Synthetic studies on ezomycins: Stereoselective route to a thymine octosyl nucleoside derivative

Article abstract of DOI:10.1021/jo801050r

(Chemical Equation Presented) The ezomycins are Streptomyces-denved antifungal natural products, belonging to the complex peptidyl nucleoside family of antibiotics. Employing D-serine as a chiral platform, we report herein a novel synthetic route to the bicyclic octosyl nucleoside core of the ezomycins. A key step in the sequence involved a stereoselective 6-exo-trig oxymercuration-oxidation of a strategic δ-hydroxy alkene derivative, toward construction of the trans-fused furopyran ring system as present in the target products. In contrast to the known carbohydrate-based synthetic routes to the above furopyranyl fragment, the present amino acid chiral template approach is expected to offer a more flexible pathway toward potential SAR-targeted structural/stereochemical modifications of this central bicyclic nucleoside component of the ezomycins.

Full text of DOI:10.1021/jo801050r

Synthetic Studies on Ezomycins: Stereoselective Route to a Thymine
Octosyl Nucleoside Derivative
Juhienah K. Khalaf, David G. VanderVelde, and Apurba Datta*
Department of Medicinal Chemistry, The UniVersity of Kansas, 1251 Wescoe Hall DriVe,
Lawrence, Kansas 66045
adutta@ku.edu
ReceiVed May 15, 2008
The ezomycins are Streptomyces-derived antifungal natural products, belonging to the complex peptidyl
nucleoside family of antibiotics. Employing D-serine as a chiral platform, we report herein a novel synthetic
route to the bicyclic octosyl nucleoside core of the ezomycins. A key step in the sequence involved a
stereoselective 6-exo-trig oxymercurationsoxidation of a strategic δ-hydroxy alkene derivative, toward
construction of the trans-fused furopyran ring system as present in the target products. In contrast to the
known carbohydrate-based synthetic routes to the above furopyranyl fragment, the present amino acid
chiral template approach is expected to offer a more flexible pathway toward potential SAR-targeted
structural/stereochemical modifications of this central bicyclic nucleoside component of the ezomycins.
Introduction
The ezomycins (Figure 1) are Streptomyces-derived antifungal
natural products, belonging to the complex peptidyl nucleoside
superfamily of antibiotics.1 Isolated and identified during the
1970s, the ezomycins are active against phytopathogenic fungi
such as Sclerotinia and Botrytis.2 The antifungal mechanism
of action of the ezomycins has however not yet been ascertained.
The structure and stereochemistry of the ezomycins were
determined by extensive spectroscopic and chemical degradation
studies.2,3 A common structural feature of the various ezomycins
is the bicyclic nucleoside disaccharide core, comprising a novel
trans-fused furopyranyl nucleoside-containing disaccharide (Fig-
ure 1). As evident, the two structurally distinct constituents of
the above ezomycin nucleoside disaccharide core are ezoam-
FIGURE 1. Structures of representative ezomycins.
* To whom correspondence should be addressed. Tel.: +1 785 864 4117.
Fax: +1 785 864 5326.
inuroic acid and the octosyl nucleoside segment. The ezoam-
inuroic acid as present in ezomycins is the first example of a
naturally occurring 3-amino-3-deoxyhexuronic acid, whereas,
the structurally rigid octosyl nucleoside component of the
ezomycins is made up of a furanoid ring trans-fused to a
pyranoid ring, forming the 3,7-anhydrooctose moiety. While the
ezomycins A1 and A2 contain cytosine as the nucleobase, in
ezomycins B1 and B2 the nucleobase is of the pseudouridine
(γ-uracil) type.
(1) For reviews on complex peptidyl nucleoside antibiotics, see: (a) Garner,
P. Synthetic approaches to complex nucleoside antibiotics. In Studies in Natural
Products Chemistry. Atta-ur-Rahman. Ed.; Stereoselective synthesis (Part A);
Elsevier: Amsterdam, 1988;, Vol. 1, 397-435. (b) Isono, K. Pharmacol. Ther.
1991, 52, 269–286. (c) Knapp, S. Chem. ReV. 1995, 95, 1859–1876. (d) Zhang,
D.; Miller, M. J. Curr. Pharm. Design 1999, 5, 73–99, and references therein.
(2) (a) Sakata, K.; Sakurai, A.; Tamura, S. Agric. Biol. Chem. 1974, 38, 1883–
1890. (b) Sakata, K.; Sakurai, A.; Tamura, S. Agric. Biol. Chem. 1977, 41, 2027–
2032, and references therein.
(3) (a) Sakata, K.; Sakurai, A.; Tamura, S. Tetrahedron Lett. 1974, 15, 4327–
4330. (b) Sakata, K.; Sakurai, A.; Tamura, S. Tetrahedron Lett. 1975, 16, 3191–
3194. (c) Sakata, K.; Sakurai, A.; Tamura, S. Agric. Biol. Chem. 1975, 39, 885–
892. (d) Sakata, K.; Sakurai, A.; Tamura, S. Agric. Biol. Chem. 1977, 41, 2033–
2039, and references therein.
In addition to the ezomycins, differently substituted, but
otherwise structurally similar bicyclic 3,7-anhydrooctose nucleo-
10.1021/jo801050r CCC: $40.75
Published on Web 07/04/2008
2008 American Chemical Society
J. Org. Chem. 2008, 73, 5977–5984 5977

Khalaf et al.
FIGURE 2. Retrosynthetic strategy and approach.
side motifs are also found in nucleoside antibiotics, such as the
octosyl acids and malayamycin A.4 Interestingly, in biological
studies, the L-cystathionine side-chain-containing ezomycins A1
and B1 were found to display antifungal activity, whereas those
lacking this pseudopeptide (e.g., ezomycins A2 and B2) were
devoid of activity.2 Although a few syntheses of the ezomycin
structural fragments have been reported,1a–c,4c,5 the total syn-
thesis or detailed structure-activity relationship (SAR) studies
of these natural products are yet to be accomplished.
The antifungal activity, challenging structural features, an as
yet undetermined mechanism of action, and limited synthetic
or medicinal chemical studies, render the ezomycins as attractive
targets for further exploration. As part of an ongoing research
investigating the antifungal peptide nucleoside antibiotics,6,7 we
report herein the preliminary results of our synthetic studies
directed at the octosyl nucleoside core of the ezomycins.
strategically located terminal olefin and the C-3′ oxygen
functionality of 2 ultimately leads to the desired trans-
furopyranyl bicyclic nucleoside 1. For the sake of convenience,
we decided to pursue the present study employing the structur-
ally simpler and commercially available bis-TMS-thymine as
the nucleobase donor.
In accordance with the above strategy, and employing a
recently developed protocol from our laboratory,6a readily
available D-serine was converted to the corresponding strategi-
cally functionalized aminobutenolide 3 (Scheme 1) in good
overall yield (53%, eight steps from D-serine). Subsequent
potassium osmate catalyzed dihydroxylation of 3 under reported
conditions resulted in the known diol 4 with high stereocontrol.6a
Partial reduction of the lactone, followed by treatment of the
resulting lactol with an excess of acetic anhydride provided the
triacetate derivative 5 (anomeric mixture) in good overall yield.
The nucleobase introduction was performed by reacting 5 with
bis-silylated thymine in the presence of TMSOTf, resulting in
a highly stereoselective formation of the nucleoside derivative
6.6a,8 Interestingly, the acidic reaction conditions employed
during the nucleobase incorporation also led to clean cleavage
of the N,O-acetonide protection, thereby eliminating the need
of an otherwise necessary additional reaction step.8,9 Aiming
for a two-carbon elongation at the C-4′ side chain, in a two-
step sequence, Dess-Martin periodinane oxidation of the
alcohol 6 to the corresponding aldehyde and its subsequent
reaction with a stabilized Wittig reagent yielded the E-R,ꢀ-
unsaturated ester 7 in good overall yield.
