ORGANIC LETTERS
Synthesis of a Conformationally Locked Version of Puromycin Amino Nucleoside
2002 Vol. 4, No. 4 589-592
Yongseok Choi,† Clifford George,‡ Peter Strazewski,§ and Victor E. Marquez*,†,⊥ Laboratory of Medicinal Chemistry, Center for Cancer Research, NCI, Frederick, Maryland 21702, Laboratory for the Structure of Matter, NaVal Research Laboratory, Washington, D.C. 20375, and Laboratoire de Syntheses de Biomole´ cules, UniVersite´ Claude Bernard-Lyon 1, 69622 Villeurbanne Cedex, France
[email protected] Received December 7, 2001
ABSTRACT
A conformationally locked carbocyclic version of puromycin amino nucleoside was synthesized via Mitsunobu coupling of a 3-azido-substituted carbocyclic moiety with 6-chloropurine without interference from the azido group reacting with triphenylphosphine. The requisite 3-azidosubstituted carbocyclic pseudosugar was prepared by a double inversion of configuration at C3′ (nucleoside numbering) involving a nucleophilic displacement with azide.
Conformationally locked carbocyclic nucleosides built on a rigid bicyclo[3.1.0]hexane template1 have a defined sugar pucker that mimics either a North-type conformation (2E) or a South-type (3E) conformation as defined in the pseudorotational cycle.2 These nucleosides have been devised to study the role of sugar conformation in the processes of recognition and binding of nucleosides, nucleotides, and oligonucleotides to their target enzymes. Recent studies from our laboratory suggest that the majority of enzymes appear †
Center for Cancer Research, NCI. Naval Research Laboratory. Universite´ Claude Bernard-Lyon 1. ⊥ Fax: 301-846-6033. (1) (a) Rodriguez, J. B.; Marquez, V. E.; Nicklaus, M. C.; Barchi, J. J. Tetrahedron Lett. 1993, 34, 6233. (b) Altman, K.-H.; Kesselring, R.; Francotte, E.; Rihs, G. Tetrahedron Lett. 1994, 35, 2331. (c) Rodriguez, J. B.; Marquez, V. E.; Nicklaus, M. C.; Mitsuya, H.; Barchi, J. J. J. Med. Chem. 1994, 37, 3389. (d) Altman, K.-H.; Imwinkelried, R.; Kesselring, R.; Rihs, G. Tetrahedron Lett. 1995, 35, 7625. (e) Marquez, V. E.; Siddiqui, M. A.; Ezzitouni, A.; Russ, P.; Wang, J.; Wagner, R. W.; Matteucci, M. D. J. Med. Chem. 1996, 39, 3739. (f) Ezzitouni, A.; Marquez, V. E. J. Chem. Soc., Perk. Trans. 1997, 1073. (g) Marquez, V. E.; Ezzitouni, A.; Russ, P.; Siddiqui, M. A.; Ford, H.; Feldman, R. J.; Mitsuya, H.; George, C.; Barchi, J. J. J. Am. Chem. Soc. 1998, 120, 2780. (h) Shin, K. J.; Moon, H. R.; George, C.; Marquez, V. E. J. Org. Chem. 2000, 65, 2172. (i) Moon, H. R.; Ford, H.; Marquez, V. E. Org. Lett. 2000, 2, 3793. (2) Altona, C.; Sundaralingam, M. J. Am. Chem. Soc. 1972, 94, 8205. ‡ §
10.1021/ol010288c CCC: $22.00 Published on Web 01/30/2002
© 2002 American Chemical Society
to have strict conformational requirements for substrate binding in which the furanose ring is in a well-defined shape.1e,g,3 Since the bicyclo[3.1.0]hexane template mimics the active, receptor-bound conformation of the nucleoside or nucleotide, it provides an accurate identification of the favored sugar conformation that results in optimal recognition by the target enzyme. This knowledge can be put to practice not only to achieve a desirable biological outcome but also to prevent an undesirable metabolic event. For example, the amino nucleoside 1b (Figure 1), a hydrolytic product of the antibiotic puromycin (1a), forms a nephrotoxic amino nucleotide. This prompted Vince and Daluge to synthesize the carbocyclic cyclopentyl analogue 2 lacking the 5′-OH group to avoid kinase activation.4 Alternatively, since we know that bicyclo[3.1.0]hexane nucleosides locked in a North-type conformation are more resistant to phosphory(3) (a) Marquez, V. E.; Russ, P.; Alonso, R.; Siddiqui, M. A.; Shin, K. J.; George, C.; Nichlaus, M. C.; Dai, F.; Ford, H. Nucleosides Nucleotides 1999, 18, 521. (b) Wang, P. Y.; Brank, A. S.; Banavali, N. K.; Nichlaus, M. C.; Marquez, V. E.; Christman, J. K.; MacKerell, A. D. J. Am. Chem. Soc. 2000, 122, 12422. (e) Marquez, V. E.; Wang, P. Y.; Nichlaus, M. C.; Maier, M.; Monoharan, M.; Christman, J. K.; Banavali, N. K.; MacKerell, A. D. Nucleosides Nucleotides Nucleic Acids 2001, 20, 451. (4) Daluge, S.; Vince, R. J. Med. Chem. 1972, 15, 171.
