Practical stereoselective synthesis of α-D-C-mannosyl-(R)-alanine

Article abstract of DOI:10.1016/S0040-4020(02)01225-5

α-D-C-Mannosyl-(R)-alanine 2 was synthesized in only four steps starting from the known acetonide protected D-ribo-hex-1-enitol 4 and N-benzoylalanine. The key C-C bond formation between the sugar and the amino acid moieties was effected through a Claisen rearrangement of the intermediate oxazole 7, derived from the ester 6.

Full text of DOI:10.1016/S0040-4020(02)01225-5

Tetrahedron 58 (2002) 9381–9386
Practical stereoselective synthesis of a-D-C-mannosyl-(R)-alanine
*
Lino Colombo, Marcello di Giacomo and Paola Ciceri
`
Dipartimento di Chimica Farmaceutica, Universita di Pavia, via Taramelli 12, I-27100 Pavia, Italy
Received 18 July 2002; revised 3 September 2002; accepted 26 September 2002
Abstract—a-D-C-Mannosyl-(R)-alanine 2 was synthesized in only four steps starting from the known acetonide protected D-ribo-hex-1-
enitol 4 and N-benzoylalanine. The key C–C bond formation between the sugar and the amino acid moieties was effected through a Claisen
rearrangement of the intermediate oxazole 7, derived from the ester 6. q 2002 Elsevier Science Ltd. All rights reserved.
C-Glycoside derivatives1 have been the subject of intensive
research effort in recent years due to their chemical stability,
protection against glycosidases, and favorable confor-
mational properties that allow the use of such compounds
as mimics of more common, naturally occurring N- and
O-glycoconjugates. Recent studies have demonstrated that
C-linked glycoconjugates bind biological targets with
nearly identical conformation and affinities as natural
O-linked glycoconjugates, in spite of a seemingly dramatic
difference in chemical structure.2
applied to the preparation of 1 (R¼Me) relied on the
Steglich variant of the Claisen transposition.13 Rearrange-
ment reactions have found a limited application in the
synthesis of C-glycosides,14 in spite of the fact that the
intramolecular and concerted character of such reactions
should entail a stereoselective formation of the C-glycosidic
bond. In this paper we report that the Steglich rearrangement
can be applied to a practical and stereoselective synthesis of
a-D-C-mannosyl-(R)-alanine 2. The use of N-benzoylala-
nine in racemic form, the conciseness of the synthetic
sequence, and the need, in all steps, of rather inexpensive
shelf-reagents contribute to the practical value of the
method. Moreover, an appreciable amount of the (S)-
diastereomer 3 could also be obtained (Fig. 1).
Within this general class of compounds, C-glycosyl
a-amino acids3 have emerged as an important class of
building blocks for the construction of C-glycosylated
peptides.4 The interest in this class of glycopeptide
mimetics stems not only from enhanced resistance to
enzymatic hydrolysis5 but also from potential superior
properties for specific therapeutic or biological applications.
It has been reported, for example, that C-linked carbo-
hydrates exert conformational restrictions on a peptide
backbone and this effect was ascribed to unfavorable steric
interactions and limitations of the conformational space of
the glycosyl side chain.6 Well defined conformational
constraints could also be imparted to a peptide chain by
the insertion of quaternary a-amino acids.7 We reasoned
that a combined effect could be derived by attaching a
sugar moiety to the a-carbon of a monosubstituted a-amino
acid.
The synthesis started from the known acetonide protected
D-ribo-hex-1-enitol 5,15 prepared in four steps from
commercially available tri-O-acetyl-D-glucal 4. Esterifica-
tion with racemic N-benzoylalanine to give the esters 6
turned out to be an unexpectedly difficult operation. Using
the experimental conditions that were shown to be effective
for the same transformation on the epimeric 3-b-OH
compound,12 the desired ester 6 was obtained in very low
yields, the rearranged N-acylurea being the major product.
Other methods for the activation of the carboxyl function
Only a few examples of C-pyranosyl a-amino acids of the
general structure 1 (Fig. 1) have been reported.8–12 All of
them are b-C-glycosyl derivatives and only in two cases,
reported by one of us,12 a quaternary carbon atom is
adjacent to the sugar anomeric center. The synthetic strategy
Keywords: C-glycosyl a-amino acids; quaternary a-amino acids; Claisen
rearrangement.
