Synthesis of a directly connected thiazole-oxazole ring system present in microcin B17

Full text of DOI:10.1021/jo951023c

778
J . Org. Chem. 1996, 61, 778-780
Sch em e 1
Syn th esis of a Dir ectly Con n ected
Th ia zole-Oxa zole Rin g System P r esen t in
Micr ocin B17
Gang Li, Philip M. Warner,* and David J . J ebaratnam*
Department of Chemistry, Northeastern University,
Boston, Massachusetts 02115
Received J une 6, 1995 (Revised Manuscript Received
October 9, 1995)
Microcin B17 (McB17) is a 43 residue peptide antibiotic
ribosomally synthesized in several strains of Escherichia
coli.1 Recently, we2 and others3 have reported that
fourteen of these residues are posttranslationally modi-
fied; two Gly-Cys and Gly-Ser dipeptides lead, respec-
tively, to two isolated thiazole and oxazole rings, whereas
two tripeptides Gly-Cys-Ser and Gly-Ser-Cys produce two
directly connected thiazole-oxazole ring systems. The
identification of thiazoles and oxazoles as the modified
forms clearly indicates that McB17 is structurally unre-
lated to the two known classes of DNA gyrase inhibitors,
namely quinolones and coumarins. Yet McB17 is a
strong DNA gyrase inhibitor leading to the inhibition of
DNA replication and, hence, cell death.4 While the exact
roles of the thiazole and oxazole rings are not yet
understood, genetic and biochemical studies indicate that
they may be structurally and functionally important.2
Thus, we undertook their synthesis.
Although a variety of methods were available for the
preparation of isolated thiazole5 and oxazole6 rings, to
the best of our knowledge, the synthesis of a directly
connected thiazole-oxazole ring system has never been
reported in the literature.7-10 Here we report the syn-
thesis of 8 (Scheme 1), an appropriately protected and
directly connected thiazole-oxazole ring system present
in McB17. Coincidentally, 4 represents a protected
version of the aforementioned Gly-Cys modified dipeptide
found in McB17.
Commercially available aminoacetonitrile hydrochlo-
ride (1) served as our starting material. Although both
N-acetyl and N-benzoyl derivatives could be used in the
synthesis described in Scheme 1, the latter were easier
to crystallize and purify throughout the series. There-
fore, 1 was converted to 2 under standard benzoylation
conditions (benzoyl chloride, pyridine, 25 °C, 0.5 h,
81%).12 Hydrogen sulfide13 treatment of an aqueous
ethanolic ammonia solution of 2 provided the desired
thionamide 3, which on condensation with ethyl bro-
mopyruvate14 in boiling methanol afforded the fully
protected thiazole amino acid 4. It should be noted that
4 was indeed a methyl ester due to complete transesteri-
fication in boiling methanol. Selective hydrolysis of the
(1) Yorgey, P.; Lee, J .; Kolter, R. Bacteriocins, Microcins and
Lantibiotics; J ames, R., Lazdunski, C., Pattus, F., Eds., Springer: New
York, 1992; pp 19-31.
(2) Yorgey, P.; Lee, J .; Ko¨rdel, J .; Vivas, E.; Warner, P.; J ebaratnam,
D.; Kolter, R. Proc. Natl. Acad. Sci. U.S.A. 1994, 91, 4519.
(3) Bayer, A.; Freund, S.; Nicholson, G.; J ung, G. Angew. Chem.,
Int. Ed. Engl. 1993, 32, 1336.
(4) Viza´n, J . L.; Herna´ndez-Chico, C.; del Castillo, I.; Moreno, F.
EMBO J . 1991, 10, 467.
(5) For a review, see: Vernin, G. In The Chemistry of Heterocyclic
Compounds, 34; Wiley: New York, 1979; pp 165-335.
(6) For reviews, see: (a) Turchi, I. J . In The Chemistry of Heterocyclic
Compounds, 45; Wiley: New York, 1986; pp 1-342. (b) Gant, T. G.;
Meyers, A. I. Tetrahedron 1994, 50, 2297.
(7) Syntheses of directly connected oxazole-oxazole,8 oxazoline-
oxazoline,6b,9 thiazole-thiazole10 and thiazoline-thiazoline11 ring sys-
tems, however, have been reported.
