Chemoselective synthesis of sialic acid 1,7-lactones

Article abstract of DOI:10.1021/jo100732j

The chemoselective synthesis of the 1,7-lactones of N-acetylneuraminic acid, N-glycolylneuraminic acid, and 3-deoxy-d-glycero-d-galacto-nononic acid is accomplished in two steps: a simple treatment of the corresponding free sialic acid with benzyloxycarbonyl chloride and a successive hydrogenolysis of the formed 2-benzyloxycarbonyl 1,7-lactone. The instability of the 1,7-lactones to protic solvents has been also evidenced together with the rationalization of the mechanism of their formation under acylation conditions. The results permit to dispose of authentic 1,7-sialolactones to be used as reference standards and of a procedure useful for the preparation of their isotopologues to be used as inner standards in improved analytical procedures for the gas liquid chromatography-mass spectrometry (GLC-MS) analysis of 1,7-sialolactones in biological media.

Full text of DOI:10.1021/jo100732j

pubs.acs.org/joc
Chemoselective Synthesis of Sialic Acid 1,7-Lactones
Pietro Allevi, Paola Rota, Raffaella Scaringi, Raffaele Colombo, and Mario Anastasia*
ꢀ
Dipartimento di Chimica, Biochimica e Biotecnologie per la Medicina, Universita degli Studi di Milano,
Via Saldini 50, I-20133 Milano (Italy)
mario.anastasia@unimi.it
Received April 15, 2010
The chemoselective synthesis of the 1,7-lactones of N-acetylneuraminic acid, N-glycolylneuraminic
acid, and 3-deoxy-D-glycero-D-galacto-nononic acid is accomplished in two steps: a simple treatment
of the corresponding free sialic acid with benzyloxycarbonyl chloride and a successive hydrogenolysis
of the formed 2-benzyloxycarbonyl 1,7-lactone. The instability of the 1,7-lactones to protic solvents
has been also evidenced together with the rationalization of the mechanism of their formation under
acylation conditions. The results permit to dispose of authentic 1,7-sialolactones to be used as
reference standards and of a procedure useful for the preparation of their isotopologues to be used as
inner standards in improved analytical procedures for the gas liquid chromatography-mass
spectrometry (GLC-MS) analysis of 1,7-sialolactones in biological media.
Introduction
N-Acetylneuraminic acid 1 (Neu5Ac), N-glycolylneura-
minic acid 2 (Neu5Gc), and 3-deoxy-D-glycero-D-galacto-
nononic acid 3 (KDN; Figure 1) are the three more repre-
sentative members of the sialic acids (Sias) family, a group of
biological important nine carbon monosaccharides,1 that at
the moment form a family with more than 60 members,
mainly differing for the substitution at the alcoholic hydro-
xyls.1a,c,2 Sias are usually linked as R-acetals to the nonredu-
cing end of the carbohydrate chains of glycolipids and
glycoproteins, where they are involved in many important
biological recognition processes as lymphocyte homing,
tumor metastasis, and pathogenic bacteria infections.1
Among the members of the Sias family, the N-acetylneur-
aminic acid 1,7-lactone 4 (Neu5Ac 1,7 L) and the N-glyco-
lyneuraminic acid 1,7-lactone 5 (Neu5Gc 1,7 L), indirectly
identified in various glycoproteins and in the membranes of
FIGURE 1. Representatives sialic acids and their 1,7-lactones.
diseased erythrocytes,3 appear of particular interest for their
unusual structure and the unconventional β geometry of
their glycosidic bond in glycoconjugated matrixes.4 The
lactones have not been isolated but indirectly identified in
glycoconjugates, by gas liquid chromatography-mass spec-
trometry (GLC-MS) analysis,3 after acidic hydrolysis of
their acetalic bond and volatilization by treatment with per-
fluorinated anhydrides. As part of a larger program, directed
to assess suitable analytical protocols for the identification
(1) (a) Varky, A.; Cummings, R. D.; Esko, J. D.; Freeze, H. H.; Stanley,
P.; Bertozzi, C. R.; Hart, G. W.; Etzler, M. E. Essentials of Glycobiology, 2nd
ed.; CSH: New York, 2009. (b) Schauer, R. Zoology 2004, 107, 49. (c)
Angata, T.; Varky, A. Chem. Rev. 2002, 102, 439. (d) Schauer, R.; Kelm, S.;
Reuter, G.; Roggentin, P.; Shaw, L. Biology of the Sialic Acid; Rosemberg,
A., Ed.; Plenum Press: New York, 1995; pp 7-67.
(3) (a) Bulai, T.; Bratosin, D.; Pons, A.; Montreuil, J.; Zanetta, J. P. FEBS
Lett. 2003, 534, 185. (b) Bratosin, D.; Palii, C.; Moicean, A. D.; Zanetta, J. P.;
Montreuil, J. Biochemie 2007, 89, 355. (c) Cebo, C.; Dambrouck, T.; Maes,
E.; Laden, C.; Strecker, G.; Michalski, J. C.; Zanetta, J. P. J. Biol. Chem.
2001, 276, 5685. (d) Cebo, C.; Vergoten, G.; Zanetta, J.-P. Biochim. Biophy-
sic. Acta 2002, 1572, 422.
(2) Zanetta, J. P.; Pons, A.; Iwersen, M.; Mariller, C.; Leroy, Y.; Timmerman,
P.; Schauer, R. Glycobiology 2001, 11, 663.
(4) Furuhata, K. Trends Glycosci. Glycotechnol. 2004, 16, 143.
5542 J. Org. Chem. 2010, 75, 5542–5548
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DOI: 10.1021/jo100732j
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Allevi et al.
JOCArticle
of Sias,5 we recently reported6 the first synthesis of the
1,7-lactone derivative 4. Herein, we noticeably expand our
initial seeding results, describing in detail our synthetic
studies about the preparation of 1,7-lactone derivatives
(4, 5, and 6) from Neu5Ac, Neu5Gc, and KDN, improving
the procedure previously reported for the preparation of the
1,7-lactone 4 and reporting some information on the stability
of these lactones.
chloride (CbzCl), a bulky, scarcely reactive, acyl chloride.11 In
fact, the extensive work of Ogura12 and of Gervay,13 on the
formation of peracylated or partially acylated 1,7- and 1,4-
lactones of Sias, under acylation conditions, showed, as a
common feature, the formation of lactones with an acylated
anomeric hydroxyl group, even in compounds containing some
free alcoholic hydroxyls. This suggested that any lactone, even-
tually obtained in the reaction promoted by CbzCl, should be a
stable and easily isolable compound, since it was protected at the
anomeric hydroxyl. On the other hand, CbzCl, being less reactive
than the common acyl chlorides, presented the advantage to not
react with the secondary and primary hydroxy groups of the sialic
acid.11 Moreover, the eventually formed esters could be cleaved
under hydrogenolysis conditions in a suitable solvent. Unfortu-
nately, in our first attempts, with the reaction of Neu5Ac 1 with
CbzCl, in DMF, no lactonization occurred, but after a long
reaction time, the benzyl ester 9 was obtained as the only
product,12b,14 probably formed by the benzyl alcohol deriving
from the decomposition of CbzCl during the long reaction time
(Scheme 2).
