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Description  |
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BACKGROUND OF THE INVENTION
The present invention is directed to novel and advantageous processes for
the synthesis of tigogenin beta-cellobioside and to certain novel
intermediates used in these processes.
Tigogenin beta-cellobioside is a known compound having utility in the
treatment of hypercholesterolemia and atherosclerosis (Malinow, U.S. Pat.
Nos. 4,602,003 and 4,602,005; Malinow et al. Steroids, vol. 48, pp.
197-211, 1986). Each patent discloses a different synthesis of this
compound from beta-cellobiose octaacetate; the first via the glycolyl
bromide heptaacetate which is coupled with tigogenin in the presence of
silver carbonate, and finally hydrolyzed; and the second direct stannic
chloride catalyzed coupling of the octaacetate with tigogenin in methylene
chloride, again followed by hydrolysis. In Malinow et al., reaction of
cellobiose octaacetate with titanium tetrabromide gave the glycosyl
bromide heptaacetate, which was coupled with tigogenin by means of
mercuric cyanide, and then hydrolyzed. All of these methods have serious
drawbacks for producing bulk material. A desirable goal, met by the
present invention, has been to devise synthetic methods which avoid toxic
and/or expensive reagents, and which cleanly produce the desired tigogenin
betacellobioside, avoiding tedious and expensive purification steps.
Schmidt, Angew. Chem. Int. Ed. Engl., v. 25, pp. 212-235 (1986) has
reviewed the synthesis and reactions of O-glycosyl trichloroacetimidates
formed by the reaction of sugars possessing a 1-OH group (but with other
hydroxy groups protected, e.g., by benzyl or acetyl) with
trichloroacetonitrile in the presence of a base. There is preferential
formation of the alpha-anomer when NaH is used as base, and preferential
formation of the beta-anomer when the base is K.sub.2 CO.sub.3. The alpha
anomer of tetrabenzylglucosyl trichloroacetimidate when coupled with
cholesterol gave anomeric mixtures which varied with catalyst
(p-toluenesulfonic acid or boron trifluoride etherate) and temperature
(-40.degree. to +20.degree. C.). On the other hand, both the alpha and
beta anomers of tetraacetylglucosyl analog reportedly yield exclusively
beta-anomeric products.
SUMMARY OF THE INVENTION
The present invention is directed to intermediate compounds of the formula
##STR1##
wherein
R is R.sup.4 CO;
R.sup.1 and R.sup.2 are taken separately, R.sup.1 is R.sup.4 CO, and
R.sup.2 is
##STR2##
R.sup.1 and R.sup.2 are taken together and are
##STR3##
R.sup.4 is (C.sub.1 -C.sub.4)alkyl.
Of particular value are those compounds wherein R.sup.4 is methyl, i.e., R
is acetyl.
The present invention is also directed to over-all processes and certain
individual process steps used for the present syntheses of tigogenin
beta-cellobioside, as follows:
(a) reacting a cellobiose heptaalkanoate of the formula
##STR4##
wherein
R is R.sup.4 CO and R.sup.4 is (C.sub.1 -C.sub.4)alkyl, with
trichloroacetonitrile in the presence of a catalytic amount of cesium
carbonate in a reaction-inert solvent at or about ambient temperature to
form an imidate of the formula (I) wherein R.sup.1 and R.sup.2 are taken
together, i.e., of the formula
##STR5##
either
(b) reacting said imidate with tigogenin in the presence of zinc bromide or
magnesium bromide etherate in the same or another reaction-inert solvent
at or about ambient temperature to form an orthoester of the formula (I)
wherein R.sup.1 and R.sup.2 are taken together, i.e., of the formula
##STR6##
wherein Tig is
##STR7##
followed by heating said orthoester in the same or another reaction-inert
solvent to form a tigogenin beta-cellobioside heptaalkanoate of the
formula
##STR8##
or
(b') reacting said imidate of the formula (III) with tigogenin in the
presence of boron trifluoride etherate in the same or another
reaction-inert solvent at or about ambient temperature to form a said
tigogenin beta-cellobioside heptaalkanoate of the formula (V); and
(c) conventionally hydrolyzing said tigogenin beta-cellobioside
heptaalkanoate to form said tigogenin beta-cellobioside.
