Synthesis of septanosides with the L-ido configuration may be achieved by inversion of configuration at C-5 of an appropriately substituted D-glucoseptanoside derivative. We have described [1] the preparation of methyl 3,4-O-isopropylidene-a-D-glucoseptanoside (la) from methyl a-D-glucoseptanoside. Selective protection of the hydroxyl group on C-2 of la would provide a suitably substituted derivative which could be converted into the L-ido derivative using an oxidation-reduction sequence or by nucleophilic displacement of an appropriate group. We describe here the selective benzoylation of la and the preparation of methyl /3-L-idoseptanoside derivatives by both oxidation-reduction and displacement of a sulfonyloxy group.
Benzoylation of la.--Treatment of la with benzoyl chloride in pyridine until all of the starting material had been consumed gave the dibenzoate lb [1] and a new product, le, which appeared between la and lb on thin-layer chromatography; lc was isolated by chromatography over silicic acid. Its IR spectrum had absorptions at 1720 and 3520 cm -~, consistent with a monobenzoate structure. Analysis of the I H NMR spectrum allowed its identification as methyl 2-O-benzoyl-3,4-O-isopropylidene-a-o-glucoseptanoside (lc). The low-field doublet of doublets in the spectrum of le corresponds to the similar multiplet assigned [1] to H-2 in the spectrum of lb. The yield of lc was 88%. Such a high yield of a monobenzoate is reminiscent of the preferential acylation of the C-2 hydroxy group in methyl 4,6-O-benzylidene-a-D-glucopyranoside [2,3]. We note also that the conformation of la is probably the same as that of its derived diacetate [1] * Corresponding author. I Septanose Carbohydrates, Part 5. For Part 4, see ref. [1].