Heteroatom Exchange
2.3
255
⊡ Scheme 56
⊡ Scheme 57
Regio- and stereo-controlled iodocyclizations of allylic trichloroacetimidates provide a route
to cis-hydroxyamino sugar from hexenopyranosides. For example, conversion of the hydroxyl
of compound 117 into the trichloroacetimidate 118 followed by IDCP-mediated intramolecular
cyclization, gives iodo-oxazoline derivative 119, which is reduced (Bu 3 SnH) and hydrolyzed
(pyridine, TsOH) to afford N-acetyldaunosamine methyl glucoside 120 ( > Scheme 57) [95].
3.4 Cyclization of Dialdehydes
Dialdehydes, obtained by periodate oxidation of appropriate cyclic polyhydroxy precursors, condensing with some nitrogen-containing reagents in basic solution has been developed into a useful synthesis of amino sugars. Thus, the dialdehyde 121, on treatment with
nitromethane in the presence of sodium methoxide produces a mixture of aci-nitro salt isomers 122, which mainly give 3-amino-3-deoxyhexopyranosides 123 and 124 after reduction
( > Scheme 58) [96].
In another instance, when the dialdehyde 125 is treated with phenylhydrazine, a cyclization
occurs, probably via the monophenylhydrazone 126 to give the phenylazo derivative 127,