Total synthesis of (-)-muscoride A

Article abstract of DOI:10.1021/jo960891m

The recently isolated cyanobacterium metabolite muscoride A was synthesized in 15 steps and in 4.3% overall yield. Novel structural features of this peptide antibiotic include the presence of a threonine-derived bioxazole core and an N-(1,1-dimethyl)allyl ('reverse prenyl') valine residue. In the context of our synthesis, efficient new strategies for the preparation of these segments were developed. The synthesis of two epimers of muscoride A allowed the unambiguous assignment of the relative and absolute configuration of the natural product by NMR and optical rotation analyses.

Full text of DOI:10.1021/jo960891m

J . Org. Chem. 1996, 61, 6517-6522
6517
Tota l Syn th esis of (-)-Mu scor id e A
Peter Wipf*,1 and Srikanth Venkatraman
Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260
Received May 14, 1996X
The recently isolated cyanobacterium metabolite muscoride A was synthesized in 15 steps and in
4.3% overall yield. Novel structural features of this peptide antibiotic include the presence of a
threonine-derived bioxazole core and an N-(1,1-dimethyl)allyl (“reverse prenyl”) valine residue. In
the context of our synthesis, efficient new strategies for the preparation of these segments were
developed. The synthesis of two epimers of muscoride A allowed the unambiguous assignment of
the relative and absolute configuration of the natural product by NMR and optical rotation analyses.
Terrestrial freshwater as well as marine organisms
have been the source of a great variety of natural
products with novel architecture and functionalities.2 The
ubiquitous incorporation of oxazole moieties in these
secondary metabolites is exemplified by the phosphatase
inhibitor calyculin A,3 the cytotoxic macrolide phorbox-
azole A,4 and the polyazole tantazole5 (Figure 1).
reverse prenyl function at the R-nitrogen of an amino acid
residue, in this case valine.14
For our recently described total syntheses of thianga-
zole and hennoxazole A,15,16 we had developed a method
for the introduction of bi-azole segments in polyfunction-
alized compounds.17 We were quite confident that this
method was applicable for the preparation of the threo-
nine-derived bioxazole core of muscoride A, but this
remained to be proven. In addition, we were interested
in establishing a straightforward general protocol for the
preparation of N-reverse-prenylated amino acids because
we envision these to become an attractive new motif in
peptide chemistry for manipulation of secondary struc-
ture and inhibition of enzymatic degradation.
In contrast to the large number of natural products
that contain isolated oxazole rings, significantly fewer
compounds with contiguously linked polyoxazoles are
known: halichondramides,6 ulapualides,7 and mycalolides8
incorporate teroxazole segments and hennoxazole A9 and
diazonamides10 contain a bioxazole core (Figure 2).
More recently, the isolation of a new bioxazole peptide
alkaloid from the freshwater cyanobacterium Nostoc
muscorum was reported by Sakakibara and co-workers
(Figure 3).11 Muscoride A (1) showed antibacterial activ-
ity and raised our interest because of its novel structural
features invoking a distant relationship to the potent
DNA gyrase inhibitor microcin B17.12 The bioxazole core
of muscoride A is formally derived from two threonine
residues, which is rather remarkable since even isolated
oxazoles derived from threonine are rare.13 Muscoride
A also appears to be the first natural product with a
To achieve a reasonable level of convergence in our
synthesis, our retrosynthetic plan disconnected mus-
coride A between the valine and proline residues (Figure
4). The tripeptide ester 4 could easily be prepared by
standard peptide coupling techniques and would serve
as the precursor for the bioxazole moiety 2. Acylation of
this segment with an active ester derivative of N-reverse-
prenylated valine 3 should directly lead to the natural
product. To avoid the potential problems in the separa-
tion of regioisomers that would likely be formed in the
dimethylallylation of valine, we chose propargylated
valine derivative 5 as a precursor to 3. We arbitrarily
selected the more readily available L-amino acids as
starting materials. Our total synthesis of (-)-muscoride
A ultimately established the stereochemistry at C(8) and
C(13) as shown in Figure 3, but the configuration at these
stereocenters had remained unresolved in the NMR-
based structure elucidation by Sakakibara et al.11
Syn th esis of th e Bioxa zole Segm en t of Mu scor id e
A. Dipeptide 7 was readily obtained from the condensa-
tion of Boc-protected L-proline with threonine methyl
ester hydrochloride in the presence of isobutyl chlorofor-
mate (IBCF, Scheme 1).18 Oxidative cyclodehydration17
of the threonine residue by side-chain oxidation with
Dess-Martin Periodinane19 followed by exposure to
X Abstract published in Advance ACS Abstracts, September 1, 1996.
(1) Alfred P. Sloan Research Fellow, 1994-1996. NSF Presidential
Faculty Fellow, 1994-1999. Camille Dreyfus Teacher-Scholar, 1995-
1997.
(2) For representative reviews, see: (a) Faulkner, D. J . Nat. Prod.
Rep. 1996, 13, 75. (b) Wipf, P. Chem. Rev. 1995, 95, 2115. (c) Honkanen,
R. E.; Boynton, A. L. In Protein Kinase C; Kuo, J . F., Ed.; Oxford
University Press: Oxford, 1994; p 305. (d) Chem. Rev. 1993, 93, 1673-
1944. (e) Michael, J . P.; Pattenden, G. Angew. Chem., Int. Ed. Engl.
