Total synthesis of bistratamide D

Article abstract of DOI:10.1021/jo981664i

The total synthesis of the macrocyclic hexapeptide bistratamide D is reported. The synthetic strategy involved assembly of enantiomerically pure oxazole, thiazole, and oxazoline segments derived from amino acids. Macrocyclization of the hexapeptidic aminooxazoline-oxazole-thiazole carboxylic acid fragment was accomplished by activation with O-(7- azabenzotriazol-1-yl)-N,N,N',N'-tetramethyl-uronium hexafluorophosphate (HATU).

Full text of DOI:10.1021/jo981664i

826
J . Org. Chem. 1999, 64, 826-831
Tota l Syn th esis of Bistr a ta m id e D†
Susan V. Downing, Enrique Aguilar, and A. I. Meyers*
Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523
Received August 17, 1998
The total synthesis of the macrocyclic hexapeptide bistratamide D is reported. The synthetic strategy
involved assembly of enantiomerically pure oxazole, thiazole, and oxazoline segments derived from
amino acids. Macrocyclization of the hexapeptidic aminooxazoline-oxazole-thiazole carboxylic acid
fragment was accomplished by activation with O-(7-azabenzotriazol-1-yl)-N,N,N′,N′-tetramethyl-
uronium hexafluorophosphate (HATU).
Sch em e 1
A large number of oxazole- and/or thiazole-containing
natural products have been isolated from marine organ-
isms, mainly sponges and ascidians, over the last two
decades. The cytotoxic and antineoplastic activities that
they exhibit, as well as the possibility of acting as metal
ion chelating metabolites, have inspired a considerable
amount of both structural and synthetic studies.1
We have focused our attention on the synthesis of some
of these natural products and have recently reported the
total synthesis of bistratamide C.2 We next turned our
attention to the synthesis of the closely related macro-
cyclic hexapeptide bistratamide D (1), isolated from a
Philippine collection of the ascidian Lissoclinum bistra-
tum.3 While oxazole-containing products are known from
nudibrach egg masses4 and from a number of sponges,5
bistratamide D is only the third example of an oxazole-
containing product from an ascidian.3,6 Furthermore,
bistratamide D is one of the more cytotoxic of the cyclic
hexapeptides isolated from L. bistratum and has been
found to induce depressant effects in mice when admin-
istered by intracerebral injection.3
A retrosynthetic approach to bistratamide D, consisting
of disconnection at each of the amide linkages and
therefore similar to the one performed for bistratamide
C2, is outlined in Scheme 1. However, the presence of the
trans-4,5-disubstituted oxazoline 3 in the bistratamide
D skeleton required that we use different protecting
groups during the synthetic process. The trans-substi-
tuted oxazoline 3 is an acid-sensitive and readily opened
moiety,7 and for these reasons the oxazoline ring closure
in earlier work on peptidic macrocycles8 was delayed until
the final step. However, there were instances where the
preconstructed trans-4,5-disubstituted oxazoline was
utilized9 successfully. We chose to follow this course since
it represented a more convergent strategy.
Synthesis of the oxazole moiety 2 could be achieved
by radical oxidation of the valine-serine-derived oxazoline
52 by a procedure recently developed in our laboratory,
allowing us to obtain oxazole 2 in 67% yield on a 2.5 g
scale without affecting the chiral center R to the 2-posi-
tion of the oxazole ring. This procedure10 involves reflux-
ing of oxazoline 5 in benzene in the presence of copper(I)
bromide, copper(II) acetate, and tert-butylperoxybenzoate
(Scheme 2). It offers a high degree of reproducibility,
being therefore much more reliable than MnO2 or NiO2
for the oxidation of oxazolines. However, by following a
procedure recently reported by Williams,11 involving use
of bromotrichloromethane and 1,8-diazabicyclo[5.4.0]-
undec-7-ene (DBU), oxazole 2 was obtained in quantita-
tive yield. Saponification with lithium hydroxide in
aqueous methanol gave the oxazole carboxylic acid 6, in
† Dedicated to Professor Teruaki Mukaiyama on the occasion of his
75th birthday.
(1) (a) Wipf, P. Chem. Rev. 1995, 95, 2115. (b) Michael, J . P.;
Pattenden, G. Angew. Chem., Int. Ed. Engl. 1993, 32, 1.
(2) Aguilar, E.; Meyers, A. I. Tetrahedron Lett. 1994, 35, 2477.
(3) Foster, M. P.; Concepcio´n, G. P.; Caraan, G. B.; Ireland, C. M.
J . Org. Chem. 1992, 57, 6671.
(4) (a) Roesener, J . A.; Scheuer, P. J . J . Am. Chem. Soc. 1986, 108,
846. (b) Matsunaga, S.; Fusetani, N.; Hashimoto, K.; Koseki, K.; Noma,
M. J . Am. Chem. Soc. 1986, 108, 847. (c) Matsunaga, S. et. al. J . Org.
Chem. 1989, 54, 1360.
(5) (a) Ichiba, T.; Yoshida, W. Y.; Scheuer, P. J .; Higa, T.; Gravalos,
D. G. J . Am. Chem. Soc. 1991, 113, 3173. (b) Kernan, M. R.; Molinski,
T. M.; Faulkner, D. J . J . Org. Chem. 1988, 53, 5014. (c) Fusetani, N.;
Yasumuro, K.; Matsunaga, S.; Hashimoto, K. Tetrahedron Lett. 1989,
2809. (d) Kato, Y.; Fusetani, N.; Matsunaga, S.; Hashimoto, K. J . Am.
Chem. Soc. 1986, 108, 2780. (e) Adamczeski, M.; Quin˜oa´, E.; Crews,
P. J . Am. Chem. Soc. 1988, 110, 1598. (f) Fusetani, N.; Sugawara, T.;
Matsunaga, S.; Hirota, H. J . Am. Chem. Soc. 1991, 113, 7811. (g)
Kobayashi, J . et. al. J . Am. Chem. Soc. 1991, 113, 7812.
(6) Rodr´ıguez, J .; Nieto, R. M.; Crews, P. J . Nat. Prod. 1993, 56,
2034.
(8) (a) Boden, C.; Pattenden, G. Tetrahedron Lett. 1994, 35, 8271.
