Chemical synthesis of UNA-isoguanine phosphoramidite

The synthesis of protected UNA-isoguanine phosphoramidite (Fig 2) was performed according to general procedure of the synthesis of UNA phosphoramidites with some modifications [18]. 2,6-Diaminopurine riboside (1) was treated with lithium nitrite and acetic acid to yield isoguanosine (2). Treatment of compound (2) with N,N-dimethylformamide dimethyl acetal resulted in the N6 protected derivative (3) which was converted with dimethoxytrityl chloride into 5′-O-4,4′-dimethoxytrityl derivative (4). Compound (4) was treated with sodium periodate followed by sodium borohydride to produce UNA derivative (5). Selective benzoylation of (5) at O2′ was achieved by addition of benzoyl chloride at decreased temperature accordingly to the previously published protocol [18]. Treatment of the resulting compound (6) with 2-cyanoethyl-N,N,N′,N′-tetraisopropylphosphordiamidite gave the final UNA-isoguanine in overall yield ca. 17%. Protocol for the synthesis of UNA-isoguanine phosphoramidite is described in S1 File. The ¹H and ¹³C NMR spectra were recorded on a 500 MHz (11.74 T) AVANCE III Bruker spectrometer. Thin layer chromatography was conducted on Merck silica gel 60 F254 glass plates. Silica gel column chromatography was performed using Merck TLC gel H 60.

Reagents: (i) LiNO₂, AcOH, H₂O; (ii) N,N-dimethylformamide dimethyl acetal, DMF; (iii) DMTCl, Py; (iv) NaIO₄, 1,4-dioxane/water; NaBH₄, 1,4-dioxane/water; (v) BzCl, DCM, -70°C; (vi) 2-cyanoethyl-N,N,N′,N′-tetraisopropylphosphordiamidite, tetrazole, MeCN.