Toward stereoselective installation of the desired C-6′ hy-
droxy functionality, conversion of the side chain acryloyl moiety
to the corresponding allylic alcohol and its subsequent epoxi-
dation were next investigated. Accordingly, DIBAL-mediated
reduction of the ester 7 resulted in the corresponding allylic
alcohol 8 in good yield. It is noteworthy that, under the reaction
conditions employed, the acetate functionalities of 7 were found
to be unaffected, and only the carbomethoxy functionality
underwent reduction to the corresponding alcohol.10 Having
obtained the desired allylic alcohol intermediate 8, its stereo-
selective conversion to the corresponding epoxy alcohol was
initiated. Unfortunately, when subjected to the Sharpless asym-
metric epoxidation (SAE),11 in the presence of either of the
chiral ligands, (+)-DET or (-)-DET, the allylic alcohol 8 failed
to undergo any desired epoxidation, instead the starting material
was recovered unchanged. The surprising failure in the above
SAE of 8 forced us to consider an alternative epoxidation of
the olefin. Accordingly, when subjected to reaction with
m-CPBA, the olefin 8 did indeed react to form the corresponding
Results and Discussion
In the literature reported syntheses of the octosyl nucleoside
core, various carbohydrate starting materials have been em-
ployed as chiral synthons to construct the bicyclic furopyranyl
structural framework.1a–c,4c,5 In a strategic deviation from the
above approaches, our plan for the present synthesis involves
de novo construction of the target bicyclic nucleoside compo-
nent, starting from a structurally simpler and more flexible chiral
amino acid building block. In recent studies, we have utilized
serine as a versatile chiral precursor in the stereoselective
synthesis of the glycosyl nucleoside amino acid cores of various
peptidyl nucleoside antibiotics.6 In continuation of the above
approach, our synthetic strategy for the construction of the
octosyl nucleoside core envisages utilization of the D-serine-
derived enantiopure aminobutenolide 3 as an appropriate chiral
template toward initial formation of the C-4′ carbon chain
extended furanosyl nucleoside derivative 2 (Figure 2). A
subsequent stereoselective 6-exo-trig cyclization involving the
(4) (a) Isono, K.; Crain, R. F.; McKloskey, J. A. J. Am. Chem. Soc. 1975,
97, 943–945. (b) Benner, J. P.; Boehlendrof, B. G. H.; Kipps, M. R.; Lambert,
N. E. P.; Luck, R.; Molleyres, L.-P.; Neff, S.; Schuez, T. C.; Stanley, P. D. WO
03/062242, CAN 139:132519. (c) For a review, see: More, J. D. Org. Prep.
Proc. Int. 2007, 39, 107–133.
(5) (a) Kim, K. S.; Szarek, W. A. Can. J. Chem. 1981, 59, 878–887. (b)
Bovin, N. V.; Zurabyan, S. E.; Khorlin, A. Y. Carbohydr. Res. 1981, 98, 25–
35. (c) Hanessian, S.; Dixit, D.; Liak, T. Pure Appl. Chem. 1981, 53, 129–148.
(d) Kim, K. S.; Szarek, W. A. Carbohydr. Res. 1982, 100, 169–176. (e)
Danishefsky, S.; Hungate, R. J. Am. Chem. Soc. 1986, 108, 2486–2489. (f)
Hanessian, S.; Kloss, J.; Sugawara, T. J. Am. Chem. Soc. 1986, 108, 2758–
2759. (g) Sakanaka, O.; Ohmuri, T.; Kozaki, S.; Suami, S. Bull. Chem. Soc.
Jpn. 1987, 60, 1057–1062. (h) Danishefsky, S. J.; Hungate, R.; Schulte, G. J. Am.
Chem. Soc. 1988, 110, 7434–7440. (i) Maier, S.; Preuss, R.; Schmidt, R. R.
Liebigs Ann. Chem. 1990, 483–489. (j) Knapp, S.; Shieh, W.-C.; Jaramillo, C.;
Trilles, R. V.; Nandan, S. R. J. Org. Chem. 1994, 59, 946–948. (k) Haraguchi,
K.; Hosoe, M.; Tanaka, H.; Tsuruoka, S.; Kanmuri, K.; Miyasaka, T. Tetrahedron
Lett. 1998, 39, 5517–5520. (l) Knapp, S.; Gore, V. K. Org. Lett. 2000, 2, 1391–
1393.
(8) Poon, K. W. C.; Liang, N.; Datta, A. Nucleosides, Nucleotides, Nucleic
Acids 2008, 27, 389–407.
(9) Poon, K. W. C.; Lovell, K. M.; Dresner, K. N.; Datta, A. J. Org. Chem.
2008, 73, 752–755.
(6) For some recent studies, see:(a) Bhaket, P.; Stauffer, C. S.; Datta, A. J.
Org. Chem. 2004, 69, 8594–8601. (b) Stauffer, C. S.; Bhaket, P.; Fothergill,
A. W.; Rinaldi, M. G.; Datta, A. J. Org. Chem. 2007, 72, 9991–9997. (c) Stauffer,
C. S.; Datta, A. J. Org. Chem. 2008, 73, 4166–4174.
(10) For an example of a similar selective reduction with DIBAL, see: Roush,
W. R.; Coffey, D. S.; Madar, D. J. J. Am. Chem. Soc. 1997, 119, 11331–11332.
(11) For a review, see: Johnson, R. A.; Sharpless, K. B. Catalytic asymmetric
epoxidation of allylic alcohols. In Catalytic Asymmetric Synthesis, 2nd ed; Ojima,
I. Ed.; Wiley-VCH: New York, 2000, 231-285, and references therein.
(7) For our synthesis of the ezoaminuroic acid component of the ezomycins,
see: Khalaf, J. K.; Datta, A. J. Org. Chem. 2005, 70, 6937–6940.
5978 J. Org. Chem. Vol. 73, No. 15, 2008

Synthetic Studies on Ezomycins
SCHEME 1
epoxide, albeit with poor stereoselectivity (∼1:1 mixture of
of the rearranged product.13 The above sequence helped install
the secondary hydroxyl bearing the desired chiral center at C-6′,
and also provided the terminal olefin functionality as a strategic
handle toward construction of the desired furopyran bicyclic
skeleton. Subsequent protection of the free secondary hydroxy
group of 12 as its TBS-ether derivative 13 followed by removal
of the acetate protection afforded the diol 14 in high yield.
Toward performing the desired bicyclic ring formation, an
intramolecular 6-exo-trig cyclization was next investigated.
Gratifyingly, when 14 was subjected to an oxymercuration-oxi-
dation protocol by initial reaction with mercuric trifluoroacetate
in refluxing acetonitrile, followed by treatment of the resulting
alkylmercury intermediate with NaBH4 amid continuous bub-
bling of oxygen,14 the corresponding bicyclic furopyranyl
nucleoside 15 was isolated as the major product, along with
minor quantities of the corresponding C-7′ methyl analogue 16.
With an aim to simplify the 1H NMR spectrum for structural
assignment, sequential silyl deprotection and acetylation of 15
provided the triacetate derivative 17 (Scheme 1). At this stage,
extensive NMR analysis of 17 unambiguously confirmed the
bicyclic structure and the assigned stereochemistry of this trans-
fused furopyranyl nucleoside. Thus, in an HMBC experiment,
the observed three-bonds connectivity between H-7′ and C-3′
confirmed the furopyran bicyclic ring formation. Similarly,
strong NOE correlations between H-3′/H-7′ and H-6′/H-7′
1
diastereoisomers by H NMR). The unsatisfactory selectivity
in the above epoxidation prompted us to investigate prior
protection of the primary hydroxyl group of 8 and study the
subsequent effect on m-CPBA epoxidation. Gratifyingly, protec-
tion of the alcohol 8 to the corresponding TBS-ether derivative
9 followed by its reaction with m-CPBA, resulted in the
stereoselective formation of the epoxide 10 along with minor
quantities of the other diastereoisomer (major/minor ) 5:1 by
1H NMR).12 The tentatively assigned stereochemistry of the
epoxide 10 was based on the assumption that the bulky
carbamoyl substituent adjacent to the olefin would direct the
epoxidizing reagent to approach from the less hindered R-face.12
True to our expectation, the above stereochemical assignment
was subsequently proven to be correct on the basis of extensive
NMR analysis of a downstream product obtained from 10
(compound 17, vide infra). At this stage, several attempts to
construct the trans-fused bicyclic furopyran framework via
acetate deprotection and subsequent 6-endo addition of the
resulting C-3′ hydroxyl group to the C-7′ of the epoxide 10
were however not successful. Following an alternative strategy,
the silyl ether protection of 10 was then removed to form the
corresponding epoxy alcohol derivative 11. In a two-step
sequence, 11 was then transformed to the corresponding
secondary allylic alcohol derivative 12 by initial conversion of
the primary hydroxyl group to iodo, followed by reductive
elimination in the presence of Zn, resulting in clean formation
(13) For an example of a similar functional group transformation, see: Yadav,
J. S.; Srihari, P. Tetrahedron: Asymmetry 2004, 15, 81–89, and references therein.
(14) For similar examples of intramolecular oxymercuration-oxidation
protocol, see:(a) Hill, C. L.; Whitesides, G. M. J. Am. Chem. Soc. 1974, 96,
1269–1278. (b) Khalaf, J. K.; Datta, A. J. Org. Chem. 2004, 69, 387–390.
