Expanding Heteroaromatic and 2-Aminosugar Chemical Space Accessible from the Biopolymer Chitin

Abstract

Herein, we report the expansion of chemical space available from chitin, accessible via the biogenic N-platforms 3A5AF, M4A2C, and di-HAF. The biologically active heteroaromatics furo[3,2-d]pyrimidin-4-one and furo[3,2-d]pyrimidin-4-amine can be selectively accessed from 3A5AF and M4A2C, respectively. The chiral pool synthon di-HAF is a viable substrate for Achmatowicz rearrangement, providing streamlined access to 2-aminosugars possessing a versatile hydroxymethyl group at C5.

1. Introduction

The nitrogen (N) atom in fine and commodity chemicals is derived from the commodity ammonia: a base chemical produced on an enormous scale using the energy-intensive Haber process [1,2,3,4]. To reduce the carbon footprint of organonitrogen chemicals, their manufacture can be conducted in a Haber-independent fashion by sourcing nitrogen from the huge quantities of biogenic nitrogen available on earth, with one accessible source being the biopolymer chitin [5,6]. Indeed, several reports describing the valorisation of chitin (or its monomer, N-acetyl-d-glucosamine; GlcNAc) into biogenic N-platforms have appeared [7,8,9,10,11,12], including the functionally rich furans 3-acetamido-5-furfuryl aldehyde (3A5F) [13], 3-acetamido-5-acetylfuran (3A5AF) [14] and dihydroxyethyl acetamidofuran (di-HAF) [15] (Scheme 1).

Over the past few years, we have showcased the utility of 3A5AF in the Haber-independent synthesis of the natural product proximicin A [16], 3-azafurans [17,18], new heteroaromatic scaffolds [19], 2-aminosugars [20] and in a diversity-oriented synthesis (DOS) programme that furnished a number of structurally distinct N-heterocycles [21]. We recently demonstrated that the inherent chirality present in chitin can be transferred to the natural product epi-leptosphaerin A via the chiral pool di-HAF platform [22]. Other research groups have shown that 3A5AF (and derivatives) [23,24] and di-HAF [25] can serve as dienes in the Diels-Alder reaction, while 3A5F has shown significant promise as a platform chemical, including as a bioconjugation handle for N-cysteine’s modification [13].

Our ongoing efforts in this area have focused on using the aforementioned N-platforms to access the chemical intermediates used within the pharmaceutical and/or agrochemical sectors. One target that fulfils these criteria is furo[3,2-d]pyrimidin-4-amine 1, a heteroaromatic scaffold found in the C-nucleoside antibiotic pyrrolosine [26,27,28] and employed in a multitude of medicinal chemistry programmes against a diverse range of targets, including spleen tyrosine kinases [29], G protein-coupled receptor 119 [30], p110δ PI3 kinase [31], and FYVE-type finger-containing phosphoinositide kinases [32]. We envisaged that the heteroaromatic ring system 1 could be accessible from 3A5AF via cyanation at C2 followed by acetamide hydrolysis to provide aminonitrile 2 followed by heteroannulation with formamidine [27,28,33,34] (Scheme 2). Although only one nitrogen atom in 1 is sourced from chitin, our aim was to source other N-atoms from N-compounds present in nature (q.v).

