Stereodivergent synthesis of the LFA-1 antagonist BIRT-377 by porcine liver esterase desymmetrization and Curtius rearrangement

Article abstract of DOI:10.1039/c4ob02303j

The LFA-1 inhibitor and leukocyte adhesion suppressor BIRT-377 was prepared in high enantiomeric excess by desymmetrization of dimethyl 2-p-bromobenzyl-2-methylmalonate, followed by condensation of the resulting carboxylic acid with 3,5-dichloroaniline, s

Full text of DOI:10.1039/c4ob02303j

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Stereodivergent synthesis of the LFA-1
antagonist BIRT-377 by porcine liver esterase
Cite this: DOI: 10.1039/c4ob02303j
desymmetrization and Curtius rearrangement†
Aaron Johnson, Matthew J. Saunders and Thomas G. Back*
The LFA-1 inhibitor and leukocyte adhesion suppressor BIRT-377 was prepared in high enantiomeric
excess by desymmetrization of dimethyl 2-p-bromobenzyl-2-methylmalonate, followed by condensation
of the resulting carboxylic acid with 3,5-dichloroaniline, saponification of the remaining ester and Curtius
rearrangement as the key steps. When Curtius rearrangement preceded the condensation step, (ent)-
BIRT-377 was similarly obtained in high ee.
Received 29th October 2014,
Accepted 26th November 2014
DOI: 10.1039/c4ob02303j
www.rsc.org/obc
Introduction
Binding of the ligand ICAM-1 to the antigen LFA-1 mediates
leukocyte adhesion, which is implicated in a number of key
immunological processes. The hydantoin BIRT-377 (1) sup-
presses leukocyte adhesion by selectively inhibiting LFA-1,
thus providing a potential therapy for certain immune dis-
orders, as well as serving as an anti-inflammatory agent.1,2
Despite the relative structural simplicity of 1, the enantio-
selective formation of its quaternary stereocenter poses a sig-
nificant challenge.3 This has been previously addressed by
means of chiral oxazolidinone intermediates,3a,h,k via alkyl-
ation of a bislactim derived from D-valine and alanine3d and
the use of chiral imidazolidinone intermediates.3g,i,j Other
methods employed chiral phase transfer catalysts in enolate
alkylations,3b,c a chiral aziridine intermediate3e and an organo-
catalytic amination procedure.3f We considered that a possible
new approach might be via appropriate derivatives of the
corresponding α,α-disubstituted amino acid 2 (Scheme 1),
since a number of methods are available for the preparation of
such compounds.4 We recently reported5 that the use of
porcine liver esterase (PLE)-mediated desymmetrization of
α,α-dialkylated malonate diesters,6 followed by stereospecific
Curtius rearrangement,7 provides an enantioselective approach
to α-quaternary amino acids, as well as their Cbz and urea
derivatives (Scheme 2). This method often delivers the pro-
ducts with high enantiomeric excesses and the active site
model of PLE proposed by Jones et al.,6b as well as analogy
with previously reported examples,5,6,8,9 enables the prediction
Scheme 1
Scheme 2
of absolute configuration of the product with a reasonable
degree of certainty. However, PLE consists of several isozymes
with different activities and substrate specificities.10 Some vari-
ation in the properties of different batches of commercial PLE
can therefore be expected if they have nonidentical isozyme
composition. The cosolvent also plays an important role in
determining the degree of enantioselectivity.11 An earlier vari-
Department of Chemistry, University of Calgary, 2500 University Drive NW,
Calgary AB, Canada, T2N 1N4. E-mail: tgback@ucalgary.ca
†Electronic supplementary information (ESI) available: 1H and 13C NMR spectra
of products. See DOI: 10.1039/c4ob02303j
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ation of this protocol provided a highly enantioselective
synthesis of the antiviral agent (−)-virantmycin and its
(+)-enantiomer.9 We now report the application of sequential
PLE desymmetrization and Curtius rearrangement to a stereo-
divergent synthesis of BIRT-377.
