Regioselective alkylation of the exocyclic nitrogen of adenine and adenosine by the Mitsunobu reaction Steven Fletcher * Department of Pharmaceutical Sciences, University of Maryland School of Pharmacy, Baltimore, MD 21201, USA article info Article history: Received 2 March 2010 Revised 18 March 2010 Accepted 23 March 2010 Available online 27 March 2010 abstract A novel synthetic route to N 6 -substitution of adenine is presented, employing the Mitsunobu reaction as the key step. A range of primary and secondary alcohols all coupled in very good to excellent yields within 30 min at 45 °C, offering a milder alternative to the traditional nucleophilic aromatic substitution of 6-chloropurine. The utility of this protocol is further demonstrated by its application to the syntheses of N 6 ,N 9 -di-substituted adenines, including the potent and selective A 1 adenosine receptor agonist N 6 - cyclopentyladenosine. Ó 2010 Elsevier Ltd. All rights reserved. Adenosine is an endogenous hormone that elicits both physio- logical and pathophysiological functions through binding to aden- osine receptors (ARs). There are four known subtypes of adenosine receptor: A 1 and A 3 , both of which inhibit the enzyme adenylate cyclase, leading to a reduction in cellular cAMP levels, and A 2A and A 2B , both of which stimulate adenylate cyclase, thereby raising the levels of cAMP. 1 The A 1 receptor plays a role in the regulation of heart rate; adenosine is an A 1 agonist, and is used in the treatment of tachycardia. The ubiquitous nature of adenosine receptors and the lack of receptor subtype selectivity have prompted the devel- opment of compounds that are selective for specific receptor sub- types. For example, it is known that functionalization of the exocyclic (N6) amino group of adenosine with heterocycles and cycloalkyls, as in N 6 -cyclopentyladenosine (1, Fig. 1), enhances the specificity for the A 1 receptor. 2 Conversely, replacement of the ribose moiety in 1 with hydrophobic groups, including methyl, ethyl and cyclopentyl, to afford N 6 ,N 9 -disubstituted adenines, such as 2 (Fig. 1), leads to the generation of potent and selective A 1 antagonists. 3 Meanwhile, N 6 -substituted adenines, such as 3 (Fig. 1), are known synthetic cytokinins 4 as well as potent antimi- crobial agents. 5 N 6 -Alkylation of adenines and adenosines is typically accom- plished by nucleophilic aromatic substitution reactions with pri- mary amines on the corresponding 6-chloropurines. 6 However, 6- chloropurine and 6-chloropurine riboside are considerably more expensive chemicals than their adenine counterparts. In addition, the conditions required for such reactions often involve an excess of the amine, high temperatures and reaction times, in some cases, upwards of 24 h. These conditions might be incompatible with cer- tain substrates. An alternative synthetic approach might be the di- rect alkylation of the exocyclic amino group but methylation of adenosine leads to reaction at the 1- and 7-positions, with no alkyl- ation of the N6 position. 7 Moreover, the attempted ethylation of adenosine also leads to a mixture of products. 7 A more common alkylation approach is described by Robins and Trip who effected the indirect N 6 -alkylation of 2 0 -deoxyadenosine by N 1 -alkylation followed by the Dimroth rearrangement to the desired N 6 -substi- tuted product. 8 More recently, Pedersen and co-workers have dem- onstrated a two-step, direct N 6 -alkylation of 2 0 -deoxyadenosine using benzotriazole as a synthetic auxiliary, although overall yields are typically moderate. 9 Alkylation of the N6 amino group has also been accomplished by the reaction of N 6 -benzoyladenosine deriv- atives with the highly reactive electrophile benzyloxymethyl chlo- ride in moderate yield, 10 and inadvertently in good yield by the in situ trapping of an allyl-protecting group during its removal. 11 As part of our research into the synthesis of modified nucleo- bases for the preparation of novel peptide nucleic acids (PNAs), we discovered that protection of the exocyclic N2 amino group in the guanine precursor 2-amino-6-chloropurine as its Boc carba- mate concomitantly activated that group as an acidic nucleophile (NuH) in the Mitsunobu reaction. 12 It was considered that the 0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2010.03.103 * Tel.: +1 410 706 6361; fax: +1 410 706 5017. E-mail address: sfletche@rx.umaryland.edu N N N N HN O OH OH HO 1 N N N N HN 2 N N N N H HN 3 Figure 1. A potent and selective agonist (1) and antagonist (2) of the A 1 adenosine receptor, and a synthetic cytokinin (3). Tetrahedron Letters 51 (2010) 2948–2950 Contents lists available at ScienceDirect Tetrahedron Letters journal homepage: www.elsevier.com/locate/tetlet