(12) For examples of m-CPBA epoxidation of acyclic chiral allylic amides
and stereoelectivities thereof, see: Roush, W. R.; Straub, J. A.; Brown, R. J. J.
Org. Chem. 1987, 52, 5127–5136.
J. Org. Chem. Vol. 73, No. 15, 2008 5979

Khalaf et al.
SCHEME 2
FIGURE 3. Diagnostic NOE correlations and coupling constants of
compound 17.
indicated a cis-relationship between these protons, thereby
confirming the assigned stereochemistry at the C-6′ and C-7′
chiral centers (Figure 3). Additionally, the observed coupling
constant between H-5′ and H-6′ (J ) 5.7 Hz) substantiated the
assigned trans-relationship between these two protons.
The observed stereoselectivity in the above intramolecular
oxymercuration (14 f 15) is probably attributable to the
sterically more favorable chairlike transition state II (Figure 4)
leading to the stereoselective formation of the desired trans-
fused furopyran bicyclic derivative 15.
In summary, employing D-serine as a chiral building block,
a novel synthetic route to an appropriately functionalized octosyl
nucleoside derivative of the ezomycins has been developed. The
present approach represents the first instance wherein the octosyl
nucleoside component has been constructed starting from a
noncarbohydrate precursor. An advantage of this de novo
approach is expected to be increased flexibility in terms of
potential SAR modifications at the carbohydrate core as well
as in the creation of non-natural stereocenter containing deriva-
tives. In future studies, extension of the present route toward
incorporation of cytosine nucleobase, and completion of the total
synthesis via glycosidic attachment of ezoaminuroic acid at C-6′
will be pursued.
FIGURE 4. Probable mechanistic pathway leading to the stereoselective
formation of compound 15.
Experimental Section
(2R,3R,4R,5R)-2-((R)-1-(Benzyloxycarbonylamino)-2-hydroxy-
ethyl)-5-(5-methyl-2,4-dioxo-3,4-dihydropyrimidin-1(2H)-yl)tet-
rahydrofuran-3,4-diyl diethanoate (6). To a stirring, room tem-
perature solution of the triacetate 56a (2.30 g, 4.80 mmol) in
anhydrous CH2Cl2 (65 mL) under N2 atmosphere, freshly prepared
bis-silylated thymine (3.25 g, 12.0 mmol) was added, and the
resulting suspension stirred for 5 min. Freshly distilled TMSOTf
(4.34 mL, 24.0 mmol) was then added to the reaction mixture,
resulting in a clear solution. After stirring at ambient temperature
for 2 h, the reaction was quenched with a saturated solution of
NaHCO3 (10 mL). The two layers were separated, and the aqueous
layer was further extracted with CHCl3 (3 × 10 mL). The combined
organic layer was then dried over anhydrous Na2SO4 and concen-
trated under vacuum to afford the crude product. Purification by
flash chromatography (hexane/EtOAc ) 2:3 to 1:5) afforded the
amino alcohol 6 as a white foamy solid (2.11 g, 87%): mp )
98-100 °C; [R]D ) -1.20 (c 1.00, CHCl3). IR (NaCl) 3308, 1749,
Toward formation of the 7′-carboxylic acid substituent,
exhaustive oxidation of the C-7′ hydroxymethylene functionality
of 15 was next undertaken. Initial attempts at one-step, selective
oxidation of the primary hydroxy group in presence of the free
C-2′ secondary hydroxy,5f were however not successful. Sub-
sequently, following a selective protection-deprotection pro-
tocol, silyl protection of the primary hydroxy group of 15 to
form the di-TBS-derivative 18, followed by acetyl protection
of the secondary 2′-hydroxy group resulted in the corresponding
fully protected bicyclic nucleoside 19 (Scheme 2). Selective
deprotection15 of the primary hydroxyl to unmask the mono-
hydroxy compound 20 and oxidation of the hydroxy group to
carboxylic acid and its subsequent esterification furnished the
fully protected thymine octosyl nucleoside derivative 21 in good
overall yield.
The assigned structure and stereochemical integrity of 21 was
ascertained by high-resolution NMR studies. Similar to the
earlier observations as in compound 17, strong NOE interactions
were observed between H-3′/H-7′ and H-6′/H-7′. Additionally,
conformity of the NMR data of 21 with structurally similar
octosyl nucleosides reported in the literature, reconfirmed the
assigned structure and stereochemistry of 21 and its precursors.
1
1697 cm-1. H NMR (400 MHz, CDCl3) δ 1.91 (s, 3H), 2.09 (s,
3H), 2.10 (s, 3H), 2.94 (br s, 1H exchangeable with D2O),
3.78-3.81 (m, 1H), 3.91-3.93 (m, 1H), 4.03-4.08 (m, 1H), 4.26
(t, J ) 5.1 Hz, 1H), 5.14 (br s, 2H), 5.42 (t, J ) 5.8 Hz, 1H), 5.55
(t, J ) 5.6 Hz, 1H), 5.78 (br d, J ) 7.9 Hz, 2H), 7.10 (br s, 1H),
7.28-7.37 (m, 5H), 9.10 (br s, 1H exchangeable with D2O). 13C
NMR (125.8 MHz, CDCl3) δ 12.4, 20.4, 20.5, 53.4, 61.7, 67.2,
70.8, 72.5, 82.0, 88.9, 112.0, 128.0, 128.3, 128.6, 136.1, 136.4,
150.4, 163.1, 169.8, 170.1. HRMS (ES+) calcd for C23H27N3O10
m/z (M + H)+, 506.1775; found, 506.1753.
(15) Nelson, T. D.; Crouch, R. D. Synthesis 1996, 1031–1069.
5980 J. Org. Chem. Vol. 73, No. 15, 2008

Synthetic Studies on Ezomycins
(2R,3R,4R,5R)-2-((R,E)-1-(Benzyloxycarbonylamino)-4-methoxy-
4-oxobut-2-enyl)-5-(5-methyl-2,4-dioxo-3,4-dihydropyrimidin-1(2H)-
yl)tetrahydrofuran-3,4-diyl diethanoate (7). Step 1. Dess-Martin
periodinane (15% in CH2Cl2 solution, 5.75 mL, 2.04 mmol) was
added dropwise to an ice-cooled solution of the amino alcohol 6
(0.856 g, 1.69 mmol) dissolved in CH2Cl2 (33 mL), and the reaction
mixture was stirred at 0 °C for another 30 min. The reaction was
then allowed to attain room temperature, and the stirring continued
for another 2 h. The reaction was quenched by the addition of a
solution of 10 mL of saturated aqueous NaHCO3 containing 1.5 g
of Na2S2O3. The layers were separated, and the aqueous layer was
extracted with CH2Cl2 (3 × 100 mL). The combined organic layer
was dried over anhydrous Na2SO4, and removal of solvent under
vacuum resulted in a colorless oily residue, which was used directly
for the subsequent reaction.
layer was extracted with CH2Cl2 (3 × 10 mL). The combined
organic extracts were dried over anhydrous Na2SO4 and concen-
trated under vacuum to give the crude product. Purification by
column chromatography (hexane/EtOAc ) 2:3) afforded the TBS
compound 9 as a white foamy solid (1.26 g, 87%): mp ) 92-94
°C; [R]D ) 2.95 (c 1.11, CHCl3). IR (NaCl) 3306, 1751, 1697 cm-1
.