Table 1 Desymmetrization of α,α-disubstituted malonate diester
with PLE
3
Cosolventa
(%)
Temperatureb
(°C)
Time
(days)
Yieldc
(%)
eed
(%)
i-PrOH 2.5
5
7.5
15
5
5
THF 2.5
5
7.5
15
t-BuOH 2.5
5
7.5
10
12.5
15
2.5
5
MeCN 2.5
5
10
12.5
RT
RT
RT
RT
0
2
2
2
2
7
1
2
2
2
2
2
2
2
2
2
2
7
1
2
2
2
2
2
2
2
2
31
64
45
47
59 (97)e
49
36
39
62
15
32
26
23
34
58
51
57
36
70
26
55
47
68
43
19
SM
64
70
68
58
75
68
50
50
44
40
68
64
62
60
50
46
73
64
58
50
30
20
30
64
58
—
Results and discussion
40
Our synthetic approach to BIRT-377 is shown in Scheme 3.
Commercially available dimethyl 2-methylmalonate was alkyl-
ated with p-bromobenzyl bromide to afford the achiral qua-
ternary malonate diester 3. In earlier studies, Björkling et al.6f
had observed that dimethyl 2-benzyl-2-methylmalonate
afforded a very low ee of only 16% with PLE. However, we
later noted that the ee increased considerably to 80% when
the 2-benzyl substituent was homologated to the corres-
ponding 2-(2-phenethyl) group.5 This suggested that the
increased size of the phenethyl substituent resulted in tighter
binding to the large hydrophobic pocket of PLE in the Jones
model.6b Similarly, we reasoned that the larger p-bromobenzyl
substituent of 3, compared with benzyl, might provide
enhanced binding to the enzyme and afford improved enantio-
selectivity. Thus, desymmetrization of 3 with PLE was investi-
gated under a variety of conditions, but produced only
modest enantiomeric excesses of up to 75% of the half-ester 4
(Table 1). In general, the process tolerated a variety of co-
solvents at concentrations in the range of 2.5–15% in an aqueous
phosphate buffer at pH 8. Similar buffers at pH 7.5 and 7, as
well as tris HCl buffer at pH 7.5 gave marginally lower ee
values. The highest enantioselectivities were obtained at 0 °C
in 5% 2-propanol (ee: 75%; isolated yield: 59%) or in 2.5%
RT
RT
RT
RT
RT
RT
RT
RT
RT
RT
0
40
RT
RT
RT
RT
RT
RT
RT
RT
15
CF3CH2OH 2.5
5
12.5
a The cosolvent was mixed with a pH 8 phosphate buffer. b RT = room
temperature. c Isolated yields of half-ester 4 are reported; SM indicates
that only starting material was recovered. d The enantiomeric excesses
(ee) were measured by 1H NMR integration of signals from
diastereomers in a CDCl3 solution containing an equimolar amount of
(R)-(+)-α-methylbenzylamine. e The product was accompanied by 38%
recovery of starting material; the yield in parentheses is the total mass
balance.
t-butanol (ee: 73%; isolated yield: 57%) as the cosolvent after
7 days.12 When the former experiment in 2-propanol was con-
ducted at room temperature for 2 days, the yield increased
slightly to 64% and the ee decreased to 70%. These conditions
were preferred for larger scale experiments. Enantiomeric
excesses of the half-ester 4 were measured by 1H-NMR
integration of a solution of the diastereomeric salts formed
by mixing the product with an equimolar amount of
(R)-(+)-α-methylbenzylamine in CDCl3.
The use of THF, acetonitrile or trifluoroethanol afforded
lower enantioselectivities, while aqueous ethanol (not shown)
provided low yields and very poor enantioselectivities under all
conditions studied. The PLE-promoted partial hydrolysis in all
solvent combinations typically slowed after the first few days
and the unreacted diester was easily recovered and recycled.
For example, the product 4 obtained in 5% 2-propanol at 0 °C
was accompanied by 38% recovery of starting material, for a
mass balance of 97%. A variety of other enzymes was also
tested. These included: α-chymotrypsin,6f,13 as well as lipases
from Thermomyces lanuginosus (lipolase), Rhizopus Niveus
and Pseudomonas Cepacia.14,15 All either failed to provide the
desired product 4 from diester 3 or afforded significantly lower
enantioselectivities.
