An efficient synthesis of (R)-3-{(R)-3-[2-O-(α-L-rhamnopyranosyl)-α-L-rhamnopyranosyl] oxydecanoyl}oxydecanoic acid, a rhamnolipid from Pseudomonas aeruginosa

Article abstract of DOI:10.1002/(sici)1099-0690(199802)1998:2<303::aid-ejoc303>3.0.co;2-u

N-iodosuccinmide/triflic acid mediated one-pot two-step glycosylation of ethyl 2,3,4-tri-O-benzyl-1-thio-α-L-rhamnopyranoside (8) with phenyl 3,4-O-(2,3-dimethoxybutane-2,3-diyl)-1-thio-α-L-rhamnopyranoside (10b) and phenacyl (R)-3-hydroxydecanoate (13) gave rhainnolipid 17. The latter was transformed in five steps into the title compound 2. Esterification of 2 with diazomethane resulted into the corresponding methyl ester derivative 1.

Full text of DOI:10.1002/(sici)1099-0690(199802)1998:2<303::aid-ejoc303>3.0.co;2-u

FULL PAPER
-Rhamnopyranosyl)-␣-
An Efficient Synthesis of (R)-3-{(R)-3-[2-O-(␣-
L
L-
rhamnopyranosyl]oxydecanoyl}oxydecanoic Acid, a Rhamnolipid from
Pseudomonas Aeruginosa
Howard I. Duynstee, Michiel J. van Vliet, Gijsbert A. van der Marel, and Jacques H. van Boom*
Leiden Institute of Chemistry, Gorlaeus Laboratories, University of Leiden,
P. O. Box 9502, NL-2300 RA Leiden, The Netherlands
Fax: (internat.) ϩ31(0)71/5274307
Received October 27, 1997
Keywords: Pseudomonas aeruginosa / Rhamnolipid / One-pot two-step glycosylation / Chemoselectivity / Esterification /
Carbohydrates
N-iodosuccinimide/triflic acid mediated one-pot two-step
glycosylation of ethyl 2,3,4-tri-O-benzyl-1-thio-α- -rhamno- transformed in five steps into the title compound 2.
pyranoside (8) with phenyl 3,4-O-(2,3-dimethoxybutane-2,3-
hydroxydecanoate (13) gave rhamnolipid 17. The latter was
L
Esterification of 2 with diazomethane resulted into the corre-
sponding methyl ester derivative 1.
diyl)-1-thio-α-L-rhamnopyranoside (10b) and phenacyl (R)-3-
Introduction
It is well established[1] that the rhamnolipids methyl (R)-
3-{(R)-3-[2-O-(α--rhamnopyranosyl)-α--rhamnopy-
ranosyl]xydecanoyl}oxydecanoate (1) and the correspond-
ing acid 2 are both produced by Pseudomonas aeruginosa.
They exhibit interesting biological properties such as anti-
bacterial, mycoplasmacidal, and antiviral activities[2]. In
addition, compound 2 may also function as an immunomo-
dulator of autoimmune diseases[3]
.
Several years ago we reported[4] the synthesis of rhamno-
lipid 1 from the building units 3,4-di-O-benzyl-2-O-chlo-
roacetyl-α--rhamnosylpyranosyl fluoride (3) and methyl
(R)-3-[(R)-3-hydroxydecanoyl]oxydecanoate (4). It is evi-
dent that the biologically interesting rhamnolipid 2 would
be accessible in a similar fashion from 3 and phenacyl (R)-
3-[(R)-3-hydroxydecanoyl]oxydecanoate (5). However, the
synthesis of the target molecule 2 on a multigram scale is
rather time-consuming due to the multi-step preparation of
rhamnopyranosyl donor 3.
Interestingly, Ley et al. revealed[5] that the fully protected
methyl α-(1Ǟ2)--trirhamnoside 6 could be assembled ster-
eospecifically in a one-pot two-step process by a sequential
condensation of HO-2 and C-1 in the cyclohexane-1,2-di-
acetal (CDA) protected ethyl 1-thio-α--rhamnopyranoside
(7) with ethyl 2,3,4-tri-O-benzyl-1-thio-α--rhamnopyrano-
side (8) and methyl 3,4-O-(1,2-dimethoxycyclohexane-1,2-
diyl)-α--rhamnopyranoside (9). Later on, Frost et al.
showed[6] that the butane-2,3-diacetal (BDA) group is an
inexpensive and efficient alternative for the CDA group in
the selective protection of vicinal diequatorial diols.
We here describe a convenient and straightforward syn-
thetic route to rhamnolipid 2 starting from 8, ethyl 3,4-O-
(2,3-dimethoxybutane-2,3-diyl)-1-thio-α--rhamnopyr-
anoside (10a) and phenacyl (R)-3-hydroxydecanoate (13).
Eur. J. Org. Chem. 1998, 303Ϫ307
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303

H. I. Duynstee, M. J. van Vliet, G. A. van der Marel, J. H. van Boom
FULL PAPER
Results and Discussion
(TfOH) with the known[7] ethyl 2,3,4-tri-O-benzyl-1-thio-α-
-rhamnopyranoside (8).
