Purine ribosides

Nebularine. Nebularine(44) is a naturaHy occurring purine riboside isolated from S.jokosukanensis (1,3,4). It is phosphorylated, and inhibits purine biosynthesis and RNA synthesis, but is not incorporated into RNA by E. coli RNA polymerase. It has also found appHcation as a transition state analogue for treatment of schistosomiasis and as a substrate for the restriction endonuclease, Hindll (138—141). 

FIGURE 16.8 (a) Phosphoglycolohydroxamate is an analog of the enediolate transition state of the yeast aldolase reaction, (b) Purine riboside, a potent inhibitor of the calf intestinal adenosine deaminase reaction, binds to adenosine deaminase as the 1,6-hydrate. The hydrated form of purine riboside is an analog of the proposed transition state for the reaction. 

Our inhibitor design strategy was based on the premise that structural modifications in the base of purine riboside that enhance purine base hydration without impairing the binding of the hydrated species to the ADA binding site would result in purine riboside (PR) analogues with high ADA inhibitory potency. Since the apparent inhibition constant (Kj (app)) is related to the hydration equilibrium constant (Keq) and the inhibitory constant for the hydrated molecule (Kj ) by... 

Figure 3. Purine riboside (PR) hydration and the effect of base modification Y.

Our next step was to assess whether the methodology used to calculate hydration free energy differences for simple carbonyl-containing compounds9 was suitable for heteroaromatic bases. Since our drug design strategy entailed analysis of purine riboside hydration, a series of azanaphthalenes was initially selected for analysis in part because of their structural similarity to purines and in part because of the extensive... 

To test whether we could accurately calculate the fold-difference of ADA inhibitory potency between purine riboside (8) and analogues of purine riboside, we selected 8-azapurine riboside (9) for our studies. Compound 9 was reported to be a 400-fold more potent ADA inhibitor relative to 8 despite differing from 8 only by the replacement of C8 with a nitrogen (Figure 8).19 The molecular reason for this enhancement in potency was not determined, but could be due either to enhanced hydration or enhanced ADA binding affinity of the hydrated species or both. To determine the reason,... 

Figure 8. Purine riboside/8-azapurine riboside and corresponding hydrates...

An alternative explanation for the 400-fold improvement in inhibitory potency exhibited by 8-azapurine riboside (9) is that the 8-aza analogue hydrates to a much larger extent than purine riboside (8). Calculation of the relative hydration free energy difference between 9-methylpurine and 8-aza-... 

Table 2 Inhibition Potency of Purine Riboside and its 8-aza Analogue...

One of the more difficult prebiotic syntheses is that of the nucleosides. Heating of ribose and purines at 100°C gives fair yields (2-20%) of a mixture of isomers of the purine ribosides, with some of the correct 13-isomer being produced.45 However, a mixture of pyrimidines and ribose gives no detectable yield of nucleosides. Whether the inability to synthesize pyrimidine ribosides is a matter of not trying the correct conditions or whether pyrimidine nucleosides were not involved in the first organisms remains to be determined. 

Peculiarly little attention was aimed at the investiga tion of other 6-alkylpurine derivatives. Recently, cyto kinin activity was reported in some 6-(arylalkynyl), 6-(arylalkenyl)-, and 6-(atylalkyl)purines.21 We have recently described the cytostatic activity of 6-(trifluo-romethyl)purine riboside.22 The corticotropin-releasing hormone antagonist activity of some 2,8,9-trisubsti-tuted-6-arylpurines has been also reported.23... 

The instability of simple purine ribosides toward alkali88 89 can be related to the fairly strong electron-withdrawing power of the ribose moiety, which directs the nucleophile to the 8-carbon. Stabilization of a nucleoside can be achieved if anion formation is possible. This is seen with inosine (hypoxanthine-9-D-riboside) (39, R = H), which is... 

Kalckar117118119 has shown that the enzymatic phosphorolysis of inosine (hypoxanthine 9-D-ribofuranoside) may give rise to the formation of a pentose phosphate, isolable as its barium salt. The phosphate was found to be non-reducing although easily hydrolyzed by either acid or alkali to equimolar quantities of phosphate and pentose. In view of these properties and the fact that it could be used for the enzymatic synthesis of purine ribosides, Kalckar has tentatively assigned to it the D-ribose 1-phosphate structure its ring structure and configuration at carbon 1 remain undetermined. 

Nucleoside phosphorylase catalyzes the reversible conversion of a purine riboside such as inosine to a purine base such as hypoxanthine and ribose-1-phosphate. Free phosphate is also required as a substrate. 

Figure 9.99 Separation of the components of the reaction studied (catalysis by nucleoside phosphorylase of a purine riboside-base conversion) by HPLC. Chromatographic conditions isocratic elution flow rate, 2 mL/min 0.02 F KH2P04 (pH 4.2) and 3% methanol. Peaks 1, uric acid 2, hypoxanthine 3, xanthine 4, inosine. (From Halfpenny and Brown, 1980.)...

Similarly, adenosine, inosine and 2 -deoxyinosine have been converted into their 8-trifluoromethyl derivatives by reaction with the copper complex formed from trifluoromethyl iodide and copper in hexamethylphosphorotriamide (80JCS(P1)2755). iV -Trifluoromethyl-purine riboside was also produced from 6-chloropurine riboside. Recently, guanosine has been shown to give the C (8)-substituted derivative (148) by reaction with a benz[a]anthracene 5,6-dioxide at pH 9.5 over 4 days at 37 °C (80CC82). 

An interesting cyclization reaction on AlCA-riboside with 2-chloroethyl isothiocyanate to give a dihydrothiazolo[3,2-a]purine riboside 19 has recently been reported. 

Improvement in fluorination. DAST used for the laboratory-scale synthesis described above is not desirable for industrial synthesis in terms of availability and safety. Thus, we examined the fluorination of 2 -activated nucleoside with triethylamine trihydrogenfluo-ride. The triflate, which was quantitatively obtained from 40, was reacted with 6 equivalents of Et3N 3HF and 3 equivalents of Et3N in ethyl acetate. Fluorination proceeded very smoothly to give 41 in 88% yield [68]. To the best of our knowledge, this is the highest reported yield in the fluorination of a purine riboside at the 2 -position. Next, we treated compound 41 with ammonia to give 42 in almost quantitative yield by simultaneous 6-amination and 3 -benzoyl deprotection, (see Scheme 7.14). 

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