K.P. Cole et al. / Bioorganic & Medicinal Chemistry xxx (2017) xxx–xxx
7
melting point reported for ester 3f was obtained using differential
scanning calorimetry (DSC).
yellow oil was purified using an IscoÒ automated chromatography
system with a 330 g silica column. The product was eluted with
2 2
CH Cl :hexanes (gradient elution 15:85 to 35:65). Fractions corre-
7
7
.2. Representative procedure for preparation of esters 3a–d
sponding to the desired product were collected and concentrated
to provide the product in 57% isolated yield and 99.3% (w/w) purity
(based on Q NMR analysis of the sample).
.2.1. tert-Butyl 2-morpholinoacetate (3a)
A 3-necked 2 L round bottom flask was equipped with mechan-
ical stirring, a reflux condenser, a low-pressure nitrogen source,
thermocouple, and heating mantle. The flask was inerted by allow-
ing the nitrogen to sweep the vessel for >30 min. To the flask was
charged 97.88 g (0.650 mol) of tert-butyl chloroacetate, which was
washed in with 50 mL of THF. Next, 57.2 mL of morpholine
7.2.4. tert-Butyl diphenylglycinate
m.p. 67.2–69.1 °C; 1H NMR (400 MHz, DMSO-d
) d 7.27 (dd,
J = 8.4, 7.2 Hz, 4H), 6.97–6.94 (m, 6H), 4.40 (s, 2H), 1.37 (s, 9H).
6
1
3
6
C NMR (100 MHz, DMSO-d ) d 169.6, 147.1, 129.2, 121.4, 120.2,
À1
80.9, 54.2, 27.7 (3C); IR (neat, ATR, cm ) 1737.3, 1588.9, 1499.9,
(
0.656 mol) was added, followed by 100 mL of THF. Lastly, triethy-
lamine (91.5 mL, 0.656 mol) was added and washed in with THF
535 mL). Agitation was started and the solution was heated to
4 °C. A slurry began to form. After 17 h of heating, GC–MS analysis
1355.0, 1221.9, 1159.4; HRMS (ESI) (m/z) [M+H]+ calc. for
18 2
C H21NO 284.1651; found 284.1653 7.5, d = 0.7 ppm.
(
6
7.3. General procedure for small batch screening for thermal
deprotection
showed that a small amount of starting chloride remained (<2%),
and the mixture was heated an additional 21 h. GC–MS shows that
the conversion was > 99% and the slurry was cooled to ambient
temperature. The solid triethylamine*HCl salt was removed by fil-
tration through a coarse-fritted Buchner funnel, and the waste cake
was washed with fresh THF (3 Â 100 mL). Most of the THF was
then removed by rotary evaporation, yielding a thick oil. EtOAc
was added (1 L) and the solution was transferred into a separatory
funnel along with water (0.5 L). The mixture was shaken and
allowed to separate. The water layer was drained off and discarded.
Brine was added (100 mL) and the mixture was shaken and
allowed to separate. The brine layer was drained off and discarded.
The organic layer was then poured into a 2 L Erlenmayer flask and
sodium sulfate (50 g) was added with occasional manual agitation
to ensure uniform drying. The solvent was removed by rotary
evaporation to yield a light yellow oil, which was dried further
by exposure to house vacuum (ꢀ20 torr) overnight. The mass after
To a 1.0 mL volume 316 stainless steel SwagelokÒ port connec-
tor reactor was charged 50 mg of tert-butyl ester and 0.50 mL of
pre-mixed solvent, nitrogen inertion was not used. The reactor
was sealed and the mass recorded. The reactor was placed in a
modified GC oven and heated to the desired temperature. The reac-
tion time was started when the GC oven temperature reached the
setpoint, and timing was stopped when the setpoint was reduced
to ambient temperature and the oven vented. The reactors were
removed from the oven and allowed to cool in a sand bath, their
mass was then recorded to check for any leakage of the individual
reactors. Generally <5% of the solution mass was lost, often none.
