Solid-phase synthesis of 1,3-diamino ketones

Article abstract of DOI:10.1016/S0040-4039(00)01262-4

A versatile solid-phase methodology for the synthesis of 1,3-diamino ketone class of cysteine protease inhibitors is reported. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)01262-4

Tetrahedron Letters 41 (2000) 7145±7149
Solid-phase synthesis of 1,3-diamino ketones
Chakrapani Subramanyam* and Shang-Poa Chang
P®zer Global Research and Development, Eastern Point Road, Groton, CT 06340, USA
Received 21 July 2000; accepted 25 July 2000
Abstract
A versatile solid-phase methodology for the synthesis of 1,3-diamino ketone class of cysteine protease
inhibitors is reported. # 2000 Elsevier Science Ltd. All rights reserved.
Cysteine proteases are ubiquitous in nature and have been implicated in the etiology of a
number of disease states.1 Over the past two decades identi®cation of selective and reversible
inhibitors for this class of enzymes has been an area of intense research.2 These investigations
have led to a number of reversible inhibitors such as peptidyl aldehydes, a-ketoamides and a-keto
heterocycles.3 Recently, researchers from SmithKline Beecham have reported the design and
synthesis of a novel class of cysteine protease inhibitors based on a 1,3-diamino ketone (1) sca€old.4
The synthesis of these compounds involved selective acylation of diamino propanol (2) with
various carboxylic acids and amino acids and ®nally oxidation of the resulting alcohol to the
corresponding ketone (Scheme 1).4
However, the need to chromatographically purify the acylated intermediates 3 and 4 at each
stage would render such a methodology unsuitable for any e€ort aimed at the synthesis of
libraries of 1,3-diamino ketones. Herein, we report a versatile methodology for the solid-phase
synthesis of the title compounds.5 Our approach relies on immobilization of a di€erentially
protected diamino propanol 5 onto a polymer support. Simultaneous modi®cation of the amino
termini in 6, followed by release from the solid support, would provide alcohol 7, which upon
oxidation would provide the title compounds 1 (Fig. 1).
* Corresponding author.
0040-4039/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)01262-4

7146
Scheme 1.
Figure 1.
The successful implimentation of this approach is shown in Schemes 2 and 3. Thus, treatment
of commercially available epoxide 8 with sulfonamide 96 in re¯uxing 2-propanol containing a
catalytic amount of pyridine7 provided alcohol 10 in excellent yield. Immobilization of 10 onto
Ellman's THP linker8 was achieved without incident to yield resin bound diamino propanol 11 in
good yield.9
With 11 in hand, we turned our attention towards selective modi®cation of either of the amino
termini. Removal of the phthalimide protecting group provided the intermediate amine which
was coupled with appropriate acids to give resin bound amides which, after treatment with
mercaptoethanol/DBU, yielded the corresponding amines 12. Reaction of 12 with either another
carboxylic acid or a sulfonyl chloride followed by release from solid support (6:1 TFA:H2O)
yielded alcohols 13a±b. Analysis of crude product by MS and NMR indicated that the product
13b was contaminated by substantial amount of the bis-sulfonylated material 14. This probably
Scheme 2.

7147
.
Scheme 3. (a) N2H4 xH2O/THF/DMF; (b) RCO2H/DIC/HOBt/DMF; (c) b-mercaptoethanol/DBU/DMF; (d)
R00CO2H/DIC/HOBt/DMF; (e) R0SO2Cl/DIEA/DCE; (f) 6:1 TFA/H2O 70±75% overall yield
arises by sulfonylation not only of the amine functionality in 12 but also of the amide. The ratio
of the desired product to the bis-sulfonylated material was also dependent on the nature of the
acid used in the ®rst coupling step. With more sterically hindered acids, the amount of 14 was
reduced considerably. The undesired formation of 14 was easily overcome by reversing the order
of amide and sulfonamide formation (Scheme 4), resulting in alcohols 13 in very good yield and
high purity.10 Finally, oxidation of 13 under the SKB protocol4 provided the desired diamino
ketones 1.
.
Scheme 4. (a) N2H4 xH2O/THF/DMF; (b) RCO2H/DIC/HOBt/DMF; (c) RSO2Cl/DIEA/DCE; (d) b-mercaptoetha-
nol/DBU/DMF; (e) R0CO2H/DIC/HOBt/DMF; (f) 6:1 TFA/H2O; (g) Jones' reagent, acetone
To help ease the puri®cation of the ®nal products, we also carried out oxidation of one of the
alcohols (13b) with Dess±Martin periodinane. Scavenging of excess periodinane reagent with the
recently disclosed thiosulfate resin11 provided the target ketone 1b in excellent yield and purity
(Scheme 5).
Scheme 5.
As shown in Table 1, a number of 1,3-diamino ketones 1 was synthesized via this SPS
approach. In all instances, the product was obtained in good overall yield and purity.12
In summary, a versatile solid-phase methodology has been developed for synthesis of the 1,3-
diamino ketone class of cysteine protease inhibitors. This method should allow simultaneous
variation of functional groups at either end of the diamino ketone sca€old and should ®nd wide
spread use in synthesis of this class of protease inhibitor libraries.

