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DPLA MANUFACTURING PROCESS FOR TADALAFIL FROM
RACEMIC OR L-TRYPTOPHAN
Introduction
Tadalafil 1 is a selective inhibitor of cGMP specific Type V phosphodiesterase (PDE5)
and it is used for treatment of erectile dysfunction (Cialis®). The pharmacological activity of Tadalafil is assigned specifically to (6R,12aR)- enantiomer. Since Tadalafil 1 possesses at C(12a)-atom R-configuration, corresponding
to configuration of the unnaturally configurated D-tryptophan, all previously reported syntheses have been using exclusively this, significantly more expensive, enantiomer as the starting material (US6140329, US6127542, Synlett 2004, 8, 1428, OPPI Briefs 2005,
37, No.1, Tetrahedron Asymmetry 2008, 19, 435-442, ibid. 2009, 20, 2090, ibid. 2009,
20, 430, Synth. Commun. 2008, 38, 4265 and Europ. J. Org. Chem. 2010, 1711.
No synthesis of Tadalafil has been ever reported using either less expensive L- or rac.- tryptophan: L-tryptophan industrial production is based on the fermentation of indole and serine using either wild-type or genetically modified bacteria. This conversion is catalyzed by the enzyme tryptophan synthase which cannot produce D-tryptophan. For the synthesis of Tadalafil 1 the required, more expensive D-tryptophan has to be
manufactured by a resolution of rac.-tryptophan prepared by a chemical method. The manufacturing cost for Tadalafil 1 could be significantly lowered if the less expensive
either L- or racemic tryptophan would be used. DPLA Manufacturing Process
DPLA process for the manufacture of Tadalafil 1 starts from less expensive either L- or
(L)- or rac. - tryptophan
According to our process the key intermediate 2 is prepared from inexpensive rac.- or L-
tryptophan in high yield and high optical purity: Treatment of rac.- or L-tryptophan with piperonal 6 in the presence of suitable chiral acid as e.g. CSA provides initially imine 4
which undergoes readily acid catalyzed epimerization at the carbon atom bearing the nitrogen function. If an appropriate solvent is used, in which the salt 3 is only limited
soluble, a process called crystallization induced asymmetric transformation converts finally all starting material 4 into the enantiomerically pure compound 3 which undergoes
in the next step stereo specific cyclization to enantiomerically pure intermediate 2. This
intermediate 2 can be easily converted into Tadalafil 1 in 2 steps as reported in
Tetrahedron Asymmetry 2008, 19, 435-442.
Depending on the starting material the compound 4 can be present in the form as
enantiomerically pure compound derived from (L)-tryptophan or as racemic compound derived from racemic tryptophan or even as a mixture containing variable amount of As a resulting agent various chiral acids in stoichiometric amount can be used. Preferably acids (1R or 1S)-10-camphorsulfonic acid or (D or L)-tartaric acid or (D or L)- dibenzoyl tartaric acid or (1R or 1S)-3-bromocamphor-8-sulfonic acid itself or in a mixture with another aliphatic or aromatic carboxylic acid, preferably glacial acetic acid, The best results have been achieved specifically with (1R or 1S)-10-camphorsulfonic acid in a suitable solvent in which the compound 3 is only limited soluble as e.g.
acetonitrile, nitromethane, lower alcohols, preferably isopropanol, n-butanol, n-pentanol, THF or chlorinated hydrocarbons as CHCl3, dichloroethylene, or dimethoxyethane and DPLA process is very efficient and allows a direct reaction of either L- or rac.-tryptophan with piperonal 6 in the presence of a stoichiometric amount of a chiral acid under
elevated temperature in a suitable solvent, followed by crystallization of the said mixture and collection of the desired diastereomeric salt 2 from the precipitate. Treatment of the
salt of 2 with suitable organic or inorganic base, provides also the enantiomerically pure
compound 2, having specifically the (1R,3R)-configuration (Scheme 1).
In another approach the chiral intermediate 2, having the (1R,3R)-configuration, can be
prepared from a stereo isomeric mixture of 2, having any possible configuration at C(1)-
and C(3)-chiral atoms, in the form as an enantiomerically pure compound or as a racemate or as a mixture of diastereomers, by adding a suitable chiral acid in stoichiometric amount in the presence of a catalytic amount of piperonal 6 (5-10 mol-%)
in a suitable solvent at elevated temperature followed by collection of the desired diastereomeric salt 2 from the precipitate.
any stereo mixture of 2
The reaction is carried out in boiling solvents as acetonitrile or nitromethane where the HX salt of the compound 2, having (1R,3R)-configuration, has only limited solubility.
Under these conditions the starting material (either in a form as enantiomerically pure compound or as racemate or diastereomeric mixture) undergoes crystallization induced asymmetric transformation providing enantiomerically pure salt of the compound 2,
having specifically only (1R,3R)-configuration. This process is possible because at elevated temperature the chiral centers at C(1)- and C(3)-atoms can be epimerized via an open structure intermediates as shown in Scheme 2. If an appropriate solvent is used, in which the salt of 2, having (1R,3R)-configuration, is only limited soluble,
crystallization induced asymmetric transformation converts finally all material into the enantiomerically pure compound 2, specifically with (1R,3R)-configuration.
DPLA process does not use any expensive reagents (rac.- or L-trypthopan) and does not require reaction condition which would not be suitable to the most manufacturers. The key step, crystallization induced asymmetric transformation, can be readily accomplished in standard equipment used for crystallization.

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