2. Materials and Methods

2.1. General Information

Commercially available starting materials, reagents, and solvents were used as received unless otherwise noted. In case anhydrous conditions were applied, the reaction was performed under an atmosphere of dry nitrogen in oven-dried (100 °C) glassware, and the solvent was dried by passing through a column of activated alumina under nitrogen using an LC Technology solvent purification system. Thin layer chromatography (TLC) was performed using F254 0.2 mm silica plates, followed by visualisation with UV irradiation at 254 nm and 366 nm and staining with ethanolic vanillin solution. Flash column chromatography was performed using 63−100 μm silica gel. The melting points were determined on Kofler hot-stage apparatus (John Morris, New Zealand) and were uncorrected. High-resolution mass spectra were recorded on a Bruker micrOTOF Q mass spectrometer (Massachusetts, USA) operated in the positive ion mode. The standard electrospray ion (ESI) source was used to generate the ions. The instrument was operated in the m/z 50−1000 range. Infrared (IR) spectra were recorded using a a PerkinElmer FTIR Spectrometer (Massachusetts, USA) with a universal attenuated total reflectance (ATR) attachment installed. Absorption maxima were expressed as the wavenumber (cm⁻¹). NMR spectra were recorded at room temperature in CDCl₃, (CD₃)₂SO, or (CD₃)₂CO solutions using a Bruker NMR spectrometer (Massachusetts, USA) operating at 400 MHz for the ¹H nuclei and 100 MHz for the ¹³C nuclei. All chemical shifts were reported in parts per million (ppm) scale and measured relative to the protium solvent in which the sample was analysed: CDCl₃ (δ 7.26 ppm for ¹H NMR and δ 77.16 ppm for ¹³C NMR), (CD₃)₂SO (δ 2.50 ppm for ¹H NMR and δ 39.52 ppm for ¹³C NMR) or (CD₃)₂CO (δ 2.05 ppm for ¹H NMR and δ 29.84 ppm for ¹³C NMR). Coupling constants, J, are reported in Hertz [Hz] where applicable. Multiplicities are reported as “s” (singlet), “d” (doublet), and “br s” (broad singlet). X-ray diffraction measurements of single crystals were performed on a Rigaku Oxford Diffraction XtaLAB-Synergy-S single-crystal diffractometer (Tokyo, Japan) with a PILATUS 200K hybrid pixel array detector using Cu Kα radiation (λ = 1.54184 Å). The data were processed with the SHELX2018-3 and Olex2 software packages [35,36,37]. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were inserted at calculated positions or located directly and either refined with a riding model or without restrictions. Mercury 2020.3.1 [38] was used to visualise the molecular structure. Crystal growth for X-ray crystallographic analysis purposes was achieved using slow evaporation or slow vapour diffusion. All other relevant data are contained in this article and the Supplementary Material.

2.1.1. 2-Hydroxyimino-3-acetamido-5-acetylfuran (4)

A solution of hydroxylamine hydrochloride (195 mg, 2.8 mmol) and sodium carbonate (148 mg, 1.4 mmol) in water (10 mL) was added dropwise to a solution of 2-formylfuran 3 [18] (488 mg, 2.5 mmol) in methanol (25 mL) at 0 °C. The solution was stirred at room temperature for 2 h, then concentrated in vacuo, diluted in ethyl acetate (80 mL), and washed with water (40 mL) and brine (40 mL). The organic layer was dried over Na₂SO₄, filtered, and concentrated in vacuo. The title compound (489 mg, 2.3 mmol, 93%; dr 10:1) was used in the next step without any purification.

2.1.2. 2-Cyano-3-acetamido-5-acetylfuran (5)

A solution of oxime 4 (250 mg, 1.2 mmol) in acetic anhydride (5 mL) was stirred at 50 °C for 16 h. The reaction mixture was concentrated in vacuo to obtain acetoxyiminofuran, which was dissolved in acetonitrile (5 mL). Triethylamine (0.50 mL, 3.6 mmol) was added, and the solution was stirred at 50 °C for 2.5 h. The solution was concentrated in vacuo and purified by flash column chromatography on silica gel eluting with ethyl acetate-light petroleum (2:3) to provide the title compound (196 mg, 1.0 mmol, 85%) as a yellow solid; mp 126.0–127.5 °C; HRMS [ESI, (M + Na)⁺]: calcd. for [C₉H₈N₂O₃ + Na]⁺ 215.0433, found 215.0427 $\nu$_max/cm⁻¹ (ATR) 3286, 3236, 3088, 2227, 1690, 1559, 1528, 1366, 1276, 1223, 1141, 928; ¹H NMR (400 MHz, acetone-d₆): δ 9.86 (1 H, br s, NH), 7.67 (1 H, s, ArH), 2.50 (3 H, s, Me), 2.17 (3 H, s, Me); ¹³C NMR (100 MHz, acetone-d₆): δ 186.7 (C), 169.2 (C), 154.4 (C), 136.8 (C), 111.7 (CH), 111.4 (C), 26.3 (Me), 23.3 (Me), 1 × C not observed.