Analysis of the glycosylation by TLC showed the pres-
It was expected[4] that removal of the 4-oxopentanoate
(levulinate) ester from the fully protected di-lipid derivative ence of 16a as the main product. After addition of the
11, prepared by esterification of (R)-3-levulinoyloxyde- phenacyl protected decanoate derivative 13, followed by an
canoic acid (12)[4] with 13[4], would afford the required lipid additional amount of the same promotor NIS cat. TfOH,
building unit 5. However, esterification of 12 with 13 under the fully protected rhamnolipid 17 was isolated in an overall
the agency of either dicyclohexylcarbodiimide (DCC) or di- yield of 55%. In this respect, it is of interest to note that a
isopropylcarbodiimide (DIC) in the presence of 4-(dimeth- similar yield was obtained using the cyclohexane-1,2-diace-
ylamino)pyridine (DMAP) led to the isolation of 11 in an tal, instead of the butane-2,3-diacetal, protected ethyl 1-
unsatisfactory yield of 19%. Moreover, it was also found thio-α--rhamnopyranoside 7 as the building unit in the
that the condensation was accompanied by the formation same one-pot approach. These results indicate that the opti-
of the unwanted products 14[4] (36%) and 15 (18%).
mal level of chemoselectivity of the iodonium-ion mediated
Scheme 1
condensation of donor 8 with acceptor 10a or 7 is not fully
met. It occurred to us, that the chemoselectivity of the first
step [i.e., formation of the α-(1ЈǞ2) linkage] could be en-
hanced by using the relatively more “disarmed”[8] phenyl
3,4-O-BDA-1-thio-α--rhamnopyranoside (10b) as ac-
ceptor. Indeed, the fully protected rhamnolipid 17 was ob-
tained in an overall yield of 67% in the NIS cat. TfOH
assisted one-pot two-step condensation of equimolar
amounts of the three compounds 8, 10b and 13. Apart from
this, it was the more gratifying to establish that 17 was iso-
lated in an excellent overall yield of 75% by executing the
same one-pot two-step condensation with a slight excess of
compound 8 (1.2 equiv.) and 13 (1.3 equiv.).
The low yield of 11, as well as the occurrence of the latter
side products may be explained by the migratory aptitude
of the 3-levulinoyl function in the acylurea adduct A gener-
ated in the reaction of 12 with DCC or DIC. Thus, mi-
gration of the levulinoyl group in A, instead of esterification
of 13 with A, will result in the mixed anhydride B which in
turn reacts with 13 to give 14 and 15. On the basis of the
above result, it was decided to employ phenacyl (R)-3-
hydroxydecanoate (13) for the introduction of the α-(1-3)-
linked (R)-3-[(R)-3-hydroxydecanoyl] oxydecanoic acid
moiety via a two-step process comprising glycosylation and
esterification. In order to achieve this goal most effectively,
we adopted the one-pot two-step protocol of Ley for the
assemblage of the fully protected phenacyl (R)-3-[2-O-(α--
Next, attention was directed to the introduction of sec-
rhamnopyranosyl)-α--rhamnopyranosyl]oxydecanoate ond (R)-3-hydroxydecanoic acid moiety. Thus, the phenacyl
(17, see Scheme 2). To this end, ethyl 3,4-O-(2,3-dimeth- group in 17 was removed under the agency of activated
oxybutane-2,3-diyl)-1-thio-α--rhamnopyranoside
(10a), zinc, and the resulting free acid derivative 18 was condensed
prepared according to Frost, was glycosylated in the pres- with 13 in the presence of DCC/DMAP to give the fully
ence of N-iodosuccinimide (NIS) and catalytic triflic acid protected target compound 19 in an overall yield of 70%.
304
Eur. J. Org. Chem. 1998, 303Ϫ307

An Efficient Synthesis of (R)-3-{(R)-3-[2-O-(α-L-Rhamnopyranosyl)-α-L-rhamnopyranosyl]
FULL PAPER
ylamino)pyridine, N-iodosuccinimide were purchased from Acros.
1-Methyl-3-nitro-1-nitrosoguanidine was purchased from Aldrich.
1-Methyl-3-nitro-1-nitrosoguanidine diazomethane generation ap-
paratus was purchased from Aldrich. Ϫ Reactions were monitored
by TLC analysis using Schleicher-and-Schüll DC Fertigfolien (F
1500 LS 254). Ϫ Compounds were visualised by UV light and by
spraying with 20% sulphuric acid in methanol followed by charring
at 140°C. Ϫ Column chromatography was performed on silica gel
60, 230Ϫ400 mesh (Merck). Ϫ Optical rotations were measured
with a Propol polarimeter. Ϫ NMR spectra were recorded with a
Jeol JNM-FX-200 (1H and 13C at 200 and 50.1 MHz, respectively),
and a Bruker WM-300 spectrometer equipped with an Aspect 2000
computer (1H and 13C at 300 and 75 MHz, respectively). Chemical
shift are given in ppm (δ) relative to tetramethylsilane as an internal
standard. Ϫ Mass spectra were recorded with a Finnigan MAT
TSQ-70 equipped with a custom-made Electrospray Interface
(ESI).
