Treatment of the bridged methyl furanosides 1, 3 and 5 with dilute acid furnishes the corresponding deoxyfuranose derivatives 2, 4 and 6. The trans-substitution pattern of the tetrahydrofuran ring was established by X-ray structural analysis of 2 and by comparison of the 1 H-NMR spectra. A new reso-
The synthesis of unnatural or modified carbohydrate derivatives has gained great attention for various reasons [1] . Many contributions in this field are also concerned with furanosides [2] . In previous publications [3] [4] of this series we described the synthesis of optically active bridged methyl furanosides 1 and 3 and the assignment of their absolute configurations. We now report on the stereