Stereoselective route to the ezoaminuroic acid core of the ezomycins

Article abstract of DOI:10.1021/jo051086n

Starting from readily available (R)-glycidol, an efficient pathway to a strategically functionalized ezoaminuroic acid derivative of the antifungal ezomycins has been developed. A key transformation in the synthesis involves regio- and stereoselective con

Full text of DOI:10.1021/jo051086n

Stereoselective Route to the Ezoaminuroic
Acid Core of the Ezomycins†
Juhienah K. Khalaf and Apurba Datta*
Department of Medicinal Chemistry, 1251 Wescoe Hall
Drive, University of Kansas, Lawrence, Kansas 66045
adutta@ku.edu
Received May 31, 2005
FIGURE 1. Representative structure of an ezomycin.
Ezoaminuroic acid is the first example of a naturally
occurring 3-amino-3-deoxyhexuronic acid. Unlike the
previously reported carbohydrate starting material based
syntheses of ezoaminuroic acid derivaties,4 we decided
to investigate a more flexible non-carbohydrate approach
toward de novo construction of the strategically func-
tionalized carbohydrate core of the target compound
(Scheme 1).
Our strategy involved utilization of commercially avail-
able (R)-glycidol toward initial formation of the enan-
tiopure lactone 2, followed by its subsequent trans-
formation to the strategically functionalized masked
ezoaminuroic acid derivative 1.
Starting from readily available (R)-glycidol, an efficient
pathway to a strategically functionalized ezoaminuroic acid
derivative of the antifungal ezomycins has been developed.
A key transformation in the synthesis involves regio- and
stereoselective conversion of the olefinic functionality of a
5,6-dihydropyran-2-one to the C-2, C-3 trans-1,2-amino
alcohol moiety as present in ezoaminuroic acid.
SCHEME 1
The ezomycins are Streptomyces-derived natural prod-
ucts with specific activity against phytopathogenic fungi
such as Sclerotina and Botritus.1 The unique structural
features and biological activity of the ezomycins are
representative of the complex peptidyl nucleoside family
of antibiotics.2 Structurally, the ezomycins (Figure 1)
include (i) an octosyl nucleoside core, (ii) ezoaminuroic
acid component, and (iii) an N-linked pseudopeptide
(cystathionine). While some of the ezomycins contain the
cytosine nucleobase, others are of the pseudouridine type.
The complex structural features and antifungal poten-
tial of the ezomycins represents an attractive target for
total synthesis and structure-activity relationship re-
lated studies. Although the syntheses of various struc-
tural fragments of ezomycins have been reported,3,4 the
total synthesis of this family of compounds has not yet
been achieved. In continuation of our studies on complex
peptidyl nucleoside antibiotics,5 we report herein an
efficient route to a strategically protected, masked ezoam-
inuroic acid structural core.
Accordingly, following a known sequence of reactions,6
silyl protection of the hydroxy group of (R)-glycidol,
regioselective opening of the epoxide with vinylcuprate,
conversion of the resulting secondary hydroxy group to
its acrylate derivative, and subsequent ring closing
metathesis with Grubbs’ first-generation catalyst pro-
vided the functionalized lactone 26 (Scheme 2) in 73%
overall yield. Osmium tetroxide catalyzed stereoselec-
(3) For some recent studies related to the octosyl nucleoside of the
ezomycins, see: (a) Knapp, S.; Gore, V. K. Org. Lett. 2000, 2, 1391-
1393 and references therein. (b) Haraguchi, K.; Hosoe, M.; Tanaka,
H.; Tsuruoka, S.; Kanmuri, K.; Miyaska, T. Tetrahedron Lett. 1998,
39, 5517-5520. (c) Knapp, S.; Shieh, W.-C.; Jaramillo, C.; Trilles, R.
V.; Nandan, S. R. J. Org. Chem. 1994, 59, 946-948. (d) Maier, S.;
Preuss, R.; Schmidt, R. R. Liebigs Ann. Chem. 1990, 483-489. (e)
Danishefsky, S. J.; Hungate, R.; Schulte, G. J. Am. Chem. Soc. 1988,
110, 7434-7440 and references therein.
† This paper is dedicated to Professor Hiriyakkanavar Junjappa on
the occasion of his 70th birthday.