(b) Kato, S.; Hamada, Y.; Shioiri, T. Tetrahedron Lett. 1986, 27, 2653.
(c) Hamada, Y.; Shibata, M.; Shioiri, T. Tetrahedron Lett. 1985, 26,
6501; (d) 1985, 26, 5159; (e) 1985, 26, 5155.
(9) (a) Schmidt, U.; Griesser, H. Tetrahedron Lett. 1986, 27, 163.
(b) Schmidt, U.; Utz, R.; Gleich, P. Tetrahedron Lett. 1985, 26, 4367.
(c) Schmidt, U.; Gleich, P. Angew. Chem., Int. Ed. Engl. 1985, 24, 569.
(10) Tavares, F.; Meyers, A. I. Tetrahedron Lett. 1994, 35, 6803.
(11) Williams, D. R.; Lowder, P. D.; Gu, Y.; Brooks, D. A. Tetrahe-
dron Lett. 1997, 38, 331.
(7) Hamada, Y.; Kato, S.; Shiori, T. Tetrahedron Lett. 1985, 26, 3223.
10.1021/jo981664i CCC: $18.00 © 1999 American Chemical Society
Published on Web 01/15/1999

Total Synthesis of Bistratamide D
J . Org. Chem., Vol. 64, No. 3, 1999 827
Sch em e 2
complished by forming mixed acyl carbonates with both
isobutyl and isopropyl chloroformates, and mixed anhy-
drides with both pivaloyl and 2,4,6-trichlorobenzoyl
chloride. However, both these routes resulted in low to
moderate yields of the desired product 12, often ac-
companied by large amounts of undesired side products.
Activation of oxazole acid 6 with (1,1′)-carbonyldiimida-
zole (CDI)16 resulted in a 78% yield of adduct 12, while
use of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide
hydrochloride (EDCI) in the presence of 1-hydroxyben-
zotriazole (HOBt) proved to be superior, finally providing
adduct 12 in 94% yield (Scheme 5). The primary amine
13 or the carboxylic acid 14 could be obtained from 12 in
almost quantitative yields by employment of well-known
deprotection protocols, as shown in Scheme 5.
Sch em e 3
To arrive at the final product, 1, two alternative routes
were explored. Coupling of the tetrapeptidic acid 14 with
oxazoline amine 9 was accomplished in 53% yield by
using EDCI in the presence of 4-(dimethylamino)pyridine
(DMAP) in dichloromethane (Scheme 6). However, amine
deprotection of 15 did not provide the amino ester 16.
Only products derived from the opening of the oxazoline
moiety were obtained. This result prompted us to explore
the alternative coupling of oxazoline acid 8 with tet-
rapeptidic amine 13. Carboxyl activation of 8 via a mixed
anhydride or the employment of dicyclohexylcarbodiimide
(DCC) with DMAP were found to be inefficient. The use
of EDCI with DMAP, 2-chloro-1,3-dimethyl-imidazo-
lidium hexafluorophosphate (CIP) with 1-hydroxy-7-
azabenzotriazole (HOAt),17 or of triphenylphosphine with
hexachloroacetone18 did produce the hexapeptide 17 in
a modest 31-40% yield (Scheme 7). However, the yield
was eventually improved to 79% by using EDCI with
HOBt in DMF.
Sch em e 4
Removal of the Cbz group in 17 proved to be one of
the critical steps of the synthesis. Any attempt to carry
out an acid-catalyzed cleavage (BBr3, TFA, bromocate-
cholborane) resulted in opening of the oxazoline moiety
rather than loss of the Cbz group. On the other hand,
catalytic hydrogenolysis under standard conditions (10%
Pd/C, Pd(OH)2, atmospheric pressure) was an ineffective
and low-yielding alternative because of the poisoning
effect of the sulfur-containing thiazole moiety. The
employment of liquid ammonia as solvent did little to
improve matters.19 This problem was eventually solved
by using higher pressure (100 psi), a more active catalyst
(Pd black), and EtOH/Et3N as the solvent system to
achieve an 83% yield of amine 18 (Scheme 8). The final
ring-closure step was successfully implemented after
ester hydrolysis of 18 using LiOH-aqueous EtOH, by
treatment of the crude amino acid with HATU and
Hunigs base in dry DMF. Isolation of the product
produced bistratamide D (1) in 48% yield.
near-quantitative yield, which is now ready to be coupled
to the thiazole 4.
The oxazoline portion 3 of the target was obtained as
follows by a slight modification of the epimerization
conditions reported by Wipf.12 Sequential treatment of
cis-oxazoline 7 with 1 M HCl, K2CO3, and basic Al2O3 in
refluxing MeOH provided the dipeptide with the allo-
threonine configuration (not shown), which upon cycliza-
tion with Burgess reagent13 gave the required trans-
oxazoline fragment 3 in good (70%) yield from cis-
oxazoline 7 (Scheme 3). trans-Oxazoline 3 was easily
transformed to the free acid 8 by basic hydrolysis or to
the amine 9 by catalytic hydrogenolysis of the Cbz group
in very good yields.
The requisite thiazole fragment 4 was prepared from
thioamide 10 by using Holzapfel’s modified Hantzsch
procedure,14 as outlined in Scheme 4. Amine deprotection
was achieved with acetyl chloride in absolute ethanol,
as described by North,15 to provide primary amine 11 in
quantitative yield.
As mentioned earlier, our first route to bistratamide
D, involving the N-Boc cleavage of hexapeptide 15, was
found to be unsuccessful. However, we felt that further
study may prove useful. For our second route described
above, a reliable method to cleave the N-Cbz group from
hexapeptide 17 had been obtained, despite the presence
To couple the three fragments, it was decided to first
join oxazole 6 with thiazole amine 11. This was ac-
(12) Wipf, P.; Miller, C. P. J . Am. Chem. Soc. 1992, 114, 10975.
(13) (a) Burgess, E. M.; Penton, H. R.; Taylor, E. A. J . Org. Chem.
1973, 38, 26. (b) Burgess, E. M.; Penton, H. R.; Taylor, E. A.; Williams,
W. M. Organic Synthesis; Wiley: New York, 1988; Collect. Vol. VI, p
788.
(14) Bredenkamp, M. W.; Holzapfel, C. W.; van Zyl, W. J . Synth.
Commun. 1990, 20, 2235.
