Total synthesis of gnetifolin F (I)

182

183

1H NMR (401 MHz, CDCl3): δ 7.49-7.19 (m, 13H, Ph), 7.00-6.93 (m, 2H, Ph), 6.89-6.83 (m, 3H, H- 10, H-13, H-14), 6.68 (dd, J = 2.0, 0.9 Hz, 1H, H-4/6), 6.65 (m, 2H, H-2’, H-5’), 6.48 (dd, J = 2.0, 0.7 Hz, 1H, H-4/6), 6.39 (d, J = 8.3 Hz, 1H, H-6’), 5.65 (s, 1H, H-8), 5.15 (s, 2H, C12-OCH2Ph), 5.09 (s, 1H, H-7’), 5.08 (s, 2H, C5-OCH2Ph), 4.90 (d, J = 11.9 Hz, 1H, C3-OCHaHbPh), 4.83 (d, J = 11.9 Hz, 1H, C3-OCHaHbPh), 4.70 (t, J = 0.7 Hz, 1H, H-7), 4.09-3.99 (m, 2H, OCH2CH2TMS), 3.89 (s, 3H, C11-OCH3), 3.69 (s, 3H, C3’-OCH3), 3.61-3.47 (m, 2H, OCH2CH3), 1.18-1.13 (m, 2H, OCH2CH2TMS), 0.72 (t, J = 7.1 Hz, 3H, OCH2CH3), 0.04 (s, 9H, Si(CH3)3);

13C{¹H} NMR (101 MHz, CDCl3): δ 174.8 (s, C-9’), 166.2 (s, COOEt), 161.5 (s, C-5), 155.8 (s, C- 3), 150.0 (s, C-11), 148.9 (s, C-3’), 148.1 (s, C-12), 147.4 (s, C-4’), 143.3 (s, C-1), 137.0 (s, C12- OCH2Ph^ipso), 136.7 (s, C5- OCH2Ph^ipso), 136.5 (s, C3-OCH2Ph^ipso), 132.1 (s, C-9), 131.8 (s, C-1’), 128.8 (d, 2×C, Ph^meta), 128.7 (d, 2×C, Ph^meta), 128.4 (d, 2×C, Ph^meta), 128.4 (d, Ph^para), 128.0 (d, Ph^para), 127.9 (d, Ph^para), 127.8 (d, 2×C, C5-OCH2Ph^ortho), 127.4 (d, 2×C, C12-OCH2Ph^ortho), 127.0 (d, 2×C, C3-OCH2Ph^ortho), 124.7 (s, C-2), 120.4 (d, C-6’), 117.4 (d, C-14), 114.0 (d, C-13), 113.1 (d, C-2’), 112.4 (d, C-5’), 108.7 (d, C-10), 101.0 (d, C-4/6), 100.8 (d, C-4/6), 84.6 (d, C-8), 71.1 (t, C12- OCH2Ph), 70.7 (t, C5-OCH2Ph), 69.7 (t, C3-OCH2Ph), 66.5 (s, H-8’), 66.3 (t, OCH2CH2TMS), 62.0 (t, OCH2CH3), 56.4 (d, H-7’), 56.3 (d, H-7), 56.2 (q, C11-OCH3), 56.0 (q, C3’-OCH3), 17.8 (t, OCH2CH2TMS), 13.7 (q, OCH2CH3), −1.3 (q, Si(CH3)3);

MS (ESI+) m/z, (%): 1803 (35, [2M + H3O]⁺), 915 (16, [M + Na]⁺), 910 (100, [M + NH4]⁺), 893 (4, [M + H]⁺), 669 (37, [M – 3-methoxy-4-(2-(trimethylsilyl)-ethoxy)benzene]⁺);

HRMS (ESI+) m/z: [M + H]⁺ calcd. for C54H57O10Si 893.3715; found: 893.3715, [M + NH4]⁺ calcd.

for C54H60NO10Si 910.3981; found: 910.3980.