However, performing the reaction of Neu5Ac 1 with a
large excess of CbzCl and triethylamine (10 mol equiv) in a
mixture of DMF-THF, we obtained the 2-benzyloxycarbo-
nylated lactone 10, in satisfactory yields (76%), after a reaction
time of 24 h at 0 °C (Scheme 2, procedure a). Successively,
taking in consideration the mechanism of the lactonization
reaction, described later, we reacted the triethylammonium salt
of Neu5Ac 1 with CbzCl and obtained a much more rapid
reaction (1 h in place of 24 h; Scheme 2, procedure b).
Results and Discussion
In our initial unsatisfactory experiments, we confirmed the
literature report according to which the direct lactonization
of free Neu5Ac 1, by reaction with dicyclohexylcarbodiimide
(DCC), results into an inseparable mixture of the starting
acid and lactones.7 This was observed even when the reaction
was performed in a pyridine solution containing 1-hydro-
xybenzotriazole hydrate (HOBT), a compound able to im-
prove yields and speed of many condensation reactions
(Scheme 1).8,9 In this case, after a 48 h reaction, the presence
of a major compound, with the polarity and the mass spectrum
of the successively prepared lactone 4, could be monitored by
TLC. However, the compound partially decomposed on TLC
and resisted all isolation attempts (Scheme 1).
SCHEME 1
The lactone 10, isolated in pure crystalline form by column
chromatography, showed the expected physicochemical
properties, with a carbonyl stretching band at 1770 cm-1
,
diagnostic for the lactone ring.12a Moreover, by reaction
with acetic anhydride in pyridine, it afforded, as expected,
the triacetylated lactone 11, possessing the appropriate
physicochemical properties.
Considering that the instability of the lactone 4 to the
common purification procedures could be due to the presence of
the free anomeric hydroxyl, which allows the reversible opening
of its pyranose ring, in successive experiments, we performed the
reaction on the Neu5Ac methyl acetal 7 (Scheme 1).10 In this
case, we isolated in low yield (36%), after column chromatogra-
phy, a lactone to which the structure 8 could be assigned on
the base of its analytical data (Scheme 1). However, since the
regeneration of its anomeric hydroxyl, in the presence of the
lactone ring was a difficult problem, we discontinued this route.
In the same time, we realized that any successful preparation of
the free lactone 4 from Neu5Ac 1, required the selective protec-
tion of the anomeric hydroxyl of the acid with a group easily
cleavable under neutral, not hydrolytic conditions, with a reac-
tion, ideally quantitative, in order to avoid any purification of the
final 1,7-lactone. Thus, we devised to attempt the lactonization of
Neu5Ac 1, activating its carboxylic group by benzyloxycarbonyl
Interesting, no trace of the possible isomeric 1,4-lactone
was found in the crude reaction mixture (thin-layer chroma-
tography (TLC), NMR). This could appear in contrast with
the Ogura et al.12 and Gervay et al.13 reports on sialic acid
lactonization, under acylation conditions, where 1,4-lactones
accompany the 1,7-isomers. However, our results can be easily
rationalized considering that all 1,4-lactones obtained by Ogura
et al. and Gervay et al. have a constantly 7-acylated hydroxyl.
This appears to suggest 1,4-lactones of the sialic acid form, in
concurrence with 1,7-isomers, when the 7-hydroxy groups have
been already esterified by the acylating agent. In our case, the
CbzCl is unable to acylate any alcoholic hydroxyl, then only the
more stable 1,7-lactone forms. However, in absence of any
additional experimental evidence, any additional rationaliza-
tion appears highly speculative.
(11) Morere, A.; Mouffouk, F.; Jeanjean, A.; Leydet, A.; Montero, J.-L.
Carbohydr. Res. 2003, 338, 2409.
(5) Allevi, P.; Femia, E. A.; Costa, M. L.; Cazzola, R.; Anastasia, M.
J. Chromatogr., A 2008, 1212, 98.
(6) Colombo, R.; Anastasia, M.; Rota, P.; Allevi, P. Chem. Commun.
2008, 5517.
(7) (a) Derevitslaya, V. A.; Kalinevich, V. M.; Kochetkov, N. K. Dockl.
Akad. Nauk SSSR 1966, 169, 1087. (b) Khorlin, A. Y.; Privalova, I. M. Khim.
Prir. Soedin 1967, 3, 191.
(12) (a) Sugiyama, N.; Sugai, K.; Yamada, N.; Goto, M.; Ban, C.;
Furuhata, K.; Takayanagi, H.; Ogura, H. Chem. Pharm. Bull. 1988, 36,
1147. (b) Furuhata, K.; Sato, S.; Anazawa, K.; Goto, M.; Takayanagi, H.;
Ogura, H. Chem. Pharm. Bull. 1987, 35, 3609. (c) Sato, S.; Furuhata, K.;
Ogura, H. Chem. Pharm. Bull. 1988, 36, 4676.
(13) (a) Gervay, J.; Ramamoorthy, P. S.; Mamuya, N. N. Tetrahedron
1997, 53, 11039. (b) Gervay, J.; Mamuya, N. N.; Barber, A. Tetrahedron Lett.
1997, 38, 1865. (c) Parrill, A. L.; Mamuya, N.; Dolata, D. P.; Gervay, J.
Glycoconjugate J. 1997, 14, 523.
ꢁ
(8) J. Cardenas, J. A.; Morales-Serna, E.; Sanchez, R.; Gavino, R.;
Lomas, L.; Guerra, N.; Negron, G. J. Arkivoc 2005, 428.
(9) Han, S.-Y.; Kim, Y.-A. Tetrahedron 2004, 60, 2447.
(10) Kun, R.; Lutz, P.; Mac Donald, D. L. Chem. Ber. 1966, 99, 611.
(14) Ogura, H.; Furuhata, K.; Sato, S.; Anazawa, K.; Itoh, M.; Shitori, Y.
Carbohydr. Res. 1987, 77, 167.
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Allevi et al.
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SCHEME 2
The lactone 10, stable in a number of protic and aprotic
solvents, by simple catalytic hydrogenolysis in anhydrous
ethyl acetate afforded the parent lactone 4, in quantitative
yields (Scheme 2). The lactone 4 is unstable in protic solvents
such as methanol or water. In fact, by simple treatment with
methanol at room temperature, it afforded the correspond-
ing Neu5Ac methyl ester 1215 (Scheme 3).
of the otherwise scarcely reactive anomeric hydroxyl. This
should facilitate the 1,7-lactonization, eliminating any pos-
sible hydrogen bond of this hydroxyl from the β-side of the
molecule and favoring the flipping of the Neu5Ac 1 ring from
the 2C5 to the 5C2 conformation. Then, a second mixed
anhydride forms and accomplishes the lactonization.
SCHEME 3
FIGURE 2. Lactonization mechanism under acylation conditions.
Moreover, we considered that an alternative mechanism
(Figure 2, path B), in which the benzyloxycarbonylation of
the anomeric hydroxyl of the lactone 4 follows the lactoniza-
tion, could not be excluded. In order to support or reject this
possibility, we performed two separate experiments. First we
treated the free lactone 4 with CbzCl, under the lactonization
conditions, and observed that no trace of the benzyloxycar-
bonylated lactone 10 forms in the reaction (Scheme 4).
Moreover, when the lactone 4 was dissolved in D2O, it
could be characterized by a complete NMR analysis, but in
some hours, it was opened to afford Neu5Ac 1, identified by
NMR, together with a minor amount of inseparable forms
which are known to accompany it to an extent depending on
the pH.16 Similarly, when the lactone 4 was dissolved in
water, in the presence of ion exchange resins (Dowex Hþ), it
was quantitatively transformed into pure Neu5Ac 1, isolated
by lyophilization (Scheme 3). These findings suggest the
improbable survival of this lactone to the acidic hydrolysis
of the glycoconjugated, performed to cleave its acetalic
bond, before its identification by GLC-MS.2,3
SCHEME 4
With the 1,7-lactone in hand, we did some experiments
directed to ascertain the mechanism of lactonization under
acylation conditions, intuitively proposed by Ogura et al.12a
for the formation of perbenzoylated Sias lactones (Figure 2,
path A). According to this mechanism, the initial formation
of a mixed anhydride occurs which permits the esterification
This confirmed the Ogura suggestion12 and excluded that
the benzyloxycarbonylation of the anomeric hydroxyl of 4
can follow the formation of the lactone ring. Additional
(15) Furuhata, K.; Sato, S.; Goto, M.; Takayanagi, H.; Ogura., H. Chem.
Pharm. Bull. 1988, 36, 1872.
(16) Klepach, T.; Carmichael, I.; Serianni, A. S. J. Am. Chem. Soc. 2008,
130, 11892.
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SCHEME 5
SCHEME 6
support to this rationalization was obtained subjecting the
acetal 7 to the lactonization reaction (Scheme 4). In this case
we obtained the corresponding lactone 8, in more satisfac-
tory yields (73%), in agreement with the view that the pro-
tected anomeric hydroxy group of Neu5Ac 1 facilitates the
lactonization.
preventive debenzylation of the hydroxyacyl group of com-
pound 13 (Scheme 5).
In effect, reacting the benzylether 13 with CbzCl, accord-
ing to our more favorable procedure, we obtained, in good
yields (78%), the 1,7-lactone 14, as a stable compound,
isolated by chromatography on silica (Scheme 5). The lac-
tone 14 showed the expected analytical data and, after
hydrogenolysis, afforded the free lactone 5 with appropriate
physicochemical properties.
At this point, being more familiar with the management of
the lactone 5, we attempted its direct preparation, i.e., by
treatment of the triethylammonium salt of Neu5Gc 2 with
CbzCl. In this way, we obtained the 1,7-lactone 15, in good
yields (75%) and in a short time (1 h). This lactone, after
regeneration of the anomeric hydroxyl group by hydroge-
nolisis, afforded the 1,7-lactone 5, identical in all respects to
that obtained previously.
Finally, we attempted the lactonization of KDN 3, which
in spite of the possible formation of a 1,5-β-lactone, requir-
ing a total equilibration of the carboxylic group of KDN 3
from the R- to the β-position, afforded the 2-benzyloxycar-
bonyl 1,7-lactone 16 as a major compound in satisfactory
yields (61%; Scheme 6). This was accompanied by a less
polar derivative, obtained in low yields (16%), to which the
structure of the 8,9-carbonate 1,7-lactone 17 could be as-
signed, on the basis of its physicochemical properties and
those of its triacetate 18, obtained from its treatment with
acetic anhydride.
This possible mechanism prompted us to react the triethy-
lammonium salt of Neu5Ac 1 with CbzCl in order to facil-
itate the initial formation of the mixed anhydride and to
accelerate the lactonization reaction of Neu5Ac, which really,
in this case, was completed in a short time (1 h; Scheme 2).
The synthesis of the 1,7-lactone 5, deriving from the
Neu5Gc 2, could be complicated by the presence of an
additional primary hydroxy group in the starting acid which
could interfere forming an hydrogen bond with the hydroxyl
group at C-7. Thus, we decided to perform the first lactoni-
zation reaction on the benzyl ether 13 of Neu5Gc (Scheme 5).
We considered also that, being that the benzyl ether 13 is a
key intermediate in the protocol proposed by Ogura et al.17
for the preparation of Neu5Gc from the commercial Neu5Ac,
we would not have extended the synthetic route to obtain the
desired lactone 5.
In fact, after the lactonization of the protected Neu5Gc 13,
the benzyl group could be cleaved in the final hydrogenoly-
sis regenerating the anomeric hydroxyl of the lactone 5.
If operative, this route could be even shorter than that
involving the direct lactonization of Neu5Gc prepared by a
The lactone 16 showed analytical data in agreement with
the proposed structure. Moreover by hydrogenolysis in the
(17) Ogura, H.; Furuhata, K.; Itoh, M. Shitori, Y. JP Patent 226539/85,
1985; EU Patent 0222172B1, 1991.
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Allevi et al.