1993, 32, 1 and references cited therein.
(3) Kato, Y.; Fusetani, N.; Matsunaga, S.; Hashimoto, K.; Fujita,
S.; Furuya, T. J . Am. Chem. Soc. 1986, 108, 2780.
(4) Searle, P. A.; Molinski, T. F. J . Am. Chem. Soc. 1995, 117, 8126.
(5) Carmeli, S.; Paik, S.; Moore, R. E.; Patterson, G.; Yoshida, W.
Y. Tetrahedron Lett. 1993, 34, 6681.
(6) Matsunaga, S.; Fusetani, N.; Hashimoto, K.; Koseki, K.; Noma,
M.; Noguchi, H.; Sankawa, U. J . Org. Chem. 1989, 54, 1360.
(7) Roesener, J . A.; Scheuer, P. J . J . Am. Chem. Soc. 1986, 108, 846.
(8) (a) Fusetani, N.; Yasumuro, K.; Matsunaga, S.; Hashimoto, K.
Tetrahedron Lett. 1989, 30, 2809. (b) Rashid, M. A.; Gustafson, K. R.;
Cardellina, J . H.; Boyd, M. R. J . Nat. Prod. 1995, 58, 1120.
(9) Ichiba, T.; Yoshida, W. Y.; Scheuer, P. J .; Higa, T.; Gravalos, D.
G. J . Am. Chem. Soc. 1991, 113, 3173.
(10) Lindquist, N.; Fenical, W.; Van Duyne, G. D.; Clardy, J . J . Am.
Chem. Soc. 1991, 113, 2303.
(11) Nagatsu, A.; Kajitani, H.; Sakakibara, J . Tetrahedron Lett.
1995, 36, 4097.
(14) Natural products incorporating reverse prenyl groups at various
indole ring positions are quite common. For example, see: (a) (Aster-
riquinone) Yamamoto, Y.; Nishimura, K; Kiriyama, N. Chem. Pharm.
Bull. 1976, 24, 1853. (b) (Okaramine A) Hayashi, H.; Takiuchi, K.;
Murao, S.; Arai, M. Agric. Biol. Chem. 1989, 53, 461. (c) (Amauromine)
Marsden, S. P.; Depew, K. M.; Danishefsky, S. J . J . Am. Chem. Soc.
1994, 116, 11143. (d) (Gypsetin) Schkeryantz, J . M.; Woo, J . C. G.;
Danishefsky, S. J . J . Am. Chem. Soc. 1995, 117, 7025.
(12) (a) Bayer, A.; Freund, S.; Nicholson, G.; J ung, G. Angew. Chem.,
Int. Ed. Engl. 1993, 32, 1336. (b) Yorgey, P.; Lee, J .; Koerdel, J .; Vivas,
E.; Warner, P.; J ebaratnam, D.; Kolter, R. Proc. Natl. Acad. Sci. U.S.A.
1994, 91, 4519.
(15) (a) Wipf, P.; Venkatraman, S. J . Org. Chem. 1995, 60, 7224.
(b) Wipf, P.; Venkatraman, S. Synlett. in press.
(16) Wipf, P.; Lim, S. J . Am. Chem. Soc. 1995, 117, 558.
(17) Wipf, P.; Miller, C. P. J . Org. Chem. 1993, 58, 3604.
(18) Anderson, G. W.; Zimmerman, J . E; Callaham, F. M. J . Am.
Chem. Soc. 1967, 89, 5012.
(13) Cyclodehydrated threonines are, however, frequently encoun-
tered at the oxazoline oxidation level.2
S0022-3263(96)00891-2 CCC: $12.00 © 1996 American Chemical Society

6518 J . Org. Chem., Vol. 61, No. 19, 1996
Wipf and Venkatraman
F igu r e 1. Some oxazole-containing secondary metabolites.
F igu r e 4. Retrosynthetic analysis.
the alternative tandem oxidative cyclodehydration of the
prolylthreonylthreonine intermediate 4, since the ma-
nipulation of this polar tripeptide in common organic
solvents was difficult.
To complete the synthesis of the C-terminal segment
of muscoride A, the methyl ester 10 was converted to the
3,3-dimethylallyl ester 11 by saponification followed by
prenylation in the presence of BOP Reagent.20 The
overall yield for the preparation of segment 11 from
proline 6 was 19.3%, and this synthesis was readily
scaled up to gram quantities.
F igu r e 2. Natural products with ter- and bioxazole moieties.
Syn th esis of th e Rever se-P r en yla ted Va lin e Resi-
d u e of Mu scor id e A. After the efficient preparation of
the heterocyclic segment 11 by the use of our recently
developed variant of the Robinson-Gabriel oxazole syn-
thesis, we required an equally efficient route to the
N-terminal amino acid residue carrying the unique
reverse-prenylation signature. As expected, our first
attempt to directly introduce this five-carbon appendage
by treatment of L-valine methyl ester with dimethylallyl
chloride required vigorous heating and led to very low
yields of the desired compound in addition to many other
side products. Fortunately, the analogous propargylation
of ester 12 was accomplished very cleanly under the
F igu r e 3. Muscoride A, a bioxazole peptide alkaloid from a
freshwater cyanobacterium.
electrophilic phosphorus reagent assembled the first
heterocycle in 68% yield.
