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Innovative Approaches to the Imidazo[4,5-<I>B</I

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Innovative Approaches to the Imidazo[4,5-<I>B</I 100 YEARS OF PROGRESS WITH LONZA 280 CHIMIA 51 (1997) Nr. 6 (Juni) Chimia 51 (1997) 280-282 Published Syntheses of 3 © Neue Schweizerische Chemische Gesellschaft ISSN 0009-4293 In the first published synthesis [2] of3, 2-(N-nitramino)pyridine 5 was prepared by nitration (HN03/H2S04) of aminopyri- Innovative Approaches to the dine 4 at ca. _100 (Scheme 1). Treatment 0 of 5 with cone. H2S04 at 0 gave a mixture Imidazo[4,5-b]pyridine Ring of 6 and 7 (55:45) which was subjected directly to catalytic hydrogenation (10% System. Development of an Pd/C in MeOH). Treatment of the result- ing mixture of diaminopyridines 8 and 9 Efficient Process for Industrial- with propionic acid in polyphosphoric acid gave a mixture of3 and 10. Disadvantages Scale Production of a Key Inter- of this route to 3 are the lack of regioselec- tivity in the nitration step and the resultant mediate for Potent Angiotensin requirement for chromatographic separa- tion of 3 and 10. II Receptor Antagonists In an alternative synthesis [3] of 3 (Scheme 2), condensation of acetylace- tone with amidine hydrochloride 11 Gerhard C. Stucky*, Jean-Paul Roduit, and Beat Schmidt gave 2-aminonicotinamide 12 in 92% yield. Hofmann rearrangement of 12 using PhI(OAch (or more conveniently NBS/ KOH [4]) gave a high yield of the urea Abstract. Two syntheses of 2-ethyl-5,7-dimethyl-3H-imidazo[4,5-b]pyridine (3), an derivative 13, which was converted to3 by important intermediate for the synthesis of several potent angiotensin II antagonists, treatment with a mixture of propionic acid have been investigated. The first route involves conversion of 1,1-bis(methylthio)-2- and propionic anhydride in the presence of nitroethene (17) to 2-amino-4,6-dimethyl-3-nitropyridine (6); catalytic hydrogenation MgCI2. Hydrochloride 11 can be prepared of 6 in propionic acid gave 3 in high yield. In the second synthesis, propionitrile is in two steps from cyanoacetic ester; this converted to imidate hydrochloride 15· HCI which is neutralised and reacted with synthesis of3 thus requires five steps from aminoacetonitrile in the presence of acetylacetone to give 3 in 55% overall yield. The commercially available starting materi- propionitrile route was scaled up to produce 3 in the pilot plant. als. Introduction In view of its importance in blood- CI pressure regulation, the renin-angiotensin ~~OH system (RAS) has been a prime target for N N=N the treatment of cardiovascular disease. N" N'K ~N One therapeutic approach which has un- ):.,JlN~ dergone intensive recent investigation is N H the attempt to block the action of angio- tensin II (AIl), the biologically active oc- tapeptide of the RAS, at the level of its 2 3 receptor; the first All antagonist to reach the market as a treatment for hypertension wasMerck's Losartan potassium (Cozaa/iJ, Scheme J 1). Several All antagonists based on the imidazo[ 4,5-b ]pyridine ring system have been described, one example being L-159,282 (2) which was reported to be hN NH2 significantly more potent in vivo than 4 Losartan [1]. The conventional approach N02 to the synthesis of 2 has been via N(3)- NH2 :1 ~ I JyN~ alkylation of 2-ethyl-5,7-dimethyl-3H- 1 N NH 2 h N NH2 ~ •.!-N h N H imidazo[4,5-b]pyridine (3), which is thus 6 8 3 of interest as a general precursor to the + + + group of potent All antagonists exempli- N NHN0 --. h 2 --. - fied by 2. 5 02Nit H2N ~ I EtCOHNit N NH , I ~ I *Correspondence: Dr. G.c. Stucky 2 N NH n2 N NHCOEt Department of Research and Development 7 LONZA AG 9 10 CH-3930 Visp 100 YEARS OF PROGRESS WITH LONZA 281 CHIMIA 5/ (1997) Nr. 6 (luni) In view of its importance as an inter- Scheme 2 mediate and the disadvantages associated with the two syntheses described above, o o 0 we decided to seek a more efficient ap- ~hCONH2 H 2 ;; N proach to 3. {NH - ,., I I )=0 HN NH2 Hel N NH ,., N _ +N~ 2 --hN H ~)LN N H Retrosynthetic Analysis 11 12 13 3 Two routes for construction of the imi- dazo[ 4,5-b]pyridine ring system were in- Scheme 3 vestigated (Scheme 3). The first makes use of l,l-bis(methylthio)-2-nitroethen (17) N02 as starting material for an alternative ap- it",'" I => lxNO,'" I => .....sis..... proach to diaminopyridine 8, the second -::::? N NH2 N NH2 relies on condensation of acetyl acetone 8 17 with aminoimidazole 14; the planned syn- 6 JxN~"" N thesis of 14 foresaw reaction of aminoace- N H tonitrile with imidate 15. ~ 3 /'yO ..... eN 1)-~ + ( H N H => NH Synthesis of 3 via Diaminopyridine 8 2 NH2 14 15 Two regioselecti ve syntheses of8 were investigated (Scheme 4). The first was based on a published synthesis [5] of 2- Scheme 4 amino-4,6-dimethyl-3-nitropyridine and involved condensation ofenamine 16 (pre- NH2 0 pared by reaction of acetylacetone with jN02 AA ammonia) with 17, an intermediate previ- NO NH ously used in an industrial synthesis of the 's Is...... 16 ~ , ~ I , antiulcer agent ranitidine [6]. This reac- h h.