Scheme 1a
Figure 1.
lation by cellular kinases,5 we propose the synthesis of 3, which corresponds to a 2′-deoxyribo version of 1b, as another option to prevent formation of a toxic nucleotide metabolite. One additional attractive feature of the proposed target 3 is that by virtue of its locked North-type conformation, the fixed pseudoequatorial orientation of the critical 3′-NH2 is expected to be equivalent to that found in the crystal structures of the parent antibiotic puromycin (1a)6 and the corresponding amino nucleoside 1b.7 The initial approach to the desired methanocarba amino nucleoside 3 was based on our previous work (Scheme 1).1g To introduce the amino group with the proper stereochemistry, inversion at C3′ (nucleoside numbering) was necessary. In North-type bicyclo[3.1.0]hexane pyrimidine nucleosides, inversion of the stereochemistry at C3′ can be easily achieved via anhydride formation resulting from the intramolecular attack of the pyrimidine C2 oxygen onto the C3′ carbon functionalized with a suitable leaving group.1g Since there is no anhydride formation with purines, the strategy involved reaction of 5 under standard Mitsunobu conditions to give the inverted benzoate ester 6. Surprisingly, this ester resisted an ensuing hydrolysis under mild conditions (NaOMe/ MeOH, 0 °C), giving instead the 6-methoxy analogue 7. It is possible that the ester group in 6 is sterically hindered by virtue of the all-cis disposition of substituents on the cyclopentane ring. Compound 6 was then heated in a mixture of aqueous dimethylamine and ethanol to give 8, which was still contaminated with the corresponding benzoate ester analogue. After further treatment of the mixture with NaOMe (0.5 M in MeOH) at room temperature, compound 8 was finally obtained as the exclusive product. The inverted configuration at C3′ was confirmed by comparing the 1H NMR coupling constants of compound 8 vis-a`-vis those of diastereoisomer 9 synthesized from 5 (Scheme 1). Conversion of the secondary hydroxyl group in 8 to the mesylate ester was followed by nucleophilic displacement (5) Agbaria, R.; Noy, R.; Ben-Zvi, Z.; Manor, E.; Candotti, F.; Morris, J. C.; Ford, H., Jr.; Marquez, V. E.; Johns, D. G. Submitted. (6) Sundaralingam, M.; Arora, S. K. J. Mol. Biol. 1972, 71, 49. (7) Padmaja, N.; Ramakumar, S.; Viswamitra, M. A. Acta Crystallogr. 1988, C44, 2176. 590
a Reagents: (i) Ph P, 6-Cl-purine, DEAD, THF; (ii) (a) TBAF, 3 THF, (b) Ph3P, BzOH, DEAD, benzene; (iii) NaOMe, MeOH, 0 °C; (iv) (a) 40% HN(Me)2, EtOH, 90 °C, (b) NaOMe, MeOH, rt; (v) (a) MsCl, TEA, DMAP, CH2Cl2, (b) NaN3, DMF, 120 °C; (vi) 10% Pd-C, THF-EtOH.