(16) Paul, R.; Anderson, G. W. J . Am. Chem. Soc. 1960, 82, 4596.
(17) Kuriyama, N.; Akaji, K.; Kiso, Y. Tetrahedron Lett. 1997, 53,
8323.
(18) Villeneuve, G. B.; Chen, T. H. Tetrahedron Lett. 1997, 38, 6489.
(19) (a) Meienhofer, J .; Kuromizu, K. Tetrahedron Lett. 1974, 15,
3259. (b) Kuromizu, K.; Meienhofer, J . J . Am. Chem. Soc. 1974, 96,
4978.
(15) North, M.; Pattenden, G. Tetrahedron 1990, 46, 8267.

828 J . Org. Chem., Vol. 64, No. 3, 1999
Downing et al.
Sch em e 5
Sch em e 6
Sch em e 8
Sch em e 9
Sch em e 7
of the thiazole moiety. Given these circumstances, it
seemed that a reexamination of the first route, involving
a change in protecting group for 14, could overcome the
earlier failure.
If amine 13 was reprotected as the N-Cbz derivative
and then the ester, 19, hydrolyzed to provide acid 20, it
seemed we should have a coupling partner in hand that
could later be N-deprotected (Scheme 9). This indeed was
shown to be the case. Coupling of amine 9 with carboxylic
acid 20 using EDCI with HOBt proceeded in good yield,
and the resulting hexapeptide 21 was smoothly hydro-
genolyzed to amine 22 using the conditions described for
17 to 18. Amine 22 was then carried on, after ester
hydrolysis, to bistratamide D in 32% yield using the
HATU/Hu¨nigs sequence outlined above. Given the greater
steric demands of this cyclization, a lower yield of
bistratamide D from this hexapeptidic fragment was
expected.

Total Synthesis of Bistratamide D
J . Org. Chem., Vol. 64, No. 3, 1999 829
to remove the alumina. The filtrate was concentrated and the
residue dissolved in THF (200 mL). Burgess reagent13 (3.16
g, 13.2 mmol) was added and the solution heated at 80 °C in
a pressure tube for 3 h. The mixture was cooled, the solvent
concentrated, and the residue purified by chromatography to
provided 2.95 g (70%) of 3 as a clear oil, whose spectroscopic
data are in agreement with those reported by Wipf:12 Rf ) 0.29
In summary, the first total synthesis of bistratamide
D has been accomplished wherein employment of EDCI
with HOBt for fragment coupling proved to be superior
to other coupling methods explored. Our convergent
approach allowed for easy access to either of two hexapep-
tidic fragments (17 and 21), cyclization of which with
HATU provided the final compound in 48% (from 17) or
32% (from 21) yields.
(hexanes/EtOAc 1:1); [R]23 +62.5 (c 2.1, CH2Cl2); 1H NMR
D
(CDCl3) δ 0.93 (d, J ) 5.9 Hz, 3H), 0.98 (d, J ) 6.9 Hz, 3H),
1.43 (d, J ) 6.2 Hz, 3H), 2.14 (m, 1H), 3.77 (s, 3H), 4.27 (d, J
) 7 Hz, 1H), 4.42 (m, 1H), 4.85 (m, 1H), 5.11 (m, 2H), 5.47 (d,
J ) 9.2 Hz, 1H), 7.35 (m, 5H); 13C NMR (CDCl3) δ 17.4, 19.0,
21.1, 32.0, 52.8, 54.6, 67.2, 74.3, 80.0, 128.3, 128.3, 128.7, 136.6,
Exp er im en ta l Section
Gen er a l a n d Sta r tin g Ma ter ia ls Used . Commercially
available copper(I) bromide was purified by refluxing in dry
THF, decanting the liquid, and repeating the procedure until
a colorless liquid was obtained. Methyl N-((triethylammonio)-
sulfonyl)carbamate (Burgess reagent), prepared according to
literature procedures,13 was stored in brown bottles under
argon at -20 °C and checked by NMR prior to each use.
Oxazoline 5 was prepared as previously described.2 cis-
Oxazoline 7 was prepared as described by Wipf.12 Thioamide
10 was synthesized following the procedure described by
Holzapfel.14 All other reagents were commercially available
(Aldrich) and used without any further purification.
2-[(S)-1-ter t-Bu tyloxyca r bon yla m in o-2-m eth ylp r op yl]-
4-ca r bom eth oxyoxa zole 2. Oxazoline 5 (1.67 g, 5.56 mmol),
dissolved in CH2Cl2 (56 mL), was cooled to 0 °C in an ice bath.
DBU (0.91 mL, 6.08 mmol) was added and then BrCCl3 (0.71
mL, 7.20 mmol), dropwise. This was allowed to stir overnight
while warming to room temperature. The mixture was washed
with saturated (aqueous) NH4Cl (2 × 27 mL), and the aqueous
phase was extracted with EtOAc (2 × 14 mL). The combined
organic phases were dried (MgSO4) and concentrated to
provide 1.66 g (100%) of 2 as a yellow solid that required no
156.4, 169.2, 171.5; IR (neat) 3370, 3221, 1721 cm-1
.
4-[2-(1-Ben zyloxyca r bon yla m in o-2-m et h ylp r op yl)]-5-
m eth yl-tr a n s-oxa zolin e Ca r boxylic Acid 8. A solution of
3 (360 mg, 1.03 mmol) in MeOH (3 mL) was treated with 2 M
NaOH (0.57 mL, 1.14 mmol) at 0 °C. After 1 h, the solvent
was evaporated in vacuo, and the residue was partitioned
between water and CH2Cl2. The organic layer was discharged,
and the aqueous layer was acidified and extracted with CH2-
Cl2 (3 × 3 mL). The combined organic extracts were evaporated
to give 324 mg (94%) of 8 that was used without further
purification: [R]23 -2.3 (c 0.80, MeOH); 1H NMR (CD3OD) δ
D
0.92 (d, J ) 6.8 Hz, 3H), 0.96 (d, J ) 6.8 Hz, 3H), 1.40 (d, J )
7.1 Hz, 3H), 2.14 (m, 1H), 3.61 (d, J ) 5.5 Hz, 1H), 4.13 (d, J
) 5.9 Hz, 1H), 5.09 (s, 2H), 5.33 (m, 1H), 7.33 (m, 5H); 13C
NMR (CD3OD) δ 17.8, 18.3, 19.6, 31.9, 60.0, 61.1, 67.8, 71.6,
128.9, 129.1, 129.5, 138.1, 158.9, 170.9, 172.1; IR (neat) 3404,
1736, 1702, 1654, 1640 cm-1
.
2-(1-Am in o-2-m eth ylp r op yl)-4-ca r bom eth oxy-5-m eth -
yl-tr a n s-oxa zolin e 9. A solution of 3 (124 mg, 0.36 mmol) in
MeOH (2 mL) was hydrogenated over 15 mg of 5% Pd/C. After
2 h, the reaction mixture was filtered over Celite and evapo-
rated to yield 75 mg (97%) of crude 9 that was used without
further purification: 1H NMR (CDCl3) δ 0.92 (m, 6H), 1.42 (d,
J ) 6.4 Hz, 3H), 2.02 (m, 1H), 3.44 (d, J ) 5.2 Hz, 1H), 3.76
(s, 3H), 4.25 (d, J ) 5.7 Hz, 1H), 4.84 (m, 1H); IR (neat) 3379,
further purification: mp 120-124 °C; Rf ) 0.68 (hexanes/
1
EtOAc 1:1); [R]23 -44.4 (c 0.8, CH2Cl2); H NMR (CDCl3) δ
D
0.84 (app t, J ) 6.4 Hz, 6H), 1.34 (s, 9H), 2.12 (m, 1H), 3.82
(s, 3H), 4.71 (dd, J ) 6.3 Hz, 9.2 Hz, 1H), 5.30 (br d, J ) 9.5
Hz, 1H), 8.13 (s, 1H); 13C NMR (CDCl3) δ 18.0, 18.8, 28.3, 32.9,
52.2, 54.4, 80.0, 133.2, 144.0, 155.4, 161.6, 165.2; IR (neat)
3353, 1714 cm-1; MS m/z 298. Anal. Calcd for C14H22N2O5: C,
56.36; H, 7.43; N, 9.39. Found: C, 56.31; H, 7.43; N, 9.31.
4-[2-[(S )-1-t er t -B u t y lo x y c a r b o n y la m in o -2-m e t h y l-
p r op yl]]-oxa zole Ca r boxylic Acid 6. Lithium hydroxide
monohydrate (95 mg, 2.26 mmol) was added to a stirred
solution of oxazole ester 2 (0.29 g, 0.97 mmol) in 4 mL of
MeOH/H2O (3:1) at 0 °C and stirred for 1 h with gradual
warming to room temperature. TLC monitoring shows com-
plete consumption of starting material. The solvents were
concentrated, and the residue was partitioned between EtOAc
(10 mL) and H2O (10 mL). The organic phase was separated,
and the aqueous phase was acidified to pH 2 with 1 M HCl
and then extracted with EtOAc (4 × 3 mL). The combined
organic phases were dried (MgSO4) and concentrated to give
274 mg (99%) of 6 as a pale yellow solid that was used without
purification: mp 154-155 °C; Rf ) 0.12 (CH2Cl2/MeOH 95:5);
[R]23D -28.9 (c 1.5, CHCl3); 1H NMR (CDCl3) δ 0.89 (d, J ) 6.7
Hz, 3H), 0.96 (d, J ) 6.9 Hz, 3H), 1.37 (s, 9H), 2.17 (m, 1H),
4.82 (app t, J ) 8.2 Hz, 1H), 6.41 (d, J ) 9.8 Hz, 1H), 8.28 (s,
1H), 12.44 (br s, 1H); 13C NMR (CDCl3) δ 18.5, 19.1, 28.5, 33.0,
54.8, 80.2, 133.4, 144.9, 156.2, 163.9, 167.1; IR (neat) 3314,
3107, 2552, 1745 cm-1; MS m/z 284; HRMS calcd for C13H20N2O5
284.13722, found 284.13720.
3303, 2965, 1730, 1654 cm-1
.
2-[(S)-1′-ter t-Bu tyloxycar bon ylam in o-2′-m eth ylpr opyl]-
4-ca r beth oxyth ia zole 4, was prepared using Holzapfel’s
procedure.14 Thioamide 10 (0.50 g, 2.15 mmol) and KHCO3
(1.72 g, 17.2 mmol) were stirred vigorously in 13 mL of DME
for 8 min at room temperature. Ethyl bromopyruvate (0.81 mL,
6.45 mmol) was added via syringe and the mixture stirred at
room temperature for 45 min before being cooled to ∼0 °C in
an ice bath. A solution of trifluoroacetic anhydride (1.21 mL,
8.58 mmol) and 2,6-lutidine (2.12 mL, 18.3 mmol) in 3.3 mL
of DME was transferred to the mixture dropwise via cannula
over a period of 10 min. After the addition was complete, the
mixture was stirred at 0 °C for 30 min and then allowed to
warm to room temperature. The solvents were concentrated,
and the residue was partitioned between H2O (25 mL) and
EtOAc (50 mL). The phases were separated, and the aqueous
phase was extracted with EtOAc (4 × 10 mL). The combined
organic phases were dried (MgSO4) and concentrated. Flash
chromatography of the residue (EtOAc/hexanes 5:1) followed
with ninhydrin stain gave, after recrystallization (CHCl3/
hexanes), 0.50 g (71%) of 4 as a white solid: mp 116-117 °C;
Rf ) 0.81 (hexanes/EtOAc 1:1); [R]23 -37.1 (c 0.5, CHCl3); 1H
D
NMR (CDCl3) δ 0.86 (d, J ) 6.8 Hz, 3H), 0.94 (d, J ) 6.7 Hz,
3H), 1.35 (t, J ) 7.1 Hz, 3H), 1.40 (s, 9H), 2.39 (m, 1H), 4.36
(q, J ) 7.1 Hz, 2H), 4.85 (m, 1H), 5.30 (br d, J ) 9 Hz, 1H),
8.04 (s, 1H); 13C NMR (CDCl3) δ 14.1, 17.0, 19.2, 28.0, 33.0,
57.8, 61.1, 79.7, 126.6, 147.1, 155.2, 161.1, 173.0; IR (CDCl3):
3350, 3097, 1712, 1477 cm-1; MS m/z 285, 229, 185; HRMS
calcd for C15H24N2O4S 328.14568, found 328.14576. Anal. Calcd
for C15H24N2O4S: C, 54.85; H, 7.36; N, 8.53. Found: C, 54.58;
H, 7.51; N, 8.49.
2-(1-Ben zyloxyca r bon yla m in o-2-m eth ylp r op yl)-4-ca r -
bom eth oxy-5-m eth yl-tr a n s-oxa zolin e 3. To cis-oxazoline 7
(4.20 g, 12.1 mmol) in 200 mL of THF was added 1 M HCl
(aqueous) (80 mL), and the mixture was stirred for 30 min.
The solution was then made alkaline (pH 9) by addition of solid
K2CO3. The THF was evaporated, the residue was extracted
with EtOAc (3 × 150 mL), and the combined organic phases
were concentrated. To the residue were added MeOH (150 mL)
and basic alumina (10.0 g, 98.0 mmol), and the mixture was
heated to reflux for 4 h. The mixture was cooled to room
temperature, sonicated for 5 min, and then vacuum filtered
2-[(S)-1-Am in o-2-m eth ylp r op yl]-4-eth oxyca r bon ylth i-
a zole 11. Acetyl chloride (2.5 mL, 35.3 mmol) was added
dropwise to absolute ethanol (17 mL) at 0 °C. Thiazole 4 (1.16
g, 3.53 mmol) was added as a solid in one portion and the
reaction allowed to stir overnight at room temperature.
Concentration of the reaction mixture gave 0.93 g (100%) of

830 J . Org. Chem., Vol. 64, No. 3, 1999
Downing et al.
11‚HCl as a white foam. Purified material was obtained by
partitioning the crude material between CH2Cl2 (5 mL) and
saturated (aqueous) NaHCO3 (5 mL). The phases were sepa-
rated, and the organic phase was dried (Na2SO4) and concen-
trated to provide the free amine 11 as a colorless oil: Rf )
3 h, water was added and the organic phase separated. The
aqueous phase was extracted with CH2Cl2 (3 × 4 mL), and
the combined organic extracts were dried (MgSO4) and con-
centrated to give 275 mg (53%) of 15: 1H NMR (CDCl3) δ 0.89-
1.03 (m, 18H), 1.41 (s, 9H), 1.41 (d, J ) 6.1 Hz, 3H), 2.16-
2.29 (m, 1H), 2.48-2.59 (m, 1H), 3.75 (s, 3H), 4.28 (d, J ) 6.8
Hz, 1H), 4.75-4.84 (m, 3H), 5.09 (d, J ) 8.6 Hz, 1H), 5.29 (dd,
J ) 6.3 and 9.2 Hz, 1H), 7.35 (d, J ) 9.2 Hz, 1H), 7.71 (d, J )
9.1 Hz, 1H), 7.99 (s, 1H), 8.13 (s, 1H); 13C NMR (CDCl3) δ 17.7,
17.8, 18.0, 18.7, 18.8, 19.5, 20.8, 28.2, 31.8, 32.4, 32.7, 52.3,
52.4, 54.3, 55.9, 74.2, 79.3, 80.7, 123.3, 135.4, 141.3, 149.6,
155.2, 160.2, 160.6, 163.8, 168.4, 171.1, 171.3.
0.18 (EtOAc); [R]23 -24.4 (c 1.06, CHCl3); 1H NMR (CDCl3) δ
D
0.90 (d, J ) 6.8 Hz, 3H), 0.99 (d, J ) 6.8 Hz, 3H), 1.40 (t, J )
7.1 Hz, 3H), 2.02 (br s, 2H), 2.27 (m, 1H), 4.19 (m, 1H), 4.42
(q, J ) 7.1 Hz, 2H), 8.11 (s, 1H); 13C NMR (CDCl3) δ 14.6, 17.0,
19.8, 34.8, 59.6, 61.6, 127.4, 147.2, 161.9, 191.2; IR (neat) 3387,
3318, 3116, 1728, 1618 cm-1; HRMS calcd for C10H16N2O2S
228.09325, found 228.09315.
N-Boc-oxazole-th iazole-eth yl Ester 12. To thiazole amine
11 (0.20 g, 0.88 mmol) in DMF (13 mL) at -10 °C were added
1-hydroxybenzotriazole (0.38 g, 2.81 mmol) and oxazole acid
6 (0.28 g, 0.98 mmol), and this was stirred at -10 °C for 20
min. EDCI (0.20 g, 1.05 mmol) was added and the mixture
stirred at room temperature for 21 h. After this time, the
reaction mixture was diluted with 26 mL of EtOAc, and 13
mL of brine was added. The phases were separated, and the
aqueous phase was extracted with EtOAc (2 × 26 mL). The
organic phases were then washed successively with 10% citric
acid (2 × 13 mL), saturated aqueous NaHCO3 (2 × 13 mL),
and brine (2 × 13 mL) and then dried (Na2SO4). Flash
chromatography of the residue (hexanes/EtOAc 2:1) gave 0.41
g (94%) of 12 as a white solid: mp 144.5-145.8 °C; Rf ) 0.40
N-Cbz-oxazolin e-oxazole-th iazole-eth yl Ester 17. Amine
13 (504 mg, 1.28 mmol) and acid 8 (428 mg, 1.28 mmol) were
dissolved in toluene, and then the solution was concentrated.
DMF (18 mL) was added, the solution cooled to -10 °C, and
HOBt (553 mg, 4.10 mmol) was added. This was stirred for
20 min at -10 °C before EDCI (294 mg, 1.54 mmol) was added.
The mixture was allowed to warm to room temperature and
stirred for 14 h, after which the solvent was evaporated under
reduced pressure and the residue partitioned between EtOAc
(40 mL) and brine (20 mL). The phases were separated, and
the aqueous phase was extracted with EtOAc (2 × 20 mL).
