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[Preprint]. 2023 May 26:2023.05.26.542506. [Version 1] doi: 10.1101/2023.05.26.542506

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Scheme 1. — Synthesis of exNA phosphoramidites (7a and 7b). Reagent and conditions: (i) TBDMSCl, Imidazole/DMF, rt, overnight; (ii) 3% DCA/CH₂Cl₂, triethylsilane, rt, 1 h, 2a:88 % (2 steps), 2b: 84 % (2 steps); (iii) IBX/ CH₃CN, 85°C, 1.5 h; (iv) CH₃PPh₃Br, ^tBuOK, THF, 0°C then rt, overnight, 4a: 75% (2 steps), 4b: 67% (2 steps); (v) 9-BBN, THF, 0°C, overnight; (vi) NaBO₃ · 4H₂O, MeOH, THF, H₂O, 0°C then rt, overnight, 5a: 62% (2 steps), 5b: ND; (vii) DMTr-Cl, pyridine, rt, 2 h; (viii) 0.1 M TBAF, THF, rt, 1 h, 6a:93% (2 steps), 6b: 12% (3 steps); (ix) 2-cyanoethyl N,N-diisopropylchlorophosphoramidite, DIPEA/CH₂Cl₂, 0°C then rt, 0.5 h, 7a: 86%, 7b: 81%.