Oxazolo[5,4-d] pyrimidines can be considered as 9-oxa-purine analogs of naturally occurring nucleic acid bases. Interest in this ring system has increased due to recent reports of biologically active derivatives. In particular, 5-aminooxazolo[5,4-d]pyrimidine-7(6H)-ones (9-oxa-guanines) have been shown to inhibit ricin. The preparation of a series of 2-substituted 5-aminooxazolo[5,4-d] pyrimidin-7(6H)-ones and related 5-thio-oxazolo[5,4-d] pyrimidines is described, including analogs suitable for further elaboration employing "click" chemistry utilizing copper-catalyzed Huisgen 1,3-dipolar cycloadditions. Two of the compounds prepared were found to inhibit ricin with IC(50) ca. 1-3 mM.Pharmac

Bai, Yan

Kerwin, Sean M.

Li, Wen Tai

Mandal, Subrata

Robertus, Jon D.

Beilstein Journal of Organic Chemistry

English

PubMed

Oxazolo[5,4-d] pyrimidines can be considered as 9-oxa-purine analogs of naturally occurring nucleic acid bases. Interest in this ring system has increased due to recent reports of biologically active derivatives. In particular, 5-aminooxazolo[5,4-d]pyrimidine-7(6H)-ones (9-oxa-guanines) have been shown to inhibit ricin. The preparation of a series of 2-substituted 5-aminooxazolo[5,4-d] pyrimidin-7(6H)-ones and related 5-thio-oxazolo[5,4-d] pyrimidines is described, including analogs suitable for further elaboration employing &quot;click&quot; chemistry utilizing copper-catalyzed Huisgen 1,3-dipolar cycloadditions. Two of the compounds prepared were found to inhibit ricin with IC(50) ca. 1-3 mM.Pharmaceutical Science