(1) (a) Sakata, K.; Sakurai, A.; Tamura, S. Agric. Biol. Chem. 1974,
38, 1883-1890. (b) Sakata, K.; Sakurai, A.; Tamura, S. Agric. Biol.
Chem. 1975, 39, 885-892. (c) Sakata, K.; Sakurai, A.; Tamura, S. Agric.
Biol. Chem. 1977, 41, 2027-2032. (d) Sakata, K.; Sakurai, A.; Tamura,
S. Agric. Biol. Chem. 1977, 41, 2033-2039. (e) Sakata, K.; Sakurai,
A.; Tamura, S. Tetrahedron Lett. 1974, 15, 1533-1536.
(4) For previous syntheses of the ezoaminuroic acid component,
see: (a) Reference 3a above. (b) Knapp, S.; Jaramillo, C.; Freeman, B.
J. Org. Chem. 1994, 59, 4800-4804. (c) Knapp, S.; Levorse, A. T.;
Potenza, J. A. J. Org. Chem. 1988, 53, 4773-4779. (d) Ogawa, T.;
Akatsu, M.; Matsui, M. Carbohydr. Res. 1975, 44, C22-24. (e)
Mieczkowski, J.; Zamojski, A. Bull. Acad. Pol. Sci. 1975, 23, 581-583.
(5) Bhaket, P.; Stauffer, C. S.; Datta, A. J. Org. Chem. 2004, 69,
8594-8601.
(2) For reviews, see: (a) Zhang, D.; Miller, M. J. Curr. Pharm.
Design 1999, 5, 73-99. (b) Knapp, S. Chem. Rev. 1995, 95, 1859-1876.
(c) Isono, K. Current progress on nucleoside antibiotics. Pharmacol.
Ther. 1991, 52, 269-286. (d) Garner, P. Synthetic approaches to
complex nucleoside antibiotics. In Studies in Natural Products Chem-
istry; Atta-ur-Rahman, Ed.; Elsevier: Amsterdam, 1988; Stereoselec-
tive synthesis (Part A), Vol. 1, pp 397-435. (e) Isono, K. J. Antibiot.
1988, 41, 1711-1739.
(6) Hansen, T. V. Tetrahedron: Assymetry 2002, 13, 547-550.
10.1021/jo051086n CCC: $30.25 © 2005 American Chemical Society
Published on Web 07/14/2005
J. Org. Chem. 2005, 70, 6937-6940
6937

SCHEME 2
FIGURE 2. Ball and stick model of compound 10 (adapted
from the X-ray crystal structure). Hydrogen atoms (except on
the pyranose ring) have been omitted for clarity.
of the fully protected ezoaminuroic acid derivative 10 as
a colorless crystalline solid.
At this stage, the assigned structure and absolute
stereochemistry of 10 could be unamibigously confirmed
by its X-ray crystallographic analysis (Figure 2). The
strategically protected functionalities of both the inter-
mediate 1 and the further oxidized product 10 render
them as suitable ezoaminuroic acid glycosyl donors for
eventual synthesis of the ezomycin nucleoside disaccha-
ride component.
In terms of efficiency and brevity, the present route
compares favorably with the other reported synthesis of
ezoaminuroic acid derivatives.4 Ready availability of the
starting glycidol in both the enantiomerically pure forms,
a simple and practical sequence of reactions, and the
amenability of the method toward easy scale-up is
expected to make the present synthetic route a method
of choice in the total synthesis and structure-activity
relationship investigations involving the ezomycin family
of antifungal antibiotics.
tive dihydroxylation of 2 (from the less hindered R-face)5
yielded the corresponding diol 3 as the only product.
Protection of the diol as the corresponding acetonide 4,
followed by partial reduction of the lactone to lactol and
subsequent O-alkylation resulted in the stereoselective
formation of the methyl glycoside 5 in good overall yield.
