3. Materials and Methods
1H-NMR spectra were obtained at 400 MHz in CDCl
3 or D
2O with chemical shift values (δ) in ppm downfield from tetramethylsilane in the case of CDCl
3 and
13C-NMR spectra were obtained at 100.61 MHz in CDCl
3 (see
Supplementary Materials). Assignments are supported by 2D correlation NMR studies. Medium pressure preparative column chromatography: silica gel 60H. Analytical TLC: Aluminium-backed silica gel 60 F254. Specific rotations ([α]
20D) were measured using an automatic polarimeter. Reagents and solvents were purified and dried according to the literature [
27]. All reactions were carried out under an inert atmosphere (argon), except when the solvents were undried. The enantiomeric excesses were determined by HPLC on a Waters 600E/U6K instrument using a Daicel Chiralpack AD-H column (see
Supplementary Materials).
[1-13C]-2-bromo-N-methoxy-N-methylacetamide (8). [1-13C]-2-bromoacetic acid (7, 510 mg, 3.64 mmol) was dissolved in dry toluene (18 mL), under argon atmosphere, and the solution was cooled to 0 °C. NHMe(OMe) (805 μL, 10.93 mmol) was added, and 10 min later a solution of PBr3 (173 μL, 1.82 mmol) in 4 mL of toluene was added dropwise. After stirring for 10 min at 0 °C, the reaction was warmed to room temperature, left stirring during 10 min and then warmed up to 60 °C. After the starting material was consumed (TLC, approximately 30 min), a saturated aqueous solution of NaHCO3 (35 mL) was added, and the aqueous phase was extracted with 3 portions of 18 mL of AcOEt. The combined organic layers were dried with MgSO4, filtered and concentrated under reduced pressure. The crude was purified by flash column chromatography (7:3 Hexane:AcOEt), and 586 mg of compound 8 was obtained as a colourless oil (Yield = 88%). 1H-NMR (400 MHz, CDCl3) δ (ppm): 4.02 (d, 2JH-C = 4.0 Hz, 2H, CH2), 3.80 (s, 3H, OMe), 3.24 (d, 3JH-C = 2.1 Hz, 3H, NMe). 13C-NMR (101 MHz, CDCl3) δ (ppm): 167.8 (C1), 61.8 (OCH3), 32.6 (NCH3), 25.3 (d, 1JC-C = 57.2 Hz, CH2). FT-IR (neat): 1618.6 (C=O). HRMS: calcd. for [C313CH8BrNNaO2]+: 204.96642; found: 204.96647.
[1-13C]-2-acetoxy-N-methoxy-N-methylacetamide (9). To a solution of glacial AcOH (293 μL, 5.12 mmol) in dry 1,2-dichloroethane (2 mL), under argon atmosphere, Et3N (713 μL, 5.12 mmol) was added dropwise. Then, a solution of 8 (586 mg, 3.20 mmol) in dry 1,2-dichloroethane (2 mL) was added, and the reaction was heated at reflux temperature. Once the starting material was consumed (TLC, approximately 1 h), 50 mL of HCl 5% was added, and the aqueous phase was extracted with 3 portions of 55 mL of CH2Cl2. The combined organic layers were dried with MgSO4, filtered and concentrated under reduced pressure, rendering 446 mg of compound 19 that was used without any further purification (colourless oil, yield = 86%). 1H-NMR (400 MHz, CDCl3) δ (ppm): 4.83 (d, 2JC-H = 4.1 Hz, 2H, AcOCH2-), 3.74 (s, 3H, OMe), 3.20 (d, 3JC-H = 2.1 Hz, 3H, NMe), 2.18 (s, 3H, CH3C(O)O). 13C-NMR (101 MHz, CDCl3) δ (ppm): 170.8 (CH3C(O)O), 168.1 (C(O)N), 61.6 (OCH3), 61.1 (d, 1JC-C = 57.0 Hz, AcOCH2), 32.4 (NCH3), 20.7 (CH3C(O)O). FT-IR (neat): 1743.3 (AcO), 1639.1 (C(O)N). HRMS: calcd for [C513CH11NNaO4]+: 185.06138; found: 185.06149.