8’-Carbethoxy-9’-oxognetifolin F 3,5,12-O-tribenzyl ether (178)

Obtained from oxidative cyclisation along with 177, when using 1 N HCl/EA for workup.

Rf 0.15 (hexane/EA 3:1);

IR (film) 𝜈̃[cm^-1]: 3600-3400 (br.), 3450 (w), 3088 (w), 3063 (w), 3032 (w), 2936 (w), 2872 (w), 2845 (w), 1779 (m), 1732 (m), 1601 (m), 1514 (s), 1454 (m), 1373 (w), 1327 (w), 1268 (s), 1232 (m), 1152 (vs), 1030 (s), 857 (w), 810 (w), 738 (m), 697 (m);

1H NMR (401 MHz, CDCl3): δ 7.46-7.20 (m, 13H, Ph), 6.99-6.94 (m, 2H, Ph), 6.88-6.83 (m, 3H, H- 10, H-13, H-14), 6.71 (d, J = 8.2 Hz, 1H, H-5’), 6.68 (d, J = 2.0 Hz, 1H, H-4), 6.62 (broad, 1H, H-2’), 6.48 (d, J = 2.0 Hz, 1H, H-6), 6.38 (broad, 1H, H-6’), 5.65 (s, 1H, H-8), 5.15 (s, 2H, C12-OCH2Ph), 5.07 (s, 3H, H-7’, C5-OCH2Ph), 4.90 (d, J = 11.9 Hz, 1H, C3-OCH^aH^bPh), 4.83 (d, J = 11.9 Hz, 1H,

184

C3-OCH^aH^bPh), 4.69 (t, J = 0.7 Hz, 1H, H-7), 3.89 (s, 3H, C11-OMe), 3.70 (s, 3H, C3’-OMe), 3.60- 3.47 (m, 2H, OCH2CH3), 0.71 (t, J = 7.1 Hz, 3H, OCH2CH3);

13C{¹H} NMR (101 MHz, CDCl3): δ 174.1 (s, C-9’), 166.1 (s, COOEt), 161.5 (s, C-5), 155.8 (s, C- 3), 150.0 (s, C-11), 148.1 (s, C-12), 146.0 (s, C-3’), 144.8 (s, C-4’), 143.3 (s, C-1), 137.0 (s, C12- OCH2Ph^ipso), 136.7 (s, C5- OCH2Ph^ipso), 136.5 (s, C3-OCH2Ph^ipso), 132.0 (s, C-9), 131.3 (s, C-1’), 128.8 (d, 2×C, Ph^meta), 128.7 (d, 2×C, Ph^meta), 128.38 (d, 2×C, Ph^meta), 128.36 (d, Ph^para), 128.0 (d, Ph^para), 127.8 (d, Ph^para), 127.8 (d, 2×C, C5-OCH2Ph^ortho), 127.3 (d, 2×C, C12-OCH2Ph^ortho), 127.1 (d, 2×C, C3-OCH2Ph^ortho), 124.6 (s, C-2), 121.1 (d, C-6’), 117.4 (d, C-14), 114.0 (d, C-13), 113.9 (d, C- 2’), 112.2 (d, C-5’), 108.7 (d, C-10), 101.0 (d, C-4/6), 100.8 (d, C-4/6), 84.6 (d, C-8), 71.1 (t, C12- OCH2Ph), 70.7 (t, C5-OCH2Ph), 69.7 (t, C3-OCH2Ph), 66.4 (s, H-8’), 61.9 (t, OCH2CH3), 56.5 (d, H- 7’), 56.3 (d, H-7), 56.2 (q, C11-OCH3), 56.0 (q, C3’-OCH3), 13.6 (q, OCH2CH3);

MS (ESI−) m/z, (%): 1583 (8, [2M − H]⁻), 837 (15, [M + HCOO]⁻), 827 (19, [M + Cl]⁻), 791 (100, [M − H]⁻);

HRMS (ESI−) m/z: [M − H]⁻ calcd. for C49H43O10 791.2862; found: 791.2860.