*
Corresponding author. Tel.: þ39-0382-507387; fax: þ39-0382-422975;
e-mail: lino.colombo@unipv.it
Figure 1.
0040–4020/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(02)01225-5

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L. Colombo et al. / Tetrahedron 58 (2002) 9381–9386
Scheme 1. Reagents and conditions: (i) N-Benzoylalanine, DCC, DMAP, DMAP–HCl, CHCl3, reflux, 9 h; (ii) PPh3, CCl4, Et3N, rt, 12 h.
Scheme 2. Reagents and conditions: (i) (1) 1 M NaOH, THF/H2O, rt, 30 min; (2) I2, rt, 48 h; (ii) 5 M HCl, THF, rt, 3 h. (Arrows indicate nOe correlations).
proved equally ineffective, producing 2-phenyl-4-benzyl-
5(4H)-oxazolone as a major product.16 After extensive
experimentation we returned to the DCC activation
protocol, modified by the addition of a mixture of a
stoichiometric amount of DMAP and a two-fold excess of
its hydrochloride salt.17 Application of the above conditions
allowed the preparation of the diastereomeric mixture of
esters 6 in an acceptable 80% yield. (Scheme1).
iodolactones 9a and 9b by NaOH hydrolysis of the
oxazolone nucleus to N-benzoyl a-amino acid carboxylates,
followed by one pot treatment with iodine. The acetonide
protecting group was then removed in order to make an
unequivocal assignment of the NMR signal corresponding
to the oxazolone methyl group. (Scheme 2)
Irradiation of the methyl signal of the major diastereoisomer
10a caused an intensity enhancement of peaks correspond-
ing to H30 (2.3%) and H50 (4.8%), thus establishing the
configuration of the iodolactone stereocenter as (R). This
result was confirmed by performing the same experiment on
the minor stereoisomer 10b, where the methyl signal
presented nOe correlations with resonances attributed to
H20 and H10.
Treatment of the esters 6 with PPh3, CCl4, and Et3N at room
temperature directly provided a 2.6:1 mixture of two
diastereomeric oxazolones 8a and 8b, deriving from the
Claisen rearrangement of the intermediate oxazole 7, which
could not be detected neither by TLC nor by any standard
spectroscopic means. The intramolecular, concerted nature
of the rearrangement reaction should lead to the stereo-
selective formation of a-C-glycosyl derivatives, epimeric at
the oxazolone stereocenter. This stereochemical result was
easily demonstrated by comparing the physical and spectro-
scopic properties of 8a and 8b with those of the epimeric
b-C-glycosyl analogous compounds previously synthesized
by us following a similar procedure.12 Furthermore, nOe
experiments carried out on advanced intermediates
confirmed the above results (see below).
It is interesting to note that an (R) configured stereocenter
would be formed if the Claisen rearrangement were to
proceed via a boat-like transition state. One can only
speculate on the factors disfavoring the usually more stable
chair-like transition state. Inspection of molecular models
suggests a steric interaction between H50 and the oxazolone
Configurational assignment of the oxazolone stereocenter
required conversion of 8a and 8b into conformationally
constrained compounnds in order to correlate, by nOe
difference experiments, the spatial arrangement of the
oxazolone methyl substituent with some proton of defined
stereochemistry in the sugar moiety. To this end 8a and 8b
were easily separated by flash chromathography (DRf¼0.13,
4:1 hexane–EtOAc) and individually converted into
Figure 2. Boat-like A and chair-like B transition states leading to 8a and
8b, respectively.

L. Colombo et al. / Tetrahedron 58 (2002) 9381–9386
9383
Scheme 3. Reagents and conditions: (i) OsO4, NMO, Me2CO–H2O, rt, 24 h; (ii) 6N, HCl, 808C, 4 h. (Arrows indicate nOe correlations).
nucleus projecting toward the bottom face of the sugar
moiety as a likely factor destabilizing the chair-like
transition state. (Fig. 2)
Merck Kieselgel 60 (230–400 mesh). Melting points were
determined on a Kofler apparatus and are uncorrected.
Optical rotations were measured at room temperature with a
Perkin–Elmer 343 polarimeter. The 1H (300 and 400 MHz)
and 13C NMR (75 and 100 MHz) spectra were recorded with
Bruker Avance 300 and 400 instruments respectively. In the
peak listing of 13C spectra abbreviations s, d, t, and q refer to
zero, one, two, and three protons attached to the carbons, as
determined by DEPT experiments. Infrared spectra were
recorded on a Perkin–Elmer FTIR 1600 series spectometer.