(11) (a) Walker, M. A.; Heathcock, C. H. J . Org. Chem. 1992, 57,
5566. (b) Pattenden, G.; Thom, S. M. Synlett 1992, 533. (c) Pattenden,
G.; Thom, S. M. J . Chem. Soc., Perkin Trans. 1 1993, 1629. (d)
Fukuyama, T.; Xu, L. J . Am. Chem. Soc. 1993, 115, 8449. (e) Parsons,
R. L., J r.; Heathcock, C. H. J . Org. Chem. 1994, 59, 4733. (f) Parsons,
J r., R. L.; Heathcock, C. H. Tetrahedron Lett. 1994, 35, 1379. (g)
Parsons, R. L., J r.; Heathcock, C. H. Tetrahedron Lett. 1994, 35, 1383.
(h) Boyce, R. J .; Pattenden, G. Synlett 1994, 587. (i) Boyce, R. J .;
Mulqueen, G. C.; Pattenden, G. Tetrahedron Lett. 1994, 35, 5705. (j)
Ehrler, J .; Farooq, S. Synlett 1994, 702. (k) Boyce, R. J .; Pattenden,
G. Tetrahedron 1995, 51, 7313. (l) Boyce, R. J .; Mulqueen, G. C.;
Pattenden, G. Tetrahedron 1995, 51, 7321.
(8) (a) Knight, D. W.; Pattenden, G.; Rippon, D. E. Synlett 1990,
36. (b) Pattenden, G. J . Heterocyc. Chem. 1992, 29, 607. (c) Wipf, P.;
Miller, C. P. J . Org. Chem. 1993, 58, 3604.
(9) (a) Evans, D. A.; Woerpel, K. A.; Hinman, M. M.; Faul, M. M. J .
Am. Chem. Soc. 1991, 113, 726. (b) Helmchen, G.; Krotz, A.; Ganz,
K.-T.; Hansen, D. Synlett 1991, 257. (c) Takacs, J . M.; Weidner, J . J .;
Newsome, P. W.; Takacs, B. E.; Chidambaran, R.; Shoemaker, R. J .
Org. Chem. 1995, 60, 3473.
(10) (a) Zee-Cheng, K. Y.; Chen, C. C. J . Heterocycl. Chem. 1970, 7,
1439. (b) McGowan, D. A.; J ordis, U.; Minster, D. K.; Hecht, S. M. J .
Am. Chem. Soc. 1977, 99, 8078. (c) Sakai, T. T.; Riordan, J . M.; Booth,
T. E.; Glickson, J . D. J . Med. Chem. 1981, 24, 279. (d) Houssin, R.;
Bernier, J .-L.; Henichart, J .-P. J . Heterocycl. Chem. 1984, 21, 681. (e)
Boger, D. L.; Menezes, R. F. J . Org. Chem. 1992, 57, 4331. (f) Kane, S.
A.; Sasaki, H.; Hecht, S. M. J . Am. Chem. Soc. 1995, 117, 9107.
(12) Klages, A.; Haack, O. Chem. Ber. 1903, 36, 1646.
(13) Cross, D. F. W.; Kenner, G. W.; Sheppard, R. C.; Stehr, C. E.
J . Chem. Soc. 1963, 2143.
(14) See the Experimental Section for details.
0022-3263/96/1961-0778$12.00/0 © 1996 American Chemical Society

Notes
J . Org. Chem., Vol. 61, No. 2, 1996 779
when the flask was set aside at room temperature overnight,
and these were collected by filtration and washed with 95%
ethanol to obtain 3 (21.1 g, 83%), mp 149-150 °C. 1H NMR
(DMSO-d6): δ 9.72 (1H, s), 9.17 (1H, s), 8.84 (1H, t, J ) 6.45
Hz), 7.45-7.92 (5H, m), 4.16 (2H, d, J ) 6.00 Hz); 13C NMR
(DMSO-d6): δ 203.45, 166.36, 133.99, 131.27, 128.12, 127.43,
49.60. Anal. Calcd for C9H10N2OS: C, 55.65; H, 5.19; N, 14.42.
Found: C, 55.80; H, 5.53; N, 14.33.
ester group (aqueous K2CO3, THF, MeOH, 25 °C, 6 h,
75%) provided 5, which was then elaborated, as described
below and in Scheme 1, to construct the oxazole ring.