Saponification-decarboxylation of 177

177 (120 mg, 134 µmol) was dissolved in dioxane (5 mL). A freshly prepared solution of KOH (490 mg, 8.7 mmol) in water (2 mL) was added. Vacuum was applied to the stirred reaction mixture until the solvent started to gently boil, followed by flushing with N2. The vaccum-N2 cycle was repeated 3 times. The reaction was then heated to reflux for 3 h. After cooling, the reaction mixture was partitioned between 30%w aqueous citric acid (20 mL) and DCM (30 mL). The aqueous layer was extracted with additional DCM (4×30 mL). The combined organic extracts were dried over Na2SO4, concentrated and resubmitted to the same saponification conditions. After three cycles of saponification/workup, the product was purified by flash chromatography (PE/EA, 11:1 to 3:1) to yield in order of elution 88.5 mg of 179a (80%) as colourless amorphous solid and 20.0 mg of 182 (17%) as a colourless amorphous solid. When first analysed by ¹H NMR, 179a contained an admixture, tentatively identified as 179b (dr 8:1). This admixture disappeared when the CDCl3 sample was remeasured after several hours.

Protected 9’-oxognetifolin F (179a)

Rf 0.52 (PE/EA 3:1);

185 IR (film) 𝜈̃[cm^-1]: 3089 (w), 3063 (w), 3032 (w), 2949 (w), 2931 (w), 2869 (w), 1766 (m), 1594 (m), 1512 (m), 1454 (w), 1420 (w), 1377 (w), 1332 (w), 1292 (w), 1250 (m), 1230 (m), 1138 (vs), 1069 (w), 1030 (s), 967 (w), 855 (m), 837 (m), 736 (m), 696 (m), 630 (w);

1H NMR (401 MHz, CDCl3): δ 7.49-7.23 (m, 13H, Ph), 7.06-7.08 (m, 2H, Ph), 6.90 (d, J = 8.2 Hz, 1H, H-13), 6.83-8.86 (m, 2H, H-10, H-14), 6.72-6.65 (m, 3H, H-2’, H-5’, H-6), 6.50 (d, J = 1.7 Hz, 1H, H-4), 6.44 (dd, J = 8.3, 2.1 Hz, 1H, H-6’), 5.58 (s, 1H, H-8), 5.17 (s, 2H, C12-OCH2Ph), 5.07 (s, 2H, C5-OCH2Ph), 4.95 (d, J = 12.0 Hz, 1H, C3-OCHaHbPh), 4.88 (d, J = 12.0 Hz, 1H, C3-OCHaHbPh), 4.86 (d, J = 1.1 Hz, 1H, H-7’), 4.15 (d, J = 7.6 Hz, 1H, H-7), 4.09-4.03 (m, 2H, OCH2CH2TMS), 3.92 (s, 3H, C11-OCH3), 3.68 (s, 3H, C3’-OCH3), 3.37 (dd, J =7.7, 1.2 Hz, 1H, H-8’), 1.20-1.14 (m, 2H, OCH2CH2TMS), 0.05 (s, 9H, Si(CH3)3);