Mass spectra were recorded on a Finnigan LCQ-DECA
mass spectrometer. Elemental analyses were performed on a
Carlo Erba Elemental Analyzer Mod. 1106.
Dihydroxylation of 8a by treatment with catalytic OsO4
under standard conditions furnished directly, as a single
isomer, the mannosyl lactone 11a, plausibly arising from
intramolecular attack of the C20-OH group of the inter-
mediate diol on the carbonyl carbon of the oxazolone. The
formation of a g-lactone was evidenced by the presence of
only one OH resonance and a low field chemical shift of the
C20-H in the proton NMR spectrum as well as by a strong IR
absorbance at 1771 cm21. The stereochemical outcome of
the dihydroxylation reaction was easily proven0 by NMR
data. A 10.2 Hz coupling constant correlating H1 an H20 is
diagnostic of a trans relationship between these two protons.
Moreover, an nOe correlation between H20 and H50
corroborated the assigned stereochemistry. (Scheme 3).
1.1.1. 1,5-Anhydro-4,6-O-isopropylidene-2-deoxy-3-O-
(2-benzamidopropionyl)-D-ribo-hex-1-enitol (6). To a
solution of the protected glycal 5 (2.224 g, 11.94 mmol) in
ethanol free CHCl3 (40 mL) were added sequentially
DMAP (1.459 g, 11.94 mmol), DMAP hydrochloride
(3.788 g, 23.88 mmol), DCC (4.927 g, 23.88 mmol), and
racemic N-benzoylalanine (2.307 g, 11.94 mmol) under a
nitrogen atmosphere. After refluxing for 3 h, one additional
equivalent of N-benzoylalanine was added followed by a 3 h
reflux period. After that time one more equivalent of both
DCC and N-benzoylalanine were added and the mixture
refluxed for 3 h. The reaction mixture was diluted with
EtOAc, filtered and evaporated. The resulting oily residue
was purified by flash chromatography (n-hexane–EtOAc
65:35) to give pure 6 as a white powder (3.452 g, 80%). For
sake of characterization, the two epimeric esters were
separately prepared from enantiomerically enriched (L)- and
(D)-benzoylalanine (92 and 85% e.e., respectively) and
purified by flash chromatography (n-hexane–EtOAc 65:35).
Final treatment of the lactone 11a with 6N HCl at 808C
brought about three discrete hydrolysis reactions involving
the acetonide, the lactone, and the benzamide functions
affording in excellent yield and purity a-C-mannosyl-(R)-
alanine 2. Repetition of the two-step sequence on the minor
oxazolone 8b proceeded uneventfully to give the epimeric
mannosylated amino acid 3 in comparable yields.
In summary, we have developed a concise, practical, and
high-yielding synthesis of a-D-C-mannosyl-(R)- and (S)-
alanine. The above mannosylated compounds will be
exploited as fucose mimics in glycoconjugated compounds
in which an appropriate scaffold is attached to the carboxyl
function of the amino acid unit. Such constructs, character-
ized by a properly positioned carboxylic function that
should play the role of the carboxylate present in the sialic
acid, will be tested as sialyl Lewis X mimics in binding
assays with selectins.18
Ester 6 from (L)-benzoylalanine: white amorphous solid.
[a]D¼þ219.1 (c 1.3, CDCl3). IR (KBr, cm21): 3325, 3000,
1
1741, 1637, 1533, 1242, 1165, 1087. H NMR (CDCl3,
300 MHz) d: 1.39 (s, 3H), 1.52 (s, 3H), 1.59 (d, J¼7.1 Hz,
3H), 3.81–3.91 (m, 1H), 3.95–4.10 (m, 3H), 4.85 (quintet,
J¼7.2 Hz, 1H), 4.94 (t, J¼6.0 Hz, 1H), 5.42 (dd, J¼6.0,
3.1 Hz, 1H), 6.51 (d, J¼6.0 Hz, 1H), 6.85 (bd, J¼7.2 Hz,
1H), 7.41–7.56 (m, 3H), 7.78–7.86 (m, 2H). 13C NMR
(CDCl3, 75 MHz) d: 172.8 (s), 166.7 (s), 147.9 (d), 133.9
(s), 131.6 (d), 128.5 (d), 127.0 (d), 99.8 (s), 97.8 (d), 69.0
(d), 65.8 (d), 63.2 (d), 61.6 (t), 48.6 (d), 28.7 (q), 19.1 (q),
18.9 (q). MS (ESI) m/z 384.1 (MþNa)þ, 745 (2MþNa)þ.