Scheme 2. Reagents and conditions: i) a. 10b (1 equiv.), 8 (1.2
equiv.), 1,2-dichloroethane/diethyl ether (1:1, v/v), NIS
cat. TfOH (75%). b. 13 (1.3 equiv.), NIS cat. TfOH
(75%). Ϫ ii) Zn in AcOH (1 h, 96%). Ϫ iii) 13 (1.2
equiv.), DCC (1.1 equiv.) and DMAP (0.6 equiv.) in
CH2Cl2 (18 h, 73%). Ϫ iv) Zn in AcOH (1 h, 96%)
Phenacyl (R)-3-[(R)-3-Levulinoyloxydecanoyl]oxydecanonate
(11), Phenacyl (R)-3-Levulinoyloxydecanate (14) and Phenacyl (R)-
3-[(E)-2-Decenoyl]oxydecanate (15): To a solution of 12[4] (310
mg, 1.08 mmol) and 13[4] (368 mg, 1.2 mmol) in CH2Cl2 (5 ml)
was added N,N-diisopropylcarbodiimide and 4-(dimethylamino)py-
ridine (67 mg, 0.55 mmol). TLC-analysis showed, after stirring for
18 h, the formation of three products. The reaction mixture was
diluted with CH2Cl2 (40 ml) washed with 1  NaHCO3 (20 ml)
and H2O, dried (MgSO4) and concentrated. The mixture was puri-
fied by silca gel column chromatography (light petroleum/ethyl
acetate, 9:1 Ǟ 6:4, v/v). Concentration of the appropriate fractions
afforded 11, known 14[4] and 15 in 19%, 36% and 18% yield, respec-
tively. 11: [α]D ϭ Ϫ1.1 (c ϭ 1.0, CHCl3). Ϫ Rf ϭ 0.3 (light petro-
1
leum/ethyl acetate, 8:2). Ϫ H NMR (CDCl3): δ ϭ 0.88 (m, 6 H, 2
ϫ CH3), 1.27 [m, 20 H, 2 ϫ -(CH2)5-], 1.59 (m, 4 H, 2 ϫ CH2γ),
2.15 (s, 3 H, CH3 Lev), 2.57 (m, 4 H, CH2 Lev, CH2αЈ), 2.74 (m,
4 H, CH2 Lev, CH2α), 5.23 (t, 1 H, CHβЈ), 5.30 (t, 1 H, CHβ),
5.35 (s, 2 H, CH2COPh), 7.48 (m, 2 H, CH2COPh), 7.60 (m, 1 H,
CH2COPh), 7.92 (d, 2 H, CH2COPh). Ϫ 13C{1H} NMR (CDCl3):
δ ϭ 13.9 (CH3), 22.5, 25.0, 29.0, 29.2, 31.6, 33.8 [2 ϫ -(CH2)6-],
28.0 (CH2 Lev), 29.6 (CH3 Lev), 37.8 (CH2 Lev), 38.6 (Cα), 39.0
(CαЈ), 66.0 (CH2COPh), 70.7 (Cβ, CβЈ), 127.6Ϫ133.7 (CH2COPh),
169.6 (CϭO Lev), 171.8 (CϭO), 192.4 (CH2COPh), 206.4 (CϭO
Lev). 14: [α]D ϭ Ϫ2.8 (c ϭ 1.0, CHCl3). Ϫ Rf ϭ 0.2 (light petro-
leum/ethyl acetate, 8:2). Ϫ 1H NMR (CDCl3): δ ϭ 0.86 (t, 3 H,
CH3), 1.27 [m, 10 H, -(CH2)5-], 1.55 (2 H, m, CH2γ), 2.18 (s, 3 H,
CH3 Lev), 2.57 (t, 2 H, CH2 Lev), 2.74 (m, 4 H, CH2 Lev, CH2α),
5.30 (t, 1 H, CHβ), 5.35 (s, 2 H, CH2COPh), 7.48 (m, 2 H,
CH2COPh), 7.60 (m, 1 H, CH2COPh), 7.92 (d, 2 H, CH2COPh).
Complete deprotection of 19 proceeded smoothly by the
following three-step process. Dephenacylation with acti-
vated zinc dust, followed by acid hydrolysis of the diacetal
group in 20, and subsequent debenzylation gave, after puri-
fication, homogeneous 2 in a yield of 76% over the three
steps. Diazomethylation of 2 proceeded quantitatively to
give the corresponding methyl ester 1. The physical and
spectral data of which were identical with those of an
authentic sample[4]
.
Conclusion
The results presented in this paper clearly show that the
one-pot synthesis of key intermediate 17 is a high yielding
and stereoselective process. The latter possibility opens the
way to a convenient multigram synthesis of the naturally
occurring biological interesting rhamnolipids 1؊2.