Texas ScholarWorks

Page 1 of(page number not for citation purposes)5Synthesis of 2-substituted 9-oxa-guanines{5-aminooxazolo[5,4-d]pyrimidin-7(6H)-ones} and9-oxa-2-thio-xanthines {5-mercaptooxazolo[5,4-d]pyrimidin-7(6H)-ones}Subrata Mandal1, Wen Tai Li1, Yan Bai2, Jon D. Robertus2and Sean M. Kerwin*1,2Full Research Paper Open AccessAddress:1College of Pharmacy, 1 University Station, University of Texas,Austin, TX, 78712, USA and 2Department of Biochemistry,1 UniversityStation, University of Texas, Austin, TX, 78712, USAEmail:Sean M. Kerwin* - skerwin@mail.utexas.edu* Corresponding authorKeywords:Annulation; Click Chemistry; Cyclization; Purine Analogs; RicinBeilstein Journal of Organic Chemistry 2008, 4, No. 26.doi:10.3762/bjoc.4.26Received: 09 June 2008Accepted: 11 July 2008Published: 25 July 2008© 2008 Mandal et al; licensee Beilstein-Institut.License and terms: see end of document.AbstractOxazolo[5,4-d]pyrimidines can be considered as 9-oxa-purine analogs of naturally occurring nucleic acid bases. Interest in this ringsystem has increased due to recent reports of biologically active derivatives. In particular, 5-aminooxazolo[5,4-d]pyrimidine-7(6H)-ones  (9-oxa-guanines)  have been shown to  inhibit  ricin.  The preparation of  a  series  of  2-substituted 5-aminooxazolo[5,4-d]pyrimidin-7(6H)-ones and related 5-thio-oxazolo[5,4-d]pyrimidines is described, including analogs suitable for further elabora-tion employing “click” chemistry utilizing copper-catalyzed Huisgen 1,3-dipolar cycloadditions. Two of the compounds preparedwere found to inhibit ricin with IC50 ca. 1–3 mM.IntroductionOxazolo[5,4-d]pyrimidines  have been reported to  possess  avariety of biological activities including kinase inhibition [1,2],adenosine receptor antagonism [3] and tumor growth inhibition[4]. In particular, 5-aminooxazolo[5,4-d]pyrimidin-7(6H)-ones[9-oxa-guanines] and related heterocycles have been shown toinhibit the ability of ricin to inactivate ribosomes [5]. Giventhese reports, and the expectation of further applications basedupon similarity to naturally occurring nucleic acid bases, thisheterocyclic ring system continues to generate interest.Approaches to the oxazolo[5,4-d]pyrimidine ring system gener-ally involve either cyclodehydration of an 5-(acylamino)-4-hydroxypyrimidine  [6-10]  or  elaboration  of  a  4-cyano-  or4-(alkoxycarbonyl)-5-aminooxazole [11-15] (Figure 1), withonly isolated reports of alternative routes [16,17]. However, fewp u b l i c a t i o n s  h a v e  d e s c r i b e d  t h e  p r e p a r a t i o n  o f5-aminooxazolo[5,4-d]pyrimidin-7(6H)-ones  [5,8,14]  or5-mercaptooxazolo[5,4-d]pyrimidin-7(6H)-ones [9-oxa-2-thio-xanthines]  [7,15],  and  in  both  cases,  the  conditions  do  notBeilstein Journal of Organic Chemistry 2008, 4, No. 26.Page 2 of(page number not for citation purposes)5Figure 1: The two general synthetic approaches to oxazolo[5,4-d]pyrimidines.Figure 2: Thermal cyclodehydration route to 9-oxo-guanine.appear  to  be  amenable  to  the  preparation  of  oxazolo[5,4-d]pyrimidines with variations at  the 2-position,  particularlythose  with  additional  functional  groups  or  increased  stericdemand.In  pursuing  8-methyl-9-oxa-guanine  [2-methyloxazolo[5,4-d]pyrimidin-7(6H)-one] as an initial lead in the design of inhib-itors of the ribosome-inactivating protein ricin [5], we set out toprepare  a  series  of  2-substituted  5-aminooxazolo[5,4-d]pyrimidin-7(6H)-ones  and  related  5-thio-oxazolo[5,4-d]pyrimidines. Here we report these studies, which have led tothe preparation of analogs suitable for further elaboration, forexample  by  “click”  chemistry  employing  copper-catalyzedHuisgen  1,3-dipolar  cycloadditions  [18].Results and DiscussionOur previous route [5] to 8-methyl-9-oxa-guanine (2a) involvedthe thermal cyclodehydration of 5-(acetylamino)-2-amino-4,6-dihydroxypyrimidine (1a) [19] (Figure 2). Unfortunately, forother 5-acylamino analogs of 1a, this route failed to afford therequired oxazolo[5,4-d]pyrimidines, presumably due to decom-position of the product at the temperatures required for cyclode-hydration. Other cyclodehydration conditions (POCl3,  PPA)were also unsuccessful.An alternative route was sought in which the oxazole ring wasformed