An efficient synthesis of the 4′-epimer of 2-fluoronoraristeromycin

Abstract

The 4′-epimer of 2-fluoronoraristeromycin was synthesized by employing bis-t-butoxycarbonyl (Boc) protected 2-fluoroadenine as a superior substrate for the Mitsunobu reaction with the appropriate cyclopentenol. Unlike the unsubstituted counterpart 2-fluoroadenine, this substrate is completely soluble in THF and resulted in a very good yield in the Mitsunobu coupling reaction as well as subsequent steps.

Introduction

The antiviral activity of carbocyclic nucleosides such as aristeromycin and neplanocin A (Fig. 1) is attributed to the inhibition of S-adenosyl-L-homocysteine hydrolase (SAHH), a hydrolytic enzyme that catalyzes the breakdown of S-adenosyl-L-homocysteine (SAH) to give adenosine and homocysteine.¹ SAH is a biofeedback inhibitor of methyltransferases, enzymes that catalyze essential biological methylation reactions. Therefore, inhibition of the enzyme that removes S-adenosyl-L-homocysteine, namely SAHH, increases the cellular level of SAH resulting in the suppression of methylation reactions.

Figure 1.

Biologically active carbocyclic nucleosides

Recently, Plasmodium falciparum, the causative agent of malaria, has been shown to possess a specific S-adenosyl-L-homocysteine hydrolase (PfSAHH).² X-ray crystal structure analysis of PfSAHH complexed with adenosine has revealed that there is a crucial structural difference between PfSAHH and the human (Homo sapiens) S-adenosyl-L-homocysteine hydrolase (HsSAHH).³ The active site of PfSAHH has an extra space located near the C2 of adenine ring (not present in HsSAHH) suggesting that substitution of the hydrogen of C2 by a functional group of the right size would fill the space and enhance selective inhibition of PfSAHH.⁴

Noraristeromycin (1), a carbocyclic nucleoside lacking the methylene group of aristeromycin (Fig. 2), possesses a significant inhibitory activity against PfSAHH (IC₅₀ = 3.1 μM) although it is not selective (selectivity index = mean IC₅₀ for HsSAHH/mean IC₅₀ for PfSAHH = 0.35).⁵ On the other hand, 2-fluoronoraristeromycin (2) is less potent (IC₅₀ for PfSAHH = 13 μM) than the parent compound 1, however, the introduction of fluorine at the 2-position of the adenine ring clearly confers selectivity (selectivity index = 4.8) as expected based on the structural differences at the active site of the hydrolases of the parasite (PfSAHH) and the host (HsSAHH).⁶ Another important observation in this class of compounds is that 3, the 4′-epimer of noraristeromycin, showed even better selectivity (selectivity index 9.6) than both noraristeromycin and its 2-fluoro analogue.⁷

Figure 2.

Lead and target compounds

In our pursuit of carbocyclic nucleosides with improved antimalarial activity and selectivity, we sought compound 4, (the 4′-epimer of 2-fluoronoraristeromycin) since it combines a 2-fluoro substituent and an α face 4′-hydroxyl group, features that led to improved selectivity in the lead compounds 2 and 3. An efficient and facile synthesis of 4 is reported here.

Chemistry

Mitsunobu reaction, which is widely used in the preparation of carbocyclic nucleosides, could be employed to couple 2-fluoroadenine and the requisite substituted cyclopentenol at the beginning of a convergent approach to the target compound.⁸ However, the yield of this reaction is severely hampered by the extremely low solubility of 2-fluoroadenine under typical Mitsunobu condition (THF, PPh₃, DIAD). In our hands, the yield of this reaction never exceeded 27%.

Recently, it was shown that bis-t-butoxycarbonyl (Boc) protected adenine couples with a variety of substituted cyclopentan(en)ols in the Mitsunobu reaction at room temperature with yields ranging from 85–96% in conveniently short reaction times.⁹ In order to extend this observation to the preparation of 4, bis-Boc-2-fluoroadenine was prepared as shown in scheme 1. Reaction of 2-fluoroadenine with di-tert-butyl dicarbonate (Boc₂O) in the presence of catalytic amount of 4-dimethylaminopyridine (DMAP) afforded tris-Boc substituted derivative 6.¹⁰ Removal of the N-9 Boc group was achieved using saturated sodium bicarbonate in methanol resulting in 7.

Scheme 1.

Preparation of 7

Next, the allylic alcohol 8¹¹ was subjected to Mitsunobu reaction in the presence of bis-Boc-2-fluoroadenine to give the coupling product 9 (Scheme 2). Unlike the Mitsunobu reaction involving the unprotected 2-fluoroadnenine, the bis-Boc protected derivative proved to be a superior substrate in that it is completely soluble in THF and the reaction was completed in 4 h at room temperature (yield 88 %). This was followed by dihydroxylation of 9 using 4-methylmorpholine N-oxide and catalytic amount of osmium tetroxide to yield the diol 10. The t-butoxycarbonyl group was then removed by a 1:1 mixture of trifluoroacetic acid and dichloromethane at room temperature.¹² The synthesis of the target compound was completed with the deacetylation of 11 using ammonia gas in methanol.

Scheme 2.

Preparation 4

Conclusion

An efficient preparation of the 4′-epimer of 2-fluoronoraristeromycin has been achieved via Mitsunobu coupling of an allylic cyclopentenol and bis-Boc-2-fluoroadenine. This convenient and high yielding reaction points to the extremely desirable properties of bis-Boc-2-fluoroadenine as an excellent substrate for Mitsunobu reactions in the synthesis of biologically important fluorinated carbocyclic nucleosides. A study of the antimalarial properties of the target compound will be forthcoming.