Again, the preferred value of R.sup.4 is methyl, i.e., R is acetyl.
As used above and elsewhere herein, the expression "reaction-inert solvent"
refers to a solvent which does not interact with starting materials,
reagents, intermediates or products in a manner which adversely affects
the yield of the desired product. In general, said solvent can comprise a
single entity, or contain multiple components.
DETAILED DESCRIPTION OF THE INVENTION
One key to the present invention is the stereo-specific conversion of
cellobiose heptaalkanoate (II) to a key intermediate, viz., the
alpha-acetimidate of the formula (III). In this conversion, the cellobiose
heptaalkanoate is reacted with at least one molar equivalent (preferably a
1-10 fold molar excess) of trichloroacetonitrile in a reaction-inert
solvent such as methylene chloride in the presence of a catalytic amount
of cesium carbonate (e.g., about 5 mol % relative to cellobiose
heptaacetate). Temperature is not critical, but the reaction is preferably
carried out at or near ambient temperature so as to avoid the cost of
heating or cooling. The present stereospecific formation of the
alpha-anomer with this catalyst is most surprising, since Schmidt,
particularly expert in this type of transformation, recommends another
alkali metal carbonate, viz., potassium carbonate as catalyst for
selective formation of the undesired beta-anomer.
The resulting alpha-imidate (III) is coupled with tigogenin in a
reaction-inert solvent in the presence of boron trifluoride etherate in
analogy to the method of Schmidt, cited above. This coupling step, which
is also conveniently accomplished at or about ambient temperature,
produces known tigogenin beta-cellobioside heptaacetate (V).
We have presently discovered that use of either zinc bromide or magnesium
bromide etherate as catalyst under otherwise similar conditions leads to
the clean formation of an intermediate orthoester of the formula (IV). If
desired, this ortho ester can be isolated. However, it is preferred to
simply heat the reaction mixture to accomplish rearrangement of this ortho
ester to intermediate tigogenin beta-cellobioside heptaacetate (V). It is
convenient to replace any alkanoyl groups lost in this process by reaction
with the appropriate alkanoic acid anhydride prior to isolation of this
intermediate.
In the final step, the heptaacetate of the formula (V) is conventionally
hydrolyzed or solvolyzed, e.g., according to the method of Malinow, cited
above; or by the method specifically exemplified below.
The present invention is illustrated by the following examples. However, it
should be understood that the invention is not limited to the specific
details of these examples.
EXAMPLE 1
alpha-O-Cellobiosyl Trichloroacetimidate Heptaacetate (III, R=acetyl)
Under N.sub.2, cellobiose heptaacetate (10 g, 0.0157 mol; prepared from the
octaacetate according to the method of Excoffier et al., Carbohydrate
Res., v. 39, pp. 368-373, 1975) was dissolved in 100 ml CH.sub.2 Cl.sub.2
in a flame dried flask and cooled to 0.degree.-5.degree. C.
Trichloroacetonitrile (4 ml) was added by syringe and then Cs.sub.2
CO.sub.3 (0.52 g, 0.00158 mol) was added as a finely ground powder. The
mixture, which was immediately allowed to warm to room temperature, was
stirred for 5 hours, then clarified by filtration over diatomaceous earth,
and the filtrate stripped, taken up in hexane/ethyl acetate and restripped
to yield 11 g of title product. Recrystallization from ethyl
acetate/hexane gave 6.1 g of purified title product, m.p.
192.degree.-194.degree. C; .sup.1 H-NMR(CDCl.sub.3, 300 MHz)delta(ppm)
8.63 (s, 1H), 6.45 (d, 1H), 5.50 (t, 1H), 5.1 (m, 3H), 4.9 (t, 1H), 4.52
(m, 2H), 4.37 (dd, 1H), 4.07 (m, 3H), 3.82 (t, 1H), 3.65 (m, lH), 2.10 (s,
3H), 2.07 (s, 3H), 1.97 (m, 15H).
Analysis: C 43.02, H 4.49, N 1.81; Calculated: C 43.06, H 4.65, N 1.79.