In 1H NMR studies, chemical shift (δ 4.45) and coupling
constant of the anomeric proton (J ) 5.6 Hz) confirmed
the assigned â-stereochemistry of the methyl glycoside
5. Hydrolysis of the acetonide linkage of 5 under mild
conditions yielded the diol 6 in 78% yield. It was
envisioned that stereochemical orientation of the hydroxy
groups of 6 should allow differentiation between the C-2
and C-3 hydroxy groups. Gratifyingly, when the diol 6
was subjected to acylation in the presence of 1.1 equiva-
lent of acetic anhydride, selective acetylation of the
equatorial hydroxyl group (more favored)7 resulted in the
C-2 acetylated product 7 in good yield. The assigned
structure of 7 was ascertained by high-resolution NMR
studies (COSY and NOE). Following a modified Mit-
sunobu procedure,8 the C-3 hydroxy group was then
converted to the corresponding azido derivative 8, with
simultaneous inversion of the stereocenter involved.
Subsequent reduction of the azido group to the corre-
sponding amine and in situ Boc-protection afforded the
amino sugar derivative 1 in good overall yield. Finally,
selective unmasking of the primary hydroxy group to
form 9 and its further conversion to the corresponding
methyl carboxylate culminated in an efficient synthesis
Experimental Section
(3R,4R,6S)-6-[(tert-Butyldiphenylsilyloxy)methyl]-3,4-di-
hydroxytetrahydropyran-2-one (3). Lactone 26 (1.76 g, 4.81
mmol) was dissolved in acetone/water (4:1, 30 mL), and to this
was added NMO (1.63 g, 12.0 mmol), followed by OsO4 (5% in
toluene, 1.47 mL, 6 mol %). The resulting solution was stirred
at room temperature for 50 min. After completion of the reaction
(TLC monitoring), 10% aqueous NaHSO3 (4 mL) was added to
the reaction mixture, which was stirred for 5 min and diluted
by addition of EtOAc (25 mL). The organic layer was separated,
and the aqueous layer was extracted with EtOAc (3 × 25 mL).
The combined organic extracts were dried over anhydrous Na2-
SO4 and concentrated under vacuum. Flash chromatography
(hexane/EtOAc ) 3:2) afforded the diol 3 as a colorless oil (1.55
g, 81%): [R]25D + 18.6 (c 1.04, CHCl3); IR (NaCl) 3421, 1736 cm-1
;
1H NMR (400 MHz, CDCl3) δ 1.08 (s, 9H), 2.18-2.36 (m, 2H),
2.97 (br s, 1H, exchangeable with D2O), 3.65 (br s, 1H, exchange-
able with D2O), 3.72 (dd, J ) 3.1, 11.4 Hz, 1H), 3.96 (dd, J )
3.6, 11.4 Hz, 1H), 4.15 (s, 1H), 4.41 (br s, 1H), 4.81-4.87 (m,
1H), 7.42-7.45 (m, 6H), 7.66-7.69 (m, 4H); 13C NMR (100.6
MHz, CDCl3) δ 19.7, 27.2, 30.0, 65.4, 66.5, 70.9, 78.6, 128.2,
130.3, 133.0, 133.4, 136.0, 174.4; HRMS (ES+) calcd for C22H29O5-
Si m/z (M + H)+ 401.1784, found 401.1787.
(7) For a similar observation, see: Bhatt, R. K.; Chauhan, K.;
Wheelan, P.; Murphy, R. C.; Falck, J. R. J. Am. Chem. Soc. 1994, 116,
5050-5056.
(8) (a) Pearson, W. H.; Hines, J. V. J. Org. Chem. 1989, 54, 4235-
4237. (b) Lal, B.; Pramanik, B. N.; Manhas, M. S.; Bose, A. K.
Tetrahedron Lett. 1977, 23, 1977-1979.