Although the coupling of 5 with DL-serine methyl ester
proceeded under standard solution phase peptide cou-
pling conditions, we have found that 6 exhibits a much
higher than expected tendency to eliminate. Thus, longer
reaction times and/or excessive use of triethylamine must
be avoided to obtain reasonable yields of the coupling
product.14 Further, this higher tendency for elimination
also made the conversion of 6 to 7 the most challenging
step in our synthetic sequence. The tosylate, mesylate,
or halides desired for intramolecular cyclization of 6 were
not useful, as elimination was the exclusive pathway.
Similar results were obtained under Mitsunobu condi-
tions for direct cyclization.15 Fortunately, the Burgess
reagent,16 as recently reported by Wipf and Miller,17
induced the desired cyclization to 7 in 60% yield. It was
important to use a somewhat higher reaction tempera-
ture and shorter reaction time than reported17 to mini-
mize elimination. With dihydrooxazole 7 in hand, we
then examined conditions for its oxidative aromati-
zation.6b,18,19 The recently reported19 CuBr2-DBU system
worked the best, thereby completing the synthesis of 8
in seven steps with an overall yield of 8.5%.
P r ep a r a t ion of Met h yl 2-(Ben za m id om et h yl)t h ia zole-
4-ca r boxyla te (4).13 Ethyl bromopyruvate (11.56 g, 59.3 mmol)
was added dropwise, over a 0.5 h period, to a 50 °C solution of
3 (11.5 g, 59.3 mmol) in methanol (100 mL) after which the
reaction was refluxed for an additional 2 h. Most of the product
crystallized when the solution was set aside at room temperature
overnight, and this was collected by filtration. The filtrate was
evaporated, redissolved in benzene, washed successively with
saturated aqueous sodium bicarbonate and water, dried over
anhydrous sodium sulfate, and evaporated. Recrystallization
of this residue from methanol provided an additional crop. The
total yield of 4 was 12.1 g (74%), mp 145-146 °C. 1H NMR
(CDCl3): δ 8.16 (1H, s), 7.78-7.84 (2H, m), 7.42-7.60 (3H, m),
7.03 (1H, br), 4.98 (2H, d, J ) 5.7 Hz), 3.95 (3H, s); 13C NMR
(CDCl3): δ 168.38, 167.55, 161.67, 146.39, 133.37, 132.04, 128.69,
128.56, 127.15, 52.49, 41.36. Mass spec (m/ z): 275 (M+), 257
(10%), 206 (12%), 170 (55%), 155 (10%), 138 (53%), 104 (100%),
76 (82%), 51 (55%). Anal. Calcd for C13H12N2O3S: C, 56.51; H,
4.38; N, 10.14. Found: C, 56.87; H, 4.65; N, 9.90.
P r ep a r a tion of 2-(Ben za m id om eth yl)th ia zole-4-ca r box-
ylic Acid (5). A solution of 4 (11.5 g, 41.7 mmol) in THF (75
mL), methanol (375 mL), and 1 M aqueous potassium carbonate
(150 mL) was stirred at room temperature for 6 h and then
acidified by addition of 1 M HCl (450 mL). The resulting mixture
was extracted with one 300 mL and then two 200 mL portions
of dichloromethane. The combined organic extracts were dried
over sodium sulfate and concentrated and the residue recrystal-
lized from 200 mL of hot acetonitrile/water (20/1, v/v) to obtain
5 (8.2 g, 75%) as a white solid, mp 214-215 °C. 1H NMR
(DMSO-d6): δ 9.47 (1H, t, J ) 6.03 Hz), 8.36 (1H, s), 7.51-7.93
Exp er im en ta l Section
Gen er a l. Unless otherwise noted, all synthetic reactions
were performed in oven-dried glassware under
a positive
atmosphere of dry nitrogen, and their progress was monitored
by thin layer chromatography (TLC), using E. Merck silica gel
60F glass plates (0.25 mm thick). Chromatographic purifications
were performed on silica gel according to the Still protocol.
Solvents were freshly distilled prior to use as follows: THF from
sodium benzophenone ketyl, methanol from magnesium meth-
oxide, and dichloromethane, DBU, pyridine, and triethylamine
from calcium hydride. All other solvents and reagents were used
as purchased from the manufacturers. The term in vacuo refers
to solvent removal using a rotary evaporator, followed by drying
under high vacuum (ca. 0.5 mmHg) for several hours. 1H NMR
spectra were obtained at 300 MHz and 13C NMR spectra at 75.2
MHz. Elemental analyses were performed by Galbraith Labo-
ratories, Inc., Knoxville, TN.