2.1.3. 5-Acetyl-3-amino-2-carboxamidofuran (6)

To a solution of 2-cyanofuran 5 (50 mg, 0.26 mmol) in dry methanol (0.6 mL), a solution of HCl was added in dioxane (4 M, 0.6 mL) at 0 °C. The reaction mixture was stirred at 0 °C for 10 min, quenched with saturated NaHCO₃ (15 mL), and then extracted with ethyl acetate (3 × 15 mL). The organic layer was dried with anhydrous Na₂SO₄, filtered, and concentrated in vacuo. The title compound (44 mg, 0.26 mmol, 100%) was obtained as a yellow solid, which was used in the next step without any purification; mp 192.5–193.5 °C; HRMS [ESI, (M + Na)⁺]: calcd. for [C₇H₈N₂O₃ + Na]⁺ 191.0427, found 191.0429; $\nu$_max/cm⁻¹ (ATR) 3427, 3352, 3152, 1648, 1610, 1570, 1359, 1319, 1194, 940, 775; ¹H NMR (400 MHz, DMSO-d₆): δ 7.30 (2 H, br s, NH₂), 6.86 (1 H, s, ArH), 5.47 (2 H, s, NH₂), 2.43 (3 H, s, Me); ¹³C NMR (100 MHz, DMSO-d₆): δ 187.3 (C), 162.0 (C), 149.3 (C), 141.4 (C), 129.0 (C), 110.2 (CH), 25.8 (Me).

2.1.4. 6-Acetylfuro[3,2-d]pyrimidin-4-one (7)

A solution of 2-carboxamidofuran 6 (20 mg, 0.12 mmol) in ethanol (4 mL) formamidine acetate (125 mg, 1.2 mmol) was added, and the reaction mixture was stirred at reflux for 7 h. The solution was concentrated in vacuo, diluted in ethyl acetate (20 mL), and washed with saturated Na₂CO₃ (10 mL) and brine (10 mL). The organic layer was dried with anhydrous Na₂SO₄, filtered, and concentrated in vacuo. The residue was purified by flash chromatography on silica gel, eluting with ethyl acetate to obtain the title compound (7.7 mg, 0.043 mmol, 36%) as a yellow oil; HRMS [ESI, (M + Na)⁺]: calcd. for [C₈H₆N₂O₃ + Na]⁺ 201.0271, found 201.0271; ¹H NMR (400 MHz, DMSO-d₆): δ 12.83 (1 H, br s, NH), 8.14 (1 H, s, ArH), 7.84 (1 H, s, ArH), 2.57 (3 H, s, Me); ¹³C NMR (100 MHz, DMSO-d₆): δ 187.7 (C), 154.5 (C), 152.1 (C), 147.3 (C), 147.1 (CH), 139.7 (C), 113.6 (CH), 26.5 (Me).

2.1.5. Methyl 4-acetamido-5-formylfuran-2-carboxylate (8)

A solution of phosphoryl chloride (0.28 mL, 3.0 mmol) was added in dry dimethylformamide (2.0 mL) at 0 °C dropwise to a solution of M4A2C [16] (366 mg, 2.0 mmol). The reaction mixture was warmed to room temperature and stirred for 3 h. Water (10 mL) was added at 0 °C, and the mixture was stirred for another 10 min. The aqueous phase was neutralised with NaHCO₃ and extracted with ethyl acetate (3 × 30 mL), dried with anhydrous Na₂SO₄, and filtered and concentrated in vacuo. The residue was purified by flash chromatography on silica gel, eluting with ethyl acetate-light petroleum (1:1) to obtain the title compound (262 mg, 1.2 mmol, 62%) as a yellow solid; mp 175.0–176.5 °C; HRMS [ESI, (M + Na)⁺]: calcd. for [C₉H₉NO₅ + Na]⁺ 234.0373, found 234.0368; $\nu$_max/cm⁻¹ (ATR) 3341, 3197, 1754, 1697, 1667, 1590, 1434, 1329, 1219, 1199, 984, 792, 769, 741; ¹H NMR (400 MHz, acetone-d₆) δ 9.87 (1 H, s, CH), 9.52 (1 H, br s, NH), 7.93 (1 H, s, ArH), 3.93 (3 H, s, Me), 2.24 (3 H, s, Me); ¹³C NMR (100 MHz, acetone-d₆) δ 181.3 (C), 169.8 (C), 158.8 (C), 147.2 (C), 140.7 (C), 135.0 (C), 113.3 (CH), 52.9 (Me), 23.9 (Me).