Scheme 3
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Condensation of the acyl chloride 5, derived from the half-
ester 4, with 3,5-dichloroaniline produced amide 6 in 92%
overall yield (Scheme 3). Saponification of the remaining ester
group was followed by Curtius rearrangement. The ester hydro-
lysis proceeded slowly, presumably due to steric hindrance,
and the best results were obtained when the reaction was
stopped before completion in order to avoid the accumulation
of side products. This afforded 58% of the required carboxylic
acid 7, along with 38% of recovered starting material, for a
mass balance of 96%. The Curtius rearrangement was effected
by a modification of the Shioiri procedure16 employing
diphenylphosphoryl azide (DPPA) and was conducted under
conditions where cyclization of the intermediate isocyanate
occurred in the same pot in 57% overall yield. Finally,
N-methylation of hydantoin 8 was performed by a literature
method3d,f,k in essentially quantitative yield. Thus the syn-
thesis of BIRT-377 (1) was achieved in seven steps from
dimethyl 2-methylmalonate in the overall yield of 17%, or 47%
when adjusted for the recovery of starting material in the steps
leading to 4 and 7, but with an ee limited to only 75% in the
PLE-mediated desymmetrization step.
Scheme 5
The same desymmetrized half-ester 4 was also converted
into (ent)-BIRT-377 (11) by a reversal of the order of steps
involving Curtius rearrangement and introduction of the 3,5-
dichloroaniline moiety, as shown in Scheme 4. Thus, 4 was
first converted to the isocyanate 9, followed by addition of
the aniline and cyclization to 10 in the presence of boron
trifluoride etherate. Poorer yields were obtained in the absence
of the Lewis acid, with other Lewis acids (e.g. CuCl, EtAlCl2,
Tb(OTf)3, LaCl3) or in the presence of bases (e.g. Et3N, pyri-
dine, LDA, K2CO3) under a variety of conditions. N-Methylation
again completed the formation of 11 in three steps from the
half-ester 4 in the overall yield of 37%.
The relatively poor enantioselectivity (maximum ee of 75%)
in the formation of 4 prompted further attempts at improve-
ment. We had previously reported5 that the poor enantio-
selectivity that had been observed by Björkling et al.6f in the
PLE-mediated desymmetrization of dimethyl 2-benzyl-2-
methylmalonate (ee 16%) also improved when the 2-methyl
group was increased in size to ethyl (ee 52%) or n-propyl
Scheme 6
(ee 79%). This may again be attributed to a tighter fit of the
latter substituents, in this case into the small hydrophobic
binding site of PLE in the Jones model. We therefore prepared
the larger 2-methylthiomethyl derivative 12, with the intention
of converting the resulting desymmetrized half-ester 13 to the
required methyl derivative 4 by reductive desulfurization with
nickel boride17 (Scheme 5). Unfortunately, 13 was obtained in
very poor ee (20–41%) under a variety of conditions.
Finally, we were pleased to discover that one recrystalliza-
tion of half-ester 4, obtained from ethyl acetate–hexane pro-
duced two crops of an enantiopure product (ee >98%) in
which none of the minor enantiomer could be detected by
NMR analysis, with a high recovery of 88% of the major enan-
tiomer (see the ESI† for spectra of the recrystallized product
and of a racemic sample for comparison, taken in the presence
of 1 equiv. of (R)-(+)-α-methylbenzylamine). The recrystallized
sample of 4 was then converted to BIRT-377 (1) and its enan-
tiomer 11 by the same procedures shown in Schemes 3 and 4,
respectively, to afford the final products with ees of >98% and
97%, respectively (Scheme 6), as determined by polarimetry
and confirmed by HPLC with a chiral column.
Conclusion
In summary, this method provides a concise and stereodiver-
gent synthesis of BIRT-377 and its enantiomer from the same
Scheme 4
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half-ester intermediate 4. Although the initial PLE-mediated CDCl3) δ 11.24 (br s, 1 H), 7.40 (d, J = 8.4 Hz, 2 H), 7.03 (d, J =
desymmetrization of 3 produced only a modest enantiomeric 8.4 Hz, 2 H), 3.76 (s, 3 H), 3.25 (d, J = 13.7 Hz, 1 H), 3.15 (d, J =
excess of at most 75% of 4, a single recrystallization of 13.7 Hz, 1 H), 1.39 (s, 3 H); 13C NMR (101 MHz; CDCl3)
this product delivered the half-ester and consequently the final δ 177.7, 172.0, 134.8, 132.0, 131.6, 121.4, 54.9, 53.0, 40.8, 19.9;
products 1 and 11 in a high state of enantiomeric purity.
mass spectrum (EI), (m/z, %) 302 (12, M+, 81Br), 300 (10, M+,
79Br), 256 (22), 254 (20), 171 (100), 169 (98); HRMS (EI) calc’d
for C12H1381BrO4: 301.9977; found: 301.9991; calc’d for
C12H1379BrO4: 299.9997; found: 299.9987; ee 75%.
