Oligodeoxynucleotide Phosphonoacetate Syntheses
A R T I C L E S
Complete conversion of the phosphorus trichloride (δ 201 ppm) to the
tris(pyrrolidino)phosphine (δ 103 ppm) was monitored by 31P NMR.
The reaction mixture was filtered to remove pyrrolidine hydrochloride,
and the precipitate was washed with anhydrous ether. The filtrate was
evaporated in vacuo, and the crude tris(pyrrolidino)phosphine was
isolated as an oil. The oil was distilled using a two-stage oil pump at
0.04 mmHg to yield 28.2 g (88%): bp 80-82 °C; 31P NMR (CD3CN)
δ 103.4 (s); electron impact mass spectrometry gave a molecular ion
of 241 m/e. The resulting tris(pyrrolidino)phosphine was then converted
to the bis(pyrrolidino)chlorophosphine by disproportionation with
phosphorus trichloride. Tris(pyrrolidino)phosphine (28.2 g, 117 mmol)
and anhydrous acetonitrile (150 mL) were placed in a dry 500 mL
round-bottom flask containing a Teflon stir bar. The flask was fitted
with a silicon rubber stopper and placed under argon. The flask was
immersed in an ice-water bath, and the solution was allowed to cool
with stirring for 30 min. Phosphorus trichloride (8.05 g, 58.5 mmol)
was added slowly by syringe to the stirred solution. The solution was
allowed to warm to room temperature, and the reaction was stirred
overnight. Complete conversion of the tris(pyrrolidino)phosphine (δ
103 ppm) to the bis(pyrrolidino)chlorophosphine (δ 161 ppm) was
monitored by 31P NMR. The product was distilled using a two-stage
oil pump at 0.04 mmHg to yield 36.2 g (92%): bp 59-62 °C; 31P
NMR (CD3CN) δ 161.3 (s); electron impact mass spectrometry gave a
molecular ion of 206 m/e.
procedure outlined for 7a to yield an unpurified product. The product
was unstable to further purification by triturating with pentanes or
hexanes. 31P NMR (CD3CN) δ 51.2 (s); electron impact mass
spectrometry gave a molecular ion of 244 m/e with a fragmentation
loss of CH2COOCH3 at 171 m/e.
Synthesis of 1,1-Dimethylcyanoethyl Bromoacetate. Bromoacetyl
bromide (108 g, 600 mmol) was added to a 1 L round-bottom flask
containing 500 mL of anhydrous toluene. 3-Hydroxy-3-methylbuty-
ronitrile (50 g, 500 mmol) was added slowly with stirring. The round-
bottom flask was fitted with a Friedrich’s condenser and a drying tube
vented to an acid trap. The reaction mixture was heated to reflux using
a mantel and refluxed overnight. The reaction was allowed to cool to
room temperature, and the mixture was concentrated in vacuo to an
oil. The oil was purified by vacuum distillation at ∼0.1 mmHg. The
oil was distilled into two fractions: an initial minor fraction that gave
a wide boiling range and a second major fraction that distilled at a
temperature range of 120-122 °C. The initial fraction was discarded,
and the constant boiling fraction gave 97.3 g of a clear, colorless liquid
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(88% yield). H NMR (CDCl3) δ 3.75 (s, 2H), 2.86 (2, 2H), 1.53 (s,
6H). Electron impact mass spectrometry gave molecular ions of 205
m/e and 207 m/e with isotopic abundances of 51% and 49%.
Synthesis of Acetic Acid, [Bis(N,N-diisopropylamino)phosphino]-
1,1-dimethyl-2-cyanoethyl Ester (14). Anhydrous THF (160 mL), bis-
(N,N-diisopropylamino)chlorophosphine (20 g, 75 mmol), and a
magnetic stir bar were added to a 500 mL round-bottom flask which
was stoppered, and the solution was allowed to be stirred until the
phosphine dissolved. After dissolution, anhydrous ether (100 mL) was
added. 1,1-Dimethyl-2-cyanoethyl bromoacetate (18.2 g, 82.5 mmol)
was placed in a 125 mL round-bottom flask, and anhydrous THF (75
mL) added. Granular zinc metal (7.2 g, 110 mmol), anhydrous ether
(50 mL), and a magnetic stir bar were placed in a 1 L three necked
round-bottom flask fitted with a Friedrich’s condenser and two 500
mL addition funnels. The phosphine solution and the 1,1-dimethyl-2-
cyanoethyl bromoacetate solution were added to the two addition
funnels. Aliquots of the phosphine solution (85 mL) and the bromoac-
etate solution (25 mL) were added to the three necked round-bottom
flask. The reaction mixture was then heated under reflux until an
exothermic reaction was noticeable (the slightly cloudy, colorless
reaction became clear and slightly yellow). The reaction was continued
at reflux by the addition of the remainder of the phosphine and
bromoacetate solutions. Once the addition was complete, the reaction
was kept at reflux for 30 min by heating, allowed to cool to room
temperature, and analyzed for completeness by 31P NMR. The starting
material at δ 135 ppm was converted to a single product at δ 48 ppm.