(3aR,6S,7aR)-6-[(tert-Butyldiphenylsilyloxy)methyl]-2,2-
dimethyldihydro-3aH-[1,3]dioxolo[4,5-c]pyran-4(6H)-one
(4). The diol 3 (1.67 g, 4.18 mmol) was dissolved in a mixture of
acetone (26 mL) and 2,2-dimethoxypropane (8 mL), and a
catalytic amount of BF3‚Et2O (0.1 mL) was added to it. The
6938 J. Org. Chem., Vol. 70, No. 17, 2005

resulting solution was stirred at room temperature for 2 h. The
reaction was quenched with NEt3 (0.2 mL), and the solvent was
removed under vacuum. Purification of the residue by flash
chromatography (hexane/EtOAc ) 9:1) yielded the acetonide 4
as a colorless oil (1.69 g, 92%): [R]25D + 18.4 (c 1.25, CHCl3); IR
due was purified by flash chromatography (hexane/EtOAc )
4:1-7:3) to afford the acetate 7 as a white semisolid (0.111 g,
78%): [R]25D -51.4 (c 1.36, CHCl3); IR (NaCl) 3485, 1742 cm-1
;
1H NMR (400 MHz, CDCl3) δ 1.08 (s, 9H), 1.74-1.78 (m, 1H),
1.93-1.98 (m, 1H), 2.10 (br s, 1H, exchangeable with D2O), 2.17
(s, 3H), 3.50 (s, 3H), 3.67-3.81 (m, 2H), 4.09-4.12 (m, 1H), 4.29
(br s, 1H), 4.74 (s, 2H), 7.40-7.43 (m, 6H), 7.70-7.72 (m, 4H);
13C NMR (100.6 MHz, CDCl3) δ 19.7, 21.6, 27.2, 34.0, 56.7, 66.5,
67.1, 71.3, 73.4, 99.4, 128.1, 130.1, 133.9, 136.0, 170.2; HRMS
(ES+) calcd for C25H34O6SiNa m/z (M + Na)+ 481.2022, found
481.2011.
(NaCl) 1753 cm-1 1H NMR (400 MHz, CDCl3) δ 1.09 (s, 9H),
;
1.41 (s, 3H), 1.52 (s, 3H), 2.02-2.16 (m, 2H), 3.79 (dd, J ) 3.9,
11.2 Hz, 1H), 3.89 (dd, J ) 4.3, 11.2 Hz, 1H), 4.62 (d, J ) 6.8
Hz, 1H), 4.66-4.73 (m, 2H), 7.42-7.47 (m, 6H), 7.68-7.71 (m,
4H); 13C NMR (100.6 MHz, CDCl3) δ 19.7, 24.5, 26.5, 27.2, 30.8,
65.4, 72.1, 73.3, 75.6, 111.1, 128.2, 130.3, 133.1, 133.4, 136.1,
168.5; HRMS (ES+) calcd for C25H33O5Si m/z (M + H)+ 441.2097,
found 441.2095.
(2R,3R,4S,6S)-3-Acetoxy-4-azido-6-[(tert-butyldiphenyl-
silyloxy)methyl]-2-methoxytetrahydro-2H-pyran (8). To an
ice-cooled solution of the acetate 7 (0.120 g, 0.262 mmol) in
anhydrous THF (6 mL) were added diisopropyl azodiacarboxy-
late (0.072 mL, 0.367 mmol) and triphenylphosphine (0.096 mg,
0.367 mmol), followed by dropwise addition of diphenylphos-
phoryl azide (0.080 mL, 0.367 mmol). After completion of
addition, the reaction mixture was brought to room temperature
and then heated to 50 °C overnight. Excess solvent was removed
under vacuum, and the residue was purified by flash chroma-
tography (hexane/EtOAc ) 6:1) to give the azide 8 as colorless
tert-Butyl[{(3aR,4R,6S,7aR)-4-methoxy-2,2-dimethyltet-
rahydro-3aH-[1,3]dioxolo[4,5-c]pyran-6-yl}methoxy]-
diphenylsilane (5). (Step 1) The acetonide 4 (1.52 g, 3.45 mmol)
was dissolved in toluene (40 mL) and cooled to -78 °C. To this
stirring solution was added DIBAL-H (1 M in toluene, 5.2 mL,
5.2 mmol) dropwise. The reaction was stirred at -78 °C for 1.5
h and then quenched by careful addition of MeOH (1.0 mL). The
reaction mixture was brought to room temperature and diluted
with EtOAc (50 mL), and saturated aqueous sodium potassium
tartrate (50 mL) was added to it. The resulting mixture was
stirred until two clear layers were seen. The organic layer was
separated and the aqueous layer extracted with EtOAc (3 × 30
mL). The combined organic layers were dried over anhydrous
Na2SO4 and concentrated, and the crude lactol (1.40 g) was
carried onto the next step without further purification.