and inversion of configuration with azide to afford compound 10. Unfortunately, we failed to obtain the final target amino nucleoside 3 under standard hydrogenation conditions that were expected to simultaneously remove the protective benzyl group (Scheme 1).8 An alternative, more versatile plan was then devised to access an entire series of 3′-amino-substituted analogues from the common azido carbobicyclic precursor 18 (Scheme 2). From the readily available compounds 11 and 12,9 inversion of configuration at C3 was achieved under standard Mit(8) Czech, B. P.; Bartsch, R. A. J. Org. Chem. 1984, 49, 4076. (9) Marquez, V. E.; Russ, P.; Alonso, R.; Siddiqui, M. A.; Hernandez, S.; George, C.; Nicklaus, M. C.; Dai, F.; Ford, H. HelV. Chim. Acta 1999, 82, 2119. Org. Lett., Vol. 4, No. 4, 2002
Scheme 2a
a Reagents: (i) TBDPSCl, imidazole, CH Cl ; (ii) Pd black, 2 2 HCO2H, MeOH; (iii) Ph3P, BzOH, DEAD, benzene, CH3CN; (iv) NaOMe, MeOH; (v) BzCl, pyridine, CH2Cl2; (vi) (a) MsCl, TEA, DMAP, CH2Cl2, 0 °C, (b) NaN3, DMF, 120 °C; (vii) TBAF, THF.
sunobu conditions via compound 13. The resulting dibenzoate ester 14 was hydrolyzed to diol 15, which after comparison with 13 confirmed that inversion of configuration had indeed taken place. Diol 15 was then singly protected as the monobenzoate ester 16, converted into the intermediate mesylate ester, and reacted with azide to afford 17 and finally the pivotal intermediate 18 following the removal of the silyl ether protecting group. A “one-pot” condensation of 18 with 6-chloropurine under Mitsunobu conditions (Scheme 3) provided an exceptional 94% yield of the coupled product 19. This demonstrates that the diethyl azodicarboxylate (DEAD) reacts preferentially with Ph3P with no interference from the azido group in forming an imino phosphorane intermediate. Amination of 19 with either dimethylamine or ammonia produced, respectively, compounds 20 and 21. Finally, reduction of the azido group in 20 and 21 provided the final target 3 and the corresponding 6-aminopurine analogue 22 (Scheme 3). Compound 21 provided adequate crystals for X-ray analysis, and the generated structure (Figure 2) validated all Org. Lett., Vol. 4, No. 4, 2002
Scheme 3a
a Reagents: (i) Ph P, 6-Cl-purine, DEAD, THF; (ii) 40% 3 HN(Me)2 in H2O, EtOH, 90 °C; (iii) 10% Pd-C, EtOH; (iv) NH4OH, dioxane, 70 °C.
Figure 2. X-ray structure of 21 with displacement ellipsoids drawn at the 30% probability level. 591
the previous spectral assignments. Furthermore, the crystal structure confirmed the North-type (P ) -20.3°, νmax ) 29.3°) conformation, almost 2° away from the ideal 2E envelope (P ) -18°) conformation. The corresponding conformational parameters for 1b are also typical of a Northtype conformation (P ) -2.9°, νmax ) 38.1°).7 Although the bicyclo[3.1.0]hexane template flattens slightly the cyclopentane ring (compare the νmax values), the pseudoequatorial disposition of the azido group in both structures is equivalent. It is expected that compound 3, in addition to providing a substrate resistant to enzymatic phosphorylation, will help elucidate the role of conformation in the mechanism of inhibition of puromycin in its mimicry of the 3′-terminus aminoacyl-t-RNA. These and other biological studies are in progress and will be reported in due course. In summary, we have synthesized a conformationally locked version of puromycin amino nucleoside (3) by
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Mitsunobu coupling of 6-chloropurine with a versatile azido carbocyclic moiety that can also provide access to a wide diversity of carbocyclic 3′-amino-substituted nucleosides. Concomitant reaction of the azido group with triphenylphosphine did not interfere with the Mitsunobu coupling. Synthesis of similar amino nucleoside analogues of the antibiotic puromycin with a South-type conformation is underway. Supporting Information Available: Experimental conditions for the syntheses of all compounds and their corresponding spectral and analytical data, plus the crystal data and structural refinements for 21, including tables of atomic coordinates, bond distances and angles, and anisotropic thermal parameters This material is available free of charge via the Internet at http://pubs.acs.org.. OL010288C
Org. Lett., Vol. 4, No. 4, 2002