Oxazole 8 was homologated by hydrolysis of the methyl
ester and coupling with L-threonine. The subsequent
oxidative cyclodehydration17 of tripeptide 9 closed the
second oxazole ring in 65% yield. This iterative forma-
tion of the bioxazole core of muscoride A was superior to
(20) Castro, B.; Dormoy, J . R.; Evin, G.; Selve, C. Tetrahedron Lett.
1975, 1219.
(19) Dess, D. B.; Martin, J . C. J . Am. Chem. Soc. 1991, 113, 7277.

Total Synthesis of (-)-Muscoride A
J . Org. Chem., Vol. 61, No. 19, 1996 6519
Sch em e 1
Sch em e 3
Sch em e 4
Sch em e 2
°C in 63% yield to give ester 17. An increase in either
reaction temperature or reaction time resulted not only
in a yield improvement but also in the formation of a
mixture of inseparable products arising from Glaser
coupling.23 Similarly, the reduction of alkyne 17 under
standard Lindlar conditions resulted in over-reduction
and decomposition. Fortunately, however, we were able
to optimize this step by the use of Pd/C in petroleum
ether in the presence of quinoline to give 92% of building
block 18.
Segm en t Con d en sa tion . The total synthesis of (6S,-
14S)-muscoride A was accomplished by simply heating
the active ester 18 and the hydrochloride salt derived
from 11 in chloroform in the presence of DMAP (Scheme
4). Due to the severe level of steric hindrance in both
the carboxyl and the amine component in this reaction,
the yield of coupling product generally did not exceed
22%. We also noticed that the amine was not stable
under the reaction conditions and slowly decomposed to
unidentified side products. Variations in solvent and
temperature provided only marginal improvements in the
efficiency of the final segment condensation. Nonethe-
less, >100 mg quantities of the natural product were
routinely accessible via this route. Synthetic (6S,14S)-
muscoride A was found to have physical data identical
to those reported11 for the natural product. To further
support the structural assignment of muscoride A, we
conditions of Hennion and Hanzel21 with dimethylpro-
pargyl chloride in the presence of catalytic copper and
cuprous chloride (Scheme 2). This mechanistically in-
teresting protocol provided N-(1,1-dimethyl)propargy-
lated valine 13 at 0 °C in 83% yield. The terminal alkyne
was partially hydrogenated using Lindlar catalyst to form
alkene 14. Attempts to hydrolyze the methyl ester of 14
at 25 °C were sluggish, but at 60 °C the saponification
was complete after 12 h. However, all attempts to couple
the resulting lithium salt with the amine hydrochloride
derived from 11 met with failure. We therefore decided
to activate the valine residue before the propargylation-
reduction sequence (Scheme 3).
Condensation of Boc-L-valine with pentafluorophenol
in the presence of DCC provided the activated PFP ester
16.22 The subsequent copper-catalyzed alkylation of 16
with 3-chloro-3-methyl-1-butyne proceeded cleanly at 0
(21) (a) Hennion, G. F.; Hanzel, R. S. J . Am. Chem. Soc. 1960, 82,
4908. (b) Barmettler, P.; Hansen, H.-J . Helv. Chim. Acta 1990, 73,
1515.
(22) Schmidt, U.; Lieberknecht, A.; Griesser, H.; Talbiersky, J . J .
Org. Chem. 1982, 47, 3261.
(23) Nakagawa, M. In The Chemistry of the Carbon-Carbon Triple
Bond; Patai, S., Ed.; Wiley: New York, 1978; p 655.

6520 J . Org. Chem., Vol. 61, No. 19, 1996
Wipf and Venkatraman
DMF was added, the reaction mixture was stirred at 25 °C
for 0.5 h, diluted with 100 mL of H2O, and extracted with CH2-
Cl2 (2 × 50 mL). The combined organic layers were washed
with H2O (50 mL), 2 M HCl (50 mL), and brine (50 mL), dried
(MgSO4), concentrated in vacuo, and chromatographed on SiO2
(EtOAc/hexanes 4:1) to yield 8.8 g (81%) of 7 as a viscous oil:
Rf 0.65 (EtOAc/hexanes 19:1); [R]D +60° (c 1.1, CHCl3, 25 °C);
prepared the (6R,14S)-diastereomer 20 by condensation
of D-valine-derived 19 with tripeptide ester 11. The two
epimers 1 and 20 had distinctively different spectroscopic
and chiroptical properties. For 1, an [R]D -93° (c 0.5,
MeOH, 25 °C) compared closely with the reported -89°
(c 0.7, MeOH, 25 °C), whereas the [R]D -54° (c 0.54,
MeOH, 25 °C) for the (6R,14S)-diastereomer 20 was
1
IR (neat) 3431, 3297, 1750, 1683, 1257, 1165, 1122 cm-1; H
1
considerably lower. The H NMR of 1 was identical to
NMR (365 K, DMSO) δ 7.32 (d, 1 H, J ) 7.2 Hz), 4.8-4.3 (b,
1 H), 4.32-4.24 (m, 2 H), 4.14-4.10 (m, 1 H), 3.65 (s, 3 H),
3.36 (m, 2 H), 2.14-2.05 (m, 1 H), 1.96-1.89 (m, 1 H), 1.86-
1.76 (m, 2 H), 1.40 (s, 9 H), 1.10 (d, 3 H, J ) 6.2 Hz); 13C NMR
δ 173.3, 172.6, 171.1, 155.2, 154.6, 80.4, 68.2, 67.5, 60.9, 59.9,
57.4, 52.3, 46.9, 31.0, 28.8, 28.2, 24.5, 23.6, 19.9; MS (EI) m/ z
(rel intensity) 330 (M+, 10); HRMS m/ z calcd for C15H26N2O6
330.1790, found 330.1801.