Results from >5% mass loss reactions were considered invalid,
and some reaction were run in duplicate. The reactors were then
opened and the liquid contents removed by pipette into a 20 mL
vial. Ethanol was added (5 mL) and the solvent was removed by
rotary evaporation. A second 5 mL portion of ethanol was added
and the concentration repeated until a solid formed. The solid
was then taken into appropriate deuterated solvent for 1H NMR
analysis. When reactions were run on substrates 3e and 3f, an ali-
quot of the liquid contents of the reactor was diluted with a 1:1 (v/
v) solution of acetonitrile and water. The resulting solution was
analyzed by HPLC and the chromatogram was visualized at
230 nm for substrate 3e and 280 nm for substrate 3f.
drying was 133.5 g, but the oil contained 0.28 M equivalents of
1
EtOAc. The estimated assay of the product oil based on the
H
NMR was 89.1%, giving a corrected yield of 118.9 g (0.591 mol,
8
1%).
7.2.2. Purification of ester 3e
Ester 3e was prepared according to the standard procedure
described above. The crude product was purified using an AnaLogix
Intelliflash 280 automated chromatography system using a 220 g
silica column with gradient elution (1:99 to 10:90 acetone/
hexanes).
7.4. General procedure for continuous flow thermal deprotection
7.4.1. Continuous deprotection
7.2.3. Procedure for ester 3f
A 20 mL PFR was constructed by coiling 81.5 m of 1.59 mm OD
To a 1 L round bottom flask fitted with a nitrogen inlet tube, a
(0.56 mm ID) stainless steel tubing. The PFR was placed in a gas
chromatography oven that was modified for use as a PFR thermo-
stat. 2.07 MPa backpressure was provided by an Equilibar back-
thermocouple and magnetic stir bar was added DMF (100 mL).
NaH (60% in mineral oil, 1.73 g, 43.3 mmol) was then added in 3
portions. A solution of diphenylamine (29 g, 171.4 mmol) in DMF
Ò
pressure regulator connected to
a nitrogen cylinder. After
(
100 mL) was next rapidly added. No immediate exotherm was
depressurization, the product stream was collected in a glass pres-
sure bottle under an atmosphere of nitrogen. The feed solution was
pumped using an IscoÒ 1000D high pressure syringe pump and the
flow rate was not calibrated. 100.0 g of ester 3a (estimated at 92.7%
w/w, 92.7 g, 0.461 mol) was dissolved in 1500 mL of 1:1 (v/v) 2-
propanol:water and ꢀ1000 mL of this solution was charged to
the syringe pump. The reactor was filled with 1:1 (v/v) 2-propa-
nol:water solvent. The oven temperature was set to 160 °C and
when the temperature had stabilized, flow from the reagent pump
was initiated at 0.847 mL/min (V/Q = 23.61 min). During the star-
tup phase, samples were collected from the stream immediately
after the reactor prior to the back pressure regulator in order to
observed, however slight off gassing of hydrogen was apparent.
The pale green suspension was stirred at 21 °C for 1.5 h. Following
this time, the reaction mixture was cooled to 0 °C using an ice bath.
A solution of tert-butylbromoacetate (6.60 mL, 43.8 mmol) in DMF
(
90 mL) was then added via dropping funnel over approximately
1
h and the temperature was maintained below 30 °C during the
addition. Following the addition, the reaction was stirred for an
additional 2 h at 21 °C. The reaction mixture was then poured onto
ice water (400 mL). EtOAc (400 mL) was added and the layers were
separated. The aqueous phase was extracted with EtOAc
(
2 Â 400 mL) and the combined organic layers were washed with
saturated sodium bicarbonate (2 Â 400 mL) then brine (400 mL).
The organic layer was dried (MgSO ), and concentrated using a
1
characterize the transition curve using H NMR with an internal
4
standard of 3,5-dimethylpyrazole. When the initial reagent charge
to the syringe pump was exhausted, flow was paused while the
pump was refilled with the remaining 3a solution; the flow was
rotary evaporator. The resulting crude residue consisted of starting
aniline and desired ester product in approximately a 2:3 ratio. The