[1-13C]-2-hydroxy-N-methoxy-N-methylacetamide (10). A solution of ester 9 (409 mg, 2.52 mmol) in dry MeOH (5.5 mL) was cooled to 0 °C under argon atmosphere, and MeONa (21 mg, 0.39 mmol) was added in small portions. The reaction was left to stir at 0 °C for 2 h, and then Dowex 50WX8 resin was added (until acidic pH). The resin was then filtered off, washed with methanol, and the solvent was removed under reduced pressure. 252 mg of compound 10 was obtained as a colourless oil, and it was used without any further purification (yield = 83%). 1H-NMR (400 MHz, CDCl3) δ (ppm): 4.31 (d, 2JH-C = 4.4 Hz, 2H, CH2OH), 3.71 (s, 3H, OMe), 3.24 (d, 3JC-H = 2.0 Hz, 3H, NMe). 13C-NMR (101 MHz, CDCl3) δ (ppm): 173.2 (C=O), 61.5 (OCH3), 59.8 (d, 1JC-C = 50.1 Hz, CH2OH), 32.50 (NMe). FT-IR (neat): 3434.4 (OH), 1614.8 (C(O)N). HRMS: calcd for [C313CH9NNaO3]+: 143.05082; found: 143.05097.
[1-13C]-2-(tert-Butyldiphenylsilyloxy)-N-methoxy-N-methylacetic acid (11). A solution of alcohol 10 (213 mg, 1.78 mmol) in dry CH2Cl2 (4 mL) was cooled to 0 °C under argon atmosphere, Et3N (494 μL, 3.55 mmol), TBDPSCl (554 μL, 2.13 mmol) and a catalytic amount of DMAP were added. The reaction was stirred at 0 °C during a 10 min period, and then it was left to stir overnight at room temperature. Then, a saturated aqueous solution of NH4Cl was added, and the aqueous phase was extracted with 3 portions of CH2Cl2. The combined organic layers were dried with MgSO4, filtered and concentrated under reduced pressure. The crude was purified by flash column chromatography (gradient: 5/95→10/90→20:80 AcOEt/Hexane), and 635 mg of compound 11 was obtained as a white solid (Yield = 100%). M.P. 54–55 °C (0.574 g). 1H-NMR (400 MHz, CDCl3) δ (ppm): 7.76–7.70 (m, 4H, Har-m), 7.45–7.35 (m, 6H, Har-o-p), 4.42 (d, 2JC-H = 4.1 Hz, 2H, CH2), 3.43 (s, 3H, OMe), 3.13 (d, 3JC-H = 2.1 Hz, 3H, NMe), 1.11 (s, 9H, t-Bu). 13C-NMR (101 MHz, CDCl3) δ (ppm): 171.8 (C=O), 135.7 (4 × Carom), 133.2 (2 × Carom-quat), 129.8 (2 × Carom-p), 127.7 (4 × Carom), 62.0 (d, 1JC-C = 56.1 Hz, CH2), 61.2 (OMe), 32.5 (NMe), 26.7 (t-Bu), 19.3 (t-Bu). FT-IR (neat): 1666.8 (C(O)N). HRMS: calcd. for [C3813C2H54N2NaO6Si2]+: 739.34797; found: 739.34843.
[2-13C]-(tert-Butyldiphenylsilyloxy)pent-3-yne-2-one (12). To a solution of propyne (1.52 M in THF, 1.76 mL, 2.5 mmol) was slowly added n-BuLi (1.6 M in hexanes, 1.6 mL, 2.4 mmol) at −78 °C. After 30 min at −78 °C, the Weinreb amide 11 (0.748 g, 2.1 mmol) in THF was added. The reaction mixture was stirred at −78 °C for 20 min, and then at 0 °C for 1 h. Saturated NH4Cl solution (10 mL) was added and the mixture extracted with CH2Cl2 (3 × 10 mL) and the combined extracts dried (MgSO4), concentrated and purified by flash chromatography (30:70 EtOAc/hexane) to afford 12 as white crystals. M.P. 54–55 °C (0.638 g, 85%). 1H-NMR (400 MHz, CDCl3) δ (ppm): 7.69–7.66 (m, 4H), 7.44–7.37 (m, 6H), 4.30 (d, J = 4.1 Hz, 2H), 1.99 (d, J = 1.4 Hz, 3H), 1.10 (s, 9H). 13C-NMR (100 MHz, CDCl3) δ (ppm): 186.1, 135.6, 132.8, 129.9, 128.7, 93.3, 78.8, 70.5 (d, J = 49.7 Hz), 26.7, 19.3, 4.2.