(Step 2). The lactol (1.40 g, 3.17 mmol) from the above reaction
was dissolved in anhydrous CH2Cl2 (30 mL), and to this solution
was added freshly prepared Ag2O (4.80 g, 20.7 mmol), followed
by MeI (4.28 mL, 69.0 mmol) under N2 and in the dark. The
reaction mixture was stirred at room temperature for 3 h under
N2. The reaction mixture was filtered, and the residue washed
with CH2Cl2 thoroughly. The combined filtrate was concentrated
under vacuum to afford the crude product. Purification by flash
chromatography (hexane/EtOAc ) 4:1) afforded the methyl
oil (0.1 g, 80%): [R]25 -19.6 (c 1.00, CHCl3); IR (NaCl) 2100,
D
1751 cm-1
;
1H NMR (400 MHz, CDCl3) δ 1.09 (s, 9H), 1.50-
1.56 (m, 1H), 2.10-2.16 (m, 4H), 3.48 (s, 3H), 3.56-3.70 (m, 3H),
3.81-3.85 (m, 1H), 4.30 (d, J ) 7.8 Hz, 1H), 4.78-4.83 (m, 1H),
7.39-7.48 (m, 6H), 7.70 (m, 4H); 13C NMR (100.6 MHz, CDCl3)
δ 19.6, 21.3, 27.2, 33.2, 56.8, 60.6, 66.1, 73.3, 73.6, 102.2, 128.2,
130.2, 133.5, 136.0, 170.2; HRMS (ES+) calcd for C25H33N3O6-
SiNa m/z (M + Na)+ 506.2087, found 506.2089.
(2R,3R,4S,6S)-3-Acetoxy-4-(tert-butoxycarbonylamino)-
6-[(tert-butyldiphenylsilyloxy)methyl]-2-methoxytetrahy-
dro-2H-pyran (1). The azide 8 (0.087 g. 0.18 mmol) was
dissolved in anhydrous EtOAc (5 mL) to which 10% Pd-C (26
mg) was added followed by di-tert-butyl dicarbonate (0.055 g,
0.252 mmol) and Et3N (20 µL, catalytic). The reaction was
allowed to stir under H2 atmosphere at room temperature for 3
h. The mixture was then filtered and the residue washed
thoroughly with EtOAc (3 × 10 mL). The combined filtrate was
concentrated under vacuum. The residue was purified by flash
chromatography (hexane/EtOAc ) 9:1) to afford the protected
glycoside 5 (1.34 g, 85% over two steps) as a colorless oil: [R]25
D
- 49.8 (c 1.00, CHCl3); 1H NMR (400 MHz, CDCl3) δ 1.09 (s,
9H), 1.40 (s, 3H), 1.56 (s, 3H), 1.85-1.92 (m, 1H), 2.04-2.08 (br
m, 1H), 3.51 (s, 3H), 3.67-3.79 (m, 2H), 3.89-3.95 (m, 2H), 4.45
(d, J ) 5.7 Hz, 1H), 4.95 (br s, 1H), 7.39-7.47 (m, 6H), 7.71-
7.73 (m, 4H); 13C NMR (100.6 MHz, CDCl3) δ 19.7, 25.9, 27.2,
28.1, 29.5, 56.8, 66.9, 70.9, 72.5, 75.4, 102.9, 109.5, 128.1, 130.1,
133.8, 136.0; HRMS (ES+) calcd for C26H36O5SiNa m/z (M +
Na)+ 479.2230, found 479.2228.
ezoaminuroic acid 1 (0.083 g, 83%) as a white semisolid: [R]25
D
-17.3 (c 1.00, CHCl3); IR (NaCl) 3352, 1744, 1715 cm-1; 1H NMR
(400 MHz, CDCl3) δ 1.07 (s, 9H), 1.34-1.44 (m, 10H), 2.07-
2.16 (m, 4H), 3.49 (s, 3H), 3.65-3.89 (m, 4H), 4.36 (d, J ) 7.7
Hz, 1H), 4.57-4.66 (m, 2H), 7.40-7.45 (m, 6H), 7.69-7.70 (m,
4H); 13C NMR (100.6 MHz, CDCl3) δ 19.7, 21.4, 27.2, 28.7, 34.8,
51.2, 56.9, 66.3, 73.8, 74.2, 80.1, 102.4, 128.1, 130.1, 133.7, 136.0,
155.8, 171.4; HRMS (ES+) calcd for C30H44NO7Si m/z (M + H)+
558.2887, found 558.2877.