the natural product, and epimer 20 displayed several
clearly different splitting patterns and chemical shifts.24
Con clu sion . We have developed a straightforward
convergent strategy for the preparation of the structur-
ally novel peptide alkaloid muscoride A. The heterocyclic
bioxazole core was efficiently prepared by consecutive
oxidative cyclodehydrations of threonine residues. This
application underscores the broad scope of our recently
developed protocols for oxazole synthesis.15-17 The cop-
per-catalyzed N-propargylation of a valine active ester
via the Hennion-Hanzel protocol followed by modified
Lindlar reduction allowed a direct access to the C-
terminal activated reverse-prenylated amino acid seg-
ment. We believe that this is a general protocol for the
preparation of novel reverse-prenylated amino acid build-
ing blocks that have potentially attractive applications
as conformationally rigidified peptide analogs.25
Due to the steric hindrance of the coupling components
11 and 18, the final segment condensation proceeded in
low yield. Nonetheless, muscoride A and its epimer 20
were readily obtained in an overall yield of 4.3% from
L-proline 6. Spectral comparisons between synthetic and
natural 1 and epimer 20 allowed the unambiguous
assignment of the relative and absolute stereochemistry
of the natural product. We are currently evaluating the
spectrum of biological activities and metal-binding prop-
erties26 of this cyanobacterium metabolite.
2-[1′-(ter t-Bu toxyca r bon yl)p yr r olid in -2′(S)-yl]-5-m eth -
yloxa zolyl-4-ca r boxylic Acid Meth yl Ester (8). A solution
of 4.6 g (14.0 mmol) of dipeptide 7 in 50 mL of dry CH2Cl2
was treated with 7.0 g (16.5 mmol) of the Dess-Martin reagent
and stirred at 25 °C for 2 h. The reaction mixture was
concentrated in vacuo and chromatographed on SiO2 (EtOAc/
hexanes 7:3) to yield 3.9 (81%) of the methyl ketone which was
immediately used without further purification.
A solution of the methyl ketone in 10 mL of dry THF was
added dropwise to a solution of 6.5 g (25.0 mmol) of Ph3P, 5.8
g (22.5 mmol) of I2, and 6 mL (41.0 mmol) of Et3N in 35 mL of
THF at -78 °C. The reaction mixture was stirred at -78 °C
for 3 h and diluted with 100 mL of H2O. The solution was
extracted with CH2Cl2 (2 × 50 mL), and the combined organic
layers were washed with H2O (50 mL), Na2S2O3 (50 mL), 2 M
HCl (50 mL), and brine (50 mL). The organic layer was dried
(MgSO4), filtered, concentrated in vacuo, and chromatographed
on SiO2 (EtOAc/hexanes 1:1) to yield 3.1 g (68%) of 8 as a
viscous oil: Rf 0.5 (EtOAc/hexanes 1:1); [R]D -60.8° (c 1.1,
CHCl3, 25 °C); IR (neat) 1700, 1621, 1162, 1101 cm-1; 1H NMR
(383 K, DMSO) δ 4.81 (dd, 1 H, J ) 8.1, 3.5 Hz), 3.80 (s, 3 H),
3.46-3.40 (m, 2 H), 2.53 (s, 3 H), 2.41-2.24 (m, 1 H), 2.02-
1.89 (m, 3 H), 1.31 (s, 9 H); 13C NMR δ 163.1, 162.7, 156.1,
155.8, 154.3, 153.7, 127.3, 127.1, 79.9, 54.7, 54.4, 51.9, 46.7,
46.4, 32.5, 31.5, 28.3, 28.1, 24.3, 23.7, 11.9; k MS (EI) m/ z
(rel intensity) 310 (M+, 5); HRMS m/ z calcd for C15H22N2O5
310.1528, found 310.1524.
Exp er im en ta l Section
Gen er a l Meth od s. All glassware was dried in an oven at
150 °C prior to use. THF and dioxane were dried by distilla-
tion over Na/benzophenone under a nitrogen atmosphere. Dry
CH2Cl2, DMF, and CH3CN were obtained by distillation from
CaH2. Other solvents or reagents were used as acquired
except when otherwise noted. Analytical thin layer chroma-
tography (TLC) was performed on precoated silica gel 60 F-254
plates available from Merck. Column chromatography was
performed using silica gel 60 (particle size 0.040-0.055 mm,
230-400 mesh). Visualization was accomplished with UV
light or by staining with a basic KMnO4 solution or Vaughn’s
reagent. NMR spectra were recorded in CDCl3 unless other-
wise noted at either 300 MHz (1H NMR) or 75 MHz (13C NMR).