(2R)-2-[13C]-(tert-Butyldiphenylsilyloxy)pent-3-yne-2-ol (
13)
. Ru(
p-cymene)[(
S,
S)TsDPEN] catalyst was prepared in accordance with the procedure described in the literature [
25]. To a degassed solution of ketone
12 (537 mg, 1.59 mmol) in dry
i-PrOH (12 mL) was added Ru(
p-cymene)[(
S,
S)TsDPEN] (72 mg, 0.0955 mmol, 0.06 eq.) in dry and degassed
i-PrOH (3 mL) and the brown solution was left to stir at room temperature under argon atmosphere. After 2 h, the solvent was removed under reduced pressure and the crude was purified by flash chromatography (5:95 EtOAc/hexane) to afford
13 (513 mg, 95%, 94%
ee). [α]
20D +7.48 (c 2.62, CH
2Cl
2).
1H-NMR (400 MHz, CDCl
3) δ (ppm): 7.70–7.65 (m, 4H), 7.44–7.37 (m, 6H), 4.43 (dm,
J = 148.6 Hz, 1H), 3.77 (dd,
J = 3.8, 10.2 Hz, 1H), 3.70–3.66 (m, 1H), 1.81 (t,
J = 1.6, 3H), 1.07 (s, 9H).
13C-NMR (100 MHz, CDCl
3) δ (ppm): 135.6, 135.5, 133.0, 132.9, 129.9, 129.8, 127.8, 127.7, 82.0 (d,
J = 12.7 Hz), 79.9, 67.9 (1H,
J = 39.9 Hz), 79.9, 63.4, 26.8, 19.3, 3.5.
(2R)-2-[13C]-Pent-3-yne-1,2-diol (14). To alcohol 13 (0.544 g, 1.6 mmol) in THF (3 mL) was added TBAF 1 M in THF (1.61 mL, 1.6 mmol) at r.t. The reaction mixture was stirred for 1h. Water (5 mL) was then added, and the aqueous phase was extracted with ethyl acetate (3 x 5 mL). The combined organic phases were concentrated, and the crude residue was purified by flash chromatography (70:30 EtOAc/hexane) to afford 14 as white crystals M.P. = 67–68 °C (0.114 g, 71%). [α]20D −21.2 (c 0.99, CH2Cl2). 1H-NMR (400 MHz, CDCl3) δ (ppm): 4.42 (dm, J = 148.7 Hz, 1H), 3.71 (ddd, J = 0.64, 3.7, 11.3, 1H), 3.63 (ddd, J = 2.4, 6.64, 11.4, 1H), 1.86 (dd, J = 1.52, 1.88 Hz, 3H). 13C-NMR (100 MHz, CDCl3) δ (ppm): 81.7 (d, J = 12.4 Hz), 78.9, 66.7 (d, J = 38.5 Hz), 63.4, 3.5. HRMS: calcd. for C4H8O2Na13C 124.0450, found: 124.0453 (M + Na).
(2R)-2-[13C]-Pent-1,2-benzoyloxy-3-yne (15). To the diol 14 (0.010 g, 0.1 mmol) in CH2Cl2 (1 mL) at 0 °C was added N-ethyldiisopropylamine (0.070 mL, 0.4 mmol), benzoyl chloride (0.035 mL, 0.3 mmol) and a catalytic amount of DMAP. The mixture was stirred at room temperature until total consumption of the starting material. A saturated solution of NaHCO3 was added and the resulting mixture was extracted with CH2Cl2, concentrated under vacuum and purified by flash chromatography (70:30 EtOAc/hexane) to give 15 as white crystals (0.028 g, 91% yield) M.P. = 70 °C. [α]20D −46.3 (c 1.35, CH2Cl2). 1H-NMR (400 MHz, CDCl3) δ (ppm): 8.07 (d, J = 7.2 Hz, 2H), 8.01 (d, J = 7.1 Hz, 2H), 7.58–7.52 (m, 2H), 7.45–7.39 (m, 4H), 5.96 (dm, J = 154 Hz, 1H), 4.65–4.63 (m, 2H), 1.87 (t, J = 1.44 Hz, 3H). 13C-NMR (100 MHz, CDCl3) δ (ppm): 166.04 (d, J = 2 Hz), 165.5 (d, J = 2 Hz), 133.2, 133.1, 129.8, 129.7, 129.63, 129.60, 128.42, 84.0 (d, J = 13 Hz), 79.5, 73.5, 72.7, 65.4 (d, J = 42.2 Hz), 63.0, 3.7. HRMS: calcd. for C18H16O4Na13C 332.0974, found: 332.0972 (M + Na).