Anal. calcd for C19H23NO6: C, 63.15; H, 6.41; N, 3.88.
Found: C, 63.28; H, 6.22; N, 3.94.
1. Experimental
1.1. General
THF was distilled from sodium/benzophenone ketyl and
MeCN from P2O5 under nitrogen. TLC was performed on
Kieselgel 60 F254 (Merck) glass plates with detection by UV
light, iodine, and a solution of (NH4)4MoO4 and Ce2SO4 in
dilute H2SO4. Flash chromatography was performed on

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L. Colombo et al. / Tetrahedron 58 (2002) 9381–9386
Ester 6 from (D)-benzoylalanine: white crystals, mp 123–
1258C (EtOAc–n-hexane). [a]D¼þ212.9 (c 1.1, CDCl3).
1H NMR (CDCl3, 300 MHz) d: 1.40 (s, 3H), 1.52 (s, 3H),
1.55 (d, J¼7.1 Hz, 3H), 3.95–4.10 (m, 3H), 4.89 (quintet,
J¼7.1 Hz, 1H), 4.99 (t, J¼6.0 Hz, 1H), 5.32 (dd, J¼6.0,
3.6 Hz, 1H), 6.49 (d, J¼6.0 Hz, 1H), 6.81 (bd, J¼7.1 Hz,
1H), 7.41–7.56 (m, 3H), 7.78–7.86 (m, 2H). 13C NMR
(CDCl3, 75 MHz) d: 172.5 (s), 166.6 (s), 147.8 (d), 134.1
(s), 131.6 (d), 128.5 (d), 126.9 (d), 99.9 (s), 97.8 (d), 68.8
(d), 65.9 (d), 63.9 (d), 61.6 (t), 48.6 (d), 28.7 (q), 18.9 (q),
18.7 (q). MS (ESI) m/z 384.1 (MþNa)þ. Anal. calcd for
C19H23NO6: C, 63.15; H, 6.41; N, 3.88. Found: C, 63.40; H,
6.18; N, 3.98.
sodium (3 ml) (reagent grade THF inhibits the reaction) was
added a 1 M aqueous solution of NaOH (2.2 mL). After
30 min, sublimed iodine (0.665 g, 2.62 mmol) was added in
one portion and the solution stirred at room temperature
under nitrogen for 24 h. Two additional equivalents of
iodine (0.443 g) were added and stirring continued for 24 h.
The reaction mixture was diluted with EtOAc (10 mL) and
washed with an equal volume of saturated NH4Cl. The
organic phase was washed with a 0.1N solution of Na2S2O3
until disappearance of the brown color. The aqueous layer
was extracted with EtOAc and the organic extracts were
combined, dried, and evaporated. The crude reaction
mixture was purified by flash chromatography (n-hexane–
EtOAc 60:40) to give pure 9a (0.380 g, 89%). The same
procedure was applied to the oxazolone 8b giving pure 9b in
77% yield
1.1.2. (4R)-4-Methyl-4-(40,60-O-isopropylidene-20-30-
dideoxy-a-D-erythro-20-hexenopyranosyl)-2-phenyl-
5(4H)-oxazolone (8a) and (4S) epimer (8b). To a solution
of 6 (1.508 g, 4.17 mmol) in anhydrous acetonitrile (14 mL)
were added triethylamine (1.6 mL, 11.26 mmol), CCl4
(9.38 mmol, 0.905 mL), and triphenylphosphine (2.196 g,
8.34 mmol) and the mixture was stirred under nitrogen at
room temperature overnight, becoming dark brown and
producing a heavy precipitate. The solvent was removed at
reduced pressure and the residue diluted with EtOAc,
washed with saturated NH4Cl and the aqueous phase
extracted with EtOAc. The organic phases were collected,
dried, and evaporated. The resulting reddish oily residue
was flash chromatographed (n-hexane–EtOAc 83:17) to
give 0.889 g of 8a (62%) and 0.342 g of 8b (24%).