JOCArticle
presence of palladium on carbon, it afforded the free lactone
6, in pure form and in quantitative yields. The formation of
the cyclic carbonate 17, even if unexpected to the light of the
previous reactions herein described, is not surprising, on the
basis of the chemical literature where the formation of cyclic
carbonates in carbohydrate chemistry by action of benzyl-
chloroformiate in the presence of bases has been reported.18
The carbonate group could derive from an inner transester-
ification of a 9-benzyloxycarbonyl sialic acid with the hy-
droxy group at C-8.
93%) as a white solid: mp 109-113 °C (decomposition, in sealed
tube); [R]2D5 = þ23 (concn 1, THF). Anal. Calcd for C11H17NO8:
C, 45.36; H, 5.88; N, 4.81. Found: C, 45.23; H, 5.92; N, 4.78. The
compound was identical in all respect with that reported.6
N-Glycolyl-β-neuraminic acid 1,7 Lactone (5). 1. Starting
from the Benzyloxyderivative 14. The 2-benzyloxycarbonyl-
N-benzyloxyacetyl-β-neuraminic acid 1,7-lactone 14 (50 mg, 0.09
mmol), dissolved in ethyl acetate (38 mL), was hydrogenated in
the presence of Pd on carbon (25 mg, 10%) for 5 h. At this time,
the catalyst was filtered and washed with anhydrous THF. The
solvent was then removed under reduced pressure to afford the
title compound 5 (27 mg; 93%) as a white solid: mp 112-116 °C
(decomposition, in sealed tube); [R]2D5 = þ7.6, (concn 1, THF).
1H NMR (DMSO-d6) δ 7.58 (d, JNH,5 = 8.4 Hz, 1H, NH); 7.33
(s, 1H, OH at C-2); 5.56 (brd, J = 2.3 Hz, 1H, OH at C-4); 5.52
(t, J = 5.5; OH at C-5); 5.21 (d, J = 6.2 Hz, 1H, OH at C-8); 4.64
(br t, J = 5.4 Hz, 1H, OH at C-9); 4.34 (br s, 1H, H-6); 4.25 (d,
In conclusion, herein we report a complete picture of the
synthetic accessibility of 1,7-lactones 4, 5, and 6 deriving
from the more representative Sias.
The availability of these lactones, and the possibility to
prepare their isotopologues, labeled at the pyranose ring,
following the procedure herein reported, should facilitate
both analytical studies directed to map their presence in
different biological tissues, by means of GLC-MS meth-
odologies, and studies directed to ascertain their possible
formation in vitro, by 13C NMR experiments.13 Further-
more, the availability of the lactones 4-6 is of interest for
a possible synthesis of glycoconjugated bonding with an
unusual β-geometry at the acetalic bond.
J
7,8 = 6.2 Hz, 1H, H-7); 3.89-3.83 (overlapping, 3H, H-4 and
COCH2OH); 3.80 (d, J5,NH = 8.4 Hz, 1H, H-5); 3.62-3.51
(overlapping, 2H, H-8 and H-9a); 3.47 (m, 1H, H-9b); 1.89-1.81
(overlapping, 2H, H-3a and H-3b). 13C NMR (DMSO-d6) δ
171.0 (CH2CONH), 169.0 (C-1), 90.2, (C-2), 77.1 (C-7), 71.2
(C-8), 70.0 (C-6), 65.7 (C-4), 61.8 (C-9), 61.1 (COCH2OH), 49.6
(C-5), 37.1 (C-3). MS (ESI negative) m/z 306.0 [M - H]-, 612.8
[2M - H]-. Anal. Calcd for C11H17NO9: C, 43.00; H, 5.58; N,
4.56. Found: C, 43.10; H, 5.43; N, 4.48.
2. Starting from Compound 15. The 2-benzyloxycarbonyl
N-glycolyl-β-neuraminic acid 1,7-lactone 15 (50 mg, 0.11 mmol),
dissolved in ethyl acetate (38 mL), was hydrogenated in the
presence of palladium on carbon (25 mg, 10%) for 2 h. At this
time, the catalyst was filtered and washed with anhydrous THF.
The solvent was then evaporated under reduced pressure to
afford the title compound 5 (32 mg; 93%) as a white solid: mp
112-116 °C (decomposition, in sealed tube); [R]2D5 = þ7.6,
(concn 1, THF). MS (ESI negative) m/z 306.0 [M- H]-, 612.8
[2M - H]-. Anal. Calcd for C11H17NO9: C, 43.00; H, 5.58; N,
4.56. Found: C, 43.18; H, 5.47; N, 4.49. The compound was
identical in all respects to that described above.
3-Deoxy-D-glycero-D-galacto-nononic Acid 1,7 Lactone (6).
The 2-benzyloxycarbonyl 3-deoxy-D-glycero-D-galacto-nononic
1,7-lactone 16 (50 mg, 0.13 mmol), dissolved in ethyl acetate
(38 mL), was hydrogenated in the presence of palladium on carbon
(10 mg, 10%) for 2 h. At this time, the catalyst was filtered, washed
with anhydrous THF, and the solvent was evaporated under
reduced pressure to afford the title compound 6 (30 mg; 91%) as
a white solid: mp 172-178 °C (decomposition, in sealed tube);
[R]2D5 = þ9.7, (concn 1, THF). 1H NMR (DMSO-d6) δ 7.06 (s, 1H,
OH at C-2); 5.22-5.17 (overlapping, 3H, OH at C-4, C-5 and C-8);
4.35 (br s, 1H, H-6); 4.09 (d, J7,8 = 8.8 Hz, 1H, H-7); 3.84 (br s, 1H,
H-4); 3.63-3.51 (overlapping, 2H, H-8 and H-9a); 3.45 (m, 1H,
H-9b); 3.36 (br s, 1H, H-5); 1.92 (dd, J3a,3b = 13.5, J3a,4 = 3.1, 1H,
H-3a); 1.73 (br d, J3b,3a = 13.5, 1H, H-3b). 13C NMR (DMSO-d6)
δ 169.4 (C-1), 90.3, (C-2), 76.5 (C-7), 71.4 (C-8), 71.1 (C-6), 68.9
(C-5), 67.5 (C-4), 61.9 (C-9), 36.7 (C-3). MS (ESI negative) m/z
248.7 [M - H]-, 498.8 [2M - H]-. Anal. Calcd for C9H14O8: C,
43.20; H, 5.64. Found: C, 43.12; H, 5.49.
2-Methyl N-acetyl-β-neuraminic Acid 1,7 Lactone (8). 1. By
Reaction of 2-Methyl N-acetyl-β-neuraminic Acid 7 with DCC.