(2R)-2-[13C]-1,2-Cyclohexylidenedioxypent-3-yne (16). To diol 14 (0.059 mg, 0.6 mmol) in DMF (2.0 mL) at room temperature, was added 1,1-dimethoxycyclohexanone (0.269 mL, 1.8 mmol) and H2SO4 (1 drop), and the reaction mixture was stirred overnight. The reaction was quenched with NaHCO3 sat. solution (2 mL) (pH ≥ 8). The mixture was extracted with Et2O (3 × 2 mL), concentrated and purified by flash chromatography (5:95 EtOAc/hexane) to afford 16 (94 mg, 89%) as a colourless oil. [α]20D −37.6 (c 1.31, CHCl3). 1H-NMR (400 MHz, CDCl3) δ (ppm): 4.67 (dm, J = 153.6 Hz, 1H), 4.10 (dd, J = 1.68, 7.8 Hz, 1H), 3.81 (ddd, J = 2.4, 7.1, 14.8 Hz, 1H), 1.85 (d, J = 1.8 Hz, 3H), 1.74–1.39 (m, 10H). 13C-NMR (100 MHz, CDCl3) δ (ppm): 110.4, 82.1 (d, J = 13.9 Hz), 75.9, 69.6 (d, J = 32.9 Hz), 65.5, 35.7, 35.4, 25.0, 23.8, 23.7, 22.8, 3.6. HRMS: calcd. for C10H15O213C 180.1100, found: 180.1101 (M-H).
(4S)-4-[13C]-4,5-Cyclohexylidenedioxy-2,3-pentadione (17). To compound 16 (106 mg, 0.6 mmol) in CCl4 (3.2 mL) and MeCN (3.2 mL) was added NaIO4 (286 mg, 1.3 mmol) in H2O (4.8 mL) and RuO2.H2O (2.0 mg, 0.015 mmol), the reaction mixture was vigorously stirred until all starting material has been consumed (TLC). The mixture was extracted with ethyl acetate (3 × 10 mL), filtred by silica gel and concentrated to afford a yellow oil 17 (103 mg, 82%). [α]20D −11.5 (c 0.8, CHCl3). 1H-NMR (400 MHz, CDCl3) δ (ppm): 5.13 (ddd, J = 5.3, 7.9, 153.5 Hz, 1H), 4.35 (t, J = 8.4 Hz, 1H), 4.0 (dd, J = 5.3, 8.9 Hz, 1H), 2.39 (s, 3H), 1.74–1.34 (m, 10H). 13C-NMR (100 MHz, CDCl3) δ (ppm): 198.1, 194.7, 111.9, 76.5, 65.5 (d, J = 34.6 Hz), 35.4, 34.7, 24.9, 24.5, 23.8, 23.7.
(4S)-4-[13C]-DPD (1). The experimental procedure described in ref. 8 was followed. 1H NMR (500 MHz, CDCl3) δ (ppm): 4.05 (ddd, J = 149.6, 7.0, 5.8 Hz, H4, cyclic form), 3.87–3.82 (m), 3.70 (dm, J = 145 Hz, H4, cyclic form), 3.65 (dm, J = 145 Hz, H4, linear form), 3.52–3.47 (m), 3.24 (ddd, J = 10.95, 5.4, 5.4 Hz), 2.04 (CH3, linear form), 1.11 (CH3, cyclic form), 1.08 (CH3, cyclic form). 13C-NMR (100 MHz, CDCl3) δ (ppm): 74.0 (C4, cyclic form), 73.8 (C4, linear form), 73.2 (C4, cyclic form).