Iodolactone 9a: white needles, mp 193–1948C (EtOAc–n-
hexane). [a]D¼þ23.3 (c 0.8, CDCl3). IR (nujol, cm21):
1765, 1652, 1538, 1264, 1199, 1142, 1110, 1054. 1H NMR
(CDCl3, 400 MHz) d: 1.49 (s, 3H), 1.55 (s, 3H), 1.68 (s,
3H), 3.60–3.77 (m, 3H), 3.84–3.92 (m, 1H), 3.97 (dd,
J¼11.2, 9.5 Hz, 1H), 5.20 (d, J¼9.1 Hz, 1H), 5.26–5.34 (m,
1H), 6.48 (s, 1H), 7.43–7.49 (m, 2H), 7.53–7.59 (m, 1H),
7.76–7.80 (m, 2H). 13C NMR ((CD3)2SO, 100 MHz) d:
173.2 (s), 166.2 (s), 132.7 (s), 132.2 (d), 128.5 (d), 127.8 (d),
100.2 (s), 79.8 (d), 76.7 (d), 71.7 (d), 71.0 (d), 62.0 (t), 59.4
(s), 31.5 (d), 28.8 (q), 19.1 (q), 18.4 (q). MS (ESI) m/z 488
(MþH)þ, 510 (MþNa)þ, 997 (2MþNa)þ. Anal. calcd for
C19H22INO6: C, 46.83; H, 4.55; N, 2.87. Found: C, 46.71;
H, 4.39; N, 2.95.
Oxazolone 8a: white crystals, mp 152–1538C (EtOAc–
Et2O). [a]D¼269.0 (c 0.9, CDCl3). IR (KBr, cm21): 2990,
1826, 1653, 1323, 1095, 1006. 1H NMR (CDCl3, 400 MHz)
d: 1.38 (s, 3H), 1.47 (s, 3H), 1.56 (s, 3H), 3.45 (q, J¼8.0 Hz,
1H), 3.62 (d, AB part of an ABX system, J¼8.0 Hz, 2H),
4.07–4.13 (m, 1H), 4.59 (dd, J¼5.1, 2.6 Hz, 1H), 5.98 (dt,
J¼10.6, 2.4 Hz, 1H), 6.19 (dq, J¼10.6, 1.4 Hz, 1H), 7.48–
7.55 (m, 2H), 7.57–7.65 (m, 1H), 8.01–8.06 (m, 2H). 13C
NMR (CDCl3, 75 MHz) d: 179.3 (s), 161.5 (s), 133.0 (d),
131.9 (d), 128.9 (d), 128.0 (d), 125.5 (s), 122.2 (d), 99.5 (s),
76.3 (d), 74.2 (s), 68.4 (d), 67.1 (d), 62.9 (t), 29.0 (q), 19.7
(q), 18.8 (q). MS (ESI) m/z 366.1 (MþNa)þ. Anal. calcd for
C19H21NO5: C, 66.46; H, 6.16; N, 4.08. Found: C, 66.72; H,
5.95; N, 4.12.
Iodolactone 9b: white prisms, mp 166–1678C (EtOAc–n-
hexane). [a]D¼þ21.1 (c 0.8, CDCl3)). IR (nujol, cm21):
1779, 1711, 1653, 1529, 1260, 1197, 1132, 1033. 1H NMR
(CDCl3, 400 MHz) d: 1.42 (s, 3H), 1.47 (s, 3H), 1.70 (s,
3H), 3.49 (ddd, J¼9.6, 9.6, 5.6 Hz, 1H), 3.58 (dd, J¼10.8,
9.6 Hz, 1H), 3.67 (dd, J¼10.8, 5.6 Hz, 1H), 4.07 (t,
J¼9.6 Hz, 1H), 4.36 (dd, J¼9.6, 2.8 Hz, 1H), 5.01 (d,
J¼4.1 Hz, 1H), 5.31 (dd, J¼4.1, 2.8 Hz, 1H), 6.50 (s, 1H),
7.46–7.51 (m, 2H), 7.54–7.60 (m, 1H), 7.78–7.82 (m, 2H).