The 2-methyl N-acetyl-β-neuraminic acid 7 (94 mg, 0.31 mmol)
was dissolved in anhydrous pyridine (1 mL) and treated with
DCC (192 mg, 0.93 mmol) containing HOBT (3 mg). The
mixture was then stirred at 23 °C for 48 h. At this time, MeOH
(1 mL) was added and stirring was continued for 2 h. Then, the
solvent was removed under vacuum and the crude residue
obtained was purified by chromatography on silica gel (eluting
with AcOEt/MeOH, 9:1, v/v) to afford the pure lactone 8 (32
mg; 36%): a glass; [R]D25 = þ15 (concn 1, CH3OH). MS (ESI
negative) m/z 304.4 [M - H]-, 609.1 [2M - H]-. Anal. Calcd for
Experimental Section
General Methods. Melting points are uncorrected. Nuclear
magnetic resonance spectra were recorded at 298 K operating at
500.13 MHz for 1H and 125.76 MHz for 13C. Chemical shifts are
reported in parts for million (ppm, δ units) relative to CDCl3
signal fixed at 7.26 ppm for 1H and at 77.0 ppm for 13C spectra,
relative to CD3OD signal fixed at 3.31 ppm for 1H and at
49.05 ppm for 13C spectra, relative to DMSO-d6 signal fixed at
2.50 ppm for 1H and at 39.43 for 13C spectra. Proton and carbon
assignments were established, if necessary, with 1H-1H and
1H-13C correlated NMR experiments. 1H NMR data are tabu-
lated in the following order: multiplicity (s, singlet; d, doublet;
br s, broad singlet; m, multiplet), coupling constant(s) in hertz,
number of protons assignment of proton(s). Optical rotations
were taken on a polarimeter equipped with a 1 dm tube; [R]D
values are given in 10-1 deg cm2 g-1 and the concentration are
given in grams/100 mL. Infrared (IR) spectra were recorded in
Nujol. Mass spectrometry was performed using a quadrupole ion-
trap mass spectrometer equipped with an electrospray (ESI) ion
source. The spectra were collected in continuous flow mode by
connecting the infusion pump directly to the ESI source. Solutions
of compounds were infused at a flow rate of 5 μL/min. The spray
voltage was set at 5.0 kV in the positive and at 4.5 kV in the negative
ion mode with a capillary temperature of 220 °C. Full-scan mass
spectra were recorded by scanning a m/z range of 100-2000.
All reactions were monitored by thin-layer chromatography
(TLC) carried out on 0.25 mm E. Merck silica gel plates (60 F254
)
using UV light, 50% sulfuric acid or 0.2% ninhydrin in ethanol,
and heat as developing agents. All flash chromatography was
performed with normal phase silica gel, following the general
protocol of Still.19
N-Acetyl-β-neuraminic acid 1,7-lactone (4). The 2-benzyloxy-
carbonyl-N-acetyl-β-neuraminic acid 1,7-lactone 10 (425 mg,
1.00 mmol), dissolved in ethyl acetate (350 mL), was hydro-
genated in the presence of Pd on carbon (200 mg, 10%) for 2 h.
At this time, the catalyst was filtered and washed with anhy-
drous tetrahydrofuran (THF). The solvent was then evaporated
under reduced pressure to afford the title compound 4 (271 mg;
(18) Hall, L. D.; Houg, L. Chem. Soc. 1963, 5301.
(19) Still, W. C.; Kahn, M.; Mitra, A. J. Org. Chem. 1978, 43, 2923–2925.
5546 J. Org. Chem. Vol. 75, No. 16, 2010

Allevi et al.
JOCArticle
C12H19NO8: C, 47.21; H, 6.27; N, 4.59. Found: C, 47.09; H, 6.33;
N, 4.50. Other physicochemical properties were identical to
those previously reported.6
tube); [R]2D5 = þ21.4, (concn 1, CH3OH). IR, (nujol) 3331, 1770
cm-1. MS (ESI negative) m/z 424.1 [M - H]-, 848.9 [2M - H]-.
Anal. Calcd for C19H23NO10: C, 53.65; H, 5.45; N, 3.29. Found:
C, 53.60; H, 5.35; N, 3.30. Other physicochemical properties
were identical to those reported above.
2. Reaction of 2-Methyl N-acetyl-β-neuraminic Acid 7 with
CbzCl. CbzCl (0.4 mL, 2.8 mmol), dissolved in THF (1.5 mL)
was added dropwise to a stirred solution of anhydrous THF
(2.5 mL) containing trietylamine (0.50 mL, 36 mmol), at 0 °C.
Then, Neu5Ac 1 (94 mg, 0.31 mmol) dissolved in DMF (3.0 mL)
was added and the mixture was stirred at 23 °C, for 24 h. At this
time, MeOH (1.0 mL) was added and stirring was continued
for 1 h. After evaporation of the solvent under high vacuum
(0.1 mmHg), a crude residue was obtained which, after purifica-
tion by flash chromatography (eluting with AcOEt/MeOH, 9:1,
4,8,9-Tri-O-Acetyl-2-benzyloxycarbonyl-N-acetyl-β-neurami-
nic Acid 1,7-Lactone (11). The 2-benzyloxycarbonyl-N-acetyl-
β-neuraminic acid 1,7-lactone 10 (213.0 mg, 0.50 mmol), dis-
solved in pyridine (1.5 mL), was treated with acetic anhydride
(0.70 mL, 7.4 mmol) containing a trace of 4-dimethylamino
pyridine, and the solution was stirred at 23 °C, for 24 h. At this
time the reaction was worked-up and the organic layers were
evaporated under reduced pressure to afford a crude compound
which, after flash chromatography (eluting with AcOEt/MeOH,
99:1, v/v), afforded the pure lactone 11 (265 mg; 96%): a glass;
[R]2D5 þ41 (concn 1, CHCl3). MS (ESI positive) m/z 574.2 [M þ
Na]þ, 606.1 [M þ Na þ MeOH]þ, 1124.9 [2M þ Na]þ. Anal.