Ϫ
13C{1H} NMR (CDCl3): δ ϭ 13.9 (CH3), 22.4, 24.9, 28.9, 29.1,
31.5, 33.8 [-(CH2)6-], 28.0 (CH2 Lev), 28.8 (CH3 Lev), 37.8 (CH2
Lev), 38.6 (Cα), 65.9 (CH2COPh), 70.5 (Cβ), 127.6Ϫ133.7
(CH2COPh), 169.7 (CϭO Lev), 171.9 (CϭO), 192.4 (CH2COPh),
206.4 (CϭO Lev). 15: [α]D ϭ Ϫ1.9 (c ϭ 1.0, CHCl3). Ϫ Rf ϭ 0.8
(light petroleum/ethyl acetate, 8:2). Ϫ 1H NMR (CDCl3): δ ϭ 0.88
(m, 6 H, 2 ϫ CH3), 1.27 [m, 20 H, 2 ϫ -(CH2)5-], 1.59 (m, 2 H,
CH2γ), 2.21 (m, 2 H, CH2γЈ), 2.79 (m, 2 H, CH2α), 5.34 (m, 3 H,
CHβ, CH2COPh), 5.81 (d, 1 H, HαЈ, JαЈ,βЈ ϭ 17.4 Hz), 5.81 (m, 1
H, HβЈ), 7.48 (m, 2 H, CH2COPh), 7.60 (m, 1 H, CH2COPh), 7.92
(d, 2 H, CH2COPh). Ϫ 13C{1H} NMR (CDCl3): δ ϭ 14.0 (CH3),
22.5, 25.0, 27.9, 28.9, 29.0, 29.2, 31.6. 32.3, 33.9 [2 ϫ -(CH2)6-], 38.8
(Cα), 66.0 (CH2COPh), 70.1 (Cβ), 120.9 (CαϭCβ), 127.6Ϫ133.7
(CH2COPh), 149.8 (CαϭCβ), 169.6 (CϭO Lev), 171.8 (CϭO),
192.4 (CH2COPh), 206.4 (CϭO Lev).
Experimental Section
General Methods and Materials: Dichloromethane, diethyl ether
and toluene were dried by distillation from P2O5 (5 g lϪ1) and
˚
stored over molecular sieves (4 A, Acros). 1,2-Dichloroethane (p.a.,
Rathburn) and tetrahydrofuran (p.a., Acros) were stored over mo-
˚
lecular sieves (4 A, Acros) and used without further purification.
Acetic acid (p.a., Baker) and chloroform (p.a., Baker) were used as
received. Trimethyl orthoformiat was refluxed for 2 h in the pres-
ence of CaH2 (5 g lϪ1), subsequently distilled. Solvents used for
column chromatography were of technical grade and distilled be-
fore use. Trifluoromethanesulfonic acid, 2,3-butanedione, N,N-di-
isopropylcarbodiimide, N,N-dicyclohexylcarbodiimide, 4-(dimeth-
Ethyl 3,4-O-(2,3-Dimethoxybutane-2,3-diyl)-1-thio-Ͱ--rhamno-
pyranoside (10a): To a solution of ethyl -thio-α--rhamnopyrano-
Eur. J. Org. Chem. 1998, 303Ϫ307
305

H. I. Duynstee, M. J. van Vliet, G. A. van der Marel, J. H. van Boom
FULL PAPER
side (3.34 g, 16.2 mmol), trimethylorthoformiaat (5.2 ml, 48 mmol)
(R)-3-[2-O-(2Ј,3Ј,4Ј-Tri-O-benzyl-Ͱ--rhamnopyranosyl)-3,4-O-
and 2,3-butanedione (1.6 ml, 18 mmol) in methanol (20 ml) was (2,3-dimethoxy-butane-2,3-diyl)-Ͱ--rhamnopyranosyl]oxydecanoic
added camphersulfonic acid (200 mg). The mixture was heated un- Acid (18): A solution of 17 (5.84 g, 5.94 mmol) in acetic acid (60
der reflux for 18 h. The cooled reaction mixture was then neutral-
ml) was stirred with activated zinc dust for 1.5 h at 20°C, filtered
ised with triethylamine and concentrated under reduced pressure. and concentated. Traces of acetic acid were removed by evapo-
Purification by chromatography (light petroleum/Et2O, 2:1 Ǟ 1:1,
v/v) provided 10a in 75% yield (5.22 g). Rf ϭ 0.3 (light petroleum/
ration with toluene. The product was chromatographed on silica
gel (light petroleum/ether, 4:1 Ǟ 1:2, v/v) to give pure 18 in 96%
Et2O, 1:1, v/v). Ϫ 1H NMR (CDCl3): δ ϭ 1.21Ϫ1.36 (m, 12 H, yield (4.92 g). Ϫ [α]D ϭ Ϫ74.6 (c ϭ 1.0, CHCl3). Ϫ Rf ϭ 0.35 (light
1
CH3 SEt, 2 ϫ CH3 BDA, H-6), 2.60 (m, 2 H, CH2 SEt), 3.23 (s, 3 petroleum/Et2O, 1:1, v/v). Ϫ H NMR (CDCl3): δ ϭ 0.86 (t, 3 H,
H, OMe BDA), 3.26 (s, 3 H, OMe BDA), 3.75 (t, 1 H, H-4, J3,4
ϭ
CH3), 1.15Ϫ1.47 (m, 22 H, -(CH2)5-, 2 ϫ CH3 BDA, H-6, H-6Ј),
9.8 Hz), 3.91 (dd, 1 H, H-3, J2,3 ϭ 3.0 Hz), 4.02 (br s, 1 H, H-2), 1.65 (br d, 2 H, CH2γ), 2.55 (m, 2 H, CHЉ), 3.20 (s, 3 H, OMe
4.14 (m, 1 H, H-5), 5.24 (s, 1 H, H-1). Ϫ 13C{1H} NMR (CDCl3): BDA), 3.24 (s, 3 H, OMe BDA), 3.58 (m, 2 H, H-4, H-4Ј), 3.70 (m,
δ ϭ 14.7 (C-6), 16.3 (CH3 SEt), 17.3, 17.5 (2 ϫ CH3 BDA), 24.9 2 H, H-5, H-5Ј), 3.95 (m, 4 H, H-2, H-2Ј, H-3, H-3Ј), 4.06 (m, 1
(CH2 SEt), 47.3, 47.7 (2 ϫ OMe BDA), 66.6, 68.4, 68.6, 71.0 ( C-
2, C-3, C-4, C-5), 84.1 (C-1), 99.5, 99.9 (2 ϫ Cq BDA).