1H NMR (400 MHz, CDCl3) δ 0.08 (s, 6H), 0.91 (s, 9H), 1.91 (s,
3H), 2.09 (s, 6H), 4.18-4.22 (m, 3H), 4.63 (br s, 1H), 5.14 (br s,
2H), 5.25 (t, J ) 5.9 Hz, 1H), 5.32-5.37 (m, 2H), 5.72 (dd, J )
6.3 and 15.4 Hz, 1H), 5.91 (br d, J ) 15.4 Hz, 1H), 5.99 (d, J )
5.4 Hz, 1H), 7.05 (s, 1H), 7.36 (br s, 5H), 8.99 (s, 1H). 13C NMR
(125.8 MHz, CDCl3) δ -5.3, 12.6, 18.4, 20.3, 20.4, 25.9, 53.5,
62.6, 67.1, 69.6, 72.3, 82.9, 87.1, 112.1, 123.2, 128.1, 128.2, 128.6,
134.3, 135.1, 136.2, 150.2, 155.8, 163.1, 169.5. HRMS (ES+) calcd
for C31H43N3O10Si m/z (M + Na)+, 668.2615; found, 668.2511.
(2R,3R,4R,5R)-2-((R)-(Benzyloxycarbonylamino)((2R,3R)-3-((tert-
butyldimethylsilyloxy)methyl)oxiran-2-yl)methyl)-5-(5-methyl-2,4-
dioxo-3,4-dihydro-pyrimidin-1-(2H)-yl)tetrahydrofuran-3,4-diyl di-
ethanoate (10). m-CPBA (0.856 g, 3.72 mmol) and NaHCO3 (0.470
g, 5.58 mmol) were added to a solution of the TBS compound 9
(1.20 g, 1.86 mmol) dissolved in anhydrous CH2Cl2 (45 mL). The
reaction mixture was stirred at room temperature overnight and then
concentrated under vacuum to give the crude product. Purification
by column chromatography (hexane/EtOAc ) 2:3 to 3:2) afforded
the epoxide 10 as a white foamy solid (1.10 g, 90%): mp ) 92-94
Step 2. The crude aldehyde obtained from the above reaction
was dissolved in anhydrous CH2Cl2 (32 mL) and cooled to 0 °C.
The Wittig reagent (0.68 g, 2.03 mmol) was then added to the
reaction mixture and stirred for 2 h. Removal of solvent under
vacuum and purification of the residue by flash chromatography
(CHCl3/MeOH ) 99:1 to 95:5) yielded the E-ester 7 as a white
foamy solid (0.891 g, 78% over two steps): mp ) 102 - 104 °C;
[R]D ) -0.66 (c 0.80, CHCl3). IR (NaCl) 3306, 1751, 1697 cm-1
.
1H NMR (400 MHz, CDCl3) δ 1.92 (s, 3H), 2.07 (s, 3H), 2.12 (s,
3H), 3.77 (s, 3H), 4.24 (t, J ) 5.4 Hz, 1H), 4.81 (br s, 1H), 5.13
(s, 2H), 5.34-5.42 (m, 2H), 5.64 (d, J ) 8.7 Hz, 1H), 5.78 (d, J
) 4.2 Hz, 1H), 6.13 (d, J ) 15.7 Hz, 1H), 6.90 - 6.96 (m, 2H),
7.36 (br s, 5H), 8.84 (s, 1H). 13C NMR (100.6 MHz, CDCl3) δ
12.8, 20.7, 20.8, 52.3, 53.4, 67.6, 69.9, 73.1, 82.3, 89.8, 112.5,
123.8, 128.4, 128.6, 128.9, 129.4, 136.4, 136.7, 142.7, 151.0, 156.5,
164.2, 166.5, 170.0, 170.1. HRMS (ES+) calcd for C26H29N3O11
m/z (M + Na)+, 582.1700; found, 582.1710.
°C; [R]D ) 2.33 (c 1.30, CHCl3). IR (NaCl) 3304, 1751, 1697 cm-1
.
1H NMR (400 MHz, CDCl3, diastereomeric mixtures 1:5) δ 0.07
(s, 6H), 0.89 (s, 9H), 1.93 (s, 3H), 2.08 (s, 3H), 2.10 (s, 3H), 3.02
(br s, 1H), 3.24 (s, 1H), 3.67 (dd, J ) 4.5 and 12.2 Hz, 1H), 3.94
(br d, J ) 11.0 Hz, 1H), 4.20-4.25 (m, 1H), 4.40-4.44 (m, 1H),
5.12 (s, 2H), 5.16 (br d, J ) 9.7 Hz, 1H), 5.37 (t, J ) 5.8 Hz,
5/6H), 5.41 (t, J ) 5.8 Hz, 1/6H), 5.48 (t, J ) 5.8 Hz, 1H), 5.55
(t, J ) 5.8 Hz, 1/6H), 5.93 (d, J ) 5.2 Hz, 5/6H), 5.97 (d, J ) 5.2
Hz, 1/6H), 7.15 (s, 5/6H), 7.21 (s, 1/6H), 7.36 (br s, 5H), 8.82 (s,
5/6H exchangeable with D2O), 8.86 (s, 1/6H). 13C NMR (125.8
MHz, CDCl3, diastereomeric mixtures) δ -5.4, 12.5, 12.4, 18.3,
20.3, 20.4, 25.8, 50.2, 52.8, 53.4, 55.3, 62.1, 62.2, 67.4, 70.0, 72.4,
81.7, 82.1, 87.8, 88.3, 112.0, 112.2, 128.1, 128.2, 128.3, 128.6,
130.2, 133.3, 135.7, 135.9, 150.2, 150.4, 156.3, 163.3, 163.4, 169.4,
169.5, 169.6. HRMS (ES+) calcd for C31H43N3O11Si m/z (M +
Na)+, 684.2565; found, 684.2583.
(2R,3R,4R,5R)-2-(5-Methyl-2,4-dioxo-3,4-dihydropyrimidin-1(2H)-
yl)-5-((R,E)-10,10,11,11-tetramethyl-3-oxo-1-phenyl-2,9-dioxa-4-
aza-10-siladodec-6-en-5-(yl)-tetrahydrofuran-3,4-diyl diethanoate
(8). The ester 7 (1.86 g, 3.33 mmol) was dissolved in anhydrous
THF (80 mL) and cooled to -78 °C. To this stirring solution,
DIBAL-H (1 M in toluene, 18.3 mL, 5.5 mmol) was added
dropwise, and the reaction was stirred for 2 h at the same
temperature. The reaction was quenched by careful addition of
MeOH (3 mL) and then allowed to attain room temperature,
followed by dilution with EtOAc (30 mL) and saturated aqueous
sodium potassium tartrate (30 mL). The resulting mixture was
stirred until two clear layers were seen. The layers were separated,
and the aqueous layer was extracted with EtOAc (3 × 20 mL).
The combined organic extract was dried over anhydrous Na2SO4
and concentrated under vacuum to give the crude product. Purifica-
tion by column chromatography (hexane/EtOAc ) 9:1) afforded
the allyl alcohol 8 as a white foamy solid (1.15 g, 71% based on
(2R,3R,4R,5R)-2-((R)-(Benzyloxycarbonylamino)((2R,3R)-3-(hy-
droxymethyl)oxiran-2-yl)methyl)-5-(5-methyl-2,4-dioxo-3,4-dihy-
dropyrimidin-1(2H)-yl)tetra-hydro-furan-3,4-diyl diethanoate (11).
To an ice-cooled solution of the epoxide 10 (1.12 g, 1.74 mmol),
in anhydrous THF (25 mL) was added TBAF (1 M in THF, 3.65
mL, 3.65 mmol) dropwise. The reaction mixture was stirred at 0
°C for 1.5 h and then quenched by water (5 mL); the two layers
were separated, and the aqueous layer was extracted with EtOAc
(3 × 10 mL). The combined organic layer was dried over anhydrous
Na2SO4 and concentrated under vacuum to give the crude product.