2(S)-(2′(R)-H yd r oxy-1′(S)-(m et h oxyca r b on yl)p r op yl-
ca r b a m oyl)-p yr r olid in e-1-ca r b oxylic Acid ter t-Bu t yl
Ester (7). A solution of 7.0 g (33.0 mmol) of Boc-L-proline 6
in 50 mL of dry CH2Cl2 at -15 °C was treated with 4 mL (36.0
mmol) of N-methylmorpholine (NMM) and 4.9 mL (38.0 mmol)
of isobutyl chloroformate (IBCF). The reaction mixture was
stirred for 10 min, and the temperature slowly rose to -5 °C.
After a solution of 6.1 g (36.0 mmol) of threonine methyl ester
hydrochloride and 4 mL (36.0 mmol) of NMM in 20 mL of dry
2′(S)-[4-(2(R)-Hyd r oxy-1(S)-(m eth oxyca r bon yl)p r op yl-
ca r ba m oyl)-5-m eth yloxa zol-2-yl]-p yr r olid in e-1′-ca r boxy-
lic Acid ter t-Bu tyl Ester (9). A solution of 3.1 g (10.0 mmol)
of oxazole 8 in 30 mL of THF/H2O (1:1) was treated with 0.4
g (10 mmol) of LiOH‚H2O and stirred at 25 °C for 1 h. The
reaction mixture was acidified to pH 2 with concentrated HCl
and extracted with CH2Cl2 (3 × 30 mL). The combined organic
layers were washed with H2O (30 mL) and brine (30 mL), dried
(Na2SO4), filtered, concentrated in vacuo, and used without
further purification.
A solution of the crude acid in 15 mL of CH2Cl2 under Ar
was cooled to -15 °C and treated with 1.2 g (12.0 mmol, 1.3
mL) of NMM and 1.6 g (12.0 mmol, 1.6 mL) of IBCF. The
reaction mixture was stirred for 10 min, and the temperature
rose to -5 °C. After a solution of 2.5 g (15.0 mmol) of Thr-
OMe and 1.7 mL (15.0 mmol) of NMM in 10 mL of dry DMF
was added, the reaction mixture was stirred at 25 °C for 50
min, diluted with H2O (50 mL), and washed with CH2Cl2 (3 ×
30 mL). The combined organic layers were extracted with 2
M HCl (30 mL), H2O (30 mL), and brine (30 mL), dried
(MgSO4), and chromatographed on SiO2 (EtOAc/hexanes 4:1)
to yield 3.0 g (75%) of 9 as a viscous liquid: Rf 0.4 (EtOAc/
hexanes 4:1); [R]D -44.4° (c 1.0, CHCl3, 25 °C); IR (neat) 3416,
1750, 1674, 1635, 1160, 1121 cm-1; 1H NMR (373 K, DMSO) δ
7.46 (d, 1 H, J ) 8.3 Hz), 4.84-4.80 (m, 1 H), 4.44-4.40 (m, 1
H), 4.23-4.17 (m, 1 H), 3.66 (s, 3 H), 3.49-3.38 (m, 2 H), 2.53
(s, 3 H), 2.33-2.26 (m, 1 H), 2.04-1.87 (m, 3 H), 1.30 (s, 9 H),
1.11 (d, 3 H, J ) 6.3 Hz); 13C NMR δ 171.3, 162.4, 162.1, 154.4,
153.9, 153.3, 153.0, 128.5, 80.0, 67.9, 67.6, 56.9, 54.6, 54.4, 52.5,
46.7, 46.3, 32.2, 31.3, 28.3, 28.1, 24.2, 23.6, 19.8, 11.7; MS (EI)-
m/ z (rel intensity) 411 (M+, 5); HRMS m/ z calcd for C19H29N3O7
411.2005, found 411.2008.
(24) 1H and 13C NMR of 1 and 20 are shown in the supporting
information.
(25) The reverse-prenylated valine residue has a strong preference
for fully extended conformations (φ ) -110 ( 20°, ψ ) -170 ( 20°),
in contrast to just N-methylated amino acid. Ramachandran plots of
H2CdCHC(Me)2-Val-NHMe and Me-Val-NHMe are shown in the
supporting information.
(26) For a study of the metal-complexation properties of teroxazole
segments of halichondramides, see: J ames, D. M.; Wintner, E.;
Faulkner, D. J .; Siegel, J . S. Heterocycles 1993, 35, 675.