H, CHγ), 4.48 (AB, 2 H, CH2 Bn), 4.73 (AB, 2 H, CH2 Bn), 4.79
(AB, 2 H, CH2 Bn), 4.80 (s, 1 H, H-1Ј), 5.35 (s, 1 H, H-1),
7.20Ϫ7.38 (m, 15H Bn). Ϫ 13C{1H} NMR (CDCl3): δ ϭ 14.0
(CH3), 16.4 (C-6), 17.7, 17.8, 18.0 (2 ϫ CH3 BDA, C-6Ј), 22.5,
24.7, 29.1, 29.4, 31.6, 33.4 [-(CH2)6-], 40.0 (Cα), 47.6, 47.8 (2 ϫ
OMe BDA), 71.6, 71.9, 75.1, (3 ϫ CH2 Bn), 67.2, 68.2, 68.5, 68.7,
73.8, 74.1, 74.7, 79.1, 80.4 (C-2,2Ј, C-3,3Ј, C-4,4Ј, C-5,5Ј, Cβ), 98.0,
98.4 (C-1,1Ј), 99.4, 99.5 (2 ϫ Cq BDA), 128.2Ϫ127.4 (CH arom.),
138.3, 138.5, (3 ϫ Cq Bn), 176.0 (CϭO).
Phenyl 3,4-O-(2,3-Dimethoxybutane-2,3-diyl)-1-thio-Ͱ--rham-
nopyranoside (10b): Phenyl -thio-α--rhamnopyranoside was con-
verted in DBA-proteced 10b as described above for the synthesis
of 10a. Yield 74%. Ϫ Rf ϭ 0.85 (light petroleum/Et2O, 3:2, v/v). Ϫ
1H NMR (CDCl3): δ ϭ 1.16Ϫ1.30 (m, 6 H, 2 ϫ CH3 BDA), 1.32
(d, 3 H, H-6, J5,6 ϭ 5.0 Hz), 3.25 (s, 3 H, OMe BDA), 3.31 (s, 3
H, OMe BDA), 3.78 (t, 1 H, H-4, J3,4 ϭ 9.9 Hz), 3.98 (dd, 1 H,
H-3, J2,3 ϭ 2.9 Hz), 4.18 (br s, 1 H, H-2), 4.26 (m, 1 H, H-5), 5.50
Phenacyl (R)-3-{(R)-3-[2-O-(2Ј,3Ј,4Ј-Tri-O-benzyl-Ͱ--rhamno-
(s, 1 H, H-1), 7.30 (m, 3 H Ph), 7.45 (m, 2 H Ph). Ϫ 13C{1H} NMR pyranosyl)-3,4-O-(2,3-dimethoxybutane-2,3-diyl)-Ͱ--rhamnopyra-
(CDCl3): δ ϭ 15.9 (C-6), 16.9, 17.2 (2 ϫ CH3 BDA), 46.9, 47.3 (2 nosyl]oxydecanoyl}oxydecanonate (19): To a solution of acid 18
ϫ OMe BDA), 67.1, 67.9, 68.5, 70.5 (C-2, C-3, C-4, C-5), 87.4 (C-
(8.80 g, 10.2 mmol) and phenacyl (R)-3-hydroxydecanoate 13 (3.43
1), 99.2, 99.6 (2 ϫ Cq BDA), 130.5, 128.3, 130.5 (CH Ph), 133.8 g, 11.16 mmol) in CH2Cl2 (120 ml) was added N,N-dicyclohexylcar-
(Cq Ph).