A larger batch of half ester 4 (2.64 g, ee 70%) was prepared
similarly in 64% yield, except that the reaction was carried out
Experimental section
General experimental
1H NMR spectra were recorded at 300 or 400 MHz, while 13C at room temperature for 2 d. Recrystallization from ethyl
spectra were obtained at 76 and 101 MHz, respectively. High acetate–hexane afforded 1.96 g of product in two crops of
resolution mass spectra were obtained using a time of flight off-white solid with [α]2D0 −5.4 (c 3.1, dichloromethane)
(TOF) analyzer with electron impact (EI) ionization or a quad- and [α]2D0 −5.4 (c 4.6, dichloromethane), respectively, that were
rupole TOF analyzer with electrospray ionization (ESI). Melting combined (88% recovery of the major enantiomer); mp
points and specific rotations are reported for products derived 71–72 °C; ee >98%.
from recrystallized half-ester 4 with ee of >98%. PLE was
obtained from a commercial source as a mixture of isozymes integration of well separated benzylic methylene signals
(lyophilized powder; 17 units mg−1). in the 1H NMR spectra of equimolar mixtures of 4 and
Dimethyl 2-(4-bromobenzyl)-2-methylmalonate (3). Diiso- (R)-(+)-α-methylbenzylamine in CDCl3.
The ees for the above products were measured by
propylamine (2.43 g, 3.37 mL, 24.1 mmol) was dissolved in
A racemic sample of 4 was prepared for comparison by dis-
100 mL of dry THF at 0 °C under argon. n-Butyllithium solving diester 3 (200 mg, 0.635 mmol) in 20 mL of methanol.
(9.62 mL; 2.5 M in hexanes, 24 mmol) was added dropwise Potassium hydroxide (355 mg, 6.35 mmol) in 10 mL of water
over 5 min, followed by dimethyl 2-methylmalonate (3.51 g, was added and the solution was stirred for 4 h at 16 °C. The
24.0 mmol) over 20 min. The mixture was stirred at room mixture was then washed with dichloromethane, acidified with
temperature for
1 h and then 1-bromo-4-(bromomethyl) 1 M HCl and extracted with dichloromethane. The combined
benzene (7.21 g, 28.9 mmol) in 50 mL of dry THF was added organic layers were dried and concentrated in vacuo to afford
by cannula. The mixture was refluxed for 16 h and was then the racemic half-ester 4 (162 mg, 85%) as a yellow oil with
quenched with brine and extracted with diethyl ether. The NMR spectra identical to the sample prepared by hydrolysis
ether extracts were dried, concentrated in vacuo and chromato- with PLE. Racemic half-ester 4 was treated with an equimolar
graphed over silica-gel (ethyl acetate–hexanes, 1 : 2) to afford amount of (R)-(+)-α-methylbenzylamine in CDCl3 to ensure
7.50 g of 2 (99%) as a pale yellow solid; mp 55–56 °C; IR (film) that the NMR signals were well separated and to correctly
1733 cm−1
2 H), 6.98 (d, J = 8.4 Hz, 2 H), 3.72 (s, 6 H), 3.17 (s, 2 H), 1.34 comparison (see the ESI†).
(s, 3 H); 13C NMR (101 MHz; CDCl3) δ 172.2, 135.2, 132.0,
Methyl (S)-2-(4-bromobenzyl)-3-chloro-2-methyl-3-oxopro-
;
1H NMR (300 MHz; CDCl3) δ 7.39 (d, J = 8.4 Hz, identify them in samples of the enantioenriched half-ester by
131.5, 121.2, 54.9, 52.7, 40.8, 19.9; mass spectrum (EI), (m/z, panoate (5). Half-ester 4 (145 mg, 0.482 mmol) and thionyl
%) 316 (30, M+, 81Br), 314 (32, M+, 79Br), 256 (89), 254 (91), 196 chloride (294 mg, 180 µL, 2.47 mmol) were refluxed in 50 mL
(22), 194 (22), 171 (100), 169 (98), 115 (51); HRMS (EI) calc’d of dichloromethane for 6 h. The mixture was then concen-
for C13H1581BrO4: 316.0133; found: 316.0124; calc’d for trated in vacuo to afford the crude acid chloride 5 as a light
C13H1579BrO4: 314.0154; found: 314.0143.
