Catalytic, enantioselective synthesis of stilbenecis-diamines: A concise preparation of (−)-Nutlin-3, a potent p53/MDM2 inhibitor

Tyler A. Davis and Jeffrey N. Johnston *
Department of Chemistry & Vanderbilt Institute of Chemical Biology Vanderbilt University, Nashville, TN 37235-1822, USA. E-mail: jeffrey.n.johnston@vanderbilt.edu; Web: http://www.johnstonchemistry.org Fax: (+1) 615 343 1234

Received 28th January 2011 , Accepted 24th February 2011

First published on 25th March 2011


Abstract

The first highly diastereo- and enantioselective additions of aryl nitromethane pronucleophiles to aryl aldimines are described. Identification of an electron rich chiral Bis(AMidine) catalyst for this aza-Henry variant was key to this development, leading ultimately to differentially protected cis-stilbene diamines in two steps. This method then became the lynchpin for an enantioselective synthesis of (−)-Nutlin-3 (Hoffmann-La Roche), a potent cis-imidazoline small molecule inhibitor of p53-MDM2 used extensively as a probe of cell biology and currently in drug development.


The attention provided to the aza-Henry reaction, also commonly referred to as the nitro-Mannich reaction, over the past decade has resulted in a range of catalysts that can provide the β-amino nitroalkane addition products with high enantioselection.1 These products are readily transformed to unsymmetrical vicinaldiamines by straightforward reduction of the nitro functional group, thereby providing straightforward access to a key synthetic building block.2Nitromethane additions have received the lion's share of attention in these reports, and several classes of nitroalkanes remain problematic. For example, the use of aryl nitromethanes in catalytic, enantioselective aza-Henry reactions has been reported twice, but both examples provided low enantioselection, and little diastereoselection (≤2:1) compared to otherwise high stereoselection with alternative nitroalkanes.3 We encountered similar challenges when approaching these additions with Bis(AMidine) catalysis but report here the successful realization of highly diastereo- and enantioselective aryl nitromethane additions to azomethine using catalyst control. The value of this work lies in its ability to furnish unsymmetrical cis-stilbene diamine derivatives, and this is further grounded by our development of the first fully stereocontrolled synthesis of the potent p53/MDM2 inhibitor (−)-Nutlin-3 (2) discovered by Hoffmann-La Roche (HLR).
ugraphic, filename = c1sc00061f-u1.gif

We initially targeted β-amino nitroalkane 1a (Table 1). Nitroalkane 4 was prepared in one step from para-chlorobenzyl bromide using the method of Kornblum,4 and imine 3 was formed from the corresponding α-amido sulfone using potassium carbonate to effect the elimination to azomethine.5 Our first attempts to promote the addition of nitroalkane 4 to imine 3 utilized a symmetrical chiral proton catalyst (5·HOTf), leading to the adduct in 13[thin space (1/6-em)]:[thin space (1/6-em)]1 dr and 64% ee (major) (Table 1, entry 1).6,7 Since the Brønsted acid salt of the BisAMidine ligand has always been the most selective catalyst in our past work, we did not anticipate the finding that the free base was equally effective;8 use of bis(amidine) 5 under otherwise identical conditions led to product with nearly equal diastereoselection and enantioselection (Table 1, entry 2). It should be noted that protonation of the Brønsted basic bis(amidine) by aryl nitromethane 4 is possible, and the aryl nitromethane salt that would form (e.g [BAM·H]+[ArCHNO2]) could itself be a catalyst in these reactions. Regardless, this observation broadened our investigation of catalysts to include both free base and 1[thin space (1/6-em)]:[thin space (1/6-em)]1 BAM:TfOH. Using unsymmetrical ligand 6, we found that the salt led to the desired addition product with good diastereoselection (7[thin space (1/6-em)]:[thin space (1/6-em)]1 dr), and slightly improved enantioselection (80% ee) (Table 1, entry 3), whereas the free base was similarly diastereoselective (6[thin space (1/6-em)]:[thin space (1/6-em)]1 dr), but less enantioselective (72% ee) (Table 1, entry 4). The binding pocket in catalyst 7 is presumably more open than those of the two previous ligands (5–6), leading to low enantioselection while diastereoselection remained high (Table 1, entries 5–6). However, the free base now provided enantioselection at a slightly improved level relative to the use of its triflic acid salt, emphasizing the need to optimize this transformation using a rather empirical approach. It was not until the Pyrrolidine BisAMidine (8a)6b was used that the enantioselection increased significantly to 85% ee (Table 1, entry 7). The behavior of this bis(amidine) was consistent with its parent HQuin-BAM (5) in that the free base performed equally well relative to the triflic acid salt (Table 1, cf. entries 7–8). Due to the increase in enantioselection upon increasing the Brønsted basicity/electron rich nature of the aromatic rings, we prepared three additional derivatives in which the quinoline periphery is substituted by methoxy groups (8b–d). These catalysts provided improvements over 8a, with 8d leading to the addition product in 13[thin space (1/6-em)]:[thin space (1/6-em)]1 dr, 91% ee, and nearly quantitative yield (Table 1, entries 10–12). A single fractional recrystallization of this material from toluene provided the adduct as a single diastereomer (>200[thin space (1/6-em)]:[thin space (1/6-em)]1) with 97% ee.