first, followed by elaboration of the pyrimidine ring.Our previous work had demonstrated that in the case of ethyl5-amino-2-methyloxazole-4-carboxylate 3a [20], direct annula-tion  with  chloroformamidine  in  DMSO  at  120  °C  or  with2-methylisothiourea sulfate neat or in ethylene glycol at 170 °Cdid not afford the oxazolo[5,4-d]pyrimidine 2 (Figure 2). Thus,a stepwise annulation [15] strategy was explored for the prepar-ation of analogs of 2 bearing different 2-positions substituents.The required 5-aminooxazoles 3b,c  were prepared from the(acylamino)cyanoacetates  5b,c,  which in  turn  were  derivedfrom ethyl cyanoglyoxylate oxime (4) by reduction followed byacylation  by  isobutyric  anhydride  or  azidoacetyl  chloride(Figure 3). Reaction of the aminooxazoles 3b,c with benzoyl-isothiocyanate afforded the thioureas 6b,c. Direct cyclization of6b,c  to  oxazolo[5,4-d]pyrimidines  2b,c  by  treatment  withammonia in methanol was unsuccessful. However, thioureas6b,c  were  converted  to  the  desired  2-substituted  theoxazolo[5,4-d]pyrimidines  2b,c  by  stepwise  methylationfollowed by cyclization in the presence of ammonia in meth-anol. Although the intermediate products after methylation werenot purified,  in the case of 6c  the 1H NMR spectrum of thecrude product following methylation retains the signals for theethoxy  group  of  6c  and  displays  a  new  resonance  for  theS-methyl group at 2.0 ppm (see Supporting Information File 2),indicating that the intermediate is the S-methylisothiourea A(Figure 3). The mechanism involved in the conversion of inter-mediates  such  as  A  to  2  is  not  clear.  It  is  unlikely  that  the5-(methylthio)oxazolo[5,4-d]pyrimidine  is  an  intermediatebecause treatment of 5-(methylthio)oxazolo[5,4-d]pyrimidine8a (see below) with ammonia in methanol fails to afford 2a.The thiourea 6b and its analog 6a, which was prepared fromaminooxazole 3a in 51% yield, were also subjected to cycliza-tion  in  the  presence  of  ethanolic  KOH  to  afford  the5-mercaptooxazolo[5,4-d]pyrimidin-7(6H)-ones 7a,b (Figure4). Further elaboration of 7a by alkylation with methyl iodide,Beilstein Journal of Organic Chemistry 2008, 4, No. 26.Page 3 of(page number not for citation purposes)5Figure 3: Preparation of 2-substituted 5-aminooxazolo[5,4-d]pyrimidin-7(6H)-ones.Figure 4: Preparation of 2-substituted 5-aminooxazolo[5,4-d]pyrimidin-7(6H)-ones and related thioethers.Beilstein Journal of Organic Chemistry 2008, 4, No. 26.Page 4 of(page number not for citation purposes)5Figure 5: Click chemistry elaboration of a 5-(propargylthio)oxazolo[5,4-d]pyrimidine.benzyl bromide, propargyl bromide, or methyl 6-O-(tolylsulf-onyl)-α-D-glucopyranoside [21] afforded the thioethers 8a–d,respectively. The structural assignment of 8a–d as thioethersrest  principally  on  their  1H  and  13C  NMR  spectra,  whichcontain resonances more commensurate with groups attached tosulfur (e.g., for 8a: 2.2 ppm in 1H and 13 ppm in 13C NMR,respectively) than to nitrogen, as would be the case for 4- or3-alkylated 5-mercaptooxazolo[5,4-d]pyrimidine-7(6H)-ones.The thioether 8c bearing a propargyl substituent was subjectedto  “click”  chemistry  coupling  with  benzyl  azide  or2-morpholinoethyl azide in the presence of catalytic CuSO4 andsodium ascorbate to afford the triazoles 9 and 10, respectively,in  good  yield  (Figure  5).  The  copper-catalyzed  Huisgencycloaddition of terminal alkynes and alkyl azides favors form-ation of the 1,4-triazole regioisomers [22], and in the case oftriazole  9,  this  regiochemistry  was  confirmed  by  1H  NOEspectra  (see  Supporting  Information  File  2).ConclusionIn  conclusion,  routes  to  functionalized  oxazolo[5,4-d]pyrimidines from 2-substituted 5-aminooxazole-4-carboxylicacid ethyl esters were developed, the key to which is the relat-ively mild conditions employed in the step-wise elaboration ofthe pyrimidine ring. Compounds 2b,c, 7a,b, 8a–d, 9, and 10were evaluated for their ability to inhibit recombinant, catalytic-ally active ricin A-chain (RTA) employing a modification of thepreviously reported assay [5]. Briefly, the synthesis of proteinfrom endogenous globin mRNA by rabbit reticulocyte lysatewas determined in the absence of RTA, in the presence of suffi-cient RTA to inhibit 90% of protein synthesis (ca. 10 pM), andin the presence of both RTA and increasing concentrations ofthe oxazolo[5,4-d]pyrimidines. The ability of these compoundsto  inhibit  RTA  and  thereby  rescue  protein  synthesis  wasdetermined. Compounds 2b (IC50 = 2.8 mM) and 9 (IC50 = 1.6mM),  displayed  some  activity;  whereas,  none  of  the  othercompounds examined showed any significant RTA inhibitoryactivity.Supporting InformationSupporting Information File 1This file includes full experimental details for all newcompounds.