brown oil, which was used directly without further purifi-
(R)-2-(4-Bromobenzyl)-3-methoxy-2-methyl-3-oxopropanoic cation: IR (film) 1788, 1746 cm−1; H NMR (300 MHz; CDCl3)
acid (4). Diester 3 (400 mg, 1.27 mmol) in 4 mL of 2-propanol δ 7.42 (d, J = 8.4 Hz, 2 H), 7.02 (d, J = 8.4 Hz, 2 H), 3.80 (s, 3 H),
was added to 76 mL of 0.2 M sodium phosphate buffer (pH 8). 3.27 (crude t, J = 14.6 Hz), 1.45 (s, 3 H); 13C NMR (101 MHz;
PLE (50 mg; lyophilized powder, 17 units mg−1) was dissolved CDCl3) δ 173.2, 169.8, 133.7, 132.0, 131.8, 121.9, 64.7, 53.4,
in a minimum amount of the buffer solution and was added 40.8, 20.1; mass spectrum (EI), (m/z, %) 320 (12, M+, 81Br), 318
to the solution of 3. The mixture was stirred for 7 d at 0 °C. (10, M+, 79Br), 256 (32), 254 (34), 171 (100), 169 (94); HRMS (EI)
It was then acidified with 1 M HCl and extracted with dichloro- calc’d for C12H1281Br35ClO3: 319.9638; found: 319.9642; calc’d
methane. The combined organic fractions were filtered for C12H1279Br35ClO3: 317.9658; found: 317.9667.
1
through Celite, dried and then extracted with 1 M KOH.
Unreacted starting material 3 (152 mg, 38%) was recovered by 2-methyl-3-oxopropanoate (6). Acid chloride
Methyl (R)-2-(4-bromobenzyl)-3-(3,5-dichlorophenylamino)-
(215 mg,
5
evaporation of the organic layer and chromatography over 0.673 mmol) and 3,5-dichloroaniline (109 mg, 0.673 mmol)
silica-gel (ethyl acetate–hexanes, 1 : 2). The aqueous layer was were stirred in dichloromethane (5 mL) at room temperature
reacidified with 1 M HCl and extracted with dichloromethane. for 14 h. The mixture was washed with saturated NaHCO3 solu-
The combined organic phases were dried and concentrated tion, followed by brine and extracted with dichloromethane.
in vacuo to provide 4 (225 mg, 59%) as a pale yellow solid: mp The combined organic layers were dried, concentrated in vacuo
1
61–63 °C; IR (film) 3300–2700, 1705 cm−1; H NMR (300 MHz; and chromatographed over silica gel (ethyl acetate–hexanes,
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1 : 2) to yield 6 (274 mg, 92%) as a pale pink oil: IR (film) 3478, (c 2.00, dichloromethane); lit.3d [α]D20 +121.4 (c 1, dichloro-
3388, 1715, 1645 cm−1 1H NMR (400 MHz; CDCl3) δ 9.62 methane).
(br s, 1 H), 7.49 (d, J = 1.8 Hz, 2 H), 7.37 (d, J = 8.4 Hz, 2 H), (R)-5-(4-Bromobenzyl)-3-(3,5-dichlorophenyl)-1,5-dimethyl-
;
7.11 (t, J = 1.8 Hz, 1 H), 6.95 (d, J = 8.4 Hz, 2 H) 3.78 (s, 3 H), imidazolidine-2,4-dione, BIRT-377 (1). Hydantoin 8 (22 mg,
3.39 (d, J = 13.5 Hz), 3.08 (d, J = 13.5 Hz), 1.56 (s, 3 H); 0.051 mmol) was N-methylated by a literature procedure3d,f,k to
13C NMR (101 MHz; CDCl3) δ 175.7, 169.3, 139.4, 135.4, 135.2, afford
1
(22 mg, 97%); mp 136–137 °C; lit.3d,e,g,k mp
1H NMR (400 MHz;
131.7, 131.3, 124.6, 121.6, 118.6, 55.5, 53.1, 44.1, 22.2; mass 135–136 °C; IR (film) 1776, 1724 cm−1
;
spectrum (EI), (m/z, %) 445 (18, M+, 81Br), 443 (16, M+, 79Br), CDCl3) δ 7.44 (d, J = 8.4 Hz, 2 H), 7.30 (t, J = 1.8 Hz, 1 H), 6.96
256 (36), 254 (32), 225 (38), 223 (42), 171 (98), 169 (100); HRMS (d, J = 8.4 Hz, 2 H), 6.86 (d, J = 1.9 Hz, 2 H), 3.11 (d, J =
(EI) calc’d for C18H1681Br35Cl2NO3: 444.9670; found 14.0 Hz), 3.09 (s, 3 H), 2.98 (d, J = 14.0 Hz, 1 H), 1.63 (s, 3 H);
444.9673; calc’d for C18H1679Br35Cl2NO3: 442.9691; found: 13C NMR (101 MHz; CDCl3) δ 173.4, 153.5, 135.1, 133.0, 132.9,
442.9699; [α]2D0 +60.1 (c 3.0, dichloromethane).
