Novel and efficient regioselective enzymatic approach to 3’-, 5’- and 3’,5’-di-O-crotonyl 2’-deoxynucleoside derivatives

Regioselective syntheses of several O-crotonyl 2’-deoxynucleoside derivatives have been efficiently achieved using a biocatalytic methodology. While Candida antarctica lipase B (CAL-B) afforded the 5’-O-acylated compounds, immobilized lipase from Pseudomonas cepacia (PSL-C) provided the 3’-O-crotonylated analogues. Since classical chemical approaches did not work appropriately due to side isomerization reactions, a mixture of both lipases was used to achieve a useful synthetic route toward diacylated nucleosides. © 2016 Elsevier Science. All rights reserved ——— * Corresponding author. Tel.: +34 985 103 448; fax: +34 985 103 448; e-mail: [email protected]. It has been described that in some cases acylation of one hydroxyl group of the sugar moiety in a nucleoside derivative can increase its biological activity compared with the unmodified analogue. 1 In this sense, lipasecatalyzed transformations have become simple and standard processes for regioselective acylation of nucleosides, since they avoid the time-consuming protection and deprotection steps required in nonenzymatic approaches. 2 Thus, in our research group we have developed efficient enzymatic reactions to obtain 5’O-acylated nucleosides using the lipase B from Candida antarctica (CAL-B), 3 or 3’-O-acylated derivatives using the immobilized lipase from Pseudomonas cepacia (PSLC). 3c,3d,4 The crotonyl group is present in different biological active compounds as COTC [2-crotonyloxymethyl-(4R,5R,6R)4,5,6-trihydroxy-2-cyclohexenone] 5 or COMC (2crotonyloxymethyl-2-cyclohexenone), 6 important antitumor agents. The activity of this type of derivatives can be ascribed to the presence of the -unsaturated ester which can undergo Michael-type additions of nucleophiles within an enzyme. 7 However, the introduction of this moiety on nucleosides has been scarcely studied. 8 Previously, 3’amino-5’-crotonylamino-3’,5’-dideoxythymidine was synthesized, 9 and preliminary biological studies have shown that inhibits the in vitro replication of HIV-1 and HIV-2. 10 Due to this compound can not be 5’phosphorilated, it may suffer a Michael-type addition from a specifically enzyme. Moreover, the presence of this moiety on nucleosides would afford excellent starting compounds for the synthesis of, e.g., β-amino acid analogues of potential interest. 11 Nevertheless, the synthesis of O-crotonyl derivatives is not trivial since it is known that in the usual conditions to obtain them (base-catalyzed process with crotonyl chloride), mixtures of desired compounds and β,γunsaturated analogues are provided due to the deconjugation of the double bond. This fact, firstly described in 1966 by Ozeki and Kusaka, 12 depends on several factors such •as the alcohol, the solvent, the amine, and the temperature. 13 The mechanism for this transformation is assumed to pass through a ketene intermediate (Scheme 1). 13,14 Cl O Base O ROH OR O Scheme 1. Proposed mechanism for the deconjugation of the double bond in the crotonylation of an alcohol. Pergamon TETRAHEDRON LETTERS Tetrahedron Letters 2