Purification by column chromatography (hexane/EtOAc ) 1:5)
afforded the epoxy alcohol as a white foamy solid 11 (0.855 g,
90%): mp ) 108-110 °C; [R]D ) 1.82 (c 1.10, CHCl3). IR (NaCl)
recovered starting material 0.18 g): mp ) 100 - 102 °C; [R]D
)
2.70 (c 1.00, CHCl3). IR (NaCl) 3308, 1751, 1697 cm-1. 1H NMR
(400 MHz, CDCl3) δ 1.92 (s, 3H), 2.10 (s, 6H), 2.45 (br s, 1H
exchangeable with D2O), 4.19 - 4.24 (2 br s, 3H), 4.58 (br s, 1H),
5.12 (s, 2H), 5.40 (d, J ) 3.2 Hz, 2H), 5.52 (br d, J ) 7.9 Hz, 1H),
5.72-5.78 (m, 2H), 5.99 (br d, J ) 15.4 Hz, 1H), 7.05 (s, 1H),
7.35 (br s, 5H), 9.24 (s, 1H exchangeable with D2O). 13C NMR
(125.8 MHz, CDCl3) δ 12.3, 20.4, 20.5, 53.5, 53.6, 62.0, 67.1, 69.7,
72.6, 82.9, 89.8, 112.0, 124.6, 128.1, 128.2, 128.6, 134.2, 136.2,
136.8, 150.3, 155.9, 163.4, 169.7, 169.9. HRMS (ES+) calcd for
C25H29N3O10 m/z (M + Na)+, 554.1751; found, 554.1729.
1
3306, 1751, 1697 cm-1. H NMR (400 MHz, CDCl3) δ 1.89 (s,
3H), 2.07 (s, 3H), 2.09 (s, 3H), 2.93 (br s, 1H exchangeable with
D2O), 3.12 (d, J ) 2.1 Hz, 1H), 3.38 (br s, 1H), 3.77 (br d, J )
12.6 Hz, 1H), 3.87 (br d, J ) 12.2 Hz, 1H), 4.26 (d, J ) 5.7 Hz,
2H), 5.11 (s, 2H), 5.48 - 5.51 (m, 1H), 5.56-5.63 (m, 2H),
5.68-5.76 (m, 1H), 7.09 (s, 1H), 7.34 (br s, 5H), 9.38 (s, 1H
exchangeable with D2O). 13C NMR (125.8 MHz, CDCl3) δ 12.3,
20.3, 20.4, 51.0, 53.2, 56.1, 60.3, 67.3, 69.9, 72.8, 81.8, 90.5, 111.9,
128.1, 128.3, 128.6, 136.1, 137.1, 150.4, 156.4, 163.6, 169.7, 169.8.
HRMS (ES+) calcd for C25H29N3O11 m/z (M + Na)+, 570.1702;
found, 570.1700.
(2R,3R,4R,5R)-2-(5-Methyl-2,4-dioxo-3,4-dihydropyrimidin-1(2H)-
yl)-5-((R,E)-10,10,11,11-tetramethyl-3-oxo-1-phenyl-2,9-dioxa-4-
aza-10-siladodec-6-en-5-yl)-tetrahydrofuran-3,4-diyl diethanoate
(9). The allylic alcohol 8 (1.19 g, 2.23 mmol) was dissolved in
anhydrous CH2Cl2 (50 mL), followed by sequential addition of
imidazole (0.61 g, 8.92 mmol), DMAP (25 mg, catalytic), and
TBSCl (1.10 g, 6.69 mmol). The reaction mixture was allowed to
stir at room temperature for 3 h and then quenched with ice-cooled
water (10 mL). The two layers were separated and the aqueous
(2R,3R,4R,5R)-2-((1R,2S)-1-(Benzyloxycarbonylamino)-2-hy-
droxybut-3-enyl)-5-(5-methyl-2,4-dioxo-3,4-dihydropyrimidin-1-
(2H)-yl)tetrahydrofuran-3,4-diyl diethanoate (12). Step 1. The
epoxy alcohol 11 (0.820 g, 1.50 mmol) was dissolved in a mixture
J. Org. Chem. Vol. 73, No. 15, 2008 5981

Khalaf et al.
of Et2O/CH3CN (3:1, 11 mL) and cooled to 0 °C. To this stirring
solution, imidazole (0.286 g, 4.20 mmol), triphenylphosphine (0.866
g, 3.30 mmol) and iodine (0.838 g, 3.30 mmol) were added
sequentially. After stirring at 0 °C for 20 min, the reaction mixture
was diluted with EtOAc (10 mL) and 5% aqueous HCl (5 mL).
The two layers were separated, and the aqueous layer was extracted
with EtOAc (3 × 5 mL). The combined organic layer was dried
over anhydrous Na2SO4 and concentrated under vacuum to give
the crude iodo derivative, which was taken to the next step without
Benzyl (1S,2S)-2-(tert-butyldimethylsilyloxy)-1-((2R,3S,4R,5R)-
3,4-dihydroxy-5-(5-methyl-2,4-dioxo-3,4-dihydropyrimidin-1(2H)-
yl)tetrahydrofuran-2-yl)but-3-enyl-carbamate (14). Powdered
K2CO3 (0.131 g, 0.95 mmol) was added to an ice-cooled solution
of the diacetate 13 (0.480 g, 0.74 mmol) dissolved in anhydrous
MeOH (25 mL). After stirring the reaction mixture at 0 °C for 3.5 h,
solvent was removed under vacuum to give the crude product.
Purification by column chromatography (hexane/EtOAc ) 1:9)
afforded the diol as a white foamy solid 14 (0.353 g, 85%): mp )
88-90 °C; [R]D ) -6.09 (c 1.20, CHCl3). IR (NaCl) 3431, 1697
1
further purification: H NMR (400 MHz, CDCl3) δ 1.93 (s, 3H),
1
cm-1. H NMR (400 MHz, CDCl3) δ 0.04 (s, 3H), 0.09 (s, 3H),
2.06 (s, 3H), 2.10 (s, 3H), 3.14-3.19 (m, 4H), 4.17 (t, J ) 5.7 Hz,
1H), 4.37 (t, J ) 7.2 Hz, 1H), 5.04 (d, J ) 8.9 Hz, 1H), 5.11 (s,
2H), 5.40 (t, J ) 5.6 Hz, 1H), 5.48 (t, J ) 5.9 Hz, 1H), 5.85 (d, J
) 4.6 Hz, 1H), 7.13 (s, 1H), 7.35 (s, 5H), 8.27 (s, 1H). 13C NMR
(100.6 MHz, CDCl3) δ 3.7, 12.9, 14.6, 20.8, 20.9, 23.1, 32.0, 51.0,
55.6, 60.2, 67.8, 70.5, 72.8, 82.0, 89.3, 112.4, 128.6, 128.8, 129.0,
136.2, 136.6, 150.7, 156.6, 163.9, 170.0, 170.1.
0.92 (s, 9H), 1.92 (s, 3H), 3.68 (br s, 1H exchangeable with D2O),
3.79 (t, J ) 9.4 Hz, 1H), 3.97 (br d, J ) 9.3 Hz, 1H), 4.34 (br s,
2H), 4.49 (d, J ) 5.3 Hz, 1H), 4.72 (br s, 1H exchangeable with
D2O), 5.12-5.37 (m, 5H), 5.79-5.87 (m, 2H), 7.16 (s, 1H),
7.28-7.36 (m, 5H), 9.84 (br s, 1H exchangeable with D2O). 13C
NMR (125.8 MHz, CDCl3) δ -5.1, -4.3, 12.5, 18.2, 25.8, 25.9,
57.5, 67.2, 71.3, 71.6, 73.8, 82.7, 90.8, 111.5, 116.5, 128.0, 128.2,
128.6, 136.2, 136.3, 137.9, 151.0, 157.2, 164.0. HRMS (ES+) calcd
for C27H39N3O8Si m/z (M + Na)+, 584.2404; found, 584.2422.