2.1.6. Methyl 4-Acetamido-5-[(hydroxyimino)methyl]furan-2-carboxylate (9)

A solution of hydroxylamine hydrochloride (83 mg, 1.2 mmol) and sodium carbonate (64 mg, 0.6 mmol) in water (4 mL) was added dropwise to a solution of 8 (211 mg, 1.0 mmol) in methanol (8 mL) at room temperature. The reaction mixture was stirred at room temperature for 1.5 h. Methanol was removed in vacuo, and the reaction mixture was diluted with ethyl acetate (40 mL) before being washed with water (20 mL) and brine (20 mL). The aqueous phase was extracted with ethyl acetate (40 mL), and the combined organic phases were dried with anhydrous Na₂SO₄, filtered, and concentrated in vacuo. The title compound (224 mg, 0.99 mmol, 99%, dr 10:1) was obtained as a yellow solid, which was used in the next step without further purification.

2.1.7. Methyl 4-Acetamido-5-cyanofuran-2-carboxylate (10)

A solution of oxime 9 (226 mg, 1 mmol) in acetic anhydride (3 mL) was stirred at 70 °C for 3 h. After the formation of acetoxyiminofuran, acetic anhydride was concentrated in vacuo. Acetonitrile (4 mL) and triethylamine (0.3 mL, 2 mmol) were added to the residue, and the resultant solution was stirred at reflux for 3 h. The reaction mixture was concentrated under reduced pressure and purified by flash chromatography on silica gel, eluting with ethyl acetate-light petroleum (1:1) to obtain the title compound (181 mg, 0.87 mmol, 87%) as a yellow solid; mp 219.5–220.5 °C; HRMS [ESI, (M + Na)⁺]: calcd. for [C₉H₈N₂O₄ + Na]⁺ 231.0376, found 231.0376; $\nu$_max/cm⁻¹ (ATR) 3285, 3087, 2229, 1737, 1696, 1533, 1320, 1226, 1149, 985, 815, 769; ¹H NMR (400 MHz, DMSO-d₆): δ 10.82 (1 H, s, NH), 7.41 (1 H, s, ArH), 3.86 (3 H, s, Me), 2.10 (3 H, s, Me); ¹³C NMR (100 MHz, DMSO-d₆): δ 168.7 (C), 156.9 (C), 145.8 (C), 134.6 (C), 116.1 (C), 112.2 (CH), 111.0 (C), 52.7 (Me), 22.9 (Me).

2.1.8. Methyl 4-Amino-5-cyanofuran-2-carboxylate (11)

To a solution of 10 (62 mg, 0.30 mmol) in dry methanol (3 mL) at 0 °C, hydrochloric acid was added (4 M in dioxane, 0.2 mL). The solution was stirred at 40 °C for 6 h. The reaction mixture was neutralised with solid NaHCO₃ at 0 °C, followed by the addition of ethyl acetate (15 mL) and water (15 mL). The layers were separated, and the aqueous phase was extracted with ethyl acetate (2 × 15 mL). The combined organic phases were dried over Na₂SO₄, filtered, and concentrated in vacuo. The crude mixture was purified by flash chromatography on silica gel, eluting with ethyl acetate-light petroleum (1:1) to obtain the title compound (43 mg, 0.26 mmol, 86%) as a yellow solid; mp 165.0–166.0 °C; HRMS [ESI, (M + Na)⁺]: calcd. for [C₇H₆N₂O₃ + Na]⁺ 189.0271, found 189.0270; $\nu$_max/cm⁻¹ (ATR) 3430, 3343, 3232, 2210, 1720, 1607, 1329, 1224, 1176, 765; ¹H NMR (400 MHz, DMSO-d₆): δ 6.85 (1 H, s, ArH), 6.17 (2 H, br s, NH₂), 3.81 (3 H, s, Me); ¹³C NMR (100 MHz, DMSO-d₆): δ 157.2 (C), 147.0 (C), 145.8 (C), 112.7 (C), 110.8 (CH), 109.1 (C), 52.4 (Me).