(2R,3R,4R,6S)-6-[(tert-Butyldiphenylsilyloxy)methyl]-2-
methoxytetrahydro-2H-pyran-3,4-diol (6). The methyl gly-
coside 5 (1.25 g, 2.74 mmol) was dissolved in a mixture of AcOH/
H2O (3:1, 20 mL) and stirred at room temperature for 17 h.
Excess solvent was removed in vacuo, and the residual product
was purified by flash chromatography (hexane/EtOAc ) 3:2),
affording the diol 6 as a colorless oil (0.889 g, 78%): [R]25D -34.8
(c 1.00, CHCl3); IR (NaCl) 3448 cm-1; 1H NMR (400 MHz, CDCl3)
δ 1.08 (s, 9H), 1.62-1.68 (m, 1H), 1.95-2.00 (m, 1H), 2.61 (s,
2H, exchangeable with D2O), 3.41-3.44 (m, 1H), 3.56 (s, 3H),
3.65-3.69 (m, 1H), 3.77-3.81 (m, 1H), 4.06-4.08 (m, 1H), 4.24-
4.25 (br s, 1H), 4.56 (d, J ) 7.9 Hz, 1H), 7.40-7.43 (m, 6H),
7.71-7.72 (m, 4H); 13C NMR (100.6 MHz, CDCl3) δ 19.7, 27.2,
33.8, 57.1, 66.6, 67.7, 71.6, 72.3, 101.7, 128.1, 130.0, 133.9, 136.1;
HRMS (ES+) calcd for C23H32O5SiNa m/z (M + Na)+ 439.1917,
found 439.1916.
(2R,3R,4R,6S)-3-Acetoxy-6-[(tert-butyldiphenylsilyloxy)-
methyl]-4-hydroxy-2-methoxytetrahydro-2H-pyran (7). The
diol 6 (0.13 g, 0.311 mmol) was dissolved in anhydrous CH2Cl2
(3 mL), and to this solution were added anhydrous pyridine (0.05
mL, 0.622 mmol), DMAP (10 mg, catalytic), followed by Ac2O
(0.035 mL, 0.374 mmol). The reaction mixture was allowed to
stir at room temperature for 2 h and then quenched by addition
of ice-cooled water (1 mL). The two layers were separated, and
the aqueous layer was extracted with CH2Cl2 (3 × 10 mL). The
combined organic extracts were washed with saturated aqueous
NaHCO3 (1 × 10 mL) and brine (1 × 10 mL), dried over
anhydrous Na2SO4, and concentrated under vacuum. The resi-
(2R,3R,4S,6S)-3-Acetoxy-4-(tert-butoxycarbonylamino)-
6-(hydroxymethyl)-2-methoxytetrahydro-2H-pyran (9). The
fully protected methylglycoside derivative 1 (0.208 g, 0.375
mmol) was dissolved in anhydrous pyridine (2 mL) and cooled
to 0 °C, followed by addition of HF-pyridine solution (0.5 mL).
The resulting solution was stirred at 0 °C for 1 h, after which
the reaction was quenched with saturated aqueous solution of
NaHCO3. The two layers were separated, and the aqueous layer
was extracted with EtOAc (3 × 10 mL). The combined organic
layer was washed with brine, dried over anhydrous Na2SO4, and
concentrated under vacuum. The residue was purified by flash
chromatography (hexane/EtOAc ) 3:7) to afford the alcohol 9
as a white solid (0.122 g, 87%): mp ) 132-135 °C; [R]25D -27.7
(c 1.20, CHCl3); IR (NaCl) 3358, 1740, 1668 cm-1; 1H NMR (400
MHz, CDCl3) δ 1.42 (br s, 10H), 2.03 (dd, J ) 3.4 and 12.4 Hz,
1H), 2.10 (s, 3H), 2.12 (br s, 1H, exchangeable with D2O), 3.52
(s, 3H), 3.56-3.73 (m, 3H), 3.86-3.89 (m, 1H), 4.39 (d, J ) 7.7
Hz, 1H), 4.59 (dd, J ) 2.6 and 10.4 Hz, 1H), 4.74 (br d, J ) 8.6
Hz, 1H); 13C NMR (100.6 MHz, CDCl3) δ 21.3, 28.7, 33.8, 51.1,
57.3, 65.2, 70.9, 73.7, 80.1, 102.7, 155.8, 171.5; HRMS (ES+)
calcd for C14H26NO7 m/z (M + H)+ 320.1709, found 320.1698.