P r ep a r a t ion of 2-Ben za m id oa cet on it r ile (2). Amino-
acetonitrile hydrochloride (1) (8.0 g, 87 mmol) was placed in a
flask equipped with an addition funnel. Pyridine (50 mL) was
carefully added to obtain a solution, and to this was added
benzoyl chloride (10.5 mL, 90 mmol) dropwise over 0.5 h. After
stirring overnight at room temperature, water (50 mL) was
carefully added; pyridinium hydrochloride dissolved while the
product precipitated as a white solid. The precipitate was
collected by filtration, washed with water, and recrystallized
from 95% ethanol to obtain 2 (11.3 g, 81%) as white crystals,
mp 140 °C (lit.12 144 °C). 1H NMR (DMSO-d6): δ 9.21 (1H, s),
7.80-7.95 (2H, m), 7.48-7.65 (3H, m), 4.32 (2H, d, J ) 3.90 Hz);
13C NMR (DMSO-d6): δ 166.60, 132.79, 131.91, 128.47, 127.29,
117.61, 27.68.
(5H, m), 4.75 (2H, d, J ) 6.00 Hz); 13C NMR (DMSO-d6):
δ
170.06, 166.57, 162.00, 146.65, 133.49, 131.64, 128.73, 128.42,
127.25, 41.03.
P r ep a r a tion of 2-(Ben za m id om eth yl)-4-[N-[1-(m eth oxy-
ca r bon yl)-2-h yd r oxyeth yl]ca r ba m oyl]th ia zole (6). To a 0
°C solution of 5 (2.62 g, 10 mmol) in THF (40 mL) were added,
sequentially, 1-hydroxybenzotriazole hydrate (HOBT) (1.77 g,
11.6 mmol), DL-serine methyl ester hydrochloride (1.62 g, 10.5
mmol), and triethylamine (3 mL, 21.7 mmol). The resulting
slurry was stirred for 5 min, and then dicyclohexylcarbodiimide
(DCC) (2.165 g, 10.5 mmol) was added rapidly. The reaction
mixture was then stirred for 4 h at 0 °C and then at room
temperature for 10 h. The resulting slurry was cooled to 0 °C,
diluted with ethyl acetate (40 mL), stirred for 15 min, and then
filtered. The filtrate was concentrated in vacuo, and the residue
was purified by silica gel flash chromatography (acetone/ethyl
acetate/hexane ) 1/1/1) and recrystallized from methanol to
obtain 6 (2.1 g, 58%). TLC: Rf (silica gel, acetone/ethyl acetate/
hexane ) 1/1/1) ) 0.34. 1H NMR (CDCl3): δ 8.19 (1H, d, J )
6.45 Hz), 7.89-7.92 (2H, m), 7.84 (1H, s), 7.63 (1H, br), 7.40-
7.55 (3H, m), 4.85 (3H, m), 4.10 (2H, d, J ) 2.4 Hz), 3.82 (3H,
s); 13C NMR (CDCl3): δ 170.82, 168.32, 167.61, 161.02, 148.26,
133.25, 132.10, 128.74, 127.27, 125.05, 63.11, 54.85, 52.89, 41.25.
P r ep a r a t ion of 2-(Ben za m id om et h yl)-4-[2′-[4′-(ca r -
bom eth oxy)-4′,5′-d ih yd r ooxa zolyl]]th ia zole (7). A solution
of methyl N-[(triethylammonio)sulfonyl]carbamate, Burgess re-
agent,16 (318 mg, 1.3 mmol) in THF (10 mL) was added dropwise
over 20 min to a room temperature solution of 6 (427 mg, 1.2
mmol) in THF (5 mL). The Pyrex tube containing this mixture
was then capped, and heated at 90 °C for 2 h. Upon cooling to
room temperature, the reaction mixture was concentrated,
purified by flash chromatography (acetone/ethyl acetate/hexane
) 1/1/1), and recrystallized from methanol to obtain 7 (179 mg,
60%) as a white crystalline solid, mp 193-194 °C. TLC: Rf
(silica gel, acetone/ethyl acetate/hexane ) 1/1/1) ) 0.29. 1H
NMR (CDCl3): δ 7.96 (1H, s), 7.75-7.80 (2H, m), 7.38-7.53 (3H,
m), 7.00 (1H, br), 4.95 (4H, d, J ) 6.00 Hz), 4.72 (1H, t, J )
11.79 Hz), 3.78 (3H, s); 13C NMR (CDCl3): δ 171.21, 167.46,
133.38, 132.08, 132.01, 128.78, 128.69, 127.25, 127.15, 125.03,
P r ep a r a tion of 2-Ben za m id oth ioa ceta m id e (3).13 Hy-
drogen sulfide gas was bubbled for 4 h through a solution of 2
(21.0 g, 0.13 mol) in 200 mL of absolute ethanol and 40 mL of
NH4OH. After bubbling argon for 2 h to drive off excess H2S,
the reaction volume was reduced to about 100 mL in vacuo.