Table 1 Development of a diastereo- and enantioselective BisAMidine-catalyzed aryl nitromethane addition to azomethineabcd
a All reactions used 1.1 equiv of 4 (0.1-0.2 mmol) in toluene (0.1 M) with a 24 h reaction time unless otherwise noted. Configuration assigned by analogy to an adduct whose absolute and relative configuration was assigned by X-ray crystallography. Diastereomer ratio (dr) and enantiomeric excess (ee) were determined by HPLC. b Isolated yield after column chromatography. c 10 mol% catalyst used, 48 h reaction time. d 2 h Reaction time.



ugraphic, filename = c1sc00061f-u3.gif
Catalyst 8d was applied to a range of variants to determine preliminary scope (Table 2). Electron rich aryl aldimines led to addition products with higher diastereoselection and similar enantioselection (Table 2, entries 2–4), whereas electron deficient aldimines provided lower, but still good diastereoselection at 14[thin space (1/6-em)]:[thin space (1/6-em)]1 or better (Table 2, entries 5–6). Several aryl nitromethane pronucleophiles were also prepared and subjected to the standard reaction conditions. These provided the adducts with high diastereo- and enantioselection as well (Table 2, entries 9–12), with the exception of the most electron deficient case (Table 2, entry 13). The acidity of the product in this case renders it more susceptible to epimerization at the nitro-substituted carbon. In almost every case examined, the addition products were crystalline and were isolated in very good yield.

Table 2 Catalyzed additions of arylnitromethanes to aldiminesa

Entry R Ar 1 drb eeb (%) Yieldb (%)
a All reactions were employed 1.1 equiv of nitroalkane in toluene (0.1 M) and 18–26 h reaction time unless otherwise noted. Configuration assigned by analogy to an adduct whose absolute and relative configuration was assigned by X-ray crystallography. b ee and dr were determined by HPLC. Isolated yield after column chromatography. c For comparison, using 10 mol% of the Takemoto or Deng thiourea at −20 °C provided the adduct (1b) in 4[thin space (1/6-em)]:[thin space (1/6-em)]1 dr/57% ee and 5[thin space (1/6-em)]:[thin space (1/6-em)]1 dr/−31% ee, respectively. d 1.2 Equiv of imine used. e Warmed to −20 °C for one hour before workup.
1c p-ClC6H4 (3) Ph (16) b 19[thin space (1/6-em)]:[thin space (1/6-em)]1 90 93
2d p-AllylO (9) 16 c 131[thin space (1/6-em)]:[thin space (1/6-em)]1 87 70
3e p-MeOC6H4 (10) 16 d 81[thin space (1/6-em)]:[thin space (1/6-em)]1 85 83
4d p-MeC6H4 (11) 16 e 38[thin space (1/6-em)]:[thin space (1/6-em)]1 91 93
5d p-FC6H4 (12) 16 f 15[thin space (1/6-em)]:[thin space (1/6-em)]1 87 87
6d p-CF3C6H4 (13) 16 g 14[thin space (1/6-em)]:[thin space (1/6-em)]1 84 99
7d p-PhC6H4 (14) 16 h 44[thin space (1/6-em)]:[thin space (1/6-em)]1 93 99
8d 2-Naphth (15) 16 i 25[thin space (1/6-em)]:[thin space (1/6-em)]1 91 99
9 3 4 a 13[thin space (1/6-em)]:[thin space (1/6-em)]1 91 97
10 3 m-BrC6H4 (17) j 13[thin space (1/6-em)]:[thin space (1/6-em)]1 89 91
11 3 2-Np (18) k 10[thin space (1/6-em)]:[thin space (1/6-em)]1 80 99
12e 33 p-MeOC6H4 (19) l 17[thin space (1/6-em)]:[thin space (1/6-em)]1 86 90
13 33 p-NO2C6H4 (20) m 2[thin space (1/6-em)]:[thin space (1/6-em)]1 76 99