2.1.9. Methyl 4-Aminofuro[3,2-d]pyrimidine-6-carboxylate (12)

To a solution of 4-amino-5-cyanofuran 11 (37 mg, 0.22 mmol) in acetic acid (0.2 mL), formamidine acetate was added (70 mg, 0.69 mmol), and the reaction mixture was stirred at reflux for 5 h. The solution was diluted with ethyl acetate (20 mL), washed with brine (10 mL), and saturated with NaHCO₃ (10 mL). The aqueous layer was extracted with ethyl acetate (2 × 10 mL), and the combined organic layers were dried over Na₂SO₄, filtered, and concentrated in vacuo. The residue was purified by flash chromatography on silica gel, eluting with ethyl acetate-light petroleum (7:3) to obtain the title compound (23 mg, 0.12 mmol, 54%) as a colourless solid; mp 218 °C (dec); HRMS [ESI, (M + H)⁺]: calcd. for [C₈H₇N₃O₃ + H]⁺ 194.0560, found 194.0564; $\nu$_max/cm⁻¹ (ATR) 3434, 3284, 3008, 2960, 1732, 1660, 1301, 1203, 1094, 969, 761; ¹H NMR (400 MHz, DMSO-d₆): δ 8.31 (1 H, s, ArH), 7.71 (2 H, br s, NH₂), 7.67 (1 H, s, ArH), 3.92 (3 H, s, Me); ¹³C NMR (100 MHz, DMSO-d₆): δ 158.6 (C), 154.5 (CH), 150.4 (C), 148.2 (C), 147.3 (C), 135.4 (C), 113.6 (CH), 52.6 (Me).

2.1.10. (R)-N-(5-(2-((tert-Butyldimethylsilyl)oxy)-1-hydroxyethyl)furan-3-yl)acetamide (15)

To a solution of di-HAF [15] (55 mg, 0.297 mmol) in acetonitrile (3.5 mL), imidazole was added (80.9 mg, 1.19 mmol, 4 eq.) with tert-butyldimethylsilyl chloride (42.5 mg, 0.282 mmol, 0.95 eq.) at room temperature. The reaction mixture was heated to 40 °C for 30 min and then concentrated in vacuo. The residue was purified by flash chromatography, eluting with light petroleum–acetone (5:1) to obtain the title compound (51 mg, 0.170 mmol, 58%), which was embedded within an inseparable mixture alongside its regioisomer (95:5); m.p. 58.6–59.8 °C; HRMS (ESI) m/z [M + Na]⁺, calcd for C₁₄H₂₅NO₄SiNa 322.1445, found 322.1435; $\nu$_max/cm⁻¹ (ATR) 3279, 2929, 2857, 1659, 1569, 1463, 1378, 1254, 1119, 1071, 837, 779; ¹H NMR (400 MHz, (CD₃)₂CO): δ 9.18 (1 H, br s, NH), 7.89 (1 H, s, CH), 6.28 (1 H, s, CH), 4.60 (1 H, q, J 5.5, CH), 4.35 (1 H, d, J 5.5, OH), 3.83 (2 H, dq, J 5.5, 6.4, CH₂), 2.02 (3 H, s, Ac), 0.87 (9 H, s, (Me)₃), 0.05 (3 H, s, Me), 0.04 (3 H, s, Me); ¹³C NMR (100 MHz, (CD₃)₂CO): δ 162.8 (C), 155.0 (C), 131.2 (CH), 126.7 (C), 102.3 (CH), 69.3 (CH), 67.0 (CH₂), 26.2 (3 × Me), 23.0 (COMe), 18.8 (C), −5.2 (Me), −5.3 (Me).

