1. Chemo-, regio- and stereoselective Mitsunobu reaction of unprotected pyrimidine bases with hydroxypyrrolidines
2. NMR spectra of cytosineN- and O-regioisomers:N-regioisomer11a:1H NMR (500 MHz, DMSO-d6, 50°C): 1.41 (bs, 9H,t-Bu); 2.03-2.19 (bm, 2H, H-4′); 3.23 (dd, 1H,Jgem= 11.2,J2′a'3. = 6.2, H-2′b); 3.32 (dt, 1H,Jgem= 10.9,J5′a4′= 7.5, H-5′b); 3.43 (ddd, 1H,Jgem= 10.9,J5′a4′= 8.2, 5.4, H-5′a); 3.60 (dd, 1H,Jgem= 11.2,J2′a3′= 7.4, H-2′a); 4.90 (p, 1H,J3′4′= 7.6,J3′2′a= 7.4,J= 6.2, H-3′); 5.71 (d, 1H,Jvic= 7.4, H-5); 6.92 (bs, 2H, NH2); 7.49 (d, 1H, Jvic= 7.4, H-6).13C NMR (125.8 MHz, DMSO-d6, 50°C): 28.14 (CH3-Boc); 28.54 (CH2- 4′); 43.96 (CH2-5′); 49.38 (CH2-2′); 54.66 (CH-3′); 78.60 (C-Boc); 93.82 (CH-5); 142.17 (CH-6); 153.49 (CO); 155.58 (C-2); 165.40 (C-4).O-regioisomer11b:1H NMR (500 MHz, DMSO-d6, 50°C): 1.40 (bs, 9H, t-Bu); 2.00 (bm, 1H, H-4′b); 2.11 (bm, 1H, H-4′a); 3.30-3.37 (bm, 2H, H-2′b and H-5′b); 3.41 (ddd, 1H, Jgem= 10.5, J5′a4′= 8.8,3.2, H-5′a); 3.53 (bdd, 1H, Jgem= 12.3,J2′a3′= 4.6, H-2′a); 5.34 (bm, 1H, H-3′); 6.10 (d, 1H, Jvic= 5.7, H-5); 6.71 (bs, 2H, NH2); 7.85 (d, 1H, Jvic= 5.7, H-6).13C NMR (125.8 MHz, DMSO-d6, 50°C): 28.16 (CH3-Boc); 30.69 (CH2-4′); 43.83 (CH2-5′); 51.69 (CH2-2′); 73.92 (CH-3′); 78.38 (C-Boc); 99.58 (CH-5); 153.61 (CO); 156.17 (CH-6); 164.17 (C-2); 165.46 (C-4)