An improved, cost effective process for the synthesis of N,N-diisopropyl-3-(2-hydroxy-5-methylphenyl)-3-phenylpropanamide; a key intermediate for the preparation of Tolterodine and its related substances were described. The process features one pot synthesis employing inexpensive reagents.
Tolterodine is chemically known as (R)-N,N-disiopropyl-3-(2-hydroxy-5-methyl phenyl)-3-phenyl propyl amine. Tolterodine acts as a muscarinic receptor antagonist. It is useful in the treatment of urinary incontinence . Tolterodine tartrate acts by relaxing the smooth muscle tissues in the walls of the bladder by blocking cholinergic receptors . Tolterodine tartrate  is marketed by Pharmacia & Upjohn in the brand name of Destrol®.
The present invention relates to a novel process for the preparation of N,N-diisopropyl-3-(2-hydroxy-5-methylphenyl)-3-phenylpropanamide (4); a key intermediate for the preparation of Tolterodine (1). Some different approaches have been published [4–8] for the preparation of N,N-diisopropyl-3-(2-hydroxy-5-methylphenyl)-3-phenylpropanamide (4). These methods involve multistep synthesis using hazardous, expensive reagents and some of the methods  involve activators like Grignard reagents, LDA, n-butyl lithium, Lewis acids. Hence there is a need to develop an alternative, plant friendly procedure for the preparation of N,N-diisopropyl-3-(2-hydroxy-5-methylphenyl)-3-phenylpropanamide (4) from 3,4-dihydro-6-methyl-4-phenylcoumarin (2) (Figure 1).
Results and discussion
Ring opening reactions of dihydrocoumarins are well known in literature [9–11]. But in the present invention, we have described a new methodology (Scheme 1 & Scheme 2) for the preparation of N,N-diisopropyl-3-(2-hydroxy-5-methylphenyl)-3-phenylpropanamide (4) by using inexpensive and commercially vailable starting materials like 3, 4-dihydro-6-methyl 4-phenylcoumarin (2), which was synthesized from p-cresol and trans-cinnamic acid .
3,4-Dihyhydro-6-methyl 4-phenylcoumarin (2) reacts with diisopropylamine (6) in presence of acetic acid gives N,N-diisopropyl-3-(2-hydroxy-5-methylphenyl)-3-phenylpropanamide (4) at room temperature. This process of compound 4 is very useful for commercialization of Tolterodine 1 in plant.
In order to optimize the yields, this reaction was studied in different solvents like Dichloromethane, Tetrahydrofuran, Acetonitrile, Toluene, Ethanol, Methanol and Diisopropylether. The reaction parameters are tabulated in Table 1. In methanol, methyl 3-(2-hydroxy-5-methylphenyl)-3-phenylpropanoate (3) was observed as an intermediate, which is not completely converted to amide (4). Hence the yield was found low. Among, diisopropylether was found to be the best suitable solvent for the preparation of N,N-diisopropyl-3-(2-hydroxy-5-methylphenyl)-3-phenylpropanamide (4), which results maximum yields of 53%.
In our continuous research, this reaction was further studied with acetic acid as a catalyst in diisopropylether. The experiments details are tabulated in Table 2. Based on the results, we observed the yield of compound 4 was increased to 75%, when 4 mole equivalents of acetic acid was used. But our attempts to increase the yield of compound 4 further by adding more equivalents of acetic acid were not successful. With lesser equivalents of acetic acid, we observed lower yields of compound 4.
Compound 4 is known in literature [4–12]. Compounds 4a-4c and 5-5c are novel. These compounds upon reduction [4–12] with LiAlH4, Vitride, Sodium borohydride gives corresponding amines, which are useful in the synthesis of Tolterodine and its related compounds.
An efficient, synthesis of N,N-diisopropyl-3-(2-hydroxy-6-methylphenyl)-3-phenylpropanamide was achieved by one pot synthesis between 3,4-dihydro-6-methyl-4-phenylcoumarin and diisopropylamine.
Solvents and reagents were obtained from commercial source and used without purification. The IR spectra (ϑmax, cm-1) were recorded in solid state KBr dispersion using Perkin Elmer FT-IR spectrometer. The 1H NMR and 13C NMR spectra were recorded on Bruker-Avance 300 MHz spectrometer. The chemical shifts were reported in δ/ ppm relative to TMS. The mass spectra were recorded on API 2000 Perkin Elmer PE-Sciex mass spectrometer. The reactions were monitored by Thin–layer chromatography (TLC). Melting points were determined by polman melting point apparatus (Model No MP96), open capillary method and are uncorrected.
General procedure for the synthesis of compounds 4-4c & 5-5c
To a solution of 3,4-dihyhydro-6-methyl 4-phenylcoumarin 2 (10 g, 42 mmol) in diisopropylether (200 mL), N,N-diisopropylamine (33.95 g, 336 mmol) and acetic acid (10 g, 168 mmol) were added at room temperature. The suspension was stirred for 16 h at room temperature. The reaction mass was concentrated, the resulting residue was crystallized with D.M.Water (50 mL) and diisopropyl ether (50 mL) mixture to gave N,N-diisopropyl-3-(2-hydroxy-5-methylphenyl)-3-phenylpropanamide 4 (10.6 g, 75% yield).
This methodology was extended to make similar analogues 4-4c and 5-5c of compound 4 and 5. The analogues were summarized in Tables 3 & 4.
Gianolli E, Giannini E, Bigini L, Piccolo O, Holmberg P, Lundholm T: Process for the preparation of N,N-diisopropyl-3-(2-hydroxy-5-methylphenyl)-3-phenylpropylamine and its salts. PCT Int Appl. 2012, 157: 294728-WO 2012098044 A1 20120726 CAN
Kobayashi K, Nishikata T, Yamamoto Y, Miyaura N: Stepwise palladium-catalyzed 1,4-addition of arylboronic acids to enones and regioselective Baeyer-Villiger oxidation for enantioselective synthesis of β -diaryl esters and (+)-(R)-tolterodine. Bull Chem Soc Jpn. 2008, 81 (8): 1019-1025. 10.1246/bcsj.81.1019. CAN 149:471296
Kompella A, Thungathuthy Srinivasa R, Adibhtla Kali S, Bhujanga R, Venkaiah C, Nannapaneni: Process for the preparation of an intermediate useful in the synthesis of tolterodine. Indian Pat Appl. 2005, IN 2003CH01028 A 20051230 CAN 147: 486225-
Open Access This article is distributed under the terms of the Creative Commons Attribution 2.0 International License (https://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Srinivas, G., Reddy, A.V.R., Prabahar, K.J. et al. Improved one-pot synthesis of N, N-diisopropyl-3-(2-Hydroxy-5-methylphenyl)-3-phenyl propanamide; a key intermediate for the preparation of racemic Tolterodine.
Sustain Chem Process2, 2 (2014). https://doi.org/10.1186/2043-7129-2-2