13C{¹H} NMR (101 MHz, CDCl3): δ 179.1 (s, C-9’), 161.3 (s, C-5), 156.1 (s, C-3), 150.2 (s, C-11), 149.5 (s, C-3’), 148.4 (s, C-12), 147.0 (s, C-4’), 144.4 (s, C-1), 137.0 (s, C12-Ph^ipso), 136.8 (s, C5- Ph^ipso), 136.7 (s, C3-Ph^ipso), 136.2 (s, C-1’), 133.2 (s, C-9), 128.8 (d, 2×C, Ph^meta), 128.7 (d, 2×C, Ph^meta), 128.5 (d, 2×C, Ph^meta), 128.3 (d, Ph^para), 128.1 (d, Ph^para), 127.9 (d, Ph^para), 127.8 (d, 2×C, C5- Ph^ortho), 127.4 (d, 2×C, C12-Ph^ortho), 127.2 (d, 2×C, C3-Ph^ortho), 125.8 (s, C-2), 118.4 (d, C-6’), 117.4 (d, C-14), 114.3 (d, C-13), 113.0 (d, C-5’), 111.7 (d, C-2’), 109.0 (d, C-10), 101.4 (d, C-6), 100.5 (d, C-4), 85.5 (d, C-8), 71.2 (t, OCH2Ph), 70.7 (t, OCH2Ph), 69.8 (t, C3-OCH2Ph), 66.5 (t, OCH2CH2TMS), 56.4 (q, C11-OCH3), 56.0 (q, C3’-OCH3), 54.1 (d, C-7), 52.7 (d, C-8’), 51.2 (d, C- 7’), 17.9 (t, OCH2CH2TMS), −1.3 (q, Si(CH3)3);

MS (ESI+) m/z, (%): 1659 (85, [2M + H2O + H]⁺), 1005 (66), 921 (100, [M + TMS(CH2)2]⁺), 859 (74, [M + K]⁺), 843 (89, [M + Na]⁺), 838 (66, [M + NH4]⁺), 821 (66, [M + H]⁺);

HRMS (ESI+) m/z: [M + H]⁺ calcd. for C51H53O8Si 821.3504; found: 821.3497; [M + NH4]⁺ calcd.

for C51H56NO8Si 838.3775; found: 838.3773; m/z: [M + Na]⁺ calcd. for C51H52NaO8Si 843.3329;

found: 843.3322.

Protected 8-epi-9’-oxognetifolin F (179b)

Minor unstable diastereomer observed in samples of 179a, spontaneously converted to 179a in CDCl3.

1H NMR (401 MHz, CDCl3): δ 7.49-7.23 (m, 13H, Ph, overlaps with 179a), 7.07-7.08 (m, 2H, Ph, overlaps with 179a), 7.01 (d, J = 2.0 Hz, 1H, H-10), 6.96 (dd, J = 8.4, 2.1 Hz, 1H, H-14), 6.89 (d, J = 8.2 Hz, 1H, H-13), 6.72-6.65 (m, 3H, H-2’, H-5’, H-6, overlaps with 179a), 6.51 (d, J = 2.2 Hz, 1H, H-4), 6.36 (dd, J = 8.1, 2.1 Hz, 1H, H-6’), 6.04 (s, 1H, H-8), 5.17 (s, 2H, C12-OCH2Ph), 5.10 (s, 2H, C5-OCH2Ph), 4.92 (d, J = 12.1 Hz, 1H, C3-OCHaHbPh), 4.87 (d, J = 1.1 Hz, 1H, H-7’), 4.82 (d, J =

186

12.0 Hz, 1H, C3-OCHaHbPh), 4.32 (d, J = 7.8 Hz, 1H, H-7), 4.08-4.02 (m, 2H, OCH2CH2TMS), 3.91 (s, 3H, C11-OCH3), 3.67 (s, 3H, C3’-OCH3), 3.39 (dd, J = 7.8, 1.1 Hz, 1H, H-8’), 1.20-1.14 (m, 2H, OCH2CH2TMS, overlaps with 179a), 0.05 (s, 9H, Si(CH3)3, overlaps with 179a);.