131.9, 131.1, 128.4, 124.5, 122.0, 65.7, 40.8, 25.3, 21.1; mass
(R)-2-(4-Bromobenzyl)-3-(3,5-dichlorophenylamino)-2-methyl- spectrum (EI), (m/z, %) 442 (3, M+, 81Br), 440 (2, M+, 79Br), (271
3-oxopropanoic acid (7). Amide 6 (240 mg, 0.539 mmol) was (100), 56 (71); HRMS (EI) calc’d for C18H1581Br35Cl2N2O2:
dissolved in 5 mL of methanol. Potassium hydroxide (302 mg, 441.9673; found: 441.9656; calc’d for C18H1579Br35Cl2N2O2:
5.39 mmol) in 6 mL of water was added and the solution was 439.9694; found: 439.9688. Elemental analysis calc’d for
stirred for 20 h at room temperature. The mixture was then C18H15BrCl2N2O2: C, 48.90; H, 3.42; N, 6.34; found: C, 49.09;
extracted with dichloromethane. The combined organic layers H, 3.42; N, 6.38. [α]2D0 +135 (c 1.50, ethanol); lit.3d [α]D25 +127.1
were dried and evaporated in vacuo to obtain recovered starting (c 1, ethanol); lit.3e [α]D25 +130.2 (c 1, ethanol); lit.3f [α]D25 +131.6
material 6 (91 mg, 38%). The aqueous layer was acidified with (c 1.0, ethanol); lit.3g [α]D25 +127.3 (c 0.78, ethanol); lit.3k
1 M HCl and extracted with ethyl acetate. The combined [α]2D5 +134.3 (c 1.0, ethanol); ee >98% based on HPLC analysis
organic layers were dried and concentrated in vacuo to (Chiralpak AD column; 250 mm × 4.6 mm; 10 µm particle size.
afford half-ester 7 (135 mg, 58%) as a pale pink solid: mp Solvent system: isocratic conditions using 95 : 5 hexanes-iPrOH
158–159 °C; IR (solid, attenuated total reflectance) 3700–2900, + 10% diethylamine; injection solvent: 9 : 1 hexanes–ethyl
1
3329, 1712, 1655 cm−1; H NMR (300 MHz; CD3OD) δ 7.58 (d, acetate. Detector: UV, 254 nm).
J = 1.9 Hz, 2 H), 7.39 (d, J = 8.4 Hz, 2 H), 7.17 (t, J = 1.9 Hz,
Methyl (S)-3-(4-bromophenyl)-2-isocyanato-2-methylpropanoate
1 H), 7.10 (d, J = 8.4 Hz, 2 H), 3.29 (d, J = 13.6 Hz, 1 H), 3.22 (d, (9). Half-ester 4 (480 mg, 1.59 mmol), diphenylphosphoryl
J = 13.6 Hz, 1 H), 1.41 (s, 3 H); 13C NMR (101 MHz; CD3OD) azide (415 mg, 325 µL, 1.51 mmol), triethylamine (177 mg,
δ 176.1, 172.4, 141.7, 137.1, 136.0, 133.1, 132.3, 124.9, 121.9, 254 µL, 1.75 mmol) and 4-(dimethylamino)pyridine (214 mg,
120.1, 56.9, 42.6, 21.0; mass spectrum (EI), (m/z, %) 431 1.75 mmol) were refluxed in toluene (40 mL) for 6 h under
(<1, M+, 81Br), 429 (<1, M+, 79Br), 387 (33), 199 (56), 197 (53), argon. The mixture was then washed with saturated NaHCO3
171 (100), 169 (90), 161 (74); HRMS (EI) calc’d for and brine, and extracted with diethyl ether. The combined
C17H1481Br35Cl2NO3: 430.9514; found: 430.9506; calc’d for organic layers were dried, concentrated in vacuo to afford the
C17H1479Br35Cl2NO3: 428.9534; found: 428.9554; [α]D20 +83 crude isocyanate 9 as a brown oil: IR (film) 2248, 1738 cm−1
(c 1.9, dichloromethane).