1-(tert-Butyldiphenylsilyloxy)but-3-yne-2-one (19). To a solution of N-diisopropylamine (1.25 mL, 8.09 mmol) in THF (13.6 mL), was added n-Buli (1.6M in hexane, 4.23 mL, 6.76 mmol) dropwise at 0 °C, and stirred at 0 °C over 10 min. At −78 °C, it was added trimethylsilylacetylene (0.99 mL, 7.1 mmol), after 30 min Weinreb amide 18 was added (2.12 g, 5.93 mmol), previously dissolved in THF (17.3 mL). The mixture was stirred at −78 °C and warmed to room temperature. Water was added (20 mL), the resulting mixture was extracted with CH2Cl2 (3 × 15 mL), dried with anhydrous MgSO4, concentrated under vacuum and purified by flash chromatography (30:70 AcOEt:hexane) to afford colourless oil 20 (0.850 g, 62%), 19 (0.107 g, 12% yield) and 27 (0.330 g, 16% yield). Data for 19: 1H-NMR (400 MHz, CDCl3) δ (ppm): 7.70–7.67 (m, 4H), 7.44–7.37 (m, 6H), 4.29 (s, 2H), 1.10 (s, 9H), 0.22 (s, 9H). Data for 20: 1H-NMR (400 MHz, CDCl3) δ (ppm): 7.68–7.65 (m, 4H), 7.45–7.38 (m, 6H), 4.34 (s, 2H), 3.26 (s, 1H), 1.10 (s, 9H). 13C-NMR (100 MHz, CDCl3) δ (ppm): 185.3, 135.5, 132.5, 130.0, 127.9, 81.2, 79.6, 70.6, 26.6, 19.3. FT-IR (film): 2094 (C≡C), 1706 (C=O). Data for 27: 1H-NMR (400 MHz, CDCl3) δ (ppm): 7.84 (d, J = 12.9 Hz, 1H), 7.69–7.67 (m, 4H), 7.43–7.34 (6H, m), 5.92 (d, J = 12.9 Hz, 1H), 4.17 (s, 2H), 1.11 (s, 9H). 13C-NMR (100 MHz, CDCl3) δ (ppm): 196.2, 148.3, 135.5, 133.3, 129.7, 127.7, 90.7, 68.9, 26.8, 19.3.
(2R)-1-(tert-butyldiphenylsilyloxy)but-3-yn-2-ol (21). Acetylenic ketone 19 (1 g, 3.1 mmol) was dissolved in THF (10 mL) at room temperature. (S)-Alpine Borane (0.5 M in THF, 9.3 mL, 4.65 mmol) was added, the mixture was stirred for 36 h. NH4Cl saturated solution (15 mL) was added, the resulting mixture was extracted one time with ethyl ether (10 mL) and then with dichloromethane (2 × 10 mL), dried with anhydrous MgSO4, concentrated under vacuum and purified by flash chromatography (30:70 AcOEt:hexane and 1:12:12 EtOAc:hexanes:CH2Cl2) to give 21 (0.675 g, 67% yield). Usually, no purification was done as it was very difficult to isolate the product. 1H-NMR (400 MHz, CDCl3) δ (ppm): 7.69–7.65 (m, 4H), 7.44–7.37 (m, 6H), 4.47–4.44 (m, 1H), 3.81 (dd, J = 10.2, 3.9 Hz, 1H), 3.74 (dd, J = 10.2, 6.5 Hz, 1H), 2.41 (d, J = 2.1 Hz, 1H), 1.08 (s, 9H). 13C-NMR (100 MHz, CDCl3) δ (ppm): 135.6, 135.5, 132.8, 132.7, 129.9, 127.87, 127.85, 81.8, 73.6, 67.4, 63.0, 26.8, 19.3. FT-IR (film): 3427 (O-H). To quantify the enantioselectivity of the reaction, a small sample of alcohol 21 was treated with (R)-MTPA-Cl (3 eq) and DMAP (3 eq) in CH2Cl2 at room temperature. Integration of 1H-NMR (400 MHz, CDCl3) peaks at δ (ppm): 2.50 (d, J = 2.2 Hz, minor) and 2.43 (d, J = 2.2 Hz, major) ppm indicated a dr of 93:7 corresponding to 86% ee.
(2R)-But-3-yne-1,2-diol (22). To the alcohol 21 (0.5 g, 1.54 mmol) in THF (5 mL) was added TBAF (1M in THF, 1.85 mL, 1.85 mmol) at room temperature. The reaction mixture was stirred until all starting material has been consumed (TLC) and then quenched with H2O (5 mL). The resulting mixture was extracted with ethyl acetate (3 × 5 mL), concentrated under vacuum and purified by flash chromatography (70:30 EtOAc:hexane) to give 22 as a colourless liquid (0.094 g, 71% yield). [α]20D = −27.4 (c 1.92, CH2Cl2). 1H-NMR (400 MHz, CDCl3) δ (ppm): 4.49–4.46 (m, 1H), 3.77 (dd, J = 11.4, 3.7 Hz, 1H), 3.70 (dd, J = 11.4, 6.5 Hz, 1H), 2.49 (d, J = 2.1 Hz, 1H). 13C-NMR (100 MHz, CDCl3) δ (ppm): 81.8, 74.1, 66.4, 63.0. FT-IR (film): 3291 (O-H), 2117 (C≡C).