7148
Table 1
Acknowledgements
The author would like to thank Dr. Christopher Poss for stimulating discussions and Drs.
Mikel P. Moyer and Steven J. Brickner for encouragement and support.
References
1. Aoyagi, T.; Umezawa, H. In Proteases and Biological Control; Reich, E.; Rifkin, D. B.; Shaw, E., Eds.; Cold
Spring Harbor Press: Cold Spring Harbor, 1975; pp. 429±454.
2. For a recent review on the synthesis of small molecule inhibitors of cysteine proteases, see: Rasnick, D. Perspect.
Drug Discovery Des. 1996, 647.
3. (a) Mehdi, S. Biorg. Chem. 1993, 21, 249. (b) Li, Z.; Patil, G. S.; Golubski, Z. E.; Hori, H.; Tehrani, K.; Foreman,
J. E.; Evelth, D. D.; Bartus, R. T.; Powers, J. C. J. Med. Chem. 1993, 36, 3472. (c) Tao, M.; Bihovsky, R.; Kauer,
J. C. Biorg. Med. Chem. Lett. 1996, 6, 3009.
4. Yamashita, D. S., et al. J. Am. Chem. Soc. 1997, 119, 11351. (b) LaLonde, J. M., et al. J. Med. Chem. 1998, 41,
4567.
5. While this work was in progress, an alternate method for solid-phase synthesis of 1,3-bis(acylamino)-2-butanones
was reported by researchers from SmithKline Beecham Pharmaceuticals and the University of California, Berkley:
Yamashita, D. S.; Dong, X.; Oh, H.-J.; Brook, C. S.; Tomaszek, T. A.; Szewczuk, L.; Tew, D. G.; Veber, D. F.
J. Comb. Chem. 1999, 1, 207. Lee, A.; Huang, L.; Ellman, J. A. J. Am. Chem. Soc. 1999, 121, 9907. Also, for an
earlier work on the synthesis of diamino alcohol-based aspartic acid protease inhibitors, using a similar approach,
see: Keck, E. K.; Ellman, J. A. J. Med. Chem. 1995, 38, 1427.
6. For use of o-nitrobenzene sulfonamide as a protecting group for amines, see: Fukuyama, T.; Jow, C. K.; Cheung,
M. Tetrahedron Lett. 1995, 36, 6373. Also for use of this protecting group in SPOS, see: Miller, S. C.; Scanlan, T.
S. J. Am. Chem. Soc. 1997, 119, 2301.
7. Baker, B. R.; Querry, M. V.; Kadish, A. F. J. Org. Chem. 1950, 15
8. The polymer-bound THP linker was purchased from Nova Biochem and alcohol 10 was immobilized as described
in: Thompson, L. A.; Ellman, J. A. Tetrahedron Lett. 1994, 35, 9333.
9. The loading was determined to be >90% of theory by cleavage of an aliquot of 11 (6:1 TFA: H2O) to provide
alcohol 10.
10. General procedure for the synthesis of 1: To a suspension of 2.4 g of resin 11 (0.63 mmol/g, 1.5 mmol) in 3:1
DMF:THF (15 mL) was added hydrazine hydrate (10 mL). After shaking at rt for 16 h, the resin was collected by
®ltration, washed (DMF, THF, MeOH) and dried. To 0.4 g of the resulting resin (0.25 mmol based on 0.63 mmol/g

7149
loading) in 2:1 DCE:DIEA (12 mL) was added 4-phenoxy benzenesulfonyl chloride (1.25 mmol, 5 equiv.). After
stirring at rt overnight, the resin was collected by ®ltration, washed and dried. It was resuspended in NMP (2 mL)
and DBU (5 mmol,) and b-mercaptoethanol (3 mmol, 12 equiv.) were added and gently stirred at rt for 20 h. The
resin was ®ltered washed and dried. DMF (4 mL) was added followed by Z-Leu-OH (5 equiv.), HOBt (5 equiv.)
and DIC (5 equiv.). The resulting mixture was shaken at rt for 20 h, ®ltered, washed and dried. To the resulting
resin was added 6:1 TFA:H2O (5 mL) and the reaction mixture was stirred for 15 min. The resin was ®ltered o€
and washed with CH3CN (5 mL). The combined ®ltrates were combined and evaporated to dryness in vacuo. The
residue was suspended in acetone (5 mL), Jones reagent (1.5 mL) was added and the mixture stirred at rt
overnight. i-PrOH (1 mL) was added and the solids ®ltered o€. The ®ltrate was concentrated in vacuo, dissolved in
DCE (5 mL) and satd. NaHCO3 (2 mL) was added. After shaking at rt for 10 min, the biphasic mixture was
loaded onto a Chemelut1 cartridge, allowed to stand for 5 min and eluted with DCE (10 mL). The combined
eluents were concentrated in vacuo and the residue puri®ed by preparative TLC (100% EtOAc) to give 0.04 g
(30% of theory) of ketone 1b as a white solid.
11. Parlow, J. J.; Case, B. L.; South, M. S. Tetrahedron 1999, 6785.
1
12. All compounds were characterized by a combination of H NMR, MS and the purity determined by a reverse
phase HPLC.