The successful development of an anti-selective addition to generate key intermediate 1a left our remaining aim to convert this masked diamine into Nutlin-3 (2). Nutlin-3 is a cis-imidazoline that can restore the pathway to apoptosis in cancerous cells where the p53 gene is wild type.9 The Nutlins have been shown to inhibit p53/MDM2,10 and the more potent enantiomer, (−)-Nutlin-3, has been used extensively as a small molecule probe of cell biology and remains in development as a chemotherapeutic.11 Its synthesis has not been reported outside of a collection of patents for the preparation of rac-Nutlin-3.12,13 In order to avoid a meso intermediate without unduly telescoping the synthesis using deprotection techniques, conversion of β-amino nitroalkane 1a into the unsymmetrical imidazoline would need to use the inherent chemoselectivity offered by this intermediate. This was accomplished by reduction of the nitro to amine using cobalt boride formed in situ,14 and subsequent acylation with acid 21 to produce Boc-protected amide 22 (Scheme 1). Straightforward deprotection of the Boc group using trifluoroacetic acid revealed the secondary amine in 88% yield. Acylation of the amine with carbonyl diimidazole led to an intermediate isocyanate that was treated with piperazinone 23.


Preparation of (−)-Nutlin-3.
Scheme 1 Preparation of (−)-Nutlin-3.

This set the stage to attempt a chemoselective cyclizative dehydration to the desired imidazoline. For this purpose we turned to the powerful dehydrating property of a phosphonium anhydride formed by the combination of triphenylphosphine oxide and triflic anhydride (Hendrickson's reagent).15 Faced with the question of chemoselectivity in the context of a mixed amide/carbamate substrate,16 we applied the Hendrickson protocol and observed a single imidazoline product that was retrieved in 88% isolated yield.

At present, the exact mass for Nutlin-3 determined by high resolution mass spectrometry is the only available analytical data for comparison. As a result, we sought evidence to support whether the desired imidazoline (Nutlin-3) or an alternative isomer (24) formed during this dehydrative cyclization. Support for the imidazoline depicted for Nutlin-3 was obtained by an HMBC (600 MHz) experiment. Among the 3JHC couplings observed was the diagnostic crosspeak to the urea carbon from H1 of the imidazoline; a crosspeak between H8 and an amidecarbonyl carbon was not observed. Nutlin-3 produced using this synthesis and catalyst 8d is levorotatory.

Conclusion

In conclusion, we have developed the first highly diastereo- and enantioselective aza-Henry additions using aryl nitromethanes. This transformation was used to prepare a key cis-stilbene diamine intermediate that served as a precursor to the potent chemotherapeutic (−)-Nutlin-3. As a consequence of this synthesis, complete spectroscopic and analytical data for synthetic intermediates and (−)-Nutlin-3 are available.17 This synthesis removes the current reliance on preparatory chromatography using a chiral stationary phase and substitutes it with a readily prepared chiral catalyst that furnishes intermediate 1a with high stereocontrol. This improved accessibility of cis-imidazolines may stimulate their broader use as probes, if not provide a practical synthesis to fuel drug development.

This work was supported by the NIH (GM 084333). We are grateful to Dr. Maren Pink (Indiana University Molecular Structure Center) for determination of absolute and relative configuration by X-ray diffraction for a related addition product. We are grateful to Clayton Marshall and Dr. Jennifer Pietenpol (VICC) for confirming the activity of both (−)- and (+)-Nutlin-3 (synthetic).

