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- Liao , Y. and Li , Z. M. 1996 . Synth. Commun. , 26 ( 9 ) : 1669 Anti-periplanar protons on the pyranoid ring usually exhibit a large value for vicinal coupling constant (7–10 Hz), whereas syn-clinal protons exhibit a small value (1–4 Hz).8 In our case, the proton which was on the same carbon with the pyrimidinyloxo group coupled with its two vicinal protons to give a quartet peak due to one anti-periplanar coupling (3Jaa =9.8 Hz) and another syn-clinal coupling (3Jea =3.7 Hz). In the stable chair conformation of the pyranoid ring which was shown in the scheme, H2 is syn-clinal to H1 anti-periplanar to H3 while H3 is anti-periplanar to both H2 and H4. Obviously, only the structure of 2-O substituted product 6 can accord with the 1H NMR result. Furthermore, 1H NMR result of 2,3-di-O-substituted product 7 (H 2: 5.32ppm, dd, 3J12=3.7Hz; 3J23=9.3Hz; H 3: 5.47ppm, t, 3J23=3J34=9.3Hz) also supported the above conclusion