[http://www.beilstein-journals.org/bjoc/content/supplementary/1860-5397-4-26-S1.doc]Supporting Information File 2Copies of 1H and 13C NMR spectra of compounds 2b,c;3a–c; 5b,c; 6a–c; A; 7a,b; 8a–d; 9; and 10.[http://www.beilstein-journals.org/bjoc/content/supplementary/1860-5397-4-26-S2.doc]AcknowledgmentsSupport from DOD/USAMRC DAMD17-03-C-0088 to JDR isgratefully acknowledged.Beilstein Journal of Organic Chemistry 2008, 4, No. 26.Page 5 of(page number not for citation purposes)5References1. Martin-Kohler, A.; Widmer, J.; Bold, G.; Meyer, T.; Séquin, U.; Traxler,P. Helv. Chim. Acta 2004, 87, 956–975. doi:10.1002/hlca.2004900892. Bauser, M.; Delapierre, G.; Hauswald, M.; Flessner, T.; D'Urso, D.;Hermann, A.; Beyreuther, B.; De Vry, J.; Spreyer, P.; Reissmüller, E.;Meier, H. Bioorg. Med. Chem. Lett. 2004, 14, 1997–2000. doi:10.1016/j.bmcl.2004.01.0823. Holschbach, M. H.; Bier, D.; Stüsgen, S.; Wutz, W.; Sihver, W.;Coenen, H. H.; Olsson, R. A. Eur. J. Med. Chem. 2006, 41, 7–15.doi:10.1016/j.ejmech.2005.07.0184. Claiborne, C. F.; Critchley, S.; Langston, S. P.; Olhava, E. J.; Peluso,S.; Weatherhead, G. S.; Vyskocil, S.; Visiers, I.; Mizutani, H.; Cullis, C.Heteroaryl compounds useful as inhibitors of E1 activating enzymes.PCT Int. Appl. WO 2008/019124 A1, February 14, 2008.Chem. Abstr. 2008, 148, 262855.5. Miller, D. J.; Ravikumar, K.; Shen, H.; Suh, J.-K.; Kerwin, S. M.;Robertus, J. D. J. Med. Chem. 2002, 45, 90–98. doi:10.1021/jm010186s6. Falco, E. A.; Elion, G. B.; Burgi, E.; Hitchings, G. H. J. Am. Chem. Soc.1952, 74, 4897–4902. doi:10.1021/ja01139a0497. Nishiwaki, T. Nature 1966, 211, 737–738. doi:10.1038/211737a08. Temple, C., Jr.; Smith, B. H.; Montgomery, J. A. J. Org. Chem. 1975,40, 3141–3142. doi:10.1021/jo00909a0309. Chern, J.-W.; Wise, D. S.; Townsend, L. B. J. Heterocycl. Chem. 1984,21, 1245–1246.10. Hurst, D. T.; Atcha, S.; Marshall, K. L. Aust. J. Chem. 1991, 44,129–134.11. Jansen, A. B. A.; Szelke, M. J. Chem. Soc. 1961, 405–411.doi:10.1039/JR961000040512. Sekiya, M.; Suzuki, J.; Kakiya, Y. Chem. Pharm. Bull. 1970, 18,1233–1238.13. Ohtsuka, Y. Bull. Chem. Soc. Jpn. 1973, 46, 506–509. doi:10.1246/bcsj.46.50614. Turchi, I. J.; Maryanoff, C. A. Synthesis 1983, 837–839. doi:10.1055/s-1983-3053515. Cabon, G.; Gaucher, B.; Gegout, A.; Heulle, S.; Masquelin, T. Chimia2003, 57, 248–254. doi:10.2533/00094290377767928016. Dang, V. T.; Stadlbauer, W. J. Heterocycl. Chem. 1996, 33,1025–1030.17. Dounchis, H. J. Org. Chem. 1972, 37, 2583–2587. doi:10.1021/jo00981a01418. Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Angew. Chem., Int. Ed. 2001,40, 2004–2021. doi:10.1002/1521-3773(20010601)40:11<2004::AID-ANIE2004>3.0.CO;2-519. Taylor, E. C.; Cain, C. K. J. Am. Chem. Soc. 1949, 71, 2282–2284.doi:10.1021/ja01175a00220. Grifantini, M.; Stein, M. L. Ann. Chim. (Rome, Italy) 1965, 55, 576–582.Chem. Abstr. 1965, 63, 13234.21. Griffith, B. R.; Krepel, C.; Fu, X.; Blanchard, S.; Ahmed, A.; Edmiston,C. E.; Thorson, J. S. J. Am. Chem. Soc. 2007, 129, 8150–8155.doi:10.1021/ja068602r22. Rostovtsev, V. V.; Green, L. G.; Fokin, V. V.; Sharpless, K. B. Angew.Chem., Int. Ed. 2002, 41, 2596–2599. doi:10.1002/1521-3773(20020715)41:14<2596::AID-ANIE2596>3.0.CO;2-4License and TermsThis is an Open Access article under the terms of theCreative Commons Attribution License(http://creativecommons.org/licenses/by/2.0), whichpermits unrestricted use, distribution, and reproduction inany medium, provided  the original work is properly cited.The license is subject to the Beilstein Journal of OrganicChemistry terms and conditions:(http://www.beilstein-journals.org/bjoc)The definitive version of this article is the electronic onewhich can be found at:doi:10.3762/bjoc.4.26

