2.2  Chemistry
The retrosynthetic analysis of the dinucleoside 1 structure suggested the coupling reaction between the tosyl derivative 17 previously described by us (Scheme 2) [14], and the readily accessible 5′-amino-2′,3′-isopropylidene adenosine [21]. The direct N-alkylation at room temperature did not afford the secondary amine in satisfactory yields and when increasing the temperature to enhance the reactivity of the primary amine, we noticed degradation of 17. To circumvent the lack of reactivity of primary amines, a synthetic method using nitrobenzenesulfonamides (Ns-amides) as both a protecting and activating group has been developed by Fukuyama [22]. The main advantage of this nosyl strategy is that both alkylation and deprotection proceed under mild conditions. Recently, the 2-nitrobenzenesulfonamide has been used successfully to synthetize transition state analogs of DNA methylation based on the coupling of cytosine analogs to adenosine [15]. In the same way, we envisaged the coupling between 17 and the 5′-nosyl adenosine 18 to obtain the dinucleoside 1 (Scheme 2). The building block 18 was prepared in 74% yield by reacting 4-nitrobenzenesulfonyl chloride [22] that has a similar reactivity to 2-nitrobenzenesulfonyl chloride as used in Ref. [15], with 5′-amino-2′,3′-isopropylidene adenosine prepared upon published procedures [23]. N-alkylation of Ns-amide 18 with 17 in the presence of K2CO3 in DMF at room temperature did not afford the expected dinucleoside 19, even at high temperature. Nevertheless, according to the literature [24], the addition of a catalytic amount of KI to the reaction mixture was beneficial to give 19 in 74% yield. Facile deprotection of 19 by treatment with a nucleophilic thiolate produced the desired secondary amine 20 in high yield. Removal of sugar protecting groups has been accomplished in acidic medium to give dinucleoside 1 in 76% yield. Likewise, the acidic treatment was applied to the intermediate Ns-amide 19 to afford the 4-nitrobenzenesulfonamide-containing dinucleoside 9 in 34% yield (Scheme 2).
Compounds 2, 3 and 4 were obtained from key compound 20 via reductive amination of the aldehyde 21 that was prepared in three steps from l-aspartic acid following a published procedure [25]. Reductive amination was conducted in the presence of sodium triacetoxyborohydride and acetic acid [26]. The resulting fully protected dinucleoside 22 was isolated in high yield (93%). Then, sugar hydroxyls and amine were deprotected by TFA treatment and afforded methyl ester derivative 3. Subsequent basic treatment with LiOH converted the methyl ester in carboxylic acid and dinucleoside 2 with α-amino acid chain similar to that of SAM was obtained. Finally, the SAM analogue 4 with an amide function instead of a carboxylic acid in α-amino acid chain was prepared from 22 upon a final treatment with 7 M methanolic ammonia solution. Dinucleosides 5, 6 and 7 were rather synthesized through N-alkylation of 20 with 1-bromobutane, 1-bromo-3-phenylpropane or methyl-4-bromobutyrate, respectively, in N-methylmorpholine in the presence of diisopropylethylamine (DIEA) at 110 °C under microwave. These specific conditions were optimized for a successful synthesis of 23, 24 and 25 with an average 50% yield. This moderate yield results from double N-alkylations (observed in mass spectrometry) and incomplete reactions. Increasing temperature of the reactions neither did drive the reaction to completion. Next, 23, 24 and 25 were deprotected upon TFA treatment to obtain N-alkyl derivatives 5, 6 and 7, respectively. Subsequently, an additional basic treatment with 2 M LiOH was applied to 7 to convert the methyl ester moiety into the carboxylic acid-ended alkyl chain of compound 8. The same synthetic route used for the preparation of nosyl-containing dinucleoside 9 was followed to synthesize compounds 10–13 with diverse Ns-amide moieties as analogs of the nosyl group (Scheme 3). The reaction of 5′-amino-2′,3′-isopropylidene adenosine [23] with four diversely substituted (OMe, CF3, Cl) and commercially available nitrobenzenesulfonyl chloride reagents afforded the corresponding N-nosyl adenosines 26–29 with 40–72% yield [[27], [28], [29], [30]]. Their subsequent coupling with 17 in the presence of K2CO3 and KI gave the corresponding dinucleosides 30–33 in moderate yields from 43 to 52%. A final TFA treatment provided the respective N-nosyl adenine dinucleosides 10–13 which were purified by reversed-phase chromatography (Yield 13–20%).
Scheme 4 depicts the synthesis of the adenine dinucleosides 14–16 from the intermediate NH-linked dinucleoside 20 (Scheme 2). The reaction of 4-chlorobenzenesulfonyl chloride with 20 in the presence of NEt3 [31,32] followed by a TFA treatment yielded 14. The treatment of 20 with 4-chloro-3-nitrobenzaldehyde or 4-chlorobenzaldehyde and sodium triacetoxyborohydride, followed by the removal of protective groups in acidic conditions resulted in dinucleosides 15 and 16, respectively. It is noteworthy that this reductive amination conducted at 40 °C increased the yields to 71% and 81%, respectively.