Method of synthesizing forskolin from 9-deoxyforskolin

A method of regioselectively and stereoselectively synthesizing forskolin (8,13-epoxy-1.alpha.,6.beta.,7.beta.,9.alpha.-tetrahydroxylabd-14-en-11-on e) from 9-deoxyforskolin (8,13-epoxy-1.alpha.,6.beta.,7.beta.-trihydroxylabd-14-en-11-one) with a good yield is described. In a preferred embodiment, it comprises an enol ether formation from 8,13-epoxy-1.alpha.,6.beta.,7.beta.-trihydroxylabd-14-en-11-one-6,7-carbon ate, oxidation of the enol ether with a suitable peroxy acid to obtain 11,12-dehydro-8,13-epoxy-11-methoxy-1.alpha.,6.beta.,7.beta.,9.beta.-tetra hydrolabd-14-ene-6,7-carbonate and hydrolysis of the latter under an acidic condition to obtain 8,13-epoxy-1.alpha.,6.beta.,7.beta.,9.alpha.-tetrahydroxylabd-14-en-11-one -6,7-carbonate. As an alternative way of protecting the two hydroxy groups at carbon-6 and carbon-7, they may also be converted to dimethyl acetal during the synthetic sequence. Four compounds produced in the synthetic scheme as intermediates, namely, 9,11-dehydro-8,13-epoxy-11-methoxy-1.alpha.,6.beta.,7.beta.-trihydroxylabd -14-ene-6,7-carbonate and 11,12-dehydro-8,13-epoxy-11-methoxy-1.alpha.,6.beta.,7.beta.,9.alpha.-tetr ahydroxylabd-14-ene-6,7-carbonate and the corresponding dimethyl acetal compounds are believed to be novel.

The present invention relates to a method of regioselectively and 
stereoselectively synthesizing forskolin 
(8,13-epoxy-1.alpha.,6.beta.,7.beta.,9.alpha.-tetrahydroxylabd-14-en-11-on 
e) from 9-deoxyforskolin 
(8,13-epoxy-1.alpha.,6.beta.,7.beta.-trihydroxylabd-14-en-11-one). 
During the isolation of forskolin (formula I) from Coleus forskohlii, a 
significant amount (about 10-50% of forskolin) of 9-deoxyforskolin 
(formula II), a biologically less active diterpene of related structure, 
is also obtained as a byproduct. Since forskolin is a pharmacologically 
useful compound, for instance as a hypotensive agent (see for example 
Bajwa et al. U.S. Pat. No. 4,134,986), it is useful to develop a facile 
method of stereoselectively synthesizing forskolin from 9-deoxyforskolin 
with a good yield. 
##STR1## 
I have developed a facile method of converting 9-deoxyforskolin to 
forskolin with a complete stereospecificity at 9-carbon and good 
protection of the existing functionalities. 
The synthetic method of this invention is amenable to large scale 
applications and hence will substantially increase the availability of 
forskolin for commercial applications. Certain compounds synthesized as 
intermediate compounds in the synthetic scheme of this invention are novel 
and of course useful, and they constitute another aspect of this 
invention. 
Those skilled in the art will appreciate that since 9-deoxyforskolin 
contains various functionalities, it is a difficult task to carry out a 
particular synthesic conversion without adversely affecting other 
functionalities and that regioselective and stereoselective conversion is 
generally a difficult task. 
The following numbering system is used throughout the specification and the 
appended claims for the forskolin skeleton: 
##STR2## 
A dashed line ( ) indicates that the substituent is projected below the 
average plane of the six-membered ring to which it is attached and is 
denoted as alpha (.alpha.), whereas a heavy line ( ) indicates that the 
substituent is projected above the average plane of said six-membered ring 
and is denoted as beta (.beta.). 
The synthetic method of this invention will be described first with 
reference to a preferred embodiment in which two adjacent hydroxy groups 
at carbon-6 and carbon-7 are protected by the formation of a carbonate 
linkage during the synthetic sequence, and then with reference to an 
alternative embodiment in which the hydroxy groups are protected by the 
formation of a dimethyl acetal linkage. The overall synthetic scheme used 
in the preferred embodiment is depicted schematically in FIG. 1. 
##STR3## 
STEP A 
Compound II is hydrolyzed to afford the compound of formula III depicted in 
FIG. 1. This hydrolysis is conducted typically in the presence of a 
sufficient amount of water, potassium carbonate and a suitable medium such 
as methanol and stirring the reaction mixture at room temperature for 
several hours. 