Notes and references

  1. Reviews: E. Marques-Lopez, P. Merino, T. Tejero and R. P. Herrera, Eur. J. Org. Chem., 2009, 2401 Search PubMed; T. Akiyama, J. Itoh and K. Fuchibe, Adv. Synth. Catal., 2006, 348, 999 CrossRef CAS; A. Ting and S. E. Schaus, Eur. J. Org. Chem., 2007, 5797 CrossRef CAS; B. Westermann, Angew. Chem., Int. Ed., 2003, 42, 151 CrossRef CAS . Selected examples: H. Adams, J. C. Anderson, S. Peace and A. M. K. Pennell, J. Org. Chem., 1998, 63, 9932 CrossRef CAS; K. Yamada, S. J. Harwood, H. Groger and M. Shibasaki, Angew. Chem., Int. Ed., 1999, 38, 3504 Search PubMed; K. R. Knudsen, T. Risgaard, N. Nishiwaki, K. V. Gothelf and K. A. Jorgensen, J. Am. Chem. Soc., 2001, 123, 5843 CrossRef CAS; T. Okino, S. Nakamura, T. Furukawa and Y. Takemoto, Org. Lett., 2004, 6, 625 CrossRef CAS; L. Bernardi, F. Fini, R. P. Herrera, A. Ricci and V. Sgarzani, Tetrahedron, 2005, 62, 375 CrossRef CAS; F. Fini, V. Sgarzani, D. Pettersen, R. P. Herrera, L. Bernardi and A. Ricci, Angew. Chem., Int. Ed., 2005, 44, 7975 CrossRef CAS; C. Palomo, M. Oiarbide, A. Laso and R. Lopez, J. Am. Chem. Soc., 2005, 127, 17622; X. Xu, T. Furukawa, T. Okino, H. Miyabe and Y. Takemoto, Chem.–Eur. J., 2006, 12, 466 CrossRef CAS; C. Rampalakos and W. D. Wulff, Adv. Synth. Catal., 2008, 350, 1785 CrossRef CAS.
  2. For reviews describing vic-diamines and their preparation, see: D. Lucet, T. Le Gall and C. Mioskowski, Angew. Chem., Int. Ed., 1998, 37, 2580 Search PubMed; V. Faugeroux and Y. Genisson, Curr. Org. Chem., 2008, 12, 751 CrossRef CAS; S. Kotti, C. Timmons and G. G. Li, Chem. Biol. Drug Des., 2006, 67, 101 CrossRef CAS; M. S. Mortensen and G. A. O'Doherty, Abstracts of Papers of the American Chemical Society, 2005, 230, U3433 CrossRef CAS . For metal-catalyzed diamination reactions of alkenes, see: F. Cardona and A. Goti, Nat. Chem., 2009, 1, 269.
  3. Enantioselective additions of aryl nitromethane additions to azomethine (both with α-imino esters): 2[thin space (1/6-em)]:[thin space (1/6-em)]1 dr, 84% ee, 64% yield – M. Rueping and A. P. Antonchick, Org. Lett., 2008, 10, 1731 Search PubMed 1. 2[thin space (1/6-em)]:[thin space (1/6-em)]1 dr, 74/77% ee, 59% yield – N. Nishiwaki, K. R. Knudsen, K. V. Gothelf and K. A. Jorgensen, Angew. Chem., Int. Ed., 2001, 40, 2992 CrossRef CAS.
  4. N. Kornblum, H. O. Larson, R. K. Blackwood, D. D. Mooberry, E. P. Oliveto and G. E. Graham, J. Am. Chem. Soc., 1956, 78, 1497 CrossRef CAS.
  5. O. Marianacci, G. Micheletti, L. Bernardi, F. Fini, M. Fochi, D. Pettersen, V. Sgarzani and A. Ricci, Chem.–Eur. J., 2007, 13, 8338 CrossRef CAS.
  6. B. Shen, D. M. Makley and J. N. Johnston, Nature, 2010, 465, 1027 CrossRef CAS; T. A. Davis, J. C. Wilt and J. N. Johnston, J. Am. Chem. Soc., 2010, 132, 2880 CrossRef CAS; J. C. Wilt, M. Pink and J. N. Johnston, Chem. Commun., 2008, 4177 RSC; A. Singh and J. N. Johnston, J. Am. Chem. Soc., 2008, 130, 5866 CrossRef CAS; B. Shen and J. N. Johnston, Org. Lett., 2008, 10, 4397 CrossRef CAS; A. Singh, R. A. Yoder, B. Shen and J. N. Johnston, J. Am. Chem. Soc., 2007, 129, 3466 CrossRef CAS; B. M. Nugent, R. A. Yoder and J. N. Johnston, J. Am. Chem. Soc., 2004, 126, 3418 CrossRef CAS.