(2R)-1,2-Cyclohexylidenedioxybut-3-yne (24). To the diol 22 (0.094 g, 1.09 mmol) in DMF (2 mL) at room temperature was added 1,1-dimethoxycyclohexane (0.665 mL, 4.37 mmol) and one drop of conc. H2SO4 and stirred overnight. NaHCO3 saturated solution was added (pH ≥ 8) followed by H2O (5 mL). The mixture was extracted with ethyl ether (3 × 8 mL), concentrated under vacuum and purified by flash chromatography (5:95 EtOAc:hexane) to afford colourless oil 24 (0.141 g, 78% yield). 1H-NMR (400 MHz, CDCl3) δ (ppm): 4.71 (ddd, J = 6.3, 6.3, 2.0 Hz, 1H), 4.16 (dd, J = 8.4, 6.3 Hz, 1H), 3.94 (dd, J = 8.0, 6.2 Hz, 1H), 2.48 (d, J = 2.2 Hz, 1H), 1.77–1.59 (m, 8H), 1.42–1.40 (m, 2H). 13C-NMR (100MHz, CDCl3) δ (ppm): 111.2, 81.6, 73.7, 69.5, 64.9, 35.6, 35.4, 27.0, 25.0, 23.8. FT-IR (film): 2119 (C≡C).
(2R)-5-[13C]-1,2-Cyclohexylidenedioxypent-3-yne (25). To a solution of N,N-diisopropylamine (0.298 mL, 2.1 mmol) in THF (3.23 mL), was added n-Buli (1.6 M in hexane, 1.06 mL, 1.7 mmol) at 0 °C and stirred for 10 min. At −78 °C, 24 (0.141 g, 0.848 mmol), previously dissolved in THF (1.15 mL) was added. After 30 min, 13CH3I (0.133 mL, 2.1 mmol) was added. The mixture was stirred at −78 °C and allowed to warm to room temperature. NH4Cl saturated solution was added (5 mL), the resulting mixture was extracted with CH2Cl2 (3 × 5 mL), dried with anhydrous MgSO4, concentrated under vacuum and purified by flash chromatography (50:50 AcOEt:hexane) to afford colourless oil 25 (0.150 g, 94% yield). 1H-NMR (400 MHz, CDCl3) δ (ppm): 4.69–4.66 (m, 1H), 4.11 (dd, J = 7.9, 6.2 Hz, 1H), 3.82 (dd, J = 7.8, 7.0 Hz, 1H), 1.85 (dd, J = 131.5, 2.0 Hz, 3H), 1.67–1.58 (m, 8H), 1.41–1.38 (m, 2H).
(4S)-1-[13C]-4,5-cyclohexylidenedioxy-2,3-pentadione (
26)
. To the alkyne
25 (0.083 g, 0.462 mmol) dissolved in CCl
4 (2.5 mL) and MeCN (2.5 mL) was added a solution of NaIO
4 (0.224 g, 1.05 mmol) in H
2O (3.8 mL) and RuO
2·H
2O (0.0015 g, 0.0115 mmol) and the reaction mixture was stirred vigorously for 15 min. The mixture was extracted with CH
2Cl
2, filtered by a short pad of silica for medium pressure chromatography and concentrated under vacuum to give the bright yellow oil
26 (0.089 g, 91% yield) with NMR data identical to that described in the literature[
5].
1H-NMR (400 MHz, CDCl
3) δ (ppm): 5.13 (dd,
J = 7.9, 5.3 Hz, 1H), 4.35 (dd,
J = 8.9, 7.9 Hz, 1H), 4.00 (dd,
J = 8.9, 5.3 Hz, 1H), 2.38 (d,
J = 129.3 Hz, 3H), 1.77–1.38 (m, 10H).
(4S)-1-[13C]-DPD (
1)
. The experimental procedure described in ref. [
8] was followed. The characterisation data are in accordance with those described in ref. [
5]