STEP B 
In order to protect the two hydroxy groups at carbon-6 and carbon-7, 
compound III is converted to a carbonate ester, namely, compound IV. Said 
carbonate ester formation is conducted typically by reacting compound III 
with 1,1'-carbonyldiimidazole in the presence of triethylamine and a 
suitable solvent such as anhydrous toluene. Typically, the reaction 
mixture is refluxed overnight to a few days. 
STEP C 
Compound IV is converted regioselectively (position-selectively) to the 
enol ether compound of formula V. This reaction is conducted typically by 
reacting compound IV with dimethyl sulfate in the presence of potassium 
hydride and a suitable medium such as anhydrous tetrahydrofuran. 
Typically, the reaction mixture is stirred at room temperature for several 
hours under nitrogen atmosphere. 
STEP D 
Compound V is stereoselectively oxidized to compound VI in which the 
orientation of the hydroxy group at carbon-9 is alpha. This oxidation is 
conducted typically by reacting compound V with a suitable peroxy acid 
such as m-chloroperbenzoic acid in the presence of anhydrous potassium 
carbonate or the like and a suitable medium such as dichloromethane. 
Typically, the reaction is conducted at room temperature for a few days. 
STEP E 
The enol ether compound VI is hydrolyzed under an acidic condition to 
afford compound VII. Hydrochloric acid is a preferred example of such acid 
catalyst. Thus, said hydrolysis is typically conducted in a medium 
prepared, for instance, from 3N aqueous HCl and tetrahydrofuran at 1:3 
volume ratio and by stirring the reaction mixture at room temperature 
overnight. 
STEP F 
Compound VII is hydrolyzed to afford compound VIII and thereby the 
protective group for the hydroxy groups at carbon-6 and carbon-7 is 
removed. It is preferrable to conduct this hydrolysis in a basic medium. 
Thus, said hydrolysis is typically conducted in the presence of sodium 
bicarbonate or the like, water and a suitable solvent such as methanol and 
by stirring the reaction mixture at room temperature for a few days. 
STEP G 
The hydroxy group at carbon-7 of compound VIII is selectively acetylated to 
afford compound I, namely, forskolin 
(7.beta.-acetoxy-8,13-epoxy-1.alpha.,6.beta.,9.alpha.-trihydroxylabd-14-en 
-11-one). Said acetylation is conducted typically by reacting compound VIII 
with acetic anhydride in a suitable solvent such as anhydrous pyridine. 
The reaction is conducted typically at ice temperature for several hours. 
As an alternative to the synthetic scheme described above in which the two 
adjacent hydroxy groups at carbon-6 and carbon-7 are protected by the 
formation of a carbonate linkage, one can also protect the hydroxy groups 
by the formation of a dimethyl acetal linkage. 
Thus, as an alternative to STEP B, one can prepare the acetal compound of 
formula IV' by reacting compound III with 2,2-dimethoxypropane in the 
presence of an acidic catalyst, preferably an organic acid such as 
p-toluene sulfonic acid or pyridinium p-toluene-sulfonate. 
##STR4## 
Typically, this reaction is conducted by dissolving compound III in a 
large excess of 2,2-dimethoxypropane, adding a small amount of pyridinium 
p-toluenesulfonate and stirring the mixture at reflux for a few days. 
Compound IV' can be converted regioselectively to the enol ether of formula 
V' in substantially the same manner as in STEP C. Compound V' is believed 
to be novel. 
##STR5## 
Compound V' can be oxidized stereoselectively to the compound of formula 
VI' in substantially the same manner as in STEP D. Compound VI' is 
believed to be novel. 
Compound VI' can be hydrolyzed to afford the compound of formula VII' in 
substantially the same manner as in STEP E, and thereafter the protective 
group of compound VII' can be removed in substantially the same manner as 
in STEP F to afford the aforementioned compound VIII. 
##STR6## 
People skilled in the art will appreciate that not only compound I, but 
also compounds V through VIII as well as V' through VII' are useful for 
preparing various pharmacologically useful derivatives therefrom. Thus, 
the present invention is not limited to the preparation of compound I, but 
it includes methods for preparing compounds V through VIII and V' through 
VII' as other aspects of the invention. 
Moreover, compounds V, VI, V' and VI' are believed to be novel and hence 
the present invention includes these compounds as still another aspect of 
the invention.

The following examples are given for illustrative purposes and are not to 
be construed as limiting the invention disclosed herein. All temperatures 
are given in degrees Celsius. 
