SYNTHESIS OF 2-DEOXY-2-HALO-L-ASCORBIC ACIDS

2-Iodo- and 2-bromo-2-deoxy-L-ascorbic acids (2 and 3) were prepared b y facile halogenation of 2-deoxy-L-ascorbic acid (6) with NIS and NBS, respectively. Likewise, chlorination with NCS produced 2-chloro-2-deo xy-L-ascorbic acid (4), but formation of 4 was accompanied by formatio n of the dichloro hemiketal 9. Direct fluorination of 6 with 1-chloro- 4-fluoro-1,4-diazoniabicyclo[2.2.2]octane bis(tetrafluoroborate) (F-TE DA-BF4) gave only the difluoro hemiketal. 10. A convenient synthesis o f 2-deoxy-2-fluoro-L-ascorbic acid (5) was achieved by an indirect rou te. Fluorination of 2-bromo-2-deoxy-L-ascorbic acid (3) with F-TEDA-BF 4 produced the bromofluoro hemiketal 16 as a mixture of diastereoisome rs. Debromination with tributyltin hydride gave 5 in good yield.