Total Synthesis of (-)-Muscoride A
J . Org. Chem., Vol. 61, No. 19, 1996 6521
2′-(1-(ter t-Bu t oxyca r b on yl)p yr r olid in -2(S)-yl)-5,5′-d i-
m eth yl-2,4′-bioxa zolyl-4-ca r boxylic Acid Meth yl Ester
(10). A solution of 3.0 g (7.3 mmol) of ester 9 in 15 mL of dry
CH2Cl2 was treated with 3.2 g (7.5 mmol) of Dess-Martin
reagent and stirred at 25 °C for 3 h. The reaction mixture
was concentrated in vacuo and chromatographed on SiO2
(EtOAc/hexanes 7:3) to yield the methyl ketone which was
immediately used for the next reaction.
phenyl ester hydrochloride that was immediately used without
further purification.
A suspension of 2.0 g (6.5 mmol) of L-valine pentafluoro-
phenyl ester hydrochloride in 10 mL of dry THF was treated
with 65 mg (0.65 mmol) of CuCl, 45 mg (0.65 mmol) of Cu
powder, and 2.1 mL (14.3 mmol) of Et3N. The reaction mixture
was cooled to 0 °C, treated dropwise with a solution of 0.75 g
(7.5 mmol) of 3-chloro-3-methyl-1-butyne in 5 mL of THF,
stirred at 0 °C for 3 h, diluted with 50 mL of ether, and filtered
through a plug of Celite. The filtrate was concentrated in
vacuo and chromatographed on SiO2 (EtOAc/hexanes 1:19) to
yield 1.4 g (63%) of alkyne 17 which was used directly for the
next step.
A solution of the methyl ketone in 15 mL of THF was added
dropwise to a solution of 4.7 g (18 mmol) of Ph3P, 4.2 g (14.6
mmol) of I2, and 4.2 mL (29 mmol) of Et3N in 30 mL of dry
THF at -78 °C. The reaction mixture was stirred at -78 °C
for 3 h and diluted with 30 mL of H2O. The solution was
extracted with CH2Cl2 (3 × 30 mL), and the combined organic
layers were washed with a 10% solution of Na2S2O3 (30 mL),
2 M HCl (30 mL), H2O (30 mL), and brine (30 mL) and dried
(Na2SO4). The organic layer was filtered, concentrated in
vacuo, and chromatographed on SiO2 (EtOAc/hexanes 2:3) to
yield 2.0 g (65%) of 10 as a pale yellow oil: Rf 0.65 (EtOAc/
hexanes 3:2); [R]D -53° (c 1.1, CHCl3, 25 °C); IR (neat) 3591,
A solution of 200 mg (0.57 mmol) of 17 in 7 mL of petroleum
ether was treated with 30 mg of 10% Pd/C and 150 µL of
quinoline, and H2 gas was bubbled through the solution for
45-60 min. The reaction mixture was filtered through a plug
of Celite, concentrated in vacuo, and chromatographed on SiO2
(EtOAc/hexanes 1:19) to yield 184 mg (92%) of 18 as a colorless
liquid: Rf 0.4 (EtOAc/hexanes 1:19); [R]D -53.7° (c 1.5, CHCl3,
1
1699, 1197, 1057 cm-1; H NMR (373 K, DMSO) δ 4.86-4.82
1
25 °C); IR (neat) 1783, 1090, 1059 cm-1; H NMR δ 5.73 (dd,
(m, 1 H), 3.62 (s, 3 H), 3.47-3.40 (m, 2 H), 2.60 (s, 3 H), 2.58
(s, 3 H), 2.28-2.24 (m, 1 H), 2.03-1.87 (m, 3 H), 1.30 (s, 9 H);
13C NMR δ 164.2, 162.7, 156.0, 154.1, 153.8, 149.9, 128.3,
124.5, 80.0, 54.8, 54.5, 52.0, 46.5, 32.6, 28.4, 28.2, 24.4, 23.8,
12.1, 11.8; MS (EI) m/ z (rel intensity) 391 (M+, 25); HRMS
m/ z calcd for C19H25N3O6 391.1743, found 391.1741.
1 H, J ) 17.6, 10.8 Hz), 5.11-5.04 (m, 2 H), 3.35 (d, 1 H, J )
4.9 Hz), 2.08-2.02 (m, 1 H), 1.66 (s, 1 H), 1.19 (s, 3 H), 1.18
(s, 3 H), 1.07 (d, 3 H, J ) 6.9 Hz), 0.97 (d, 3 H, J ) 6.8 Hz);
13C NMR δ 173.3, 145.5, 141.2, 139.7, 137.9, 113.0, 60.7, 54.5,
32.7, 27.1, 26.8, 19.3, 17.6; MS (CI, isobutane) m/ z (rel
intensity) 352 ([M + 1]+, 100).
2′-(1-(ter t-Bu t oxyca r b on yl)p yr r olid in -2(S)-yl)-5,5′-d i-
m et h yl-2,4′-b ioxa zolyl-4-ca r b oxylic Acid 3-Met h yl-b u t -
2-en yl Ester (11). A solution of 1.9 g (4.9 mmol) of bioxazole
10 in 10 mL of THF/H2O (1:1) was treated with 400 mg (10
mmol) of LiOH‚H2O and stirred at 25 °C for 2 h. The reaction
was quenched by addition of concentrated HCl, and the pH
was adjusted to 1. The solution was extracted with CH2Cl2 (3
× 30 mL), and the combined organic layers were dried
(MgSO4), filtered, and concentrated in vacuo. The crude acid
was used without further purification.