Calcd for C25H29NO13: C, 54.45; H, 5.30; N, 2.54. Found: C,
54.50; H, 5.15; N, 2.35. Other physicochemical properties were
identical to those previously reported.6
Reaction of the N-acetyl-β-neuraminic Acid 1,7-Lactone with
Methanol. The N-acetyl-β-neuraminic acid 1,7-lactone 4 (50.0
mg, 0.17 mmol) was dissolved in MeOH and the solution
was kept at 23 °C for 24 h. Then, the solvent was removed,
under reduced pressure, to afford the pure N-acetyl-β-neurami-
nic acid methyl ester 12 (51 mg; 92%) as a white solid: mp
182-184 °C; [R]2D5 = -32.2, (concn 1, H2O). MS (ESI positive)
m/z 346.1 [M þ Na]þ. Anal. Calcd for C12H21NO9: C, 44.58; H,
6.55; N, 4.33. Found: C, 44.67; H, 6.45; N, 4.39. The ester
was identical in all respects with an authentic sample of our
laboratory.15
Reaction of the N-Acetyl-β-neuraminic Acid 1,7-Lactone with
Water and Dowex 50WX8 (Hþ). The N-acetyl-β-neuraminic
acid 1,7-lactone 4 (50 mg, 0.17 mmol), dissolved in H2O
(3 mL), was treated with acidic resin (Dowex 50WX8, Hþ)
and the mixture was stirred for 24 h, at 23 °C. Then, the resin
was filtered and washed with water. The solution was then
lyophilized to afford pure Neu5Ac 1 (48 mg; 91%) as a white
solid: mp 185-187 °C; [R]2D5 = -32.2, (concn 1, H2O). MS (ESI
positive) m/z 332.2 [M þ Na]þ. The compound was identical in
all respects to an authentic sample of commercial Neu5Ac.
2-Benzyloxycarbonyl-N-benzyloxyacetylneuraminic Acid 1,7
Lactone (14). Triethylamine (0.19 mL, 1.34 mmol) was added
to a well stirred solution of the known17 N-benzyloxyacetyl-β-
neuraminic acid 13 (45 mg; 0.11 mmol) in THF and DMF (1.0
mL in 1.5 mL) and the reaction mixture was stirred for 5 min, at
0 °C. Then, CbzCl (0.15 mL, 1.07 mmol) dissolved in THF (1.5
mL) was added dropwise and the solution was stirred at 23 °C,
for 1 h. At this time, MeOH (1 mL) was added and the stirring
was continued for 15 min. The solvent was then evaporated
under vacuum (0.1 mmHg) to afford a crude residue which, after
chromatography (eluting with AcOEt/MeOH, 9:1, v/v), af-
forded the lactone 14 (45 mg; 78%): a glass; [R]D = þ9.5,
(concn 1, CH3OH). 1H NMR (CD3OD) δ 7.40-7.27 (over-
lapping, 10H, Ph); 5.18 (AB system, 2H, CH2Ph); 4.65 (br s, 1H,
H-6); 4.60 (s, 2H, OCH2Ph); 4.48 (d, J7,8 = 9.1 Hz, 1H, H-7);
4.09 (br m, 1H, H-4); 4.06 (br s, 1H, H-5); 4.01 (s, 2H, COCH2-
OPh); 3.96 (ddd, J8,7 = 9.1, J8,9b = 4.3 Hz, J8,9a = 2.7 Hz, 1H,
H-8); 3.79 (ABX system, J9a,9b = 11.7, J9b,8 = 4.3, J9a,8 = 2.7 Hz,
2H, H-9a and H-9b); 2.16 (ABX system, J3a,3b = 14.1, J3a,4 = 3.3,
J3b,4 = 2.4 Hz, 2H, H-3a e H-3b). 13C NMR (CD3OD) δ 171.9
(CH2CONH), 167.7 (C-1), 153.5 (PhCH2OCO), 138.6, 136.3,
129.8, 129.7, 129.6, 129.4, 129.3, 129.2 (10C-Ph), 94.8, (C-2),
79.6 (C-7), 74.6 (COCH2OCH2Ph), 73.1 (C-6), 71.9 (C-8), 71.4
(PhCH2OCO), 70.0 (COCH2OCH2Ph), 67.2 (C-4), 63.3 (C-9),
52.1 (C-5), 37.0 (C-3). MS (ESI positive) m/z 554.2 [M þ Na]þ.
Anal. Calcd for C26H29NO11: C, 58.75; H, 5.50; N, 2.64. Found: C,
58.60; H, 5.25; N, 2.70.
v/v), afforded the pure lactone 8 (67 mg; 73%): a glass; [R]D25
=
þ15 (concn 1, CH3OH). MS (ESI negative) m/z 304.4 [M - H]-,
609.3 [2M - H]-. Anal. Calcd for C12H19NO8: C, 47.21; H, 6.27;
N, 4.59. Found: C, 47.12; H, 6.29; N, 4.55. Other physicochem-
ical properties were identical to those previously reported.6
N-Acetyl-β-neuraminic Acid Benzyl Ester (9). To a well stirred
solution of CbzCl (0.44 mL, 3.1 mmol) in DMF (1.6 mL), solid
N-acetyl-β-neuraminic acid (100 mg, 0.32 mmol) was added at
0 °C. Then the mixture was stirred at 23 °C for 72 h. At this time,
MeOH (1.0 mL) was added and the stirring was continued for
15 min. After evaporation of the solvent under high vacuum
(0.1 mmHg), the crude residue obtained was chromatographed
(eluting with CH2Cl2/MeOH, 8:2, v/v) to afford the pure
compound 9 (107 mg; 84%) as a white solid: mp 184-185 °C;
[R]2D5 = -41, (concn 1, H2O). MS (ESI positive) m/z 422.4 [M þ
Na]þ, 821.8 [2M þ Na]þ. Anal. Calcd for C18H25NO9: C, 54.13;
H, 6.31; N, 3.51%. Found: C, 54.00; H, 6.10; N, 3.42. Other
physicochemical properties were identical to those previously
reported12b and of an authentic sample of our laboratory.
2-Benzyloxycarbonyl-N-acetyl-β-neuraminic Acid 1,7-Lac-
tone (10). 1. Procedure a. CbzCl (4.0 mL, 28.0 mmol) dissolved
in THF (15 mL) was added to a stirred solution of anhydrous
THF (25 mL) containing triethylamine (5.0 mL, 36.0 mmol), at
0 °C. Then, Neu5Ac 1 (900 mg, 2.91 mmol) dissolved in DMF
(30 mL) was added and the mixture was stirred at 23 °C, for 24 h.
