Fluorescent charge-neutral analogue of xanthosine: Synthesis of a 2 '-deoxyribonucleoside bearing a 5-aza-7-deazaxanthine base

A concise route is described to prepare the 5-aza-7-deazapurine 2 ' -deoxyr iboside (4), which presents the puADA hydrogen-bonding pattern, analogous t o the hydrogen-bonding pattern presented by 2 ' -deoxyxanthosine (2). The r oute begins with the commercially available 1-alpha -chloro-2-deoxy-3-5-bis toluoyloxyribofuranose (10), which proves to be a versatile point of entry to beta -2 ' -deoxyribofuranosides. In the first step, 2-nitroimidazole (8) is coupled with 10 to yield intermediate 11. Reduction of the nitro group to an amino group yields 12, which is treated with phenyl isocyanatoformate to complete the nucleobase to yield 13. Removal of the toluoyloxy protecti ng groups of 13 yields the target nucleoside 4 in 40% overall yield in four steps. In an alternative strategy, convergent coupling of 14 with 10 under basic conditions was attempted but found to yield the heterocycle glycosyl ated at the undesired position. Compound 13 displays potentially useful flu orescence properties. After excitation at 250 nm, a solution of 13 in MeCN shows a fluorescence emission with a maximum at 410 Dm. Furthermore, 13 is neutral at physiological pH, a property that it shares with natural nucleob ases but not xanthosine itself, which is an acid with a pK(a) of ca. 5.6. F urthermore, as part of the design, 4 is made capable of presenting an unsha red pair of electrons to the DNA minor groove.