The combined organic extracts were washed successively with
10% citric acid (2 × 10 mL), saturated aqueous NaHCO3 (2 ×
10 mL), and brine (2 × 10 mL), dried (MgSO4), and concen-
trated. Radial chromatographic purification of the crude
residue provided 718 mg (79%) of 17 as a light yellow foam:
(hexanes/EtOAc 1:1); [R]23 -49.0 (c 1.20, CHCl3); 1H NMR
D
(CDCl3) δ 0.91-1.00 (m, 12H), 1.34-1.42 (m, 12H), 2.15 (m,
1H), 2.57 (m, 1H), 4.38 (q, J ) 7.1 Hz, 2H), 4.75 (m, 1H), 5.15
(br d, J ) 9.1 Hz, 1H), 5.26 (m, 1H), 7.51 (d, J ) 9.1 Hz, 1H),
8.06 (s, 1H), 8.11 (s, 1H); 13C NMR (CDCl3) δ 14.5, 18.1, 18.3,
18.9, 19.8, 28.5, 32.8, 33.2, 54.4, 56.2, 61.6, 80.3, 127.1, 135.7,
141.5, 147.7, 155.5, 160.4, 161.4, 164.1, 171.4; IR (CDCl3) 3325,
3127, 1716, 1596, 1505 cm-1; HRMS calcd for C23H34N4O6S
494.21991, found 494.22019. Anal. Calcd for C23H34N4O6S: C,
55.85; H, 6.93; N, 11.33. Found: C, 55.93; H, 6.87; N, 11.27.
Am in ooxa zole-th ia zole-eth yl Ester 13. Acetyl chloride
(2.51 mL, 35.3 mmol) was added dropwise to absolute ethanol
(20 mL) at 0 °C. 12 (666 mg, 1.35 mmol) was added to the
solution, and the reaction was allowed to stir at room tem-
perature overnight. Evaporation of the solvents in vacuo
provided 579 mg of 13‚HCl (100%) as a white solid. A portion
of the compound was dissolved in CH2Cl2, and aqueous
saturated NaHCO3 was added; the organic layer was sepa-
rated, dried over Na2SO4, filtered and evaporated to provide
Rf ) 0.16 (hexanes/EtOAc 1:1); [R]25 -5.53 (c 1.03, CHCl3);
D
1H NMR (CDCl3) δ 0.80-0.97 (m, 18H), 1.30 (t, J ) 7.1 Hz,
3H), 1.41 (d, J ) 6.1 Hz, 3H), 2.04-2.19 (m, 2H), 2.46-2.55
(m, 1H), 4.15 (d, J ) 7.4 Hz, 1H), 4.31 (q, J ) 7.1 Hz, 2H),
4.73 (m, 1H), 4.95-5.07 (m, 3H), 5.23 (dd, J ) 7.1, 9.1 Hz,
1H), 5.59 (d, J ) 8.6 Hz, 1H), 7.10 (d, J ) 9.1 Hz, 1H), 7.25
(m, 5H), 7.58 (d, J ) 9.1 Hz, 1H), 8.00 (s, 1H), 8.10 (s, 1H); 13
C
NMR (CDCl3) δ 14.1, 17.7, 17.9, 18.0, 18.6, 18.7, 19.4, 21.6,
31.3, 31.9, 32.9, 52.1, 54.7, 56.0, 61.2, 66.8, 74.3, 80.5, 126.8,
127.9, 128.0, 128.3, 135.4, 136.0, 141.4, 147.2, 155.9, 160.1,
161.1, 162.7, 168.7, 170.8, 171.1; IR (film) 3302, 1721, 1659,
1596 cm-1. This material was used without further purifica-
tion.
Am in ooxa zolin e-oxa zole-t h ia zole-et h yl E st er 18. A
solution of 17 (97 mg, 0.14 mmol) in 4 mL of EtOH/Et3N (3:1)
was hydrogenated over Pd black at 100 psi. After 5 h, the
reaction mixture was sonicated for 5 min and filtered through
Celite. The solvents were concentrated, and the crude residue
was chromatographied to give 65 mg (83%) of 18: Rf ) 0.33
(CH2Cl2/MeOH 90:10); 1H NMR (CDCl3) δ 0.86-1.03 (m, 18H),
1.37 (t, J ) 7.1 Hz, 3H), 1.48 (d, J ) 6.2 Hz, 3H), 2.02-2.25
(m, 4H), 2.51-2.60 (m, 1H), 3.43 (d, J ) 5.1 Hz, 1H), 4.19 (d,
J ) 7.7 Hz, 1H), 4.38 (q, J ) 7.1 Hz, 2H), 4.76 (m, 1H), 5.05
(dd, J ) 6.4, 9.1 Hz, 1H), 5.27 (dd, J ) 7.1, 9.2 Hz, 1H), 7.14
(d, J ) 9.1 Hz, 1H), 7.59 (d, J ) 9.2 Hz, 1H), 8.05 (s, 1H), 8.12
(s, 1H); 13C NMR (CDCl3) δ 17.3, 17.4, 18.0, 18.2, 18.8, 19.0,
19.7, 21.8, 32.0, 32.2, 32.9, 52.3, 55.3, 56.1, 61.4, 74.5, 80.3,
127.0, 135.6, 141.4, 147.3, 160.2, 161.3, 163.0, 171.2, 171.3,
171.6.
13 as the free amine, Rf ) 0.20 (EtOAc); [R]25 -31.4 (c 1.01,
D
1
CHCl3); H NMR (CDCl3) δ 0.87-0.99 (m, 12H), 1.33 (t, J )
7.2 Hz, 3H), 1.76 (bs, 2H), 2.05 (m, 1H), 2.55 (m, 1H), 3.46 (d,
J ) 5.7 Hz, 1H), 4.35 (q, J ) 7.2 Hz, 2H), 5.25 (dd, J ) 7.0
and 9.2 Hz, 1H), 7.54 (d, J ) 9.2 Hz, 1H), 8.02 (s, 1H), 8.09 (s,
1H); 13C NMR (CDCl3) δ 14.2, 17.7, 18.0, 18.9, 19.5, 33.0, 33.3,
55.7, 56.0, 61.3, 126.8, 135.2, 141.0, 147.3, 160.3, 161.1, 167.0,
171.2; IR (film) 3568, 3405, 1728, 1668, 1596 cm-1
.