13C NMR ((CD3)2SO, 100 MHz) d: 174.2 (s), 167.6 (s),
133.7 (s), 132.9 (d), 129.4 (d), 128.5 (d), 100.7 (s), 81.9 (d),
77.4 (d), 72.4 (d), 71.3 (d), 62.6 (t), 57.2 (s), 29.9 (d), 29.6
(q), 25.9 (q), 19.8 (q). MS (ESI) m/z 488 (MþH)þ, 510
(MþNa)þ. Anal. calcd for C19H22INO6: C, 46.83; H, 4.55;
N, 2.87. Found: C, 46.85; H, 4.31; N, 2.91.
Oxazolone 8b: white solid, mp 84–868C. [a]D¼2121.8 (c
1.0, CDCl3). IR (KBr, cm21): 2930, 1790, 1649, 1450,
1103. 1H NMR (CDCl3, 400 MHz) d: 1.45 (s, 3H), 1.53 (s,
3H), 1.65 (s, 3H), 3.72–3.87 (m, 2H), 4.01 (dd, J¼9.5,
4.1 Hz, 1H), 4.18 (ddt, J¼8.3, 1.5, 2.7 Hz, 1H), 4.45 (q,
J¼2.5 Hz, 1H), 5.55 (dt, J¼10.5, 2.4 Hz, 1H), 6.00–6.05
(m, 1H), 7.47–7.54 (m, 2H), 7.57–7.64 (m, 1H), 7.99–8.05
(m, 2H). 13C NMR (CDCl3, 75 MHz) d: 179.0 (s), 160.2 (s),
132.8 (d), 132.1 (d), 128.8 (d), 128.1 (d), 125.6 (s), 123.1
(d), 99.7 (s), 76.7 (d), 73.1 (s), 68.2 (d), 67.4 (d), 63.2 (t),
29.2 (q), 20.9 (q), 19.0 (q). MS (ESI) m/z 366.1 (MþNa)þ.
Anal. calcd for C19H21NO5: C, 66.46; H, 6.16; N, 4.08.
Found: C, 66.63; H, 5.99; N, 3.90.
1.1.4. 2-(30-Deoxy-30-iodo-a-D-mannopyranosyl)-2-benz-
amido-(2R)-propionic acid 10 ,20-lactone (10a) and (2S)
epimer (10b). A 0.3 M solution of iodolalactone 9a
(0.135 g, 0.28 mmol) in THF was treated with a 5 M
solution of aqueous HCl (0.9 mL) and left aside at room
temperature for 3 h. The solution was reduced to a volume
of ca. 1 mL, diluted with 3 mL of saturated brine and
extracted three times with EtOAc. The combined organic
layers were dried and evaporated, giving 10a as a white
solid (0.110 g, 89%). The same procedure was applied to the
iodolactone 9b giving pure 10b in 90% yield.
1.1.3. 2-(40,60-O-Isopropylidene-30-deoxy-30-iodo-a-D-
mannopyranosyl)-2-benzamido-(2R)-propionic acid
10,20-lactone (9a) and (2S) epimer (9b). To a solution of
the oxazolone 8a (0.300 g, 0.87 mmol) in THF distilled on
Iodolactone 10a: white needles, mp 192–1948C (MeOH–
EtOAc). [a]D¼þ52.4 (c 0.6, MeOH). IR (nujol, cm21):
3483, 3166, 1766, 1625, 1314, 1126, 1079, 1020. 1H NMR

L. Colombo et al. / Tetrahedron 58 (2002) 9381–9386
9385
((CD3)2SO, 400 MHz) d: 1.45 (s, 3H), 3.51 (m, 1H, after
exchange with D2O: dd, J¼12.0, 2.3 Hz), 3.57–3.71 (m,
3H), 4.17 (dd, J¼11.3, 10.1 Hz, 1H), 4.61 (d, J¼6.9 Hz,
1H), 4.86 (t, J¼4.8 Hz, 1H, exchanges with D2O), 5.03 (dd,
J¼10.1, 6.9 Hz, 1H), 5.87 (d, J¼7.1 Hz, 1H, exchanges with
D2O), 7.56 (m, 2H), 7.59 (m, 1H), 7.89–7.83 (m, 2H), 9.10
(s, 1H). 13C NMR ((CD3)2SO, 100 MHz) d: 175.7 (s), 167.4
(s), 133.1 (s), 132.9 (d), 129.2 (d), 128.6 (d), 84.3 (d), 82.3
(d), 76.7 (d), 68.7 (d), 61.6 (t), 60.9 (s), 36.6 (d), 19.2 (q).
