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New syntheses of mono- and di-O-methyl derivatives of methyl α-l -rhamnopyranoside
Ca@ohy&ate
0
Research, 56 (1977) 191-194
ElsevierScieatiticPublishingCompany, Amsterdam;.- Printed in Belgium
Mote
Rlew syntheses of -methyl
of mono-
and di-0-methyi
Runot~ TOUAN, &EFANKAR&XONYI, AND Institute
deriwatiwes
a-L-rhamnopyranoside F~ADOSLAVPALUVE~K
of Chemistry, Sfowk Academy of Sciences, 809 33 Bratislaca (Ctechosloookia)
(Received August 1% 1976; accepted for publication, September lOth, 1976)
In our studies of pectinic polymers of the bark of the white willow (S&X alba L.), several degradation procedures afforded fragments containing high proportions of r_-rhamnose residues’. On methylation analysis of these fragments, the resulting methyl ethers of methyl a-L-rhamnopyranoside were subjected to g.l.c.-m.s., but our mass spectra dit3ered2 from those published’, and authentic reference compounds were therefore required. Syntheses of some methyl ethers of methyl a-L-rhamnopyranoside require multistep procedures4, but others are unknown. We now report on a convenient method for the preparation of some of these derivatives, which involves catalysed methylation by diazomethane5. The use of stannous chloride dihydrate, titanium tetrachloride, and cerium trichloride not only gave high yields but also very different isomer distributions. A detailed study of the reaction mechanism was not undertaken, the primary purpose being the synthesis of partially methylated derivatives of methyl a-L-rhamnopyranoside. Methanolic methyl cr+rhamnopyranoside containing catalyst (mhr) was treated with diazomethane at room temperature. The resulting methyl 2- and 3-Umethyl-a-r..-rhamnopyranosides were isolated by chromatography; the respective yields for each catalyst were as follows: SnCl, - 2H,O, 38 and 60% ; CeCl, , 65 and 32%; TiCl,, 12 and 85%. Likewise, methylation of methyl 4-0-methyl-cc+rhamnopyranoside6 gave a mixture of methyl 3,4- and 2,4-di-0-methyl-z-L-rhamnopyranosides, the respective yields of which for each catalyst were as follows: SnC12 *2H20, 6.5 and 34% ; CeCl, , 32 and 65%; TiCl,, 81 and 18%. The location of the methoxyl groups in methylated derivatives of methyl a-L-rhamnopyranoside can be assigned on the basis of n.m.r. data (Table I), since there is an upfield shift of the signal for a proton attached to a carbon atom carrying a metboxyl group and, in the acetylated derivatives, a downfield shift of the signal for the proton in the HCOAc group. The catelysts used for monomethylation of the cis-glycol system of methyl a&rhamnopyranoside and its 4-O-methyl derivative effect selectivit;r similar to that observed in methylations of nucleosides ‘, but the yields were substantially higher.
4.55bdd 4.95-5.30m 4.68d
4.58d 4.lld
4,13d 4.71d
(CD’&
CD&
CDC&
CDCll
CD&
CDClj
CDC&
CDCIS
CD&
CDCIJ
4”
3,4
3,4+
2,4
2,4’
2,3’
2,3”
2,X4
5.3lq
3.55q
3.63q 3.43q
3.5oq 3.09t
5.00t
3.23t
2.91t
3.8Oq
3.08t 3.03t
5.09q
3.120
3.690
3.980
3.780
3.80m
3.75m
H-5
3.520
3.680
3.75m
3.660
3.550
3.590
3.590
3.18btd 3.690
3.10t
4.9R
3.54q
3.41q
3.8Oq
3.58q
3.63btd 3.30
3.6lq
3.44q
5.23q
4.OOq
3.93q
4.17t
5.06t
H-4
1.29d
1.20d
3.60p
4.13O
OH
2.96O 3.89#
3.31; 3.45; 2.14 3.48
3.35; 3.50; 3.58
3.34; 3.39; 2,ll 3.54
3035;3.49; 3.54
3.34; 3.47; 3.4R; 3.53
3.36; 3.40; 2.06 3.51
-
-
3.08
-
2.83
-
2.78
2,04; 2.13 -
-
2.08; 2.14 -
-
2004; 2.08 -
-
-
Oh
1.31md 3.38; 3.45; 3.48
1.31d
1.29d
1,34d
1.29d
3.34; 3.46
3.34; 3.56
1.33d
3.33; 3.48
1.33d
3.38; 3.46
3.39 3.46
3.34 3.45
3.31
OMe
I.216
lS5d
1.206
1.31d
l.24md
Me
1.6
1,7
1.6
1.8
108
1.7
1.8
~1.6
1.6
1,9
1.7
1.8
1.8
1.5
J1,2
3.2
9.1
9.5
I
I
3.3
9.5
9.2
9.1
9.1
3.3
3.8
3.4
3.2
9.0 I
9.5
3.1 3.4 f
9.3
9.0
f
33,4
3.2
2.9
N3.0
3.Of
J&3
9.1
9.5
I
9.5
9.2
9.1
9.1
9.0 I
9.5
9.3
9.1
f .r
3409
Coupling constmts (Hz)
6.2
615
H5.9
6.5
6.5
6.0
6.5
6.2
6.3
6.5
6.5
6.5
6.5
5.6
&WI,
‘Sweep width, 100 Hz. bKcy: d, doublet; t, triplet; q, quartet; o, octet; m, multiplet; b, broad. OAnasterisk (+) denotes the ncctylated’dcrivativc. “Further splitting due to the second-order effect, C3-Proton bs. IFirst-order coupling no? observed, #2-Protons s, “In CD&, the flrst-order spectrum was not observed.