Phenacyl
bodiimide (2.52 g, 12.0 mmol) and 4-(dimethylamino)pyridine (744
mg, 5.0 mmol). After stirring for 18 h the reaction mixture was
diluted with diethyl ether (400 ml) and the dicyclohexylurea was
removed by filtration. The filtrate was washed with water (100 ml),
dried (MgSO4) and concentrated. Purification was preformed by
column chromatography (light petroleum/ether, 4:1 Ǟ 1:2, v/v) to
afford 19 in 73% yield (8.56 g, 7.4 mmol). [α]D ϭ Ϫ50.1 (c ϭ 1.0,
(R)-3-[2-O-(2Ј,3Ј,4Ј-Tri-O-benzyl-Ͱ--rhamnopyra-
nosyl)-3,4-O-(2,3-dimethoxybutane-2,3-diyl)-Ͱ--rhamnopyra-
nosyl]oxydecanoate (17): Ethyl 2,3,4-tri-O-benzyl-1-thio-α--rham-
nopyranoside (8) (5.74 g, 12.0 mmol) and phenyl 3,4-O-(2,3-di-
methoxybutane-2,3-diyl)-1-thio-α--rhamnopyranoside (10b) (3.70
g, 10.0 mmol) were dissolved in 1,2-dichloroethane ether (1:1,v/v,
1
CHCl3). Ϫ Rf ϭ 0.8 (light petroleum/Et2O, 1:1, v/v). Ϫ H NMR
˚
100 ml). Powdered molecular sieves (4 A, 5g) were added and the
(CDCl3): δ ϭ 0.87 (t, 6 H, 2 ϫ CH3), 1.15Ϫ1.38 (m, 32 H, 2 ϫ
-(CH2)5-, 2 ϫ CH3 BDA ,H-6, H-6Ј), 1.53 (br d, 2 H, CH2γ), 1.62
(br d, 2 H, CH2γЈ), 2.79 (m, 4 H, CHα, CH2αЈ), 3.20 (s, 3 H, OMe
BDA), 3.24 (s, 3 H, OMe BDA), 3.57 (m, 2 H, H-4, H-4Ј), 3.78 (m,
2 H, H-5, H-5Ј), 3.96 (m, 4 H, H-2, H-2Ј, H-3, H-3Ј), 4.06 (m, 2
H, CH(, CHγЈ), 4.50 (AB, 2 H, CH2 Bn), 4.68 (AB, 2 H, CH2 Bn),
4.78 (AB, 2 H, CH2 Bn), 4.79 (s, 1 H, H-1), 5.27 (d, 2 H,
CH2COPh), 5.37 (s, 1 H, H-1Ј), 7.20Ϫ7.35 (m, 15H Bn), 7.43 (dd,
3 H, CH2COPh), 7.91 (d, 2 H, CH2COPh). Ϫ 13C{1H} NMR
(CDCl3): δ ϭ 13.9 (2 ϫ CH3), 16.4 (C-6), 17.6, 17.8, 18.0 (2 ϫ CH3
BDA, C-6Ј), 22.6, 24.7, 25.0, 29.0, 29.1, 29.5, 31.6, 33.6 [2 ϫ
-(CH2)6-], 38.6, 40.4 (Cα,αЈ), 47.5, 47.8 (2 ϫ OMe BDA), 71.5,
71.9, 75.0, (3 ϫ CH2 Bn), 67.1, 68.1, 68.5, 68.7, 70.5, 73.7, 74.6,
79.0, 80.4 (C-2,2Ј, C-3,3Ј, C-4,4Ј, C-5,5Ј, Cβ,βЈ), 65.9 (CH2COPh),
97.9, 99.1 (C-1,1Ј), 99.3, 99.4 (2 ϫ Cq BDA), 127.2Ϫ133.6 (CH,
arom.), 133.8 (Cq, CH2COPh) 138.3, 138.5, (3 ϫ Cq Bn), 169.6,
176.0 (2 ϫ CϭO), 191.5 (CϭO, CH2COPh).
mixture were kept for 15 min. at 0°C. N-iodosuccinimide (2.70 g,
12.0 mmol) and triflic acid (108 µl, 1.2 mmol) was added. After
stirring for 5 min, to the reaction mixture was added a solution of
phenacyl (R)-3-hydroxydecanoate 13 (3.98 g, 13.0 mmol) in 1:1 1,2-
dichloroethane ether (v/v, 30 ml). A second portion of N-iodosucci-
nimide (2.70 g, 12.0 mmol) and triflic acid (108 µl, 1.2 mmol) was
added and stirring was continued for 10 min. The reacion mixture
was filtered and the filtrate was diluted with CH2Cl2, washed with 1
 Na2S2O3, 1  NaHCO3, H2O, dried (MgSO4) and concentrated.