••.. NH N NH N NH tion gave only moderate yield of pyridine 3 2 2 17 6, apparently due to the lack of reactivity 6 8 between 16 and 17. Our planned alterna- tive synthesis of 6 had in fact been de- 2 scribed shortly before we began this work; jN0 ~ ~ the reported yields [7] for the conversions H N NH oft7 to 18and 18 t06 were 45% and52%, 2 2 respecti vel y. After careful optimisation, 18 we discovered that treatment of 17 with NH3 under pressure and at lower temper- ature than that previously reported gave Scheme 5 18 in an improved yield of 77%, and that carrying out the reaction between 18 and H2' Pd-C excess acetylacetone (which could be re- cycled) in 2-methoxyethanol gave 6 in •• 93% yield. The purity of 6, which could propionic acid ~~)lN+N~ thus be isolated in 72% yield based on 17, N H cat. H2S04 was such that it could be subjected to 3 catalytic hydrogenation without prior pu- rification. Hydrogenation of 6 to diaminopyrid- ine 8 has been described [8] using 28 wt.- temperature (120-130°) and gave a mix- reported transformations, all of which were % of 10% Pd/C in methanol. In an attempt ture of 8 and 3. This result provided the improved considerably. The main draw- to convert 6 directly to 3 we first investi- basis of a practical and efficient one-pot back of this synthesis is the relati vely high gated its hydrogenation using propioni- process for the direct conversion of 6 to 3 cost of key intermediate 18. trile as solvent. This reaction proceeded in 76% yield by means of hydrogenation smoothly using 3 wt.-% of 10% Pd/C to in propionic acid with a catalytic amount give 8 which crystallised directly from of H2S04 to accelerate the conversion of 8 The Propionitrile Route to 3 propionitrile in excellent yield (98%) and to 3 (Scheme 5). purity (> 99%). We then turned to the In summary, imidazo[ 4,5-b]pyridine 3 Our alternative approach to 3 (Scheme hydrogenation of 6 in propionic acid; this was prepared in 55-60% overall yield 3) foresaw preparation of 14 via reaction reaction proceeded smoothly at elevated from 17 via a combination of previously of aminoacetonitrile with imidate 15; we 100 YEARS OF PROGRESS WITH LONZA 282 CHIMIA 5/ (1997) Nr. 6 (Juni) anticipated that the open-chain intermedi- mation and increase the yield of 3 to 55- mediates was developed in the laboratory ate 19 would cyclise spontaneously 60%. The excess acetylacetone could be and scaled up to produce 3 in the pilot (Scheme 6). recycled. plant. Particular advantages of the process Pinner reaction of propionitrile with The procedure was simplified and im- developed are its brevity, its high overall Hel in dry methanol at 15° gave the hy- proved by development of an alternative yield, and the limited amounts of waste drochloride of 15 which could be neutral- method to prepare 15, namely by addition which are generated. ised with aqueous base to give 15. During of a suspension of its hydrochloride in the neutralisation step, the maintenance of methanol to a slurry of the sodium enolate Experimental Part the pH in the range 9-10 was critical: at of acetylacetone in methanol. The advan- higher pH, 15 suffered reconversion to tages of this method were: 1. The losses of Preparation of15' HCI. Gaseous HCl (323 g, propionitrile, at lower pH, rapid hydroly- 15 due to its solubility in water were 9 mol) was bubbled into a soln. of propionitrile sis of 15 to methyl propionate was the eliminated. 2. The ideal basicity of the (248 g, 4.5 mol) in CH30H (288 g, 9.0 mol) at predominant reaction. Initial attempts to enolate of acetylacetone meant that the such a rate that the temperature could be main- tained at 0-5°. The suspension was stirred for prepare 19 or 14 by reaction of 15 with losses of 15 due to the side reactions (see further 2 h at 5° and stored at 0° until use. aminoacetonitrile gave complex mixtures. above) which occurred in aqueous milieu Preparation of aminoacetonitrile. To a soln. Spectroscopic analysis indicated that 14 were eliminated. 3. The acetylacetone of NaOH (121 g, 3 mol) in CH30H (750 ml) had been formed but had decomposed to formed during the liberation of 15 can be aminoacetonitrile hydrochloride (278 g, 3 mol) give mainly polymeric material.
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