Water (5 mL) was added and the flask slightly heated to dissolve
any solid material that had settled. Yellow crystals appeared
(15) Wipf, P.; Miller, C. P. Tetrahedron Lett. 1992, 33, 6267 and
references cited therein.
(16) Atkins, G. M.; Burgess, E. M. J . Am. Chem. Soc. 1968, 90, 4744.
(17) Wipf, P.; Miller, C. P. Tetrahedron Lett. 1992, 33, 907.
(18) Evans, D. L.; Minster, D. K.; J ordis, U.; Hecht, S. M.; Mazzu,
A. L.; Meyers, A. I. J . Org. Chem. 1979, 44, 497.
(19) Barrish, J . C.; Singh, J .; Spergel, S. H.; Han, W.-C.; Kissick, T.
P.; Kronenthal, D. R.; Mueller, R. H. J . Org. Chem. 1993, 58, 4494.

780 J . Org. Chem., Vol. 61, No. 2, 1996
Notes
53.12, 52.78, 45.07, 41.35, 41.20. Anal. Calcd for C16H15N3-
O4S: C, 55.68; H, 4.46; N, 12.38. Found: C, 55.64; H, 4.38; N,
12.17.
flash chromatography (acetone/ethyl acetate/hexane ) 1/1/1) to
obtain 8 (79 mg, 66%) as a white crystalline solid, mp 215-216
°C. TLC: Rf (silica gel, acetone/ethyl acetate/hexane ) 1/1/1)
) 0.62. 1H NMR (DMSO-d6): δ 9.51 (1H, t, J ) 5.58 Hz), 8.96
(1H, s), 8.42 (1H, s), 7.90-7.95 (2H, m), 7.48-7.62 (3H, m), 4.81
(2H, d, J ) 6.00 Hz), 3.45 (3H, s); 13C NMR (DMSO-d6): δ 171.55,
166.59, 160.93, 157.07, 145.41, 141.08, 133.43, 133.18, 131.67,
128.44, 127.26, 123.07, 51.86, 41.04. Anal. Calcd for
P r ep a r a t ion of 2-(Ben za m id om et h yl)-4-[2′-[4′-(ca r -
bom eth oxy)oxa zolyl]]th ia zole (8).19 To a suspension of
CuBr2 (155 mg, 0.7 mmol) in dry, deoxygenated dichloromethane
(15 mL) was added hexamethylenetetramine (HMTA) (97 mg,
0.7 mmol) followed by DBU (106 mg, 0.7 mmol). The resulting
warm, dark brown solution was cooled in a water bath for 10
min, and to this was added solid 7 (120 mg, 0.35 mmol). After
21 h at room temperature, the solvent was removed in vacuo
and the residue partitioned with equal amounts (v/v) of ethyl
acetate and saturated aqueous NH4Cl/concentrated NH4OH (1:
1). The aqueous layer was extracted further with ethyl acetate
(3 × 10 mL), and the combined extracts were washed succes-
sively with saturated NH4Cl/concentrated NH4OH (1:1, 3 × 10
mL), 10% citric acid, aqueous NaHCO3, and brine. The solution
was dried over Na2SO4, concentrated in vacuo, and purified by
C
16H13N3O4S: C, 55.97; H, 3.82; N, 12.24. Found: C, 55.64; H,
3.91; N, 12.39.
Su p p or tin g In for m a tion Ava ila ble: 13C NMR spectra for
compounds 5 and 6 (4 pages). This material is contained in
libraries on microfiche, immediately follows this article in the
microfilm version of the journal, and can be ordered from the
ACS; see any current masthead page for ordering information.
J O951023C