Benzyl (2R,3R,3aS,5R,6R,7S,7aR)-6-(tert-butyldimethylsilyloxy)-
3-hydroxy-5-(hydroxymethyl)-2-(5-methyl-2,4-dioxo-3,4-dihydro-
pyrimidin-1(2H)-yl)hexa-hydro-2H-furo[3,2-b]pyran-7-ylcarbam-
ate (15) and Benzyl (2R,3R,3aS,5R,6R,7S,7aR)-6-(tert-butyldimethyl-
silyloxy)-3-hydroxy-5-methyl-2-(5-methyl-2,4-dioxo-3,4-dihydropy-
rimidin-1(2H)-yl)hexahydro-2H-furo [3,2-b]pyran-7-ylcarbamate
(16). Step 1. To a stirred solution of the diol 14 (0.938 g, 1.67
mmol) in CH3CN (100 mL), mercuric(II) trifluoroacetate (0.912 g,
5.28 mmol) was added, and the mixture was refluxed overnight.
The reaction was cooled to room temperature and diluted by the
addition of EtOAc (50 mL) and brine (50 mL). The resulting
biphasic mixture was stirred at room temperature for 3 h. The
organic layer was separated and the aqueous layer was extracted
with EtOAc (3 × 40 mL). The combined organic extract was dried
over anhydrous Na2SO4, and solvent was removed in vacuo to give
a white foamy solid which was used as such for the subsequent
reaction.
Step 2. To a well-stirred solution of NaBH4 (0.20 g, 5.29 mmol)
in DMF (9 mL) at room temperature, oxygen (O2) gas was bubbled
for 1 h. To this mixture was added dropwise over 2 h, a DMF
solution (9 mL) of the crude mercuric compound (0.828 g) obtained
from the earlier step, with continuous bubbling of O2. After being
stirred for a further 8 h, the reaction mixture was filtered through
celite, the residue was washed thoroughly with EtOAc (4 × 20
mL), and the filtrate was concentrated under vacuum. Purification
of the crude residue by flash chromatography (hexane/MeOH/
EtOAc ) 18:2:80) yielded the bicyclic diol 15 and minor quantities
of the corresponding methylated product 16.
15 (major product). Obtained as a white foamy solid (0.501 g,
52%): mp ) 156-158 °C; [R]D ) -3.12 (c 0.96, CHCl3). IR
(NaCl) 3339, 1697 cm-1. 1H NMR (400 MHz, CD3OD) δ 0.09 (s,
3H), 0.20 (s, 3H), 0.91 (s, 9H), 1.69 (s, 3H), 3.26 - 3.28 (m, 2H),
3.57-3.71 (m, 3H), 3.81-3.85 (m, 2H), 4.20 (d, J ) 5.1 Hz, 1H),
4.33-4.35 (br m, 1H), 4.24 (br s, 1H), 5.04-5.14 (m, 2H), 5.63
(s, 1H), 7.25-7.36 (m, 6H). 13C NMR (125.8 MHz, CD3OD) δ
-5.1, -4.3, 12.7, 19.0, 26.4, 54.7, 62.9, 68.2, 71.9, 74.0, 74.7,
74.8, 80.2, 95.0, 111.5, 129.4, 129.7, 138.1, 138.4, 152.2, 158.7,
166.5. HRMS (ES+) calcd for C27H39N3O9Si m/z (M + Na)+,
600.2353; found, 600.2346.
Step 2. The crude iodo compound obtained from the above
reaction was dissolved in anhydrous MeOH (85 mL). To this stirring
solution, AcOH (15 mL) and Zn (3.9 g, 60 mmol) were added,
and the reaction was heated to 37 °C. After stirring at the same
temperature overnight, solvent was removed under vacuum and the
residue was diluted with EtOAc (50 mL) and filtered. The filtrate
was washed sequentially with saturated aqueous solution of
NaHCO3 and saturated solution of Na2S2O3. The organic layer was
dried over Na2SO4 and concentrated under vacuum to give the crude
product. Purification by column chromatography (CHCl3/MeOH )
99:1 to 95:5) afforded the allylic alcohol 12 as a white foamy solid
(0.590 g, 74% over two steps): mp ) 90-92 °C; [R]D ) -2.13 (c
1
1.00, CHCl3). IR (NaCl) 3308, 1751, 1695 cm-1. H NMR (400
MHz, CDCl3) δ 1.90 (s, 3H), 2.09 (s, 6H), 3.00 (br s, 1H
exchangeable with D2O), 4.00 (t, J ) 7.6 Hz, 1H), 4.24-4.29 (m,
1H), 4.55 (br s, 1H), 5.14 (br s, 2H), 5.23 (d, J ) 10.4 Hz, 1H),
5.36 - 5.40 (m, 2H), 5.50-5.53 (m, 1H), 5.59 (d, J ) 8.9 Hz,
1H), 5.84-5.92 (m, 1H), 5.95 (d, J ) 5.6 Hz, 1H), 7.14 (s, 1H),
7.35 (br s, 5H), 8.94 (br s, 1H exchangeable with D2O). 13C NMR
(125.8 MHz, CDCl3) δ 12.4, 20.4, 20.5, 56.1, 67.3, 70.2, 71.3, 72.3,
82.2, 87.5, 112.1, 116.8, 117.8, 127.9, 128.2, 128.3, 128.6, 135.8,
136.2, 136.8, 150.5, 157.1, 163.4, 169.8, 170.1. HRMS (ES+) calcd
for C25H29N3O10 m/z (M + Na)+, 554.1751; found, 554.1719.
(2R,3R,4R,5R)-2-(5-Methyl-2,4-dioxo-3,4-dihydropyrimidin-1(2H)-
yl)-5-((5S,6S)-8,8,9,9-tetramethyl-3-oxo-1-phenyl-6-vinyl-2,7-dioxa-
4-aza-8-siladecan-5-yl)tetra-hydrofuran-3,4-diyl diethanoate (13).
The allylic alcohol 12 (0.550 g, 1.036 mmol) was dissolved in
anhydrous DMF (20 mL), and to this solution were added
sequentially imidazole (0.353 g, 5.18 mmol), DMAP (25 mg,
catalytic), and TBSCl (0.624 g, 4.14 mmol). The reaction mixture
was allowed to stir at room temperature overnight. The solvent was
concentrated under high vacuum, and the resulting residue was
dissolved in CH2Cl2 (20 mL) and poured into 10 mL of H2O. The
two layers were separated, and the aqueous layer was extracted
with CH2Cl2 (3 × 5 mL). The combined organic layer was dried
over anhydrous Na2SO4 and concentrated to give the crude product.
Purification by column chromatography (hexane/EtOAc ) 1:1)
afforded the TBS-protected product 13 as a white solid (0.521 g,
78%): mp ) 140-142 °C; [R]D ) -2.63 (c 1.00, CHCl3). IR
1
(NaCl) 3219, 1753, 1697 cm-1. H NMR (400 MHz, CDCl3) δ
0.04 (s, 3H), 0.10 (s, 3H), 0.93 (s, 9H), 1.99 (s, 3H), 2.05 (s, 3H),
2.08 (s, 3H), 3.87 (t, J ) 9.4 Hz, 1H), 4.02-4.05 (m, 1H), 4.47 (d,
J ) 5.4 Hz, 1H), 5.10-5.30 (m, 5H), 5.42 (t, J ) 6.2 Hz, 1H),
5.46-5.49 (m, 1H), 5.78-5.87 (m, 1H), 6.06 (d, J ) 6.1 Hz, 1H),
7.09 (s, 1H), 7.33-7.41 (m, 5H), 8.30 (br s, 1H). 13C NMR (125.8
MHz, CDCl3) δ -5.1, -4.4, 12.6, 18.2, 20.4, 25.9, 57.4, 67.3, 71.2,
71.4, 71.9, 80.2, 86.7, 112.3, 116.9, 128.0, 128.2, 128.3, 128.6,
134.9, 136.1, 137.4, 150.1, 156.8, 163.0, 169.2, 169.4. HRMS
(ES+) calcd for C31H43N3O10Si m/z (M + Na)+, 668.2616; found,
668.2601.