EXAMPLE 1 
8,13-Epoxy-1.alpha.,6.beta.,7.beta.-trihydroxylabd-14-en-11-one 
To a stirred solution of 2.0 g of 
7-acetoxy-1.alpha.,6.beta.-dihydroxy-8,13-epoxylabd-14-en-11-one in 50 ml 
of reagent grade methanol (undried) was added 1.0 g of potassium 
carbonate. The mixture was stirred at room temperature for 3.0 hours and 
thereafter partitioned between ether (50 ml) and water (50 ml). The 
organic phase was removed and the aqueous phase was extracted with ether 
(3.times.50 ml). The combined organic phases were dried over magnesium 
sulfate. Filtration and removal of solvent in vacuo gave 1.768 g of 
crystals, mp 76.degree.-78.degree. C. (recrystallized, mp 
83.degree.-85.degree. C.). 
ANALYSIS: Calculated for C.sub.20 H.sub.32 O.sub.5 : 68.15%C, 9.15%H. 
Found: 68.42%C, 9.42%H. 
EXAMPLE 2 
8,13-Epoxy-1.alpha.,6.beta.,7.beta.-trihydroxylabd-14-en-11-one-6,7-carbona 
te 
To a solution of 300 mg of 
8,13-epoxy-1.alpha.,6.beta.,7.beta.-trihydroxylabd-14-en-11-one in 25 ml 
of dry toluene and 1 ml of triethylamine was added 169 mg of 
1,1'-carbonyldiimidazole. The solution was heated to reflux under nitrogen 
with stirring. A new spot began to appear on TLC (thin layer 
chromatography) above the starting material. 
After 72 hours, no further change in TLC was observed. The reaction mixture 
was diluted with 10 ml of toluene and filtered, and the filtrate was 
concentrated in vacuo. The residue was chromatographed on silica gel using 
3:1 hexane/ethyl acetate eluent to obtain 95.1 mg of solid, homogeneous by 
TLC. 
ANALYSIS: Calculated for C.sub.21 H.sub.30 O.sub.6 : 66.64%C, 7.99%H. 
Found: 66.38%C, 8.31%H. 
EXAMPLE 3 
9,11-Dehydro-8,13-epoxy-11-methoxy-1.alpha.,6.beta.,7.beta.-trihydroxylabd- 
14-ene-6,7-carbonate 
To a suspension of potassium hydride (383 mg of 25% KH in oil) in 75 ml of 
dry THF (tetrahydrofuran) under nitrogen were added successively 0.15 ml 
of dimethyl sulfate and a solution prepared from 302 mg of 
8,13-epoxy-1.alpha.,6.beta.,7.beta.-trihydroxylabd-14-en-11-one-6,7-carbon 
ate and 2.0 ml of dry THF. 
The reaction mixture was stirred at room temperature for 3.0 hours. The 
mixture was then poured into 75 ml of aqueous ammonium hydroxide solution. 
The flask was washed with ether and the washings were added to the 
mixture. After stirring for 5 minutes, the mixture was extracted with 
ether. The combined organic phases were washed once with water and once 
with brine, and dried over anhydrous magnesium sulfate. The solvent was 
removed in vacuo and the residue chromatographed on silica using 4:1 
hexane/ethyl acetate eluent, to provide 193.1 mg of foam, mp 
123.degree.-125.degree. C., homogenous by TLC. 
ANALYSIS: Calculated for C.sub.22 H.sub.32 O.sub.6 : 67.32%C, 8.22%H. 
Found: 66.92%C, 8.44%H. 
EXAMPLE 4 
11,12-Dehydro-8,13-epoxy-11-methoxy-1.alpha.,6.beta.,7.beta.,9.alpha.-tetra 
hydroxylabd-14-ene-6,7-carbonate hemihydrate 
To a solution of 
9,11-dehydro-8,13-epoxy-11-methoxy-1.alpha.,6.beta.,7.beta.-trihydroxylabd 
-14-ene-6,7-carbonate (100 mg) in 20 ml of dichloromethane were added 
successively a trace (about 3 mg) of anhydrous potassium carbonate and 65 
mg of 85% meta-chloroperbenzoic acid. The mixture was stirred at room 
temperature for 48 hours, then poured into a mixture of 10 ml of saturated 
aqueous sodium bisulfite solution and 10 ml of saturated aqueous sodium 
bicarbonate solution. The aqueous phase was extracted with 
dichloromethane, and the combined organic phases were dried over anhydrous 
magnesium sulfate/potassium carbonate and concentrated in vacuo. The 
residual solid was recrystallized from hexane/ethyl acetate to provide 
74.2 mg of product, mp 242.degree.-244.degree. C., homogenous by TLC. 
ANALYSIS: Calculated for C.sub.22 H.sub.32 O.sub.7.1/2H.sub.2 O: 63.29%C, 
7.96%H. Found: 63.14%C, 8.41%H. 