N-Boc-oxa zole-th ia zole ca r boxylic Acid 14. Lithium
hydroxide monohydrate (19 mg, 0.43 mmol) was added to a
solution of 12 (100 mg, 0.20 mmol) in 4 mL of EtOH/H2O (3:
1). After 1 h, the solvents were concentrated, and the residue
was partitioned between H2O and CH2Cl2. The organic phase
was separated, and the aqueous layer was acidified and
extracted with CH2Cl2 (3 × 3 mL). The combined organic
extracts were evaporated to give 90 mg (96%) of 14 as a solid:
N-Cbz-oxa zole-th ia zole-eth yl Ester 19. Potassium car-
bonate (140 mg, 1.01 mmol) was added to a stirred solution of
13 (399 mg, 1.01 mmol) in 2 mL of CH2Cl2/H2O (1:1). Benzyl
chloroformate (0.17 mL, 1.21 mmol) was added via syringe,
and the mixture was stirred for 12 h at room temperature.
The phases were separated, and the aqueous phase was
extracted with CH2Cl2 (3 × 5 mL). The combined organic layers
were washed with brine (5 mL), dried (MgSO4), and chromato-
graphed (hexanes/EtOAc 5:3) to provide 454 mg (85%) of 19
as a colorless solid: mp 169.7-170.7 °C; Rf ) 0.35 (hexanes/
mp 164-166 °C; Rf ) 0.21 (CH2Cl2/EtOAc 95:5); [R]25 -54.4
D
1
(c 1.02, CHCl3); H NMR (CDCl3) δ 0.81-1.07 (m, 12H), 1.39
(s, 9H), 2.10-2.24 (m, 1H), 2.48-2.63 (m, 1H), 4.75 (bs, 1H),
5.12-5.35 (m, 2H), 7.62 (d, J ) 9.1 Hz, 1H), 8.13 (s, 2H), 10.14
(bs, 1H); 13C NMR (CDCl3) δ 17.9, 18.1, 18.6, 19.5, 28.2, 32.5,
32.9, 54.2, 56.0, 80.1, 128.2, 135.2, 141.6, 146.8, 155.3, 160.4,
163.6, 164.1, 171.4; IR (film) 3310, 1709, 1693, 1678, 1666
cm-1. This material was used in the next step without further
purification.
EtOAc 1:1); [R]23 -34.1 (c 1.7, CHCl3); 1H NMR (CDCl3) δ
D
0.81-1.02 (m, 12H), 1.31 (t, J ) 7.0 Hz, 3H), 2.10-2.20 (m,
1H), 2.49-2.56 (m, 1H), 4.33 (q, J ) 6.9 Hz, 2H), 4.78 (app t,
J ) 7.1 Hz, 1H), 5.06 (m, 2H), 5.22 (t, J ) 8.1 Hz, 1H), 5.40
(d, J ) 8.8 Hz, 1H), 7.24-7.28 (m, 5H), 7.43 (d, J ) 8.6 Hz,
1H), 8.00 (s, 1H), 8.06 (s, 1H); 13C NMR (CDCl3) δ 9.8, 13.6,
13.8, 14.1, 15.1, 28.1, 28.5, 50.2, 51.5, 56.8, 62.7, 122.4, 123.6,
N-Boc-oxazole-th iazole-oxazolin e-m eth yl Ester 15. EDCI
(150 mg, 0.78 mmol) was added to a solution of 9 (186 mg,
0.87 mmol) and 14 (365 mg, 0.78 mmol) in CH2Cl2 (5 mL) at
0 °C. A catalytic amount of DMAP was then added. The
reaction was allowed to warm to room temperature, and after

Total Synthesis of Bistratamide D
J . Org. Chem., Vol. 64, No. 3, 1999 831
1
124.0, 131.0, 131.5, 136.8, 142.9, 151.4, 155.6, 156.7, 158.9,
) 0.45 (CH2Cl2/MeOH 90:10); H NMR (CDCl3) δ 0.86-1.00
166.7; IR (neat) 3321, 3148, 3100, 1725 cm-1
.
(m, 18H), 1.38 (d, J ) 6.3 Hz, 3H), 1.72 (bs, 2H), 2.06-2.23
(m, 2H), 2.44-2.48 (m, 1H), 3.72 (s, 3H), 3.80 (d, J ) 6.0 Hz,
1H), 4.23 (d, J ) 6.8 Hz, 1H), 4.72-4.87 (m, 2H), 5.26 (dd, J
) 2.5 and 6.6 Hz, 1H), 7.41 (d, J ) 9.0 Hz, 1H), 7.67 (d, J )
8.8 Hz, 1H), 7.96 (s, 1H), 8.09 (s, 1H); 13C NMR (CDCl3) δ 13.3,
13.4, 14.4, 14.4, 15.0, 16.4, 27.4, 28.5, 28.7, 47.9, 48.0, 51.3,
69.8, 74.9, 118.9, 130.7, 136.6, 145.1, 155.9, 156.1, 164.0, 166.3,
166.9; IR (neat) 3401, 3307, 2963, 1744, 1659 cm-1. This
material was taken directly to the next step.
N-Cbz-oxa zole-th ia zole Ca r boxylic Acid 20. Lithium
hydroxide monohydrate (28 mg, 0.67 mmol) was added to a
stirred solution of 19 (155 mg, 0.29 mmol) in 3.7 mL of MeOH/
H2O (4.5:1) at 0 °C and allowed to stir for 2 h with gradual
warming to room temperature, after which the previously
cloudy solution had cleared. The solvents were concentrated,
and the residue was partitioned between water and CHCl3.