16 (minor product). Obtained as a white foamy solid (0.065 g,
7%): mp ) 120-124 °C; [R]D ) 1.90 (c 1.0, CHCl3). IR (NaCl)
1
3339, 1701 cm-1. H NMR (400 MHz, CDCl3) δ 0.13 (s, 3H),
0.27 (s, 3H), 0.95 (s, 9H), 1.22 (d, J ) 5.9 Hz, 3H), 1.85 (s, 3H),
2.93 (br s, 1H exchangeable with D2O), 3.90 (br s, 1H), 4.01 (d, J
) 6.2 Hz 1H), 4.25-4.28 (m, 2H), 4.48 (d, J ) 3.8 Hz, 1H),
5.09-5.19 (m, 3H), 5.62 (br s, 1H), 7.00 (s, 1H), 7.32 (m, 5H),
9.34 (br s, 1H). 13C NMR (125.8 MHz, CDCl3) δ -5.0, -4.6, 12.2,
14.1, 16.7, 18.0, 22.7, 25.8, 29.4, 29.7, 53.8, 55.4, 66.9, 70.9, 72.4,
73.0, 73.2, 74.4, 98.6, 111.2, 127.7, 128.2, 128.4, 128.5, 128.7,
5982 J. Org. Chem. Vol. 73, No. 15, 2008

Synthetic Studies on Ezomycins
136.1, 139.7, 150.1, 156.5, 163.5. HRMS (ES+) calcd for
C27H39N3O8Si m/z (M + Na)+, 584.2404; found, 584.2408.
(2R,3R,3aR,5R,6R,7S,7aR)-7-(Benzyloxycarbonylamino)-5-(etha-
noyloxymethyl)-2-(5-methyl-2,4-dioxo-3,4-dihydropyrimidin-1(2H)-
yl)hexahydro-2H-furo[3,2-b]-pyran-3,6-diyl diethanoate (17).
Step 1. To a stirred, ice-cooled solution of the diol 15 (0.042 g,
0.07 mmol) in anhydrous THF (3 mL), was added TBAF (1 M in
THF, 0.1 mL, 0.1 mmol). The reaction mixture was stirred at the
same temperature for 1.5 h, followed by quenching of the reaction
by addition of H2O (0.5 mL). The two layers were separated and
the aqueous layer was extracted with EtOAc (3 × 1 mL). The
combined organic layer was dried over anhydrous Na2SO4 and
concentrated under vacuum to give the crude triol, which was used
as such for the subsequent reaction.
(3 × 5 mL). The combined organic extracts were washed with
saturated aqueous NaHCO3, dried over anhydrous Na2SO4, and
concentrated under vacuum to give the crude product. Purification
by column chromatography afforded the acetate 19 as a white foamy
solid (0.215 g, 88%): mp ) 120-122 °C; [R]D ) 0.9 (c 0.64,
CHCl3). IR (NaCl) 3306, 1699 cm-1. 1H NMR (400 MHz, CDCl3)
δ 0.02 (s, 6H), 0.15 (br s, 3H), 0.27 (br s, 3H), 0.90 and 0.95 (2s,
18H), 1.90 (s, 3H), 2.17 (s, 3H), 3.73 (br s, 2H), 3.80 (t, J ) 6.2
Hz, 1H), 4.22-4.35 (m, 4H), 5.13-5.19 (m, 3H), 5.36 (d, J ) 5.5
Hz, 1H), 5.55 (br s, 1H), 6.94 (s, 1H), 7.35 (s, 5H), 8.58 (br s,
1H). 13C NMR (201.2 MHz, CDCl3) δ -5.1, -5.0, -4.9, -4.4,
12.5, 18.2, 18.6, 21.0, 25.8, 26.0, 26.1, 53.5, 62.2, 67.0, 68.0, 71.9,
74.1, 80.0, 96.5, 111.6, 128.3, 128.4, 128.7, 136.4, 139.4, 150.0,
156.6, 163.5, 170.9. HRMS (ES+) calcd for C35H55N3O10Si2 m/z
(M + Na)+, 756.3324; found, 756.3342.
Step 2. The triol from the above reaction was dissolved in
anhydrous CH2Cl2 (2 mL), followed by sequential addition of
anhydrous pyridine (0.018 mL, 0.222 mmol), DMAP (10 mg,
catalytic), and Ac2O (0.021 mL, 0.222 mmol). After stirring at room
temperature for 2 h, the reaction was quenched by addition of ice-
cooled water (0.5 mL). The two layers were separated and the
aqueous layer was extracted with CH2Cl2 (3 × 1 mL). The
combined organic extracts were washed with saturated aqueous
NaHCO3, dried over anhydrous Na2SO4, and concentrated under
vacuum to give the crude product. Purification by column chro-
matography (hexane/EtOAc ) 2:3) afforded the triacetate 17 as a
white foamy solid (0.036 g, 89% over two steps): mp ) 150-152
(2R,3R,3aR,5R,6R,7S,7aR)-7-(Benzyloxycarbonylamino)-6-(tert-
butyldimethyl silyloxy)-5-(hydroxymethyl)-2-(5-methyl-2,4-dioxo-
3,4-dihydropyrimidin-1(2H)-yl-hexahydro-2H-furo[3,2-b]pyran-3-
yl ethanoate (20). The di-TBS derivative 19 (0.200 g, 0.273 mmol)
was dissolved in a mixture of CH2Cl2/MeOH (1:1, 12 mL) and
cooled to 0 °C. To the stirring solution was added camphor sulfonic
acid (0.031 g, 0.133 mmol) and the mixture was stirred at the same
temperature for 3 h. Removal of solvent under reduced pressure
and purification of the crude residue by column chromatography
afforded the monohydroxy product 20 as a white foamy solid (0.159
g, 94%): mp ) 140-142 °C; [R]D ) 2.70 (c 0.60, CHCl3). IR
°C; [R]D 1.13 (c 0.80, CHCl3). IR (NaCl) 3329, 1747, 1697 cm-1
.
1
(NaCl) 3263, 1697 cm-1. H NMR (400 MHz, CDCl3) δ 0.13 (br
1H NMR (400 MHz, CDCl3) δ 1.88 (s, 3H), 2.08 (s, 3H), 2.17 (s,
3H), 2.23 (s, 3H), 4.10-4.30 (m, 4H), 4.35 (br s, 1H), 4.58 (br s,
1H), 5.10-5.18 (m, 3H), 5.35-5.37 (m, 1H), 5.43 (br s, 1H), 5.88
(br s, 1H), 6.89 (s, 1H), 7.31 (s, 5H), 9.11 (s, 1H). 13C NMR (201.2
MHz, CDCl3) δ 12.3, 20.8, 20.9, 51.4, 62.3, 67.1, 68.2, 71.5, 73.9,
74.1, 74.9, 97.4, 111.6, 128.2, 128.3, 128.5, 136.1, 139.5, 150.3,
156.4, 163.5, 169.0, 170.7, 170.9. HRMS (ES+) calcd for
C27H31N3O12 m/z (M + H)+, 590.1986; found, 590.1982.
s, 3H), 0.27 (br s, 3H), 0.94 (s, 9H), 1.89 (s, 3H), 2.17 (s, 3H),
2.30 (s, 1H exchangeable with D2O), 3.60 (br s, 1H), 3.83-3.95
(m, 2H), 4.10 (br s, 1H), 4.20-4.28 (m, 2H), 4.43 (br s, 1H),
5.07-5.13 (m, 2H), 5.16 (br d, J ) 5.2 Hz, 1H), 5.44 (br d, J )
4.4 Hz, 1H), 5.81 (br s, 1H), 6.94 (s, 1H), 7.34 (s, 5H), 9.06 (br s,
1H exchangeable with D2O). 13C NMR (125.8 MHz, CDCl3) δ
-5.2, -4.6, 12.3, 17.8, 20.8, 25.6, 25.7, 29.7, 53.4, 62.9, 66.8,
68.8, 69.1, 71.6, 73.3, 73.7, 79.0, 96.4, 111.4, 128.1, 128.2, 128.5,
136.1, 139.3, 150.1, 156.5, 163.4, 170.8. HRMS (ES+) calcd for
C29H41N3O10Si m/z (M + Na)+, 642.2459; found, 642.2453.