EXAMPLE 5 
8,13-Epoxy-1.alpha.,6.beta.,7.beta.,9.alpha.-tetrahydroxylabd-14-en-11-one- 
6,7-carbonate 
To a solution of 13 mg of 
11,12-dehydro-8,13-epoxy-11-methoxy-1.alpha.,6.beta.,7.beta.,9.alpha.-tetr 
ahydroxylabd-14-en-6,7-carbonate hemihydrate in 6 ml of tetrahydrofuran was 
added 2 ml of 3N aqueous hydrochloric acid. The mixture was stirred at 
room temperature for 18 hours at which time TLC showed complete conversion 
of the starting material. The mixture was poured into 25 ml of saturated 
aqueous sodium bicarbonate solution and extracted with ether. The combined 
organic phases were dried over anhydrous magnesium sulfate and 
concentrated to provide 12 mg of 
8,13-epoxy-1.alpha.,6.beta.,7.beta.,9.alpha.-tetrahydroxylabd-14-en-11-one 
-6,7-carbonate as a solid. 
EXAMPLE 6 
8,13-Epoxy-1.alpha.,6.beta.,7.beta.,9.alpha.-tetrahydroxylabd-14-en-11-one 
To a solution of 
8,13-epoxy-1.alpha.,6.beta.,7.beta.,9.alpha.-tetrahydroxylabd-14-en-11-one 
-6,7-carbonate (5.0 mg) in 5 ml of methanol was added 1 ml of saturated 
aqueous sodium bicarbonate solution. The mixture was stirred at room 
temperature for 48 hours, diluted with water and extracted with ether. The 
combined organic phases were dried over anhydrous magnesium 
sulfate/potassium carbonate and concentrated to provide 4.2 mg of 
8,13-epoxy-1.alpha.,6.beta.,7.beta.,9.alpha.-tetrahydroxylabd-14-en-11-one 
. 
EXAMPLE 7 
7.beta.-Acetoxy-8,13-epoxy-1.alpha.,6.beta.,9.alpha.-trihydroxylabd-14-en-1 
1-one 
To a solution of 
8,13-epoxy-1.alpha.,6.beta.,7.beta.,9.alpha.-tetrahydroxylabd-14-en-11-one 
(27 mg) in 4 ml of anhydrous pyridine cooled to 0.degree., 2 ml of acetic 
anhydride was added with stirring. Stirring was continued at 0.degree. for 
5 hours, at which time no starting material was visible by TLC 2:1 
hexane/ethyl acetate. The mixture was diluted with 50 ml of water, which 
caused a white solid to precipitate. The crystals were collected, washed 
with a small amount of water and dried in vacuo to provide 25 mg of white 
crystals. These were shown by NMR to be 
7.beta.-acetoxy-8,13-epoxy-1.alpha.,6.beta.,9.alpha.-trihydroxylabd-14-en- 
11-one contaminated with less than 10% of 
1.alpha.,7.beta.-diacetoxy-8,13-epoxy-6.beta.,9.alpha.-dihydroxylabd-14-en 
-11-one. Recrystallization from hexane provided 17.6 mg of 
7.beta.-acetoxy-8,13-epoxy-1.alpha.,6.beta.,9.alpha.-trihydroxylabd-14-en- 
11-one, pure by TLC. 
EXAMPLE 8 
8,13-Epoxy-1.alpha.,6.beta.,7.beta.-trihydroxy-labd-14-en-11-one-6,7-dimeth 
yl acetal 
To a solution of 250 mg of 
8,13-epoxy-1.alpha.,6.beta.,7.beta.-trihydroxylabd-14-en-11-one in 50 ml 
of 2,2-dimethoxypropane was added one crystal of pyridinium 
p-toluene-sulfonate. The solution was heated to reflux with stirring under 
a CaSO.sub.4 drying tube. After 72 hours no starting material was observed 
by TLC. The mixture was allowed to cool to room temperature and thereafter 
poured into saturated aqueous sodium bicarbonate solution and extracted 
with 4.times.50 ml of ether. The combined organic extracts were dried over 
anhydrous magnesium sulfate, filtered and concentrated in vacuo. The 
residual solid was chromatographed on silica using hexane/ethyl acetate 
(4:1) as eluent, to give 179 mg of product as a foam, homogeneous by TLC 
(1:1 hexane/ethyl acetate). 
ANALYSIS: Calculated for C.sub.23 H.sub.36 O.sub.5 : 70.37%C, 9.24%H. 
Found: 69.78%C, 9.16%H.