Protected 9’-oxognetifolin F hydrate (182)

Minor side product, formed during the cyclisation leading to 179a in 2-17% yield. Stable in CDCl3. Rf 0.05 (hexane/EA 3:1);

1H NMR (401 MHz, CDCl3): δ 7.42-7.19 (m, 13H, Ph), 6.99 (d, J = 1.7 Hz, 1H, H-10), 6.92-6.90 (m, 2H, Ph), 6.86-6.87 (m, 2H, H-13, H-14), 6.65 (d, J = 8.3 Hz, 1H, H-5’), 6.56 (d, J = 2.0 Hz, 1H, H- 2’), 6.41 (d, J = 2.0 Hz, 1H, H-4/6), 6.40 (dd, J = 8.3, 2.0 Hz, 1H, H-6’), 6.20 (d, J = 2.0 Hz, 1H, H- 4/6), 5.14 (s, 2H, C12-OCH2Ph), 4.92 (d, J = 6.2 Hz, 1H, H-8), 4.88 (s, 2H, C5-OCH2Ph), 4.87 (d, J

= 12.0 Hz, 1H, C3-OCHaHbPh), 4.79 (d, J = 12.0 Hz, 1H, C3-OCHaHbPh), 4.75 (d, J = 9.6 Hz, 1H, H-7’), 4.25 (dd, J = 9.0, 6.2 Hz, 1H, H-7), 4.06-4.02 (m, 2H, OCH2CH2TMS), 3.87 (s, 3H, C11- OCH3), 3.80 (dd, J = 9.5, 9.2 Hz, 1H, H-8’), 3.67 (s, 3H, C3’-OCH3), 1.18-1.14 (m, 2H, OCH2CH2TMS), 0.04 (s, 9H, Si(CH3)3);

13C{¹H} NMR (101 MHz, CDCl3): δ 173.8 (s, C-9’), 160.4 (s, C-5), 155.7 (s, C-3), 149.9 (s, C-11), 148.8 (s, C-3’), 147.4 (s, C-12), 147.1 (s, C-4’), 143.6 (s, C-1), 137.2 (s, Ph^ipso), 136.9 (s, Ph^ipso), 136.8 (s, Ph^ipso), 135.4 (s, C-9), 133.3 (s, C-1’), 128.73 (d, 2×C, Ph^meta), 128.67 (d, 2×C, Ph^meta), 128.4 (d, 2×C, Ph^meta), 128.2 (d, Ph^para), 128.0 (d, Ph^para), 127.76 (d, Ph^para), 127.73 (d, 2×C, Ph^ortho), 127.5 (d, 2×C, Ph^ortho), 127.1 (d, 2×C, Ph^ortho), 126.7 (s, C-2), 120.3 (d, C-6’), 119.4 (d, C-14), 114.0 (d, C-13), 112.9 (d, C-2’/5’), 112.4 (d, C-2’/5’), 110.4 (d, C-10), 101.5 (d, C-4/6), 100.4 (d, C-4/6), 76.0 (d, C- 8), 71.2 (t, OCH2Ph), 70.2 (t, OCH2Ph), 69.6 (t, OCH2Ph), 66.2 (t, OCH2CH2TMS), 56.4 (q, OCH3), 56.0 (q, OCH3), 53.6 (d, C-7), 51.8 (d, C-8’), 49.6 (d, C-7’), 18.0 (t, OCH2CH2TMS), −1.3 (q, Si(CH3)3);

MS (ESI−) m/z, (%): 1675 (8, [2M − H]⁻), 837 (100, [M − H]⁻), 529 (9);

HRMS (ESI−) m/z: [M − H]⁻ calcd. for C51H53O9Si 837.3453; found: 837.3454.

9’-Oxognetifolin F 3,5,12-O-tribenzyl ether (181)

187 Side product from saponification of 177, formed during acidic workup using 1 N aqueous HCl or 1% TFA in DCM.

Rf 0.12 (PE/EA 3:1);

IR (film) 𝜈̃[cm^-1]: 3600-3250 (br.), 3062 (w), 3032 (m), 2928 (w), 2854 (w), 1764 (m), 1601 (m), 1513 (s), 1454 (m), 1376 (w), 1330 (w), 1263 (s), 1234 (m), 1212 (m), 1142 (vs), 1070 (w), 1030 (s), 851 (w), 811 (w), 737 (m), 697 (m), 629 (w);