2.1.11. N-((6R)-6-(((tert-Butyldimethylsilyl)oxy)methyl)-2-hydroxy-5-oxo-5,6-dihydro-2H-pyran-3-yl)acetamide (16)

To a solution of 15 (150 mg, 0.501 mmol) in dichloromethane-methanol (12 mL: 3 mL) at 0 °C, m-CPBA was added (77%, 123.5 mg, 0.551 mmol, 1.1 eq.) and the reaction mixture was stirred at 0 °C for 2 h. Na₂CO₃ (15 mL) was added to the solution and stirred for 30 min. A saturated solution of NaHCO₃ (15 mL) was added to the reaction mixture and stirred for another 30 min. The mixture was extracted with ethyl acetate (200 mL), washed with cold Na₂CO₃-NaHCO₃ (1:1, 4 × 50 mL), cold brine (2 × 30 mL), dried (Na₂SO₄), and filtered and concentrated in vacuo. The residue was purified by flash column chromatography on silica gel, eluting with light petroleum-ethyl acetate (1:1) to obtain the title compound (130 mg, 0.412 mmol, 82%, dr 1:0.6) as a colourless solid; m.p. 145.8–146.8 °C; HRMS (ESI) m/z [M + Na]⁺, calcd for C₁₄H₂₅NO₅SiNa 338.1394, found 338.1385; $\nu$_max/cm⁻¹ (ATR) 3290, 2930, 2858, 1719, 1642, 1628, 1518, 1472, 1369, 1228, 1205, 1121, 1043, 958, 872, 825, 780, 729, 708 ¹H NMR (400 MHz, (CD₃)₂CO) Major diastereomer: δ 9.21 (1 H, br s, NH), 6.79 (1 H, s, CH), 5.60 (1 H, s, CH), 4.41 (1 H, q, J 2.6, CH), 4.00 (2 H, qd, J 4.5, 2.5 CH₂), 2.13 (3 H, s, Ac), 0.86 (9 H, s, (Me)₃), 0.05 (3 H, s, Me), 0.03 (3 H, s, Me); ¹³C NMR (100 MHz, (CD₃)₂CO) 195.2 (C), 171.0 (C), 153.5 (C), 108.3 (CH), 89.2 (CH), 76.0 (CH), 64.1 (CH₂), 26.2 ((Me)₃), 24.6 (Ac), 18.90 (C(Me)₃), −5.1 (SiMe), −5.2 (SiMe); Minor diastereomer: (400 MHz, (CD₃)₂CO) δ 8.93 (1 H, br s, NH), 6.91 (1 H, s, CH), 5.48 (1 H, s, CH), 4.21 (1 H, dd, J 2.8, CH), 4.00 (2 H, qd, J 4.4, 2.7, CH₂), 2.17 (3 H, s, Ac), 0.87 (9 H, s, (Me)₃), 0.08 (3 H, s, Me), 0.05 (3 H, s, Me); ¹³C NMR (100 MHz, (CD₃)₂CO): δ 194.8 (C), 171.1 (C), 154.3 (C), 109.0 (CH), 89.6 (CH), 80.6 (CH), 64.8 (CH₂), 26.1 ((Me)₃), 24.7 (Ac), 18.87 (C(Me)₃), −5.29 (SiMe), −5.34 (SiMe).

2.1.12. (R)-N-(6-(((tert-Butyldimethylsilyl)oxy)methyl)-2,5-dioxo-5,6-dihydro-2H-pyran-3-yl)acetamide (17)