4.1Cd
4.68d
4.633
4.64d
5.2lq 3.73q
4
3.619 4.42q
3*
5.03d
4.71d
CD&
3.30
3.30
CSDJN”
3.43q
3.8lq
3
4.7ld
4,58d
21
CD&
UW&O
Solvelrt
2
grOllJ)SC
Locntiofl ofnletllyl
N.M.I. DATA FOR MRTHYLATEDDERlVATiVES OF METHYL CC-L-RHAMNOPYRANOSIDE’
TABLE I
”
G tis
.-. AND 3. ‘ihtksmr>~~npui~Iisbed results. v. KOVh%,v. KW&OV,~~.EL~RACSON~I, ANEi R. T~iw~,Carbolay&.Res., SS-(1977J inpress. K. Hetx.,K. R. SFERLING,AXD H. F. GR*' lJTz?dAcE3R,carbohy~ Rzs.,3 (1969)7p-97. R. A. LAIDLAW AND E. G. V. PERc!vAL,A&. Curbo/zy&.Chem., 7 (1952) 17-21. lbf. 3. ROBXNS, A. S. K. LEE,AND F. A. NORRIS,Cadwhy&. Res., 41(1975) 304-307, K. BUI-LER,B. I?.LLOYD, AND M. STXEY, J. Chem. Sm., (1955) 153H536. R. TOMAN, Chem. Zaesti, in press.*
1 R. Tow
2 3
4 5 6 7
0
Research, 56 (1977) 191-194
ElsevierScieatiticPublishingCompany, Amsterdam;.- Printed in Belgium
Mote
Rlew syntheses of -methyl
of mono-
and di-0-methyi
Runot~ TOUAN, &EFANKAR&XONYI, AND Institute
deriwatiwes
a-L-rhamnopyranoside F~ADOSLAVPALUVE~K
of Chemistry, Sfowk Academy of Sciences, 809 33 Bratislaca (Ctechosloookia)
(Received August 1% 1976; accepted for publication, September lOth, 1976)
In our studies of pectinic polymers of the bark of the white willow (S&X alba L.), several degradation procedures afforded fragments containing high proportions of r_-rhamnose residues’. On methylation analysis of these fragments, the resulting methyl ethers of methyl a-L-rhamnopyranoside were subjected to g.l.c.-m.s., but our mass spectra dit3ered2 from those published’, and authentic reference compounds were therefore required. Syntheses of some methyl ethers of methyl a-L-rhamnopyranoside require multistep procedures4, but others are unknown. We now report on a convenient method for the preparation of some of these derivatives, which involves catalysed methylation by diazomethane5. The use of stannous chloride dihydrate, titanium tetrachloride, and cerium trichloride not only gave high yields but also very different isomer distributions. A detailed study of the reaction mechanism was not undertaken, the primary purpose being the synthesis of partially methylated derivatives of methyl a-L-rhamnopyranoside. Methanolic methyl cr+rhamnopyranoside containing catalyst (mhr) was treated with diazomethane at room temperature. The resulting methyl 2- and 3-Umethyl-a-r..-rhamnopyranosides were isolated by chromatography; the respective yields for each catalyst were as follows: SnCl, - 2H,O, 38 and 60% ; CeCl, , 65 and 32%; TiCl,, 12 and 85%. Likewise, methylation of methyl 4-0-methyl-cc+rhamnopyranoside6 gave a mixture of methyl 3,4- and 2,4-di-0-methyl-z-L-rhamnopyranosides, the respective yields of which for each catalyst were as follows: SnC12 *2H20, 6.5 and 34% ; CeCl, , 32 and 65%; TiCl,, 81 and 18%. The location of the methoxyl groups in methylated derivatives of methyl a-L-rhamnopyranoside can be assigned on the basis of n.m.r. data (Table I), since there is an upfield shift of the signal for a proton attached to a carbon atom carrying a metboxyl group and, in the acetylated derivatives, a downfield shift of the signal for the proton in the HCOAc group. The catelysts used for monomethylation of the cis-glycol system of methyl a&rhamnopyranoside and its 4-O-methyl derivative effect selectivit;r similar to that observed in methylations of nucleosides ‘, but the yields were substantially higher.