  7. See the Supporting Information for methods used to assign absolute and relative configuration for addition products 1. The absolute configuration is also consistent with all other cases using nitroalkanes and bis(amidine) catalysts (ref. 6) and with suggestions of the absolute configuration made by HLR (ref. 17).
  8. Our first exception to this was recently published: M. C. Dobish and J. N. Johnston, Org. Lett., 2010, 12, 5744 Search PubMed.
  9. Overview: C. C. Harris, Proc. Natl. Acad. Sci. U. S. A., 2006, 103, 1659 Search PubMed . Reviews: P. M. Fischer and D. P. Lane, Trends Pharmacol. Sci., 2004, 25, 343 CrossRef CAS; G. L. Bond, W. W. Hu and A. J. Levine, Curr. Cancer Drug Targets, 2005, 5, 3 Search PubMed.
  10. S. W. Lowe, E. Cepero and G. Evan, Nature, 2004, 432, 307 CrossRef CAS.
  11. Over 1000 publications have cited the HLR paper in the intervening six years: L. T. Vassilev, B. T. Vu, B. Graves, D. Carvajal, F. Podlaski, Z. Filipovic, N. Kong, U. Kammlott, C. Lukacs, C. Klein, N. Fotouhi and E. A. Liu, Science, 2004, 303, 844 Search PubMed . Additional selected references: D. Carvajal, C. Tovar, H. Yang, B. T. Vu, D. C. Heimbrook and L. T. Vassilev, Cancer Res., 2005, 65, 1918 CrossRef CAS; D. C. Fry, B. Graves and L. T. Vassilev, Methods Enzymol., 2005, 399, 622 Search PubMed; D. C. Fry, B. J. Graves and L. T. Vassilev, Protein-Protein Interact. (2nd Ed.), 2005, 893 CrossRef CAS; L. T. Vassilev, Cell Cycle, 2004, 3, 419 CAS.
  12. See Supporting Information† for details of the HLR synthesis.
  13. N. Kong, E. A. Liu and B. T. Vu, WO 03/051360 A1, 2003 Search PubMed.
  14. T. Satoh, S. Suzuki, Y. Suzuki, Y. Miyaji and Z. Imai, Tetrahedron Lett., 1969, 4555 CrossRef CAS.
  15. J. B. Hendrickson and S. M. Schwartzman, Tetrahedron Lett., 1975, 16, 277 CrossRef; J. B. Hendrickson and M. S. Hussoin, J. Org. Chem., 1989, 54, 1144 CrossRef CAS; J. B. Hendrickson and M. S. Hussoin, J. Org. Chem., 1987, 52, 4137 CrossRef.
  16. Leading references: S.-L. You and J. W. Kelly, Org. Lett., 2004, 6, 1681 Search PubMed; S. L. You, H. Razavi and J. W. Kelly, Angew. Chem., Int. Ed., 2003, 42, 83 CrossRef CAS; M. J. Petersson, I. D. Jenkins and W. A. Loughlin, Org. Biomol. Chem., 2009, 7, 739 CrossRef CAS; M. J. Petersson, I. D. Jenkins and W. A. Loughlin, J. Org. Chem., 2008, 73, 4691 RSC.
  17. Absolute configuration has been suggested in the patent literature: G. J. Haley, N. Kong, E. A. Liu and B. T. Vu, WO 2005/123691 Al, 2005 Search PubMed.

Footnote

Electronic supplementary information (ESI) available: Complete experimental details and analytical data for all new compounds, as well as a summary of the HLR synthesis of rac-Nutlin-3. See DOI: 10.1039/c1sc00061f

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