1H NMR (300 MHz; CDCl3) δ 7.44 (d, J = 8.4 Hz, 2 H), 7.05 (d,
;
(R)-5-(4-Bromobenzyl)-3-(3,5-dichlorophenyl)-5-methylimid- J = 8.4 Hz, 2 H), 3.78 (s, 3 H), 3.11 (d, J = 13.5 Hz, 1 H), 2.85 (d,
azolidine-2,4-dione (8). Diphenylphosphoryl azide (99 µL, J = 13.5 Hz, 1 H), 1.55 (s, 3 H); 13C NMR (101 MHz; CDCl3)
126 mg, 0.46 mmol) was added to a mixture of 7 (100 mg, δ 173.2, 134.2, 131.8, 131.7, 130.9, 121.8, 65.2, 53.5, 45.7, 29.9,
0.232 mmol), K2CO3 (35 mg, 0.25 mmol) and Ag2CO3 (64 mg, 27.1. The crude product was employed directly in the next
0.23 mmol) in dry dioxane (10 mL). The mixture was stirred at step without further purification.
room temperature under argon for 1 h, followed by 22 h
(S)-5-(4-Bromobenzyl)-3-(3,5-dichlorophenyl)-5-methylimid-
at reflux. It was washed with 2 M HCl, saturated NaHCO3 and azolidine-2,4-dione (10). Boron trifluoride diethyl etherate
brine. The mixture was extracted with ethyl acetate. The (36 µL, 42 mg, 0.30 mmol) was added to a mixture of crude 9
organic layers were combined, dried, concentrated in vacuo (88 mg, 0.30 mmol) and 3,5-dichloroaniline (48 mg,
and chromatographed over silica-gel (ethyl acetate–hexanes, 0.30 mmol) in dry toluene (15 mL). The mixture was stirred
1 : 2) to afford 8 as a solid foam, (56 mg, 57%); IR (film)) 3276, under argon at room temperature for 3 h, followed by reflux
1771, 1714 cm−1 1H NMR (400 MHz; CDCl3) δ 7.44 (d, J = for 20 h. The reaction was then quenched with water
;
8.4 Hz, 2 H), 7.35 (t, J = 1.8 Hz, 1 H), 7.15 (broad s, 1 H), 7.06 and extracted with ethyl acetate. The organic layers were dried,
(d, J = 8.4 Hz, 2 H), 6.97 (d, J = 1.8 Hz, 2 H), 3.13 (d, J = concentrated in vacuo and chromatographed over silica-gel
13.7 Hz, 1 H), 2.89 (d, J = 13.7 Hz, 1 H), 1.60 (s, 3 H); 13C NMR (ethyl acetate–hexanes, 1 : 2) to afford hydantoin 10 (45 mg,
(101 MHz; CDCl3) δ 174.2, 155.0, 135.2, 132.9, 132.8, 131.8, 38% over two steps) as a colourless oil with NMR spectra iden-
131.7, 128.6, 124.6, 122.0, 62.9, 43.6, 23.4; mass spectrum tical to those of the (R)-enantiomer 8; [α]2D0 −119 (c 1.30,
(EI), (m/z, %) 428 (18, M+, 81Br), 426 (12, M+, 79Br), 259 (23), dichloromethane).