At this time, MeOH (1.5 mL) was added and the stirring was
continued for 15 min. Then, the solvent was removed under high
vacuum (0.1 mmHg) to afford, after chromatography on silica
(eluting with AcOEt/MeOH, 9:1, v/v), the pure lactone 10
(940 mg; 76%) as a white solid: mp 122-124 °C (decompo-
sition, in sealed tube); [R]2D5 =þ22.0, (concn1, CH3OH). 1HNMR
(CD3OD) δ7.39-7.34 (m, 5H; Ph); 5.18 (AB system, 2H, CH2Ph);
4.63 (br s, 1H, H-6); 4.45 (d, J7,8 = 9.1 Hz, 1H, H-7); 4.10 (br m,
1H, H-4); 4.01 (br s, 1H, H-5); 3.95 (ddd, J8,7 = 9.1, J8,9b = 4.5,
J8,9a = 2.9 Hz, 1H, H-8); 3.79 (system ABX, J9a,9b = 11.7, J9a,8 2.9,
J9b,8 4.5 Hz, 2H, H-9a and H-9b); 2.27 (dd, J3a,3b = 13.8, J3a,4 =
3.5, 1H, H-3a); 2.14 (dd, J3b,3a = 13.8, J3b,4 = 1.8, 1H, H-3b); 2.01
(s, 3H, CH3CONH). 13C NMR (CD3OD) δ 172.9 (CH3CONH),
168.0 (C-1), 153.6 (PhCH2OCO), 136.4, 129.8, 129.7, 129.3 (5C-
Ph), 94.9, (C-2), 79.8 (C-7), 73.2 (C-6), 72.0 (C-8), 71.3 (PhCH2-
OCO), 67.5 (C-4), 63.4 (C-9), 52.6 (C-5), 37.0 (C-3), 22.4
(CH3CONH). IR, (nujol) 3331, 1770 cm-1. MS (ESI negative)
m/z 424.1 [M - H]-, 848.9 [2M - H]-. Anal. Calcd for C19H23-
NO10: C, 53.65; H, 5.45; N, 3.29. Found: C, 53.54; H, 5.39; N, 3.34.
2. Procedure b. Triethylamine (5.0 mL, 36.0 mmol) was added
to a stirred solution of Neu5Ac 1 (900 mg, 2.91 mmol) in THF
and DMF (20 and 25 mL) and the stirring was continued at 0 °C,
for 5 min. Then, CbzCl (4.0 mL, 28.0 mmol), dissolved in THF
(15 mL), was added dropwise and the mixture was stirred at
23 °C, for 1 h. Then, MeOH (1.5 mL) was added and the stirring
was continued for 15 min. After evaporation of the solvent
under high vacuum (0.1 mmHg), a crude residue was obtained
which, after purification by flash chromatography (eluting with
AcOEt/MeOH, 9:1, v/v), afforded the pure lactone 10 (1.15 g;
82%) as a white solid: mp 122-124 °C (decomposition, in sealed
J. Org. Chem. Vol. 75, No. 16, 2010 5547

Allevi et al.
JOCArticle
2-Benzyloxycarbonyl-N-glycolyl-β-neuraminic Acid 1,7-Lac-
tone (15). Triethylamine (3.5 mL; 1.34 mmol) was added to a
well stirred mixture of N-glycolyl-β-neuraminic acid 2 (94 mg;
0.29 mmol) in THF and DMF (3.0 mL in 4.0 mL) and the
reaction mixture was stirred at 0 °C for 5 min. Then, CbzCl (0.41
mL, 2.80 mmol) dissolved in THF (2.5 mL) was added dropwise
to the reaction mixture which was stirred for 1 h, at 23 °C. At this
time, MeOH (1 mL) was added and the stirring was continued
for 15 min. After evaporation of the solvent, under high vacuum
(0.1 mHg), a crude residue was obtained that, after flash
chromatography (eluting with AcOEt/MeOH, 9:1, v/v) afforded
the lactone 15 (97.0 mg; 75%): a glass; [R]2D5 = þ7.6, (concn 1,
CH3OH). 1H NMR (CD3OD) 7.38-7.33 (5H, m, Ph); 5.18 (AB
system, 2H, CH2Ph); 4.67 (br s, 1H, H-6); 4.48 (d, J7,8 = 9.1 Hz,
1H, H-7); 4.12 (br m, 1H, H-4); 4.08 (br s, 1H, H-5); 4.03 (s, 2H,
COCH2OH); 3.96 (ddd, J8,7 = 9.1, J8,9b = 4.4, J8,9a = 2.9 Hz,
819.7 [2M - H]-. Anal. Calcd for C18H18O11: C, 52.69; H, 4.42.
Found: C, 52.90; H, 4.19. Additional elution (using AcOEt/MeOH
9:1, v/v) afforded the pure lactone 16 (698 mg; 61%): mp 135-136;
[R]2D5 = þ2.0 (concn 1, CH3OH). 1H NMR (CD3OD) δ 7.40-7.33
(overlapping, 5H, Ph); 5.17 (AB system, 2H, CH2Ph); 4.66 (br s,
1H, H-6); 4.36 (d, J7,8 = 9.2 Hz, 1H, H-7); 4.10 (br m, 1H, H-4);
3.99 (ddd, J8,7 = 9.2, J8,9b = 4.5 Hz, J8,9a = 2.5 Hz, 1H, H-8); 3.79
(ABX system, J9a,9b=11.7, J9a,8 = 2.5, J9b,8 = 4.5 Hz, 2H, H-9a
and H-9b); 3.67 (br s, 1H, H-5); 2.29 (dd, J3a,3b = 13.6, J3a,4 = 3.3
Hz, 1H, H-3a); 2.08 (br d, J3b,3a = 13.6 Hz, 1H, H-3b). 13C NMR
(CD3OD) δ 168.4 (C-1), 153.6 (PhCH2OCO), 136.3, 129.7, 129.3
(5C- Ph), 95.1, (C-2), 78.9 (C-7), 74.9 (C-6), 71.8 (C-8), 71.3
(PhCH2OCO), 70.8 (C-5), 69.0 (C-4) 63.5 (C-9), 36.6 (C-3). MS
(ESI negative) m/z 382.7 [M - H]-, 766.7 [2M - H]-. Anal. Calcd
for C17H20O10: C, 53.13; H, 5.25. Found: C, 53.00; H, 5.36.