Synthesis of 2-Substituted 9-Oxa-Guanines {5-Aminooxazolo 5,4-D Pyrimidin-7(6H)-Ones} and 9-Oxa-2-Thio-Xanthines{5-Mercaptooxazolo 5,4-D Pyrimidin-7(6H)-Ones}

Oxazolo[5,4-d]pyrimidines can be considered as 9-oxa-purine analogs of naturally occurring nucleic acid bases. Interest in this ring system has increased due to recent reports of biologically active derivatives. In particular, 5-aminooxazolo[5,4-d]pyrimidine-7(6H)-ones (9-oxa-guanines) have been shown to inhibit ricin. The preparation of a series of 2-substituted 5-aminooxazolo[5,4-d]pyrimidin-7(6H)-ones and related 5-thio-oxazolo[5,4-d]pyrimidines is described, including analogs suitable for further elaboration employing “click” chemistry utilizing copper-catalyzed Huisgen 1,3-dipolar cycloadditions. Two of the compounds prepared were found to inhibit ricin with IC50 ca. 1–3 mM

Subrata Mandal

Wen Tai Li

Yan Bai

Jon D. Robertus

Sean M. Kerwin

Directory of Open Access Journals

Synthesis of 2-substituted 9-oxa-guanines {5-aminooxazolo[5,4-d]pyrimidin-7(6H)-ones} and 9-oxa-2-thio-xanthines {5-mercaptooxazolo[5,4-d]pyrimidin-7(6H)-ones}

https://repositories.lib.utexas.edu/bitstream/2152/43324/1/2008_07_Mandal.pdf

Synthesis of 2-Substituted 9-Oxa-Guanines {5-Aminooxazolo 5,4-D Pyrimidin-7(6H)-Ones} and 9-Oxa-2-Thio-Xanthines{5-Mercaptooxazolo 5,4-D Pyrimidin-7(6H)-Ones}

Abstract

Similar works

Full text

Available Versions

Texas ScholarWorks

Directory of Open Access Journals