The cooled reaction mixture was concentrated in vacuo to a viscous
oil. The resulting viscous oil was extracted three times with anhydrous
hexanes which converted the oil to a solid. The solid was then dissolved
in acetonitrile, and this solution was extracted twice with anhydrous
hexanes. The acetonitrile solution was analyzed by 31P NMR for absence
of the product at δ 48 ppm and discarded. All hexane fractions were
combined and concentrated in vacuo to a slightly yellow oil, redissolved
in anhydrous hexanes, and placed in a freezer overnight. The hexane
solution was decanted into a clean, dry 500 mL round-bottom flask,
and the hexanes were removed by evaporation to give 16.4 g (88%
yield) of a slightly yellow oil. The product could be further purified
by recystalization in anhydrous pentanes to yield a crystalline solid.
1H NMR (CDCl3) δ 3.48 (m, 4H), 2.99 (s, 2H), 2.80 and 2.78 (d, 2H),
1.30 (m, 24H). Electron impact mass spectrometry gave a molecular
ion of 371 m/e with a fragmentation loss of CH2COOC(CH3)2CH2CN
at 231 m/e. 31P NMR (CD3CN) δ 48.1 ppm.
Synthesis of Acetic Acid, [Bis(N,N-diisopropylamino)phosphino]-
methyl Ester (7a). Granular zinc metal (7.2 g, 110 mmol) and a
magnetic stir bar were placed in a 1 L three neck round-bottom flask
equipped with a Fredrich’s condenser and two 500 mL addition funnels.
Bis(N,N-diisopropylamino)chlorophosphine (20 g, 75 mmol) and methyl
bromoacetate (11.6 g, 82.5 mmol) were each dissolved in 200 mL of
anhydrous ether. These two solutions were placed in dropping funnels,
and the reflux condenser was fitted with a dry argon line. Approximately
one-third of each solution was added to the flask, and the mixture was
heated with a heat gun until the Reformatsky reaction was initiated.
Once initiated, the reaction was continued by the constant addition of
the two solutions. After the addition was complete, the reaction was
kept at reflux until the starting material was consumed, as monitored
by 31P NMR. The reaction mixture was decanted from the unreacted
zinc, and the ether was removed in vacuo using a rotory evaporator.
The product was isolated by triturating with pentanes to yield 11.6 g
(54%). 1H NMR (CDCl3) δ 3.68 (s, 3H), 3.55 (m, 4H), 2.93 and 2.91
(d, 2H), 1.30 (m, 24H); 31P NMR (CD3CN) δ 49 (s); electron impact
mass spectrometry gave a molecular ion of 304 m/e with a fragmentation
loss of CH2COOCH3 at 231 m/e.
Synthesis of Acetic Acid, [Bis(N,N-diethylamino)phosphino]-
methyl Ester (7b). Starting with granular zinc (13.1 g, 200 mmol),
bis(N,N-diethylamino)chlorophosphine (21.1 g, 100 mmol), and methyl
bromoacetate (14.1, 100 mmol), we synthesized the product using the
procedure outlined for 7a to yield 2.2 g (9%) of 7b. 1H NMR (CDCl3)
δ 3.66 (s, 3H), 2.93 and 2.91 (d, 2H), 2.72 (q, 8H), 1.26 (t, 12H); 31P
NMR (CD3CN) δ 50.9 (s); electron impact mass spectrometry gave a
molecular ion of 248 m/e with a fragmentation loss of CH2COOCH3
at 175 m/e.
Synthesis of Acetic Acid, [Bis(N,N-dimethylamino)phosphino]-
methyl Ester (7c). Starting with granular zinc (13.1 g, 200 mmol),
bis(N,N-dimethylamino)chlorophosphine (15.5 g, 100 mmol), and
methyl bromoacetate (14.1, 100 mmol), we synthesized the product
using the procedure outlined for 7a to yield an unpurified product. The
product was unstable to further purification by triturating with pentanes
or hexanes. 31P NMR (CD3CN) δ 51.0 (s); electron impact mass
spectrometry gave a molecular ion of 192 m/e with a fragmentation
loss of CH2COOCH3 at 119 m/e.
Synthesis of Acetic Acid, [O-Methyl(N,N-diisopropylamino)-
phosphino]methyl Ester (8a). Compound 7a (2.0 g, 6.5 mmol) was
placed in a 100 mL round-bottom flask and dissolved in anhydrous
acetonitrile (50 mL). Anhydrous methanol (210 mg, 6.5 mmol) and
tetrazole (360 mg, 5.2 mmol) were added, and the flask was stirred for
24 h. The reaction mixture was neutralized by the addition of 5 mL of
Synthesis of Acetic Acid, [Bis(pyrrolidino)phosphino]methyl
Ester (7d). Starting with granular zinc (13.1 g, 200 mmol), bis-
(pyrrolidino)chlorophosphine (20.7 g, 100 mmol), and methyl bro-
moacetate (14.1, 100 mmol), we synthesized the product using the
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J. AM. CHEM. SOC. VOL. 125, NO. 4, 2003 947