(2S,4S,5R,6R)-Methyl 5-Acetoxy-4-(tert-butoxycarbon-
ylamino)-6-methoxytetrahydro-2H-pyran-2-carboxylate
(10). (Step 1) To a mixture of CH3CN-CCl4-H2O (1.1 mL;
J. Org. Chem, Vol. 70, No. 17, 2005 6939

1:1:10) were added NaIO4 (0.098 g, 0.46 mmol) and RuCl3‚3H2O
(0.005 g, 0.023 mmol) sequentially. The mixture was stirred at
room temperature for 45 min and then added into an ice-cooled
solution of the alcohol 9 (0.073 g, 0.23 mmol) in CH3CN (1 mL),
followed by addition of a second portion of NaIO4 (0.048 g, 0.23
mmol). The resulting mixture was stirred at 0 °C for 1 h and
filtered through Celite, and the Celite layer was then washed
with EtOAc. The combined organic filtrate was washed once with
water, and the aqueous layer was re-extracted with EtOAc (3 ×
10 mL). The combined organic extracts were dried over anhy-
drous Na2SO4 and concentrated under vacuum. The crude acid
(∼0.05 g) thus obtained was carried to the next step without
further purification.
(Step 2) [CAUTION: Diazomethane (CH2N2) is an explosive
and a highly toxic gas. Explosions may occur if the substance is
dried and undiluted. All operations involving diazomethane
should be carried out in an efficient fume hood following
appropriate precautions.] To a biphasic solution of KOH (0.60
g) in H2O (6 mL) and ether (6 mL) at 0 °C was added N-methyl-
N′-nitro-N-nitrosoguandine (MNNG, 0.20 g) in one lot. The
organic layer turned bright yellow. The ethereal layer was
decanted into an ice-cooled Erlenmeyer flask containing KOH
pellets. The aqueous layer was washed with ether (3 × 5 mL),
and the ethereal layers were combined. The CH2N2 thus
prepared was added to a stirred solution of the crude acid (0.05
g in 2 mL of ether) from step 1, and stirred for 30 min. Excess
CH2N2 was removed by bubbling nitrogen into the reaction
mixture for 15 min, followed by removal of solvent under
vacuum. Purification by flash chromatography (hexane/EtOAc
7:3) yielded the protected ezoaminuroic acid 1 as a colorless
crystalline solid (0.047 g, 59% over two steps): mp ) 158-160
°C; [R]25 -26.2 (c 0.65, CHCl3); IR (NaCl) 3362, 1749, 1742,
D
1668 cm-1
;
1H NMR (400 MHz, CDCl3) δ 1.42 (s, 9H), 1.58-
1.68 (m, 1H), 2.10 (s, 3H), 2.45 (br d, J ) 10.9 Hz, 1H), 3.53 (s,
3H), 3.79 (s, 3H), 3.85-3.96 (m, 1H), 4.15 (d, J ) 11.6 Hz, 1H),
4.41 (d, J ) 7.5 Hz, 1H), 4.65 (dd, J ) 2.6 and 10.1 Hz, 1H),
4.75 (d, J ) 8.5 Hz, 1H); 13C NMR (100.6 MHz, CDCl3) δ 21.3,
28.7, 30.1, 34.8, 50.9, 52.9, 57.5, 71.8, 73.0, 80.3, 102.6, 155.7,
170, 171.3; HRMS (ES+) calcd for C15H26NO8 m/z (M + H)+
348.1658, found 348.1661.
Supporting Information Available: General experimen-
tal methods and copies of 1H and 13C NMR spectra for all new
compounds and X-ray data (CIF). This material is available
free of charge via the Internet at http://pubs.acs.org.
JO051086N
6940 J. Org. Chem., Vol. 70, No. 17, 2005