A solution of the acid in 5 mL of CH2Cl2 was treated with
870 mg (10.0 mmol) of prenyl alcohol, 1.8 mL (15.0 mmol) of
Et3N, and 3.3 g (7.5 mmol) of BOP Reagent and stirred at 25
°C for 12 h. The reaction was quenched by the addition of
H2O (30 mL) and extracted with CH2Cl2 (3 × 30 mL). The
combined organic layers were washed with H2O (30 mL) and
2 M HCl (30 mL), dried (Na2SO4), filtered, concentrated in
vacuo, and chromatographed on SiO2 (EtOAc/hexanes 2:3) to
yield 1.4 g (72%) of 11 as a colorless oil: Rf 0.45 (EtOAc/
hexanes 2:3); [R]D -53° (c 1.5, CHCl3, 25 °C); IR (neat) 1734,
1703, 1202, 1163, 1098 cm-1; 1H NMR (373 K, DMSO) δ 5.49-
5.40 (m, 1 H), 4.88-4.84 (m, 1 H), 4.78 (d, 2 H, J ) 7.0 Hz),
3.49-3.43 (m, 2 H), 2.62 (s, 3 H), 2.60 (s, 3 H), 2.32-2.25 (m,
1 H), 2.05-1.91 (m, 3 H), 1.77 (bs, 3 H), 1.75 (bs, 3 H), 1.32 (s,
9 H); 13C NMR δ 164.0, 163.6, 162.4, 155.7, 154.0, 150.3, 149.9,
139.2, 128.6, 124.5, 118.5, 80.0, 61.9, 54.8, 54.5, 46.5, 32.6, 28.2,
25.8, 23.8, 18.1, 12.2, 11.8; MS (EI) m/ z (rel intensity) 445
(M+, 25); HRMS m/ z calcd for C23H31N3O6 445.2212, found
445.2234.
2(S)-((ter t-Bu t oxyca r b on yl)a m in o)-3-m et h yl-b u t yr ic
Acid 2,3,4,5,6-P en ta flu or op h en yl Ester (16). A solution
of 4.15 g (20.0 mmol) of acid 15 in 20 mL of dry CH2Cl2 was
treated with 3.6 g (19.5 mmol) of pentafluorophenol, 4.12 g
(20.0 mmol) of DCC, and 516 mg (4.2 mmol) of DMAP. The
reaction mixture was stirred at 25 °C for 12 h, concentrated
in vacuo, and chromatographed on SiO2 (EtOAc/hexanes 1:19)
to yield 5.5 g (72%) of 16 as an amorphous solid: Rf 0.25
(EtOAc/hexanes 1:19); mp 63-64 °C; [R]D -17.6° (c 1.0, CHCl3,
25 °C); IR (neat) 1782, 1707, 1246, 1167 cm-1; 1H NMR δ 5.01
(d, 1 H, J ) 8.8 Hz), 4.57 (dd, 2 H, J ) 8.7, 4.5 Hz), 2.38-2.30
(m, 1 H), 1.47 (s, 9 H), 1.09 (d, 3 H, J ) 6.7 Hz), 1.03 (d, 3 H,
J ) 6.8 Hz); 13C NMR δ 168.8, 155.5, 141.2, 139.7, 137.9, 80.5,
58.7, 31.2, 28.3, 19.0, 17.5; MS (CI, isobutane) m/ z (rel
intensity) 384 ([M + 1]+, 10).
2(R)-(1′,1′-Dim eth yla llyla m in o)-3-m eth ylbu tyr ic Acid
2,3,4,5,6-P en ta flu or op h en yl Ester (19). According to the
procedures for the synthesis of 18 from 16, 5.5 g (14.3 mmol)
of Boc-D-valine pentafluorophenyl ester were converted to 900
mg (18%) of 19: [R]D +52.1° (c 1.45, CHCl3, 25 °C).
2′(S)-{1-[1,1-Dim eth ylallylam in o)-3-m eth ylbu tyr yl]pyr -
r olid in -2(S)-yl}-5,5′-d im et h yl-2,4′-b ioxa zolyl-4-ca r b oxy-
lic Acid 3-Meth yl-bu t-2-en yl Ester (Mu scor id e A, 1). A
solution of 1.4 g (3.14 mmol) of bioxazole 11 in 35 mL of Et2O
saturated with HCl gas was stirred at 21 °C for 30 min. The
reaction mixture was concentrated in vacuo to yield 920 mg
(77%) of crude hydrochloride that was used without further
purification.