257 (32), 171 (100), 169 (94); HRMS (EI) calc’d for
(S)-5-(4-Bromobenzyl)-3-(3,5-dichlorophenyl)-1,5-dimethylimid-
C17H1381Br35Cl2N2O2: 427.9517; found: 427.9521; calc’d for azolidine-2,4-dione, (ent)-BIRT-377 (11). Hydantoin 10 was
C17H1379Br35Cl2N2O2: 425.9537; found: 425.9544. [α]2D0 +120 treated by the same procedure as its enantiomer 8 to afford 11
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in 97% yield, with spectra identical to those of 1; mp them in samples of the enantioenriched half-ester by
134–135 °C; [α]2D0 −128 (c 1.00, ethanol); ee 95% based on comparison.
specific rotation; 97% based on HPLC analysis performed as
for 1.
Acknowledgements
Dimethyl 2-(4-bromobenzyl)-2-(methylthiomethyl)malonate
(12). Sodium hydride (199 mg, 60% dispersion in mineral oil,
5.0 mmol) was dissolved in dry THF (100 mL) at 0 °C under
argon. Commercially available dimethyl 2-(4-bromobenzyl)-
support. M. J. S. was an Undergraduate Research Participant
malonate (1.00 g, 3.32 mmol) in dry THF (15 mL) was added
dropwise and the mixture was stirred for 2 h. Chloromethyl
methyl sulfide (818 mg, 8.47 mmol) in THF (2 mL) was added
We thank the Natural Sciences and Engineering Research
Council of Canada and the University of Calgary for financial
(January–April, 2014).
and the mixture was refluxed for 16 h. The reaction was
quenched with saturated NH4Cl solution and the mixture was
References
extracted with diethyl ether. The combined organic layers were
dried (MgSO4), concentrated in vacuo and chromatographed
over silica-gel (ethyl acetate–hexanes, 1 : 4) to afford 506 mg
1 For the discovery of BIRT-377, see: T. A. Kelly,
D. D. Jeanfavre, D. W. McNeil, J. R. Woska Jr., P. L. Reilly,
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1
(42%) of 12 as a pale yellow oil; IR (film) 1733 cm−1; H NMR
(300 MHz; CDCl3) δ 7.39 (d, J = 8.4 Hz, 2 H), 7.01 (d, J = 8.4 Hz,
2 H), 3.34 (s, 6 H), 3.31 (s, 2 H), 2.92 (s, 2 H), 2.10 (s, 3 H);
13C NMR (400 MHz; CDCl3) δ 170.4, 134.7, 131.71, 131.69,
121.4, 59.4, 52.9, 37.1, 36.7, 17.1; mass spectrum (EI), (m/z, %)
362 (5, M+, 81Br), 360 (4, M+, 79Br), 269 (15), 267 (14), 191 (100),
159 (91); HRMS (EI) calc’d for C14H1781BrO4S: 362.0010; found:
362.0008; calc’d for C14H1779BrO4S: 360.0031; found: 360.0046.
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(2 mL) was added to sodium phosphate buffer (18 mL, pH 8),
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room temperature. It was then acidified with 1 M HCl and
extracted with dichloromethane. The organic fractions were
filtered through Celite and then extracted with 1 M KOH. The
aqueous layer was reacidified with 1 M HCl and extracted with
dichloromethane. The combined organic phases were dried
(MgSO4) and concentrated in vacuo. Flash chromatography
over silica-gel (ethyl acetate–hexanes, 1 : 9, containing 1%
acetic acid) afforded 32 mg (67%) of 13 as a pale yellow oil; IR
(film) 1742, 1700 cm−1; 1H NMR (300 MHz; CDCl3) δ 10.74 (br
s, 1 H), 7.40 (d, J = 8.4 Hz, 2 H), 7.03 (d, J = 8.4 Hz, 2 H), 3.80
(s, 3 H), 3.35 (d, J = 13.8 Hz, 1 H), 3.24 (d, J = 13.8 Hz, 1 H),
3.04 (d, J = 13.4 Hz, 1 H), 2.96 (d, J = 13.4 Hz, 1 H), 2.14 (s,
3 H); 13C NMR (400 MHz; CDCl3) δ 174.7, 171.5, 134.1, 131.9,
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196 (48), 194 (52), 115 (100); HRMS (EI) calc’d for
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C13H1579BrO4S: 345.9874; found: 345.9866.
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an equimolar mixture of 13 and R-(+)-α-methylbenzylamine in
benzene-d6. With other cosolvents and additives, the ee was in
the range 20–41%.
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4. Racemic half-ester 13 was treated with an equimolar
amount of (R)-(+)-α-methylbenzylamine to ensure that the
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