Purification of the residual oil on silica gel (100:0 Ǟ 95:5, toluene/
ethyl acetate, v/v) furnished compound 17. Yield 7.38 g (75%). Ϫ
[α]D ϭ Ϫ83.2 (c ϭ 1.0, CHCl3). Ϫ Rf ϭ 0.35 (light petroleum/Et2O,
1
1:1, v/v). Ϫ H NMR (CDCl3): δ ϭ 0.86 (t, 3 H, CH3), 1.13Ϫ1.43
[m, 22 H, -(CH2)5-, 2 ϫ BDA ,H-6, H-6Ј], 1.62 (br d, 2 H, CH2γ),
2.73 (m, 2 H, CHα), 3.21 (s, 3 H, OMe BDA), 3.25 (s, 3 H, OMe
BDA), 3.61 (m, 2 H, H-4, H-4Ј), 3.74 (m, 2 H, H-5, H-5Ј), 3.95 (m,
4 H, H-2, H-2Ј, H-3, H-3Ј), 4.15 (m, 1 H, CHγ), 4.50 (AB, 2 H,
CH2 Bn), 4.64 (AB, 2 H, CH2 Bn), 4.81 (AB, 2 H, CH2 Bn), 4.78
(R)-3-{(R)-3-[2-O-(2Ј,3Ј,4Ј-Tri-O-benzyl-Ͱ--rhamnopyrano-
(s, 1 H, H-1Ј), 5.35 (s, 1 H, H-1), 5.37 (s, 2 H, CH2COPh), syl)-3,4-O-(2,3-dimethoxybutane-2,3-diyl)-Ͱ--rhamnopyrano-
7.20Ϫ7.38 (m, 15H Bn), 7.46 (dd, 3 H, CH2COPh), 7.92 (d, 2 H, syl]oxydecanoyl}oxydecanoic Acid (20): Compound 20 was pre-
CH2COPh). Ϫ 13C{1H} NMR (CDCl3): δ ϭ 13.8 (CH3), 16.4 (C- pared in an indentical way as described for the synthesis of acid
6), 17.5, 17.7, 17.8 (2 ϫ CH3 BDA, C-6Ј), 22.3, 24.4, 28.8, 29.2, 18. Purification was preformed by column chromatography (light
31.5, 33.1 [(CH2)6], 39.8 (Cα), 47.3, 47.6 (2 ϫ OMe BDA), 65.5 petroleum/ether, 4:1 Ǟ 1:1, v/v) to afford 20 in 96% yield. Ϫ [α]D ϭ
(CH2COPh), 71.3, 71.6, 72.4, (3 ϫ CH2 Bn), 67.0, 67.9, 68.4, 68.5, Ϫ74.0 (c ϭ 1.0, CHCl3). Ϫ Rf ϭ 0.35 (light petroleum/Et2O, 1:1,
73.6, 73.8, 74.4, 78.8, 80.2 (C-2,2Ј, C-3,3Ј, C-4,4Ј, C-5,5Ј, Cβ), 97.9,
98.2 (C-1,1Ј), 99.2, 99.3 (2 ϫ Cq BDA), 127.2Ϫ133.4 (CH arom.), 17.6, 17.9, 18.0 (2 ϫ CH3 BDA, C-6Ј), 22.5, 24.6, 28.9, 29.1, 29.3,
v/v). Ϫ 13C{1H} NMR (CDCl3): δ ϭ 13.9 (2 ϫ CH3), 16.3 (C-6),
133.9 (Cq, CH2COPh), 138.1, 138.2, 138.3 (3 ϫ Cq Bn), 170.2 (Cϭ
O), 191.6 (CϭO, CH2COPh).
29.5, 31.6, 32.5, 34.1 [2 ϫ -(CH2)6 -], 38.6, 39.5 (Cα,αЈ), 47.6, 47.8
(2 ϫ OMe BDA), 71.6, 71.9, 75.0, (3 ϫ CH2 Bn), 66.6, 68.1, 68.7,
306
Eur. J. Org. Chem. 1998, 303Ϫ307

An Efficient Synthesis of (R)-3-{(R)-3-[2-O-(α-
L-Rhamnopyranosyl)-α-L-rhamnopyranosyl]
FULL PAPER
70.8, 72.3, 73.8, 74.6, 79.0, 80.4 (C-2,2Ј, C-3,3Ј, C-4,4Ј, C-5,5Ј, [m, 26 H, 2 ϫ -(CH2)5-, H-6, H-6Ј], 1.54 (br s, 2 H, CH2γ), 1.62
Cβ,βЈ), 96.6, 98.1 (C-1,1Ј), 99.2, 100.0 (2 ϫ Cq BDA), 126.8Ϫ128.1
(CH arom.), 138.3, 138.5 (3 ϫ Cq Bn), 171.0, 171.9 (2 ϫ CϭO).
(br s, 2 H, CH2γЈ), 2.54 (m, 4 H, CH2α,CH2αЈ), 3.38 (t, 1 H, H-4,
H-4Ј J3,4.J3Ј4Ј ϭ 9.5 Hz ), 3.67 (m, 5 H, H-2, H-3, H-3Ј, H-5, H-
5Ј), 3.97 (dd, 1 H, H-2Ј), 4.05 (m, 2 H, CHβ), 4.83 (d, 1 H, H-1Ј),
4.92 (m, 1 H, H-1), 5.25 (m, 1 H, CHβЈ). Ϫ 13C{1H} NMR
(MeOD): δ ϭ 14.47 (2 ϫ CH3), 17.95, 17.97 (C-6,6Ј), 23.48, 25.56,
25.98, 30.07, 30.09, 30.20, 30.54, 32.69, 32.74, 33.94, 34.87 [2 ϫ
-(CH2)6-], 39.76, 40.95 (Cα,αЈ), 69.74, 69.95, 71.61, 71.69, 71.95,
72.05, 73.55, 74.04, 74.73, 80.16 (C-2,2Ј, C-3,3Ј, C-4,4Ј, C-5,5Ј,
Cβ,βЈ) 98.56, 103.83 (C-1,1Ј), 172.0, 174.0 (2 ϫ CϭO). Ϫ MS
(ESI); m/z: 673 [M ϩ Na]ϩ.
(R)-3-{(R)-3-[2-O-(2Ј,3Ј,4Ј-Tri-O-benzyl-Ͱ--rhamnopyra-
nosyl)-Ͱ--rhamnopyranosyl]oxydecanoyl}oxy-(R)-decanoic Acid:
Compound 20 (6.32 g, 6.12 mmol) was dissolved in 4:1 acetic acid-
water (v/v, 80 ml) and stirred for 2 h at 100°C, then concentrated,
and toluene (3 ϫ 75 ml) was evaporated from the residue. Purifi-
cation of the remaining oil on silica gel (toluene/ethyl acetate/
AcOH, 100:0/0.5 Ǟ 75:25:0.5, v/v/v) afforded the title compound
(4.88 g, 87%). Ϫ [α]D ϭ Ϫ53.6 (c ϭ 1.0, CHCl3). Ϫ Rf ϭ 0.28
Ϫ
1H NMR
Methyl (R)-3-{(R)-3-[2-O-(Ͱ--Rhamnopyranosyl)-Ͱ--rham-
nopyranosyl]oxydecanoyl}oxydecanoate (1): A yellow solution of
diazomethane in tetrahydrofuran was prepared using a 1-methyl-3-
nitro-1-nitrosoguanidine diazomethane generation apparatus. The
diazomethane solution was transferred by pipette into a solution
of carboxylic acid 2 (239 mg, 0.19 mmol). The excess of diazometh-
ane was quenched by dropwise addition of AcOH and the solution
was concentrated under reduced pressure to afford rhamnolipid (1).
The physical and spectral data of compound 1[4] is in every aspect
identical with an authentic sample prepared earlier by us.
(toluene/ethyl acetate/AcOH, 85:15:0.5, v/v/v).
(CDCl3): δ ϭ 0.88 (t, 6 H, 2 ϫ CH3), 1.10Ϫ1.30 (m, 26 H, 2 ϫ
-(CH2)5-, H-6, H-6Ј), 1.48 (br s, 2 H, CH2γ), 1.52 (br s, 2 H,
CH2γЈ), 2.15 (m, 2 H, CH2α), 2.32 (d, 2 H, CH2αЈ), 3.26 (t, 1 H,
H-4Ј), 3.62Ϫ3.68 (m, 3 H, H-4, H-2Ј, H-5Ј), 3.67Ϫ3.88 (m, 3 H,
H-5, H-3, H-3Ј), 4.04 (s, 1 H, H-2) 4.31 (br s, 2 H, CHβ), 4.62 (AB,
2 H Bn), 4.70 (AB, 2 H Bn), 4.83 (AB, 2 H Bn), 4.91 (d, 1 H, H-
1), 4.97 (m, 1 H, H-1Ј), 5.46 (br s, 1 H, CHβЈ), 7.23Ϫ7.44 (15H
Bn). Ϫ 13C{1H} NMR (CDCl3): δ ϭ 14.0 (2 ϫ CH3), 17.5, 17.9
(C-6,6Ј) 22.5, 24.1, 25.0, 29.0, 29.1, 29.3, 29.7, 31.2, 31.7, 34.4
[2 ϫ -(CH2)6-], 38.7, 39.6 (Cα,αЈ), 67.2, 68.7, 69.7, 70.4, 70.6, 74.3,
75.8, 79.6, 80.1, 80.4 (C-2,2Ј, C-3,3Ј, C-4,4Ј, C-5,5Ј, Cβ,βЈ), 72.1,
72.5, 75.2 (3 ϫ CH2 Bn) 93.2, 100.9 (C-1,1Ј), 127.0Ϫ127.9 (CH
arom.), 138.4, 138.5, 138.8 (3 ϫ Cq Bn), 171.3, 173.6 (2 ϫ CϭO).
[1] [1a]
M. K. Johnson, D. Boese-Marazzo, Infect. Immunol. 1980,
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[1d]
(R)-3-{(R)-3-[2-O-(Ͱ- -rhamnopyranosyl)-Ͱ- -rham-
nopyranosyl]oxydecanoyl}oxydecanoic Acid (2): Palladium on car-
bon (10%) was added to a solution of (R)-3-{(R)-3-[2-O-(2Ј,3Ј,4Ј-
tri-O-benzyl-α--rhamnopyranosyl)-α--rhamnopyra-
nosyl]oxydecanoyl}oxy-(R)-decanoic acid (4.36 g, 4.76 mmol) in a
mixture of 2-propanol/water/acetic acid (20:1:1, v/v/v, 50 ml). The
mixture was vigorous stirred for 48 h under hydrogen atmosphere.
The reaction mixture was filtered and the filtrate was concentrated.
The residual oil was applied onto a column of silica gel and elution
effected with CH2Cl2/MeOH/AcOH (100:0:0.5 Ǟ 85:15:0.5, v/v/v)
to give pure 2 (2.84 g, 92%), as a white solid. Ϫ [α]D ϭ Ϫ44.6 (c ϭ
1.0, CHCl3). Ϫ Rf ϭ 0.31 (CH2Cl2/MeOH/AcOH, 85:15:0.5, v/v/
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J. L. Montchamp, F. Tian, M. E. Hart, J. W. Frost, J. Org.
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[97327]
1
v). Ϫ H NMR (CDCl3): δ ϭ 0.89 (t, 6 H, 2 ϫ CH3), 1.22Ϫ1.30
Eur. J. Org. Chem. 1998, 303Ϫ307
307