To a stirring suspension of Celite (1 g) and pyridinium chlorochromate (502 mg, 2.33 mmol, 2 eq.) in dry dichloromethane (20 mL), 16 (367 mg, 1.16 mmol) was added in one portion at 0 °C. The mixture was warmed to room temperature and stirred for 5 h. The mixture was filtered through a plug of Celite and concentrated in vacuo. The residue was purified by flash column chromatography on silica gel, eluting with light petroleum-ethyl acetate (2:1) to obtain the title compound as a colourless solid (241 mg, 0.770 mmol, 66%), m.p. 112–113.6 °C; ${\left[\alpha \right]}_D^{23}$ +56.0 (c 0.1, CH₂Cl₂); HRMS (ESI) m/z [M + Na]⁺, calcd for C₁₄H₂₃NO₅SiNa 336.1238, found 336.1240; $\nu$_max/cm⁻¹ (ATR) 3302, 2957, 2930, 2884, 2858, 1708, 1662, 1626, 1485, 1378, 1332, 1299, 1250, 1209, 1109, 1075, 1022, 997, 918, 879, 836, 778, 733; ¹H NMR (400 MHz, CDCl₃): δ 8.30 (1 H, br s, NH), 7.63 (1 H, s, CH), 4.88 (1 H, dd, J 1.7, CH), 4.03 (2 H, dq, CH₂), 2.24 (3 H, s, Ac), 0.78 (9 H, s, C(Me)₃, 0.00 (3 H, s, Si-Me), −0.02 (3 H, s, Si-Me); ¹³C NMR (100 MHz, CDCl₃) δ 192.5 (C), 169.4 (C), 160.2 (C), 138.3 (C), 117.2 (CH), 85.1 (CH), 64.7 (CH₂), 25.6 (CMe₃), 25.0 (Ac), 18.1 (C), −5.6 (Si-Me), −5.7 (Si-Me).

2.1.13. N-((5R,6R)-6-(((tert-Butyldimethylsilyl)oxy)methyl)-5-hydroxy-2-oxo-5,6-dihydro-2H-pyran-3-yl)acetamide (18)

A solution of 17 (120 mg, 0.382 mmol) in dichloromethane-methanol (1.8:2.6 mL) was cooled to −78 °C and cerium trichloride (4.7 mg, 0.0191, 5 mol%) was added, followed by sodium borohydride (22 mg, 0.574 mmol, 1.5 eq.). The mixture was stirred at −78 °C for 3 h and then extracted with ethyl acetate (80 mL). The organic layer was washed with water (2 × 20 mL) and brine (20 mL) before being dried (Na₂SO₄), filtered and concentrated in vacuo. The residue was purified by flash column chromatography on silica gel, eluting with light petroleum-ethyl acetate (3:1) to obtain the title compound as a colourless solid (dr 93:7) (100 mg, 0.317 mmol, 83%), m.p. 144.8–146.0 °C; HRMS (ESI) m/z [M + Na]⁺, calcd for C₁₄H₂₅NO₅SiNa 338.1394, found 338.1394; $\nu$_max/cm⁻¹ (ATR) 3322, 2955, 2930, 2858, 1725, 1671, 1552, 1472, 1384, 1340, 1242, 1191, 1123, 1087, 1006, 974, 941, 895, 835, 776, 745, 668; ¹H NMR (400 MHz, CDCl₃) Major diastereomer: δ 7.83 (1 H, s, NH), 7.65 (1 H, d, J 6.8, CH), 4.54 (1 H, dd, J 2.5, CH), 4.41 (1 H, dq, J 2.5, 1.1, CH), 4.03 (2 H, dq, J 6.5, 5.3, CH₂), 2.14 (3 H, s, Ac), 0.91 (9 H, s, C(Me)₃), 0.121 (3 H, s, Si-Me), 0.117 (3 H, s, Si-Me), OH not observed; ¹³C NMR (100 MHz, CDCl₃): δ 169.4 (C), 162.1 (C), 127.1 (C), 121.2 (CH), 79.8 (CH), 61.9 (CH₂), 61.8 (CH), 25.9 (3 x Me), 24.7 (Ac), 18.3 (C(Me)₃), −5.3 (Si-Me), −5.4 (Si-Me).

3. Results and Discussion

The 3A5AF-derived furfural 3 [18] was converted to the oxime 4 (no purification), which, upon dehydration, allowed 2-cyano-3A5AF 5 to be obtained at an excellent yield over the three steps (Scheme 3). In our experience, the acetamide hydrolysis of 3A5AF derivatives does not proceed well under aqueous conditions, often leading to extensive amounts of degradation due to the elevated temperatures required. We have found anhydrous acid-mediated methanolysis to be more reliable, given that it proceeds at lower temperatures. The methanolysis of the acetamide was successful in providing 6, but NMR spectroscopic analysis that the revealed hydrolysis of the nitrile also occurred, promoted by the water produced upon the reaction of HCl with MeOH. Despite this unpredicted result, we subjected 6 to heteroannulation with formamidine to provide furo[3,2-d]pyrimidin-4-one with 7. While not the originally intended target, furo[3,2-d]pyrimidin-4-ones have found utility in medicinal programmes targeting the kinesin spindle protein [39] and ubiquitin proteasome system 7 (USP7) [40].