The organic layer was discharged, and the aqueous layer was
acidified and extracted with CHCl3 (3 × 5 mL). The combined
organic extracts were evaporated to give 145 mg (100%) of 20
that was used without further purification: 1H NMR (CDCl3)
δ 0.75-1.05 (m, 12H), 2.09-2.21 (bs, 1H), 2.43-2.55 (bs, 1H),
4.78 (bs, 1H), 4.96-5.19 (m, 2H), 5.24 (bs, 1H), 5.59 (bs, 1H),
7.25 (bs, 5H), 7.59 (bs, 1H), 8.12 (bs, 1H), 8.15 (bs, 1H); 13C
NMR (CDCl3) δ 13.4, 13.7, 14.2, 15.0, 28.0, 28.4, 50.3, 51.6,
62.7, 123.7, 123.9, 130.7, 131.4, 137.3, 142.5, 151.5, 155.8,
Bistr a ta m id e D (1). Lithium hydroxide monohydrate (12
mg, 0.29 mmol) was added to a solution of 18 (80 mg, 0.14
mmol) in 3.4 mL of EtOH/H2O (4:1) at 0 °C. After 3 h, the
solution was neutralized with 1 M HCl, and then the solvents
were evaporated to dryness by azeotropic distillation with
toluene. The solid residue was dissolved in DMF (26 mL) and
cooled to -10 °C, and diisopropylethylamine (0.05 mL, 0.31
mmol) and HATU (56 mg, 0.15 mmol) were added. The mixture
was stirred at -10 °C for 2 h and then at room temperature
for 3 days. After this time, the solvents were concentrated in
vacuo, and the residue was partitioned between brine (25 mL)
and EtOAc (30 mL). The phases were separated, and the
aqueous phase was extracted with EtOAc (2 × 13 mL). The
combined organic extracts were washed successively with 10%
aqueous citric acid (2 × 13 mL), saturated aqueous NaHCO3
(2 × 13 mL), and brine (2 × 13 mL), dried (Na2SO4), and
concentrated. Flash chromatography (EtOAc) afforded 38 mg
(48%) of 1 as a colorless solid, whose NMR data are in
agreement with that reported:3 Rf ) 0.25 (EtOAc); [R]25D -29.8
159.2, 166.7; IR (neat) 3401, 3314, 3117, 2965, 1714 cm-1
.
N-Cbz-oxa zole-th ia zole-oxa zolin e-m eth yl Ester 21. To
amine 9 (75 mg, 0.35 mmol) and acid 20 (141 mg, 0.29 mmol)
in DMF (4 mL) at -10 °C was added HOBt (126 mg, 0.93
mmol), and the mixture was stirred for 20 min. EDCI (73 mg,
0.38 mmol) was added, and stirring was continued for 10.5 h
at room temperature. The reaction mixture was then diluted
with EtOAc (8 mL), brine (4 mL) was added, the phases were
separated, and the aqueous phase was extracted with EtOAc
(2 × 8 mL). The organic phases were washed successively with
10% citric acid (2 × 4 mL), saturated aqueous NaHCO3 (2 ×
4 mL), and brine (2 × 4 mL) and dried (Na2SO4). Flash
chromatography of the residue (EtOAc/hexanes 5:4) provided
168 mg (83%) of 21 as a white foam that consisted of a mixture
(approximately 1:1) of two rotomers: Rf ) 0.06 (hexanes/EtOAc
1:1); [R]23D -1.25 (c 1.6, CHCl3); 1H NMR (CDCl3) δ 0.80-0.99
(m, 18H), 1.33-1.37 (m, 3H), 2.12-2.23 (m, 1H), 2.41-2.50
(m, 2H), 3.69 (s, 3H), 4.20 (d, J ) 6.9 Hz, 0.47H), 4.24 (d, J )
6.7 Hz, 0.52H), 4.72-4.85 (m, 3H), 5.00-5.11 (m, 2H), 5.26
(dd, J ) 3.0, 6.3 Hz, 1H), 5.36 (d, J ) 9.2 Hz, 0.48H), 5.48 (d,
J ) 9.1 Hz, 0.52H), 7.20-7.32 (m, 5H), 7.38 (d, J ) 9.0 Hz,
1H), 7.67 (d, J ) 9.1 Hz, 0.48H), 7.73 (d, J ) 9.3 Hz, 0.53H),
7.96 (s, 1H), 8.10 (s, 1H); 13C NMR (CDCl3) δ 13.2, 13.4, 13.5,
14.1, 14.2, 14.3, 14.8, 14.9, 16.3, 27.3, 28.0, 28.1, 28.4, 28.5,
47.8, 48.0, 50.3, 51.3, 62.7, 69.7, 69.8, 74.6, 118.7, 118.8, 123.5,
123.6, 123.8, 124.0, 131.0, 131.4, 136.9, 145.2, 151.4, 155.5,
156.0, 156.1, 158.9, 159.1, 163.4, 164.0, 166.1, 166.2, 166.6,
166.9; IR (neat) 3394, 3297, 2966, 1725 cm-1. Anal. Calcd for
(c 0.60, CHCl3) (lit.3 [R]25 -31 (c 0.33, CHCl3)); λmax ) 230
D
nm (lit.3 λmax ) 232 nm); 1H NMR (CDCl3) δ 0.87-1.06 (m,
18H), 1.57 (d, J ) 6.3 Hz, 3H), 2.18-2.25 (m, 1H), 2.34-2.43
(m, 2H), 4.09 (dd, J ) 2.3, 9.1 Hz, 1H), 4.76 (m, 1H), 4.96 (m,
2H), 5.27 (dd, J ) 5.2, 7.4 Hz, 1H), 7.89 (d, J ) 7.4 Hz, 1H),
8.03 (d, J ) 9.9 Hz, 1H), 8.10 (s, 1H), 8.15 (s, 1H), 8.62 (d, J
) 7.2 Hz, 1H); 13C NMR (CDCl3) δ 16.2, 17.6, 17.7, 18.1, 18.1,
19.0, 21.8, 31.1, 33.2, 34.4, 51.8, 53.0, 56.4, 74.0, 82.4, 123.6,
135.7, 140.9, 148.5, 159.1, 160.0, 163.2, 167.5, 169.3, 170.4;
IR (neat) 3387, 2966, 16779, 1539 cm-1
.
Ack n ow led gm en t. We thank Professor C. M. Ire-
land and Dr. T. Foderaro for providing us with a copy
of the NMR of the isolated natural product. A postdoc-
toral fellowship (E. A.) from Ministerio de Educacio´n y
Ciencia (Spain) and financial support by the National
Institute of Health (S. V. D.) are gratefully acknowl-
edged.
C
34H44N6O8S: C, 58.61; H, 6.36; N, 12.06. Found: C, 58.41;
H, 6.44; N, 12.06.
Am in ooxa zole-th ia zole-oxa zolin e-m eth yl Ester 22. A
solution of 21 (101 mg, 0.15 mmol) in 3.3 mL of EtOH/Et3N
(3:1) was hydrogenated over Pd black at 100 psi. After 5 h,
the reaction mixture was sonicated for 5 min and filtered
through Celite. The solvents were concentrated, and the crude
residue was chromatographed to give 57 mg (70%) of 18: Rf
Su p p or tin g In for m a tion Ava ila ble: NMR spectra for
new compounds. This material is available free of charge via
the Internet at http://pubs.acs.org.
J O981664I