MS (ESI) m/z 448 (MþH)þ, 470 (MþNa)þ. Anal. calcd for
C16H18INO6: C, 42.97; H, 4.06; N, 3.13. Found: C, 42.91;
H, 4.17; N, 3.11.
3.8 Hz, 1H), 4.18 (d, J¼10.6 Hz, 1H), 4.34–4.37 (m, 1H),
4.93 (dd, J¼10.6, 5.1 Hz, 1H), 6.15 (s, 1H), 7.46–7.51 (m,
2H), 7.55–7.60 (m, 1H), 7.77–7.81 (m, 2H). 13C NMR
(CDCl3, 100 MHz) d: 175.0 (s), 167.2 (s), 133.9 (s), 132.6
(d), 129.1 (d), 128.6 (d), 99.8 (s), 78.1 (d), 75.9 (d), 75.1 (d),
68.0 (d), 67.3 (d), 63.5 (t), 57.5 (s), 29.7 (q), 24.3 (q), 19.7
(q). MS (ESI) m/z 378 (MþH)þ, 390 (MþNa)þ, 777
(2MþNa)þ. Anal. calcd for C19H23NO7: C, 60.47; H, 6.14;
N, 3.71. Found: C, 60.42; H, 6.32; N, 3.81.
1.1.6. 2-(a-D-C-Mannopyranosyl)-(R)-alanine hydro-
chloride (2) and 2S epimer (3). A suspension of lactone
11a (0.216 g, 0.57 mmol) in 6N HCl (4 mL) in a screw-cap
vial was heated at 808C for 4 h. The suspended solid
dissolved after ca. 30 min. The pale yellow solution was
cooled to room temperature and the precipitated benzoic
acid extracted with methylene chloride (2£4 mL). The
aqueous phase was evaporated and dessicated under vacuum
to a constant weight to afford 2 (0.160 g, 92%) as a white
solid monohydrated salt, as revealed by the elemental
analysis. The same procedure was applied to the lactone 11b
to give 3 (93%).
Iodolactone 10b: white prisms, mp 98–1008C (MeOH–
EtOAc). [a]D¼þ105.1 (c 0.4, MeOH). IR (nujol, cm21):
3378, 1784, 1643, 1308. 1H NMR ((CD3)2SO, 400 MHz) d:
1.52 (s, 3H), 3.42–3.51 (m, 2H), 3.52–3.66 (m, 2H), 4.35
(dd, J¼11.3, 9.1 Hz, 1H), 4.43 (d, J¼6.4 Hz, 1H), 4.68 (t,
J¼5.2 Hz, 1H, exchanges with D2O), 5.04 (dd, J¼9.2,
6.5 Hz, 1H), 5.77 (d, J¼7.5 Hz, 1H, exchanges with D2O),
7.47 (m, 2H), 7.56 (m, 1H), 7.85 (m, 2H), 8.46 (s, 1H). 13C
NMR ((CD3)2SO, 75 MHz) d: 174.5 (s), 166.8 (s), 133.5 (s),
131.8 (d), 128.5 (d), 127.9 (d), 82.8 (d), 80.9 (d), 74.1 (d),
68.4 (d), 60.9 (t), 58.4 (s), 35.6 (d), 23.2 (q)). MS (ESI) m/z
448 (MþH)þ, 470 (MþNa)þ. Anal. calcd for C16H18INO6:
C, 42.97; H, 4.06; N, 3.13. Found: C, 43.02; H, 4.01; N,
3.09.
(2R) Epimer 2: white hygroscopic solid, dec. .2008C.
[a]D¼þ33.1 (c 0.8, MeOH). IR (nujol, cm21): 3359, 2361,
1735, 1376, 1051. 1H NMR (D2O, 400 MHz) d: 1.53 (s, 3H),
3.54 (dd, J¼3.5, 12.4 Hz, 1H), 3.72 (dd, J¼2.7, 4.8 Hz, 1H),
3.77–3.84 (m, 2H), 3.92 (dd, J¼3.6, 9.6 Hz, 1H), 3.96 (dd,
J¼9.1, 12.4 Hz, 1H), 4.05 (d, J¼9.6 Hz, 1H). 13C NMR
(D2O, 100 MHz) d: 172.7 (s), 80.2 (d), 71.4 (d), 70.6 (d),
68.7 (d), 65.2 (d), 62.9 (s), 59.2 (t), 17.4 (q). MS (ESI) m/z
252.1 (M2Cl)þ. Anal. calcd for C9H18NO7. H2O: C, 35.36;
H, 6.59; N, 4.58. Found: C, 35.01; H, 6.71; N, 4.26. (2S).