A solution of 400 mg (1.05 mmol) of activated L-valine 18
in 2 mL of CHCl3 was treated with 600 mg (1.50 mmol) of the
amine hydrochloride derived from 11, 322 µL (2.25 mmol) of
Hu¨nig’s base, and 130 mg (1 mmol) of DMAP. The reaction
mixture was heated at reflux for 24 h, concentrated in vacuo,
and chromatographed on SiO2 (EtOAc/hexanes 1:1) to yield
119 mg (22%) of 1 as a viscous oil: Rf 0.6 (EtOAc/hexanes 1:1);
[R]D -92.8° (c 0.5, MeOH, 25 °C); IR (neat) 1733, 1713, 1651,
1202, 1174, 1057 cm-1; 1H NMR (500 MHz) δ 5.75 (dd, 1 H, J
) 17.6, 10.8 Hz), 5.47-5.45 (m, 1 H), 5.20-5.18 (m, 1 H), 4.96
(d, 1 H, J ) 17.5 Hz), 4.91 (d, 1 H, J ) 10.6 Hz), 4.82 (d, 2 H,
J ) 7.3 Hz), 3.67-3.61 (m, 2 H), 2.95 (d, 1 H, J ) 5.1 Hz),
2.66 (s, 3 H), 2.63 (s, 3 H), 2.24-2.14 (m, 3 H), 2.02-1.99 (m,
1 H), 1.9-1.7 (b, 1 H), 1.78 (s, 3 H), 1.76 (s, 3 H), 1.8-1.6 (m,
1 H), 1.12 (s, 3 H), 1.10 (s, 3 H), 0.98 (d, 3 H, J ) 6.6 Hz), 0.84
(d, 3 H, J ) 6.6 Hz); 13C NMR (125 MHz) δ 176.1, 163.2, 162.5,
155.8, 154.1, 150.1, 148.0, 139.2, 128.6, 124.7, 118.6, 111.1,
61.9, 59.2, 54.6, 54.3, 46.8, 32.0, 30.7, 28.1, 26.2, 25.9, 25.1,
19.8, 18.2, 17.6, 12.3, 11.9; MS (EI) m/ z (rel intensity) 512
(M+, 22); HRMS m/ z calcd for C28H40N4O5 512.2998, found
512.2985.
2′(R)-{1-[1,1-Dim eth ylallylam in o)-3-m eth ylbu tyr yl]pyr -
r olid in -2(S)-yl}-5,5′-d im et h yl-2,4′-b ioxa zolyl-4-ca r b oxy-
lic Acid 3-Meth yl-bu t-2-en yl Ester (20). A solution of 200
mg (0.56 mmol) of activated D-valine 19 in 2 mL of 1,2-
dichloroethane was treated with 100 mg (0.25 mmol) of the
amine hydrochloride derived from 11 and 130 mg (1.1 mmol)
of DMAP. The reaction mixture was heated at reflux for 24
h, concentrated in vacuo, and chromatographed on SiO2
(EtOAc/hexanes 1:1) to yield 25 mg (18%) of 20 as a viscous
oil: Rf 0.6 (EtOAc/hexanes 1:1); [R]D -54° (c 0.58, MeOH, 25
°C); IR (neat) 1712, 1646, 1169, 1097, 1055 cm-1; 1H NMR (500
MHz, major rotamer) δ 5.70 (dd, 1 H, J ) 9.0, 5.0 Hz), 5.48-
5.44 (m, 1 H), 5.17-5.14 (m, 1 H), 5.01 (dd, 1 H, J ) 18.0, 1.0
Hz), 4.99 (dd, 1 H, J ) 10.6, 1.0 Hz), 4.82 (d, 2 H, J ) 7.1 Hz),
2(S)-(1′,1′-Dim eth yla llyla m in o)-3-m eth ylbu tyr ic Acid
2,3,4,5,6-P en ta flu or op h en yl Ester (18). A solution of 6.0
g (15.6 mmol) of ester 16 was treated for 30 min with 60 mL
of a solution of saturated HCl in ether. The reaction mixture
was concentrated in vacuo to yield 4.1 g of L-valine pentafluoro-

6522 J . Org. Chem., Vol. 61, No. 19, 1996
Wipf and Venkatraman
3.73-3.68 (m, 1 H), 3.58-3.53 (m, 1 H), 2.94 (d, 1 H, J ) 5.0
Hz), 2.67 and 2.66 (2s, 6 H), 2.27-2.14 (m, 3 H), 2.05-2.0 (m,
2 H), 1.77 (s, 3 H), 1.76 (s, 3 H), 1.7-1.6 (m, 1 H), 1.09 (s, 3
H), 1.04 (s, 3 H), 0.93 (d, 3 H, J ) 6.7 Hz), 0.89 (d, 3 H, J )
6.7 Hz); 13C NMR (125 MHz, major rotamer) δ 175.6, 163.3,
162.5, 155.6, 154.2, 149.9, 147.8, 139.2, 128.6, 124.7, 118.6,
111.7, 61.9, 59.3, 54.5, 54.3, 46.6, 32.0, 30.8, 27.5, 27.1, 25.9,
24.6, 20.0, 18.2, 17.6, 12.2, 11.8; MS (EI) m/ z (rel intensity)
512 (M+, 10); HRMS m/ z calcd for C28H40N4O5 512.2998, found
512.3015.
edges support from Merck & Co., Pharmacia-Upjohn,
and Zeneca Pharmaceuticals.
Su p p or t in g In for m a t ion Ava ila b le: 1H and 13C NMR
spectra for muscoride A, 20, and synthetic intermediates;
Ramachandran plots for N-reverse-prenylated valine methyl
amide and N-methyl valine methyl amide (21 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.
Ack n ow led gm en t. This work was supported by the
National Institutes of Health. P.W. gratefully acknowl-
J O960891M