The original plan contained limitations as we did not anticipate the facile hydrolysis of nitrile in 5. In pursuit of the original target, furo[3,2-d]pyrimidin-4-amine scaffold, the chitin-derived amino acid derivative methyl 4-acetamidofuran-2-carboxylate (M4A2C) [16] was chosen as the substrate, anticipating that the lower-electron withdrawing ester would reduce the reactivity of the nitrile, making it less susceptible to hydrolysis (Scheme 4A). The Vilsmeier formylation of M4A2C provided furfural 8, which was converted to 2-cyanofuran 10 (via oxime 9) in an excellent overall yield. The impact of the ester on M4A2C was compelling; the methanolysis selectively cleaved the acetamide in 9, leaving the cyano group intact. Finally, heteroannulation with formamidine gave furo[3,2-d]pyrimidin-4-amine 12. In products 7 and 12, only one nitrogen atom was sourced from chitin. However, hydroxylamine can be considered biogenic; it is a product of the nitrification process and is widely distributed throughout nature [41]. Formamidine is available from the natural product cyanamide [42,43], which is itself biosynthesised from the amino acid l-canavanine [44] (Scheme 4B). It is also possible that biogenic formamidine could be produced upon the conversion of the urea to thiourea, followed by reductive desulfurisation [45].

The synthesis of furo[3,2-d]pyrimidin-4-amine 12 and furo[3,2-d]pyrimidin-4-one 7 from 3A5AF expanded the heteroaromatic chemical space available from chitin. We would also like to report our preliminary results on the oxidative ring expansion of the chiral pool synthon, di-HAF. We previously reported the synthesis of enantioenriched 2-aminosugars from 3A5AF [20]; however, this method contains some drawbacks (Scheme 5A). The introduction of artificial chirality using a Noyori reduction was cumbersome (50 °C, 1 week reaction time) in providing furfuryl alcohol 13, as upon oxidative ring expansion, it gave N-acetyl-l-rednose (RedNAc). From here, several 2-aminosugars were available, but they all contained a methyl group at C5 that restricted modifications at this site. It was anticipated that these drawbacks could be overcome if the chitin-derived, chiral pool synthon di-HAF successfully underwent an Achmatowicz reaction; not only would the natural chirality present in chitin be transferred to the product (thus eliminating the need for a Noyori reduction), but the resultant 2-aminosugar 14 would possess a versatile hydroxymethyl group at the C5 position (Scheme 5B). Moreover, 14 is a novel 2-aminosugar scaffold that could be challenging to prepare from N-acetyl-d-glucosamine (GlcNAc) [46,47,48,49].

To facilitate the easy handling of the anticipated products, di-HAF was monosilylated at the primary alcohol to provide 15, alongside small quantities of its regioisomer that could not be separated by column chromatography (Scheme 6). The Achmatowicz rearrangement proceeded smoothly to obtain the somewhat unstable 2-aminosugar 16 at a good yield. The oxidation of the lactol helped to stabilise the scaffold, affording dione 17. Luche reduction in the C4-ketone provided 18 with good diastereoselectivity, and the structure of the major syn-diastereomer was confirmed by X-ray crystallographic analysis (Figure 1).

To conclude, further biogenic N-chemical space has been accessed from the chitin-derived platforms 3A5AF, M4A2C, and di-HAF. The high-value heteroaromatic scaffolds furo[3,2-d]pyrimidin-4-one 7 and furo[3,2-d]pyrimidin-4-amine 12 can be selectively obtained from the biogenic N-platforms 3A5AF and M4A2C, respectively. Moreover, the chiral di-HAF scaffold is a viable substrate for Achmatowicz rearrangement, generating new, enantioenriched 2-aminosugar chemicals possessing chiral centres that are traceable back to the chitin biopolymer, with all products possessing a versatile 5-hydroxymethyl handle at C5.