1.1.5. 2-(40,60-O-Isopropylidene-a-D-mannopyranosyl)-
2-benzamido-(2R)-propionic acid 10,20-lactone (11a) and
2S epimer (11b). To a 0.2 M solution of the oxazolone 8a
(0.200 g) in 5% aqueous acetone was added 4-methyl-
morpholine N-oxide (0.102 g, 0.87 mmol) and 0.58 mL of a
0.1 M solution of OsO4 in tert-butyl alcohol (0.06 mmol).
The pale yellow solution was stirred at room temperature
under nitrogen for 24 h. The solvent was evaporated at
reduced pressure and the residue filtered through a short pad
of silica gel eluting with EtOAc to give 0.216 g of 11a
(98%). The same procedure was adopted for the preparation
of the epimeric lactone 11b (95%).
Epimer 3: white hygroscopic solid, dec. .2058C.
[a]D¼þ71.0 (c 0.5, MeOH). IR (nujol, cm21): 3359,
1735, 1622, 1376, 1054. 1H NMR (D2O, 400 MHz) d:
1.58 (s, 3H), 3.56 (dd, J¼3.4, 12.5 Hz, 1H), 3.72 (dd, J¼2.9,
5.0 Hz, 1H), 3.79–3.85 (m, 2H), 3.97 (d, J¼9.1 Hz, 1H),
3.98 (dd, J¼9.0, 12.5 Hz, 1H), 4.05 (dd, J¼3.5, 9.1 Hz, 1H).
13C NMR (D2O, 100 MHz) d: 172.4 (s), 80.1 (d), 72.4 (d),
70.7 (d), 68.9 (d), 65.1 (d), 61.9 (s), 59.2 (t), 19.9 (q). MS
(ESI) m/z 252.1 (M2Cl)þ. Anal. calcd for C9H18NO7. H2O:
C, 35.36; H, 6.59; N, 4.58. Found: C, 35.12; H, 6.84; N,
4.38.
Lactone 11a: white crystals, mp 251–2528C (EtOAc–
Et2O). [a]D¼þ8.3 (c 0.9, acetone). IR (KBr, cm21): 3402,
1
1771, 1658, 1537, 1060. H NMR (CDCl3, 400 MHz) d:
1.44 (s, 3H), 1.51 (s, 3H), 1.58 (s, 3H), 2.61 (bs, 1H,
exchanges with D2O), 3.73 (dd, J¼10.5, 10.0 Hz, 1H),
3.78–3.86 (m, 1H), 3.96 (dd, J¼10.5, 5.6 Hz, 1H), 4.21 (dd,
J¼11.2, 4.0 Hz, 1H), 4.37 (dd, J¼5.0, 4.0 Hz, 1H), 4.52 (dd,
J¼10.6, 5.0 Hz, 1H), 5.28 (d, J¼10.6 Hz, 1H), 6.40 (bs,
1H), 7.43–7.48 (m, 2H), 7.53–7.58 (m, 1H), 7.80–7.85 (m,
2H). 13C NMR (CDCl3, 100 MHz) d: 174.8 (s), 166.6 (s),
133.9 (s), 132.7 (d), 129.2 (d), 128.5 (d), 99.8 (s), 77.8 (d),
73.4 (d), 71.3 (d), 67.6 (d), 67.5 (d), 63.6 (t), 60.8 (s), 29.7
(q), 19.7 (q), 15.2 (q). MS (ESI) m/z 378 (MþH)þ, 390
(MþNa)þ, 777 (2MþNa)þ. Anal. calcd for C19H23NO7: C,
60.47; H, 6.14; N, 3.71. Found: C, 60.52; H, 6.31; N, 3.82.
Acknowledgements
The authors thank MURST (COFIN 2000, prot.
MM03155477 ‘Synthesis of mimics and analogs of
bioactive natural compounds’) and University of Pavia for
financial support.
References
Lactone 11b: white crystals, mp 235–2368C (EtOAc–
Et2O). [a]D¼244.0 (c 0.8, CDCl3). IR (nujol, cm21): 3439,
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