EXAMPLE 2
Orthoester Derived From alpha-O-Cellobiosyl Trichloroacetimidate
Heptaacetate and Tigogenin (IV, R.sup.4 =CH.sub.3)
Title product of the preceding Example (1.2 g, 1.54 mmol), tigogenin (0.5
g, 1.2 mmol) and molecular sieves (0.5 g, 3A type) were combined in 20 ml
of CH.sub.2 Cl.sub.2 at room temperature. After stirring for 10 minutes,
ZnBr.sub.2 (0.21 g, 0.93 mmol) was added and the mixture stirred for 1.25
hours, filtered over diatomaceous earth, the filtrate washed with 0.5M
HCl, H.sub.2 O and brine, dried over MgSO.sub.4, stripped, and the residue
slurried in hexane to yield present title product as a white solid, 0.55
g, m.p. 187.5.degree.-188.6.degree. C.; tlc Rf 0.3 (3:1 CHCl.sub.3 :ethyl
acetate).
Analysis: C, 61.14; H, 7.54. Calculated: C, 61.49; H, 7.60.
Alternatively, title product was simply formed in situ by the same method,
omitting the filtration and subsequent isolation steps. The formation of
title product was monitored by tlc.
This ortho ester product was also produced when magnesium bromide etherate
was used in place of ZnBr.sub.2.
EXAMPLE 3
Tigogenin beta-Cellobioside Heptaacetate (V, R=acetyl)
Method A
Title product of the preceding Example was formed in 20 ml of CH.sub.2
Cl.sub.2 from title product of Example 1 (1.15 g, 1.47 mmol) according to
the procedure of the preceding Example. Monitoring by tlc demonstrated
complete conversion to the orthoester within 2 hours. The ortho ester was
then converted to present title product by heating the reaction mixture at
reflux for 18 hours, then cooling to room temperature, adding acetic
anhydride and allowing the reaction to stir for 3 hours to replace
partially lost acetyl groups. To isolate and purify title product, the
reaction mixture was filtered, and the filtrate washed with H.sub.2 O and
brine, dried (MgSO.sub.4), stripped and the residue chromatographed on
silica gel using 4:1 CHCl.sub.3 : ethyl acetate as eluant. The yield of
purified title product was 0.8 g (59%), identical with the known product.
Alternatively, following treatment with acetic anhydride, the reaction
mixture was filtered, washed with 0.5 N HCl, water and brine, dried
(MgSO.sub.4), stripped to an oil and the residue crystallized from
isopropyl ether, 0.46 g (34%). Additional product (0.09 g, 7%) was
obtained from mother liquors by stripping and chromatography according to
the preceding paragraph.
Method B
A mixture of tigogenin (4.7 g, 0.0113 mol) and flame dried molecular seives
(3A type, 10 g) and 100 ml hexane was added to a solution of title product
of Example 2 (0.014 mol) in 100 ml of CH.sub.2 Cl.sub.2, and the mixture
stirred 18 hours at room temperature, then cooled to 0.degree.-5.degree.
C. BF.sub.3 .multidot.(C.sub.2 H.sub.5)O (0.43 ml, 0.0055 mol) in 10 ml
CH.sub.2 Cl.sub.2 was added dropwise over 30 minutes. After 2 hours solid
NaHCO.sub.3 (5 g) was added, and the mixture stirred for 10 minutes,
filtered, the filtrate washed 2x saturated NaHCO.sub.3 and 1x brine, dried
(MgSO.sub.4) and stripped to solids which were twice recrystallized from
absolute alcohol to yield 5.32 g of purified title product.
EXAMPLE 4
Tigogenin beta-Cellobioside
Under N.sub.2, and under anhydrous conditions, title product of the
preceding Example (7.8 g, 7.53 mmol) was dissolved in 78 ml of CH.sub.3
OH: tetrahydrofuran 1:1 by volume. Sodium methoxide (0.020 g, 0.37 mmol)
was added in one portion and the mixture heated to reflux for 1 hour.
Tetrahydrofuran was removed by distillation to a head temperature
62.degree. C. Fresh methanol (80 ml) was added and distillation continued
to a head temperature of 65.degree. C. Water (8 ml) was added and the
mixture reheated to reflux, seeded, digested at reflux for 2.5 hours,
cooled slowly with stirring to room temperature, stirred overnight and
present title product recovered by filtration, 4.21 g, identical with the
known product.
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