

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