General
The carboxylic acid
was prepared according to the method of Rozantsev [
3] and purified by recrystallization from tetrahydrofuran (THF). All reagents were purchased from Aldrich Chemical Co. and solvents were purchased from Fisher Scientific. Purification of solvents was performed as follows: toluene and benzene were washed with concentrated sulfuric acid and distilled; ether and THF were distilled from lithium aluminum hydride; pyridine and triethylamine from calcium hydride; chloroform was distilled from phosphorus pentoxide. NMR spectra were measured on a Varian Gemini 300 broadband spectrometer. Analysis of purity for compounds
,
, and
was made by HPLC analysis (C-18 (5 µm, 2.6 x 100 mm), mobile phase 90% sodium acetate (50 mM, pH 6), 10% MeOH, UV detection (254 nM), flow rate 2.0 mL/min (retention times:
, 2.68 min,
, 3.60 min,
, 3.51 min) and by comparison of their melting points to literature values.
Method 1 – The acid
(1.84 g, 10 mmol) was suspended in dry toluene (10 mL), to which 11 mL of a 70% solution of sodium bis[2-methoxyethoxy]-aluminum hydride was added dropwise. The solution was heated at reflux for two hours and quenched by addition to 100 mL of a 5% solution of sodium hydroxide. The organic phase was separated and dried over anhydrous sodium sulfate and the toluene removed
in vacuo to yield 1.6g (88%)
(mp 75-76 °C; lit. 75-77 °C [
4])
Method 2 – The acid
(1.2 g, 6.5 mmol) and triethylamine (0.69 g, 6.8 mmol) were dissolved in ether (50 mL) and cooled in an ice-salt bath. Ethyl chloroformate (0.71 g, 6.5 mmol) was added and the reaction was stirred for 1.5 h. The reaction mixture was filtered to remove triethylamine hydrochloride, the ether filtrate was washed with cold HCl (5%, 10 mL), water (10 mL), saturated NaHCO
3 (10 mL), and brine (10 mL). The ether layer was dried over Na
2SO
4, filtered and concentrated
in vacuo to yield
, 1.04 g (63%). This product was suspended in ethanol (16.8 mL), to which was added NaBH
4 (560 mg, 14.8 mmol) in two equal portions. The reaction was stirred for 30 minutes, the ethanol removed
in vacuo, the product dissolved in water (8 mL), and extracted with chloroform (3 X 25 mL). The organic layer was washed with saturated NaHCO
3 (10 mL) and water (10 mL), dried over Na
2SO
4, and concentrated
in vacuo. Separation on silica gel (1:1 ethyl acetate/hexanes) afforded three fractions corresponding to compounds
,
and
in a ratio of 1:3:1. After separation the yield of
(mp 75-76 °C; lit. 75-77 °C [
4]) was 0.22 g (18%, based on
, 66% based on recovered
and
).
Method 3 – The acid
(4.2 g, 23 mmol) and triethylamine (2.36 g, 23 mmol) were dissolved in THF (40 mL). With cooling (ice-salt bath), ethylchloroformate (2.95 g, 27 mmol) in THF (7 mL) was slowly added and the mixture stirred for 30 minutes. The triethylammonium chloride salts were filtered off and washed with THF (10 mL). The filtrate and washings were added dropwise to NaBH
4 (2.36 g, 62 mmol) in water (30 mL). This reaction was allowed to stir overnight. The mixture was acidified with 10% HCl to destroy excess NaBH
4 and then the solution was saturated with solid NaCl, causing the separation of the THF and aqueous layers. The THF layer was removed, and the aqueous layer extracted repeatedly with methylene chloride (15 mL aliquots) until no more yellow color was extracted into the organic phase. The THF/methylene chloride extract was dried over MgSO
4 and concentrated to dryness
in vacuo to give compound
, yield 2.13g (55%) (mp 75-76 °C; lit. 75-77 °C [
4]).
Method 4 – The acid
(1.84 g, 10 mmol) was suspended in dry benzene (25 mL) and cooled to 0-5 °C. Dry pyridine (1 mL) and then thionyl chloride (0.9 mL) were added dropwise. The reaction was stirred at room temperature for 30 min and then concentrated
in vacuo. The residue (acid chloride
) was dissolved in THF (10 mL), the flask flushed with nitrogen, cooled to –78 °C, and maintained at that temperature as lithium tri-
tert-butoxyaluminohydride (2.1 eq, 1 eq to neutralize the pyridine hydrochloride) was added dropwise. The reaction was stirred at room temp for one hour. The mixture was quenched by the addition of NH
4Cl (10%, 10 mL), the layers separated and the aqueous layer extracted with CH
2Cl
2 (10 mL). The combined organic layers were dried, filtered and concentrated to yield the crude aldehyde
. The product
was purified by chromatography (silica gel, 3:1 hexane/ethyl acetate). Compound
yield: 1.01g (60%) (mp 77-79 °C; lit. 78-79 °C [
4])
Method 5 – To a solution of
(1.84 g, 10 mmol) in dry THF (10 mL) was added, dropwise, a solution of LAH in THF (1.0 M, 10 mL), and the mixture stirred at room temp for 12 hours. The reaction was quenched by the addition of water (0.4 mL), 15% sodium hydroxide (0.4 mL) and water (1.2 mL), filtered, and concentrated
in vacuo to yield 1.65g of
(97%) (mp 75-76 °C; lit. 75-77 °C [
4]). Scale-up proved problematic as the same procedure when repeated using 2 g or 4 g of
resulted in declining yields of 76% and 41%, respectively. In the latter case the product was a mixture of the nitroxide
and a compound whose
1H-NMR was consistent with the corresponding amine
.