4,5-Di-O-Acetylated-2-benzyloxycarbonyl-8,9-O-carbonate-
3-deoxy-D-glycero-D-galacto-nononic Acid 1,7-Lactone (18). The
2-benzyloxycarbony-8,9-O-carbonate-3-deoxy-D-glycero-D-galacto-
nononic acid 1,7-lactone 17 (213.0 mg, 0.52 mmol) was dissolved in
pyridine (1.5 mL) and treated with acetic anhydride (0.70 mL, 0.52
mmol), containing a trace of 4-dimethylaminopyridine, at 23 °C, for
12 h. Then MeOH (0.5 mL) was added and the solution was
concentrated under reduce pressure to afford a crude residue which
was recovered with ethyl acetate and washed in the sequence with an
aqueous solution of HCl, with water, with an aqueous NaHCO3
solution, and again with water. Elimination of the solvent, under
reduced pressure, afforded a crude residue which, after flash
chromatography (eluting with hexane/AcOEt, 6:4, v/v), af-
forded the triacetate 18 (246 mg; 96%): mp 116-117 °C; [R]D23
þ41 (concn 1, CHCl3). 1H NMR (CDCl3) δ 7.41-7.32 (5H, Ph);
5.24 (m, 1H, H-8); 5.18 (AB system, 2H, CH2Ph); 5.16 (br m, 1H,
H-4); 4.93 (br s, 1H, H-5); 4.71 (t, J9a,9b = J9a,8 = 9.4 Hz, 2H,
H-9a); 4.66 (dd, J9b,9a = 9.4, J9b,8 = 5.6 Hz, 1H, H-9b); 4.56 (br
s, 1H, H-6), 4.53 (d, J7,8 = 9.3 Hz, 1H, H-7); 2.39-2.32 (m, 2H,
H-3a and H-3b); 2.16 (s, 3H, COCH3 at C-5); 2.07 (s, 3H,
COCH3 at C-4). δ 13C NMR (CDCl3) 169.1 (COCH3 at C-5),
168.6 (COCH3 at C-4), 163.4 (C-1), 153.5 (CO at C-8 and C-9),
152.1 (PhCH2OCO), 133.8, 129.0, 128.7, 128.3 (5C- Ph), 93.3,
(C-2), 77.0 (C-7), 72.7, (C-8), 71.7 (C-6), 71.0 (OCOCH2), 67.4
(C-5), 66.9 (C-9), 66.7 (C-4), 33.4 (C-3), 20.7 (2 ꢀ COCH3). MS
(ESI positive) m/z 516.9 [M þ Na]þ. Anal. Calcd for C22H22O13:
C, 53.45; H, 4.49. Found: C, 53.26; H, 4.39.
1H, H-8); 3.79 (ABX system, J9a,9b = 11.7, J9b,8 = 4.5, J9a,8
=
2.9 Hz, 2H, H-9a and 9b); 2.23 (dd, J3a,3b = 13.8, J3a,4 = 3.5 Hz,
1H, H-3a); 2.16 (dd, J3b,3a = 13.8, J3b,4 = 2.2 Hz, 1H, H-3b).
13C NMR (CD3OD) δ 174.5 (CH2CONH), 167.8 (C-1), 153.5
(PhCH2OCO), 136.4, 129.8, 129.7, 129.3 (5C-Ph), 94.9 (C-2),
79.7 (C-7), 73.2 (C-6), 72.0 (C-8), 71.4 (PhCH2OCO), 67.3 (C-4),
63.4 (C-9), 62.6 (COCH2OH), 52.1 (C-5), 37.1 (C-3). IR, (nujol)
3331, 1759 cm-1. MS (ESI positive) m/z 464.1 [M þ Na]þ. Anal.
Calcd for C19H23NO11: C, 51.70; H, 5.25; N, 3.17. Found: C,
51.50; H, 5.45; N, 3.37.
2-Benzyloxycarbonyl-3-deoxy-D-glycero-D-galacto-2-nononic
Acid 1,7-Lactone (16) and 2-Benzyloxycarbonyl-8,9-O-carbo-
nate-3-deoxy-D-glycero-D-galacto-2-nononic Acid 1,7-Lactone
(17). Triethylamine (5.1 mL, 37.0 mmol) was added to a solution of
2-keto-3-deoxy-D-glycero-D-galacto-2-nononic acid 3 (800 mg;
2.98 mmol) in DMF and THF (20 mL in 35 mL) at 0 °C. After a
5 min stirring, CbzCl (4.1 mL, 29.0 mmol) dissolved in THF (12.0
mL) was added dropwise and the mixture was stirred at 23 °C, for
1 h. At this time, MeOH (1 mL) was added and the stirring was
continued for 15 min. After evaporation of the MeOH-THF
solution, the residual DMF was eliminated under high vacuum
(0.1 mmHg) to afford a crude residue which was purified by flash
chromatography to afford first (eluting with AcOEt/hexane 8:2,
v/v) the lactone 17 (195 mg; 16%): a glass; [R]2D3 þ38,4 (concn 1,
1
CHCl3). H NMR (CD3OD) δ 7.44-7.33 (5H, Ph); 5.23-5.18
(AB system, 2H, CH2Ph); 5.11 (m, 1H, H-8); 4.91 (t, J9a,9b = 8.7,
J
9b,8=5.6 Hz, 1H, H-9a); 4.77 (d, J7,8 = 6.3 Hz, 1H, H-7); 4.66 (dd,
9a,9b = J9a,8 = 8.7 Hz, 1H, H-9b); 4.37 (s, 1H, H-6); 4.12 (m, 1H,
J
H-4); 3.76 (br s, 1H, H-5); 2.34 (dd, J3a,3b = 13.7, J3,4 = 3.3 Hz,
Acknowledgment. This work was supported financially by
ꢀ
the Italian MiUR (Ministero dell’Universita e della Ricerca).
1H, H-3a); 2.16 (dd, J3a,3b=13.7, J3,4 = 2.0 Hz, 1H, H-3b). 13
C
NMR (CD3OD) δ 167.3 (C-1), 165.5 (CO at C-8 and C-9), 153.6
(PhCH2OCO), 136.6, 129.8, 129.7, 129.3 (5C- Ph), 95.2, (C-2), 78.2
(C-7), 76.8 (C-8), 74.9 (C-6), 71.5 (OCOCH2), 70.0 (C-5), 68.7 (C-
4), 67.4 (C-9), 36.4 (C-3). MS (ESI negative) m/z 409.7 [M - H]-,
Supporting Information Available: Spectroscopic data of
compounds 4, 5, 6, 8, 9, 10, 11, 12, 14, 15, 16, 17, and 18. This
material is available free of charge via the Internet at http://
pubs.acs.org.
5548 J. Org. Chem. Vol. 75, No. 16, 2010