For the synthesis of 4-methylcoumarin-7-yl cY-L-idopyranosiduronic acid, a convenient, sensitive substrate' for a-L-iduronidase (EC 3.2.1.76), we required quantities of suitable L-idopyranose derivatives. During the course of this work, a novel synthesis of L-idose (1) was reported*, and has now been included, in modified form, in a treatise of reliable synthetic methods in carbohydrate chemistry3. This novel method was recommended because it involved fewer steps and gave better yields than previous syntheses, but we have found that the syrupy L-idose obtained thereby is impure and now describe a modified method for the synthesis of pure 2,3,4-tri-O-acetyl-1,6-anhydro-P-L-idopyranose.
In the reported2v3 synthesis, the D-gluco-+L-ido transformation was achieved by an intramolecular displacement at C-5 during the synthesis of 5,6-anhydro-1,2-0-isopropylidene-P-L-idofuranose
(2). Hydrolysis was then claimed to give 1 in essentially quantitative yield. The stereoisomeric gluco-epoxide 3 gives4 a mixture of products on hydrolysis, and the difference in behaviour of 2 and 3 was ascribed' to conformational features present in the acyclic transition-state involved in the reaction of 3.
We experienced difficulties in two steps in this synthesis of L-idose. First, as previously reported5, reaction of the 5-sulphonate 4 with sodium methoxide gave 2 contaminated with -6% of an isomer; pure 2 could only be obtained by chromatography, which is undesirable for large-scale preparations. Further, when pure 2 was hydrolysed with dilute acid according to the published2 method (25mM H2S04, 45", 2.5 h) and the resulting syrupy product was subjected to ion-exchange chromatography of the borate complexe@, six components were detected (Table I, entry la). The expected product, L-idose (1) was a minor component, and the major product (62%) was 1,2-0-isopropylidene+L-idofuranose (5). The L-sorbose (6, 3%) probably arose from 1'. Component 8 was tentatively identified as 2,5-anhydro-D-glucose, and component 9 was identified as 3,6-anhydro-L-idose (see below). This result was reproducible (Table I, entry lb). During chromatog-000%6215/84/a 03.00