1H NMR (401 MHz, CDCl3): δ 7.47-7.24 (m, 13H, Ph), 7.10-7.06 (m, 2H, Ph), 6.90 (d, J = 8.2 Hz, 1H, H-13), 6.87-8.82 (m, 2H, H-10, H-14), 6.75 (d, J = 8.2 Hz, 1H, H-5’), 6.72 (d, J = 2.0 Hz, 1H, H- 6), 6.67 (d, J = 1.9 Hz, 1H, H-2’), 6.50 (d, J = 1.9 Hz, 1H, H-4), 6.40 (dd, J = 8.2, 2.0 Hz, 1H, H-6’), 5.59 (s, 1H, H-8), 5.49 (br. s, 1H, 4’-OH), 5.17 (s, 2H, C12-OCH2Ph), 5.07 (s, 2H, C5-OCH2Ph), 4.96 (d, J = 11.9 Hz, 1H, C3-OCHaHbPh), 4.88 (d, J = 11.9 Hz, 1H, C3-OCHaHbPh), 4.85 (d, J = 1.1 Hz, 1H, H-7’), 4.16 (d, J = 7.6 Hz, 1H, H-7), 3.92 (s, 3H, C11-OCH3), 3.70 (s, 3H, C3’-OCH3), 3.37 (dd, J =7.7, 1.3 Hz, 1H, H-8’);

13C{¹H} NMR (101 MHz, CDCl3): δ 179.1 (s, C-9’), 161.3 (s, C-5), 156.1 (s, C-3), 150.2 (s, C-11), 148.4 (s, C-12), 146.4 (s, C-3’), 144.4 (s, C-1), 144.3 (s, C-4’), 137.0 (s, C12-Ph^ipso), 136.8 (s, C5- Ph^ipso), 136.7 (s, C3-Ph^ipso), 135.7 (s, C-1’), 133.1 (s, C-9), 128.8 (d, 2×C, Ph^meta), 128.7 (d, 2×C, Ph^meta), 128.5 (d, 2×C, Ph^meta), 128.3 (d, Ph^para), 128.1 (d, Ph^para), 127.9 (d, Ph^para), 127.8 (d, 2×C, C5- Ph^ortho), 127.4 (d, 2×C, C12-Ph^ortho), 127.2 (d, 2×C, C3-Ph^ortho), 125.8 (s, C-2), 119.0 (d, C-6’), 117.4 (d, C-14), 114.4 (d, C-13/5’), 114.3 (d, C-13/5’), 110.8 (d, C-2’), 109.0 (d, C-10), 101.4 (d, C-6), 100.5 (d, C-4), 85.5 (d, C-8), 71.2 (t, OCH2Ph), 70.7 (t, OCH2Ph), 69.8 (t, C3-OCH2Ph), 56.4 (q, C11- OCH3), 55.9 (q, C3’-OCH3), 54.1 (d, C-7), 52.7 (d, C-8’), 51.4 (d, C-7’);

MS (ESI−) m/z, (%): 1439 (6, [2M − H]⁻), 765 (14, [M + HCOO]⁻), 755 (22, [M + Cl]⁻), 719 (100, [M − H]⁻);

HRMS (ESI−) m/z: [M − H]⁻ calcd. for C46H39O8 719.2650; found: 719.2647.

Reduction of 179a with DIBAL

A solution of DIBAL (51 µL, 51 µmol, 1 M in toluene) was added to a solution of 179a (8.4 mg, 10 µmol) in dry THF (0.5 mL) at –78 °C. The mixture was stirred at –78 °C for 15 min., followed by gradual warming to –10 °C over 30 min. The reaction was quenched by a few drops of water. The mixture was partitioned between saturated potassium sodium tartrate (10 mL) and EA (50 mL). The organic layer was washed with water (10 mL) and brine (10 mL), dried over Na2SO4, concentrated