Tetrabutylammonium Fluoride → HCID 131722672, compound

Participants

Reaction Steps

All three products exhibited sugar 1H NMR signals consistent with the hemiacetal structure. This result demonstrates the selectivity in lithiation of 7, but the formation of other products suggests that competing halogen-lithium exchange occurred, too. Deoxygenation of 8 using the same procedure as for reduction of 5 yielded, similarly as in the case of 5, an 1:1 mixture of α/β nucleosides in a low yield. On the other hand, acetylation of 8 proceeded in a quantitative yield to give 9 (only β-anomer was obtained). Reduction of 9 using Et3SiH/BF3.Et2O/CH2Cl2 at 0° C.-rt proceeded in 82% yield but again without selectivity (10α/10β 1:1). The anomeric assignments were based on, besides Δδ of methyl groups, on the well known upfield shift of the 1′-H signal for the β-anomer compared to the α-anomer (Tam et al., J. Org. Chem. 1979, 44, 4854-4862; Sokolova et al., Carbohydr. Res. 1981, 93, 19-34). 10β was deprotected to 11 in two steps (1 M TBAF, followed by refluxing 80% acetic acid). Derivative 11 was highly resistant to displacement of 2-bromo substituent with methoxide and benzylate; only KH/BnOH/DMF at 140° C. afforded 12 in a good yield, identical in every aspect with the compound synthesized from 2-(benzyloxy)-5-iodopyridine 3. This comparison proved unequivocally that 5-pyridyl regioisomer 8 was the main product obtained from 2,5-dibromopyridine 7.

Provenance & Evidence