Weygand/Hilgetag Preparative Organic Chemistry

234 Formation of carbon-halogen bonds
neutralization with sodium carbonate solution, affords bis-(l,2-dibromoethyl)
ether. 1005 In these cases the necessary hydrogen halide is produced during the
reaction.
3. Replacement of acyloxy groups by halogen (halo sugars)
The OH groups of hemiacetals, alcohols, and aliphatic hydroxy carboxylic
acids can usually be replaced by halogen under mild conditions. This reaction
is most important in sugar chemistry, particularly for preparation of the very
reactive acetohalogeno sugars that are required for the synthesis of many
glycosides and glycosylamines. 1006 A glycosidic acyloxy group is very easily
replaced while the other acyloxy groups remain intact. Most procedures give
CH2Oac CH2Oac
acO\pL|/H acoNpl_^H acoN^L^X
Oac Oac Oac
Acetohalo-^-D-glucose Pentaacetyl-/?-D-glucose Acetohalo-a-D-glucose
ac = CH3CO or C6H5CO.
the more stable a-halo product independently of whether an acylated oc- or
/3-sugar forms the starting material. Acetochloro sugars are obtained by treating
an acylated sugar with acetyl chloride and ZnCl2, 1007 with PC15 and
AICI3, 1008 with AICI3 alone, 1009 with TiCl4 in alcohol-free anhydrous
CHCI3, 1010 or with 1,1-dichloromethyl methyl ether. 1011
A particularly simple process, giving the /?-product, is treatment of a solution
or suspension of a sugar acetate with dry HC1.
Acetochloro-^-D-glucose: 1012 HC1 (50 g) is led during 40 min into a stirred mixture of
penta-O-acetyl-/?-D-glucose (20 g) and dry ether (250 ml) at 5°, whereupon most of the
pentaacetylglucose dissolves. The mixture is kept for 2 days in a closed flask at 0°, whereafter
the resulting clear solution is concentrated in a vacuum. A total of 65 % of crude product is
thus obtained and after recrystallization from dry ether has m.p. 98-99°, [