Method 6 – The acid chloride
was prepared as described under
Method 4 and then the solvents removed
in vacuo. A solution of sodium ethoxide prepared from 0.25 g of sodium metal and absolute ethanol (8 mL) was added. The resulting mixture was heated at reflux for 2 hr, cooled to room temp and the salts removed by filtration. The filtrate was concentrated to dryness, dissolved in CH
2Cl
2 (50 mL), washed with 5% aq HCl, 5% aq NaOH, and 5 % NH
4Cl (10 mL each), dried (MgSO
4), filtered and concentrated to dryness to yield
7 (1.2 g, 70%) (mp 109-111 °C; lit [
3] 109.5-111 °C). The crude product was dissolved in dry THF (7 mL) and added to LAH (1M, 7 mL). The mixture was stirred at rm temp for 2 h and then quenched by the addition of water (0.3 mL), 15% aq NaOH (0.3 mL) and water (0.9 mL). The reaction mixture was then filtered and concentrated
in vacuo. The residue was dissolved in CH
2Cl
2 (50 mL), washed with sodium hydroxide (5%, 2 x 10mL) hydrochloric acid (5%, 2 x 10mL), dried (MgSO
4), filtered and concentrated
in vacuo. The hydrochloric acid extract was made basic by the addition of aqueous 10% sodium hydroxide, extracted with CH
2Cl
2 (3 x 20 mL), dried (MgSO
4), filtered and concentrated
in vacuo. The latter extract contained the product
(Yield: 0.27g, 17%).
NMR data - [Note: all nitroxides were reduced to hydroxylamines for NMR analysis with 1.1 equivalents Na2S2O4 in a 1:1 mixture of D2O:d6-acetone.]
Carboxylic acid :1H-NMR (1:1 d6-acetone, D2O) δ ppm 1.57 (s, 6 H, 2 -CH3), 1.67 (s, 6 H, 2 -CH3), 6.52 (s, 1H, =CH-); 13C-NMR (1:1 d6-acetone, D2O) δ 27.17 (2 C, 2 -CH3), 27.35 (2 C, 2 -CH3), 68.36 (1 C, quaternary), 70.87 (1C, quaternary), 137.0 (=C-CO2H), 148.8 (=CH-), 167.4 (=C-CO2H).
Alcohol : 1H-NMR (1:1 d6-acetone, D2O) δ ppm 1.45 (s, 6 H, 2 -CH3), 1.47 (s, 6 H, 2 -CH3), 4.31 (s, 1 H, -CH2-), 4.32 (s, 1 H, -CH2-), 5.81 (s, 1H, =CH-); 13C-NMR (1:1 d6-acetone, D2O) δ 23.84 (2 C, 2 -CH3), 24.80 (2 C, 2 -CH3), 58.21 (1 H, -CH2-OH), 70.13 (1 C, quaternary), 72.20 (1C, quaternary), 128.5 (=C-CH2OH), 144.5 (=CH-).
Aldehyde : 1H-NMR (1:1 d6-acetone, D2O) δ ppm1.44 (s, 12 H, 4 -CH3), 7.11 (s, 1H, =CH-), 9.69 (-CHO); 13C-NMR (1:1 d6-acetone, D2O) δ ppm 24.37 (2 C, 2 -CH3), 24.52 (2 C, 2 -CH3), 68.47 (1 C, quaternary), 68.58 (1C, quaternary), 146.31 (=C-CHO), 156.66 (=CH-), 191.02 (-CHO).
Mixed anhydride : 1H-NMR (1:1 d6-acetone, D2O) δ ppm 1.473 (s, 6 H, 2 -CH3), 1.51 (t, 3H, -CH2CH3, J = 7.1 Hz), 1.55 (s, 6 H, 2 -CH3), 4.42 (q, 2 H, -OCH2CH3, J = 7.1 Hz), 6.92 (1H, =CH-).
Ester : 1H-NMR (1:1 d6-acetone, D2O) δ ppm 1.47 (s, 6H, 2-CH3), 1.50 (t, 3H, J=7.1 Hz, CH3CH2O), 1.54 (s, 6H, 2-CH3), 4.41 (q, 2H, J=7.1 Hz, CH3CH2O), 6.91 (s, 1H, CH); 13C-NMR (1:1 d6-acetone, D2O) δ ppm 13.68 (CH3CH2O), 24.32 (2 C, 2 -CH3), 24.41 (2 C, 2 -CH3), 60.98 (CH3CH2O), 68.05 (1 C, quaternary), 69.72 (1 C, quaternary), 136.7 (=C-CO2 ), 146.6 (=CH-), 164.8 (-COOEt).