(2R,3R,3aR,5S,6R,7S,7aR)-Methyl-7-(benzyloxycarbonylamino)-
6-(tert-butyldimethylsilyloxy)-3-(ethanoyloxy)-2-(5-methyl-2,4-di-
oxo-3,4-dihydropyrimidin-1(2H)-yl)hexahydro-2H-furo[3,2-b]pyran-
5-carboxylate (21). Step 1. Dess-Martin periodinane (15% in
CH2Cl2 solution, 0.86 mL, 0.302 mmol) was added to an ice-cooled
solution of the alcohol 20 (0.146 g, 0.232 mmol) in CH2Cl2 (10
mL), and the reaction mixture was stirred at 0 °C for 30 min. The
reaction was then allowed to attain room temperature and stirring
continued for another 2 h. The reaction was quenched by the
addition of a solution of 6 mL of saturated NaHCO3 containing
0.3 g of Na2S2O3. The layers were separated, the aqueous layer
was extracted with CH2Cl2 (3 × 10 mL), and the combined organic
extract was washed sequentially with 5.0 mL each of saturated
aqueous NaHCO3, H2O, and brine. Drying over anhydrous Na2SO4
and removal of solvent under vacuum resulted in a colorless oily
residue, which was used directly for the subsequent reaction.
Step 2. The crude aldehyde (0.12 g) as obtained above was
dissolved in tert-butanol (16.5 mL) followed by addition of a
solution of 2-methyl-2-butene (1.20 mL, 11.1 mmol). To the
resulting mixture at room temperature was added dropwise a
solution of NaClO2 (0.205 g, 2.27 mmol) and NaH2PO4 (0.212 g,
1.76 mmol) in H2O (4 mL). The reaction mixture was stirred at
room temperature for 30 min, partitioned with 2 mL of H2O, and
the layers were separated. The aqueous layer was extracted with
EtOAc (3 × 6 mL). The combined organic extract was dried over
anhydrous Na2SO4 and solvent was removed in vacuum to give
the acid derivative as a colorless viscous oil, which was used as
such for the subsequent esterification.
Benzyl (2R,3R,3aS,5R,6R,7S,7aR)-6-(tert-butyldimethylsilyloxy)-
5-((tert-butyl-dimethylsilyloxy)methyl)-3-hydroxy-2-(5-methyl-2,4-
dioxo-3,4-dihydropyrimidin-1(2H)-yl)hexahydro-2H-furo[3,2-b]py-
ran-7-ylcarbamate (18). To a room temperature solution of the diol
15 (0.362 g, 0.63 mmol) in anhydrous CH2Cl2 (15 mL), was added
sequentially imidazole (0.13 g, 1.9 mmol), 4-DMAP (5 mg,
catalytic), and TBSCl (0.12 g, 1.8 mmol). After stirring for 3 h,
the reaction was quenched by the addition of water (10 mL). The
two layers were separated and the aqueous layer was extracted with
CH2Cl2 (3 × 10 mL). The combined organic extract was dried over
anhydrous Na2SO4 and concentrated under vacuum to give the crude
product. Purification by column chromatography (hexane/EtOAc
) 2:3) afforded the primary TBS protected product 18 as a white
foamy solid (0.366 g, 84%): mp ) 120-122 °C; [R]D ) -0.70 (c
1
1.00, CHCl3). IR (NaCl) 3336, 1701 cm-1. H NMR (400 MHz,
CDCl3) δ 0.08 (s, 6H), 0.13 (br s, 3H), 0.27 (br s, 3H), 0.91 (2s,
18H), 1.89 (s, 3H), 2.67 (s, 1H exchangeable with D2O), 3.74 (br
s, 2H), 3.88 (t, J ) 5.6 Hz, 1H), 4.17-4.33 (m, 4H), 4.53 (d, J )
4.2 Hz, 1H), 5.13 (s, 2H), 5.22 (br s, 1H), 5.48 (br s, 1H), 7.03 (s,
1H), 7.35 (br s, 5H), 8.74 (br s, 1H exchangeable with D2O). 13C
NMR (201.2 MHz, CDCl3) δ -5.1, -4.3, 12.4, 18.1, 18.6, 26.0,
26.1, 53.8, 62.8, 67.1, 68.6, 72.4, 73.2, 73.6, 79.3, 98.4, 111.4,
128.4, 128.8, 139.6, 150.2, 156.6, 163.7. HRMS (ES+) calcd for
C33H53N3O9Si2 m/z (M + Na)+, 714.3218; found, 714.3223.
(2R,3R,3aR,5R,6R,7S,7aR)-7-(Benzyloxycarbonylamino)-6-(tert-
butyldimethyl-silyloxy)-5-((tert-butyldimethylsilyloxy)methyl)-2-(5-
methyl-2,4-dioxo-3,4-dihydro-pyrimidin-1(2H)-yl)hexahydro-2H-
furo[3,2-b]pyran-3-yl ethanoate (19). To a solution of the di-TBS
alcohol 18 (0.230 g, 0.333 mmol) in anhydrous CH2Cl2 (10 mL)
was added sequentially, anhydrous pyridine (0.067 mL, 0.717
mmol), DMAP (10 mg, catalytic), and Ac2O (0.064 mL, 0.792
mmol). After stirring at room temperature for 2 h, the reaction was
quenched by the addition of ice-cooled water (2 mL). The two layers
were separated and the aqueous layer was extracted with CH2Cl2
Step 3. [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
J. Org. Chem. Vol. 73, No. 15, 2008 5983

Khalaf et al.
precautions]. To a biphasic solution of KOH (0.500 g) in H2O (1
mL) and ether (1.0 mL) at 0 °C was added N-methyl-N′-nitro-N-
nitrosoguanidine (MNNG, 50% in H2O, 0.5 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 (0.11 g in 2 mL of ether) obtained
from step 2, 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 chro-
matography (hexane/EtOAc 7:3) yielded the protected octosyl
nucleoside derivative 21 as a white solid (0.101 g, 67% over three
steps): mp ) 142-144 °C; [R]D 2.83 (c 0.40, CHCl3). IR (NaCl)
-5.4, -4.4, 12.3, 17.8, 21.0, 25.4, 25.6, 52.3, 53.6, 67.0, 69.9,
71.2, 73.4, 74.1, 78.2, 97.7, 111.5, 111.6, 128.2, 128.5, 128.7, 136.0,
139.7, 150.2, 150.3, 156.7, 163.2, 163.3, 168.4, 171.4. HRMS
(ES+) calcd for C30H41N3O11Si m/z (M + Na)+, 670.2408; found,
670.2398. HPLC: Agilent, Zorbax SB-C18 5.0 µm, 10%-95%
CH3CN/H2O, 0.5 mL/min, 225 nm, retention time ) 14.19 min.
Acknowledgment. We thank the Herman Frasch Foundation
(American Chemical Society) for financial support. A predoc-
toral research fellowship to J.K.K. from the American Founda-
tion for Pharmaceutical Education (2006-2007) is also grate-
fully acknowledged.
1
3329, 1697 cm-1. H NMR (500 MHz, CDCl3) δ 0.08 (br s, 3H),
Supporting Information Available: General experimental
details and copies of NMR spectra (1H and 13C) of all the new
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
0.25 (br s, 3H), 0.89 (s, 9H), 1.89 (s, 3H), 2.22 (s, 3H), 3.77 (s,
3H), 4.24 (br s, 1H), 4.31 (br d, J ) 10 Hz, 1H), 4.54 (s, 1H),
4.55-4.58 (m, 2H), 4.99-5.13 (m, 2H), 5.16-5.19 (m, 1H), 5.30
(d, J ) 6.0 Hz, 1H), 5.78 (br d, J ) 5.2 Hz, 1H), 6.89 (s, 1H), 7.32
(s, 5H), 8.87 and 8.96 (2s, 1H). 13C NMR (125.8 MHz, CDCl3) δ
JO801050R
5984 J. Org. Chem. Vol. 73, No. 15, 2008