4.55bdd 4.95-5.30m 4.68d
4.58d 4.lld
4,13d 4.71d
(CD’&
CD&
CDC&
CDCll
CD&
CDClj
CDC&
CDCIS
CD&
CDCIJ
4”
3,4
3,4+
2,4
2,4’
2,3’
2,3”
2,X4
5.3lq
3.55q
3.63q 3.43q
3.5oq 3.09t
5.00t
3.23t
2.91t
3.8Oq
3.08t 3.03t
5.09q
3.120
3.690
3.980
3.780
3.80m
3.75m
H-5
3.520
3.680
3.75m
3.660
3.550
3.590
3.590
3.18btd 3.690
3.10t
4.9R
3.54q
3.41q
3.8Oq
3.58q
3.63btd 3.30
3.6lq
3.44q
5.23q
4.OOq
3.93q
4.17t
5.06t
H-4
1.29d
1.20d
3.60p
4.13O
OH
2.96O 3.89#
3.31; 3.45; 2.14 3.48
3.35; 3.50; 3.58
3.34; 3.39; 2,ll 3.54
3035;3.49; 3.54
3.34; 3.47; 3.4R; 3.53
3.36; 3.40; 2.06 3.51
-
-
3.08
-
2.83
-
2.78
2,04; 2.13 -
-
2.08; 2.14 -
-
2004; 2.08 -
-
-
Oh
1.31md 3.38; 3.45; 3.48
1.31d
1.29d
1,34d
1.29d
3.34; 3.46
3.34; 3.56
1.33d
3.33; 3.48
1.33d
3.38; 3.46
3.39 3.46
3.34 3.45
3.31
OMe
I.216
lS5d
1.206
1.31d
l.24md
Me
1.6
1,7
1.6
1.8
108
1.7
1.8
~1.6
1.6
1,9
1.7
1.8
1.8
1.5
J1,2
3.2
9.1
9.5
I
I
3.3
9.5
9.2
9.1
9.1
3.3
3.8
3.4
3.2
9.0 I
9.5
3.1 3.4 f
9.3
9.0
f
33,4
3.2
2.9
N3.0
3.Of
J&3
9.1
9.5
I
9.5
9.2
9.1
9.1
9.0 I
9.5
9.3
9.1
f .r
3409
Coupling constmts (Hz)
6.2
615
H5.9
6.5
6.5
6.0
6.5
6.2
6.3
6.5
6.5
6.5
6.5
5.6
&WI,
‘Sweep width, 100 Hz. bKcy: d, doublet; t, triplet; q, quartet; o, octet; m, multiplet; b, broad. OAnasterisk (+) denotes the ncctylated’dcrivativc. “Further splitting due to the second-order effect, C3-Proton bs. IFirst-order coupling no? observed, #2-Protons s, “In CD&, the flrst-order spectrum was not observed.
4.1Cd
4.68d
4.633
4.64d
5.2lq 3.73q
4
3.619 4.42q
3*
5.03d
4.71d
CD&
3.30
3.30
CSDJN”
3.43q
3.8lq
3
4.7ld
4,58d
21
CD&
UW&O
Solvelrt
2
grOllJ)SC
Locntiofl ofnletllyl
N.M.I. DATA FOR MRTHYLATEDDERlVATiVES OF METHYL CC-L-RHAMNOPYRANOSIDE’
TABLE I
”
G tis
.-. AND 3. ‘ihtksmr>~~npui~Iisbed results. v. KOVh%,v. KW&OV,~~.EL~RACSON~I, ANEi R. T~iw~,Carbolay&.Res., SS-(1977J inpress. K. Hetx.,K. R. SFERLING,AXD H. F. GR*' lJTz?dAcE3R,carbohy~ Rzs.,3 (1969)7p-97. R. A. LAIDLAW AND E. G. V. PERc!vAL,A&. Curbo/zy&.Chem., 7 (1952) 17-21. lbf. 3. ROBXNS, A. S. K. LEE,AND F. A. NORRIS,Cadwhy&. Res., 41(1975) 304-307, K. BUI-LER,B. I?.LLOYD, AND M. STXEY, J. Chem. Sm., (1955) 153H536. R. TOMAN, Chem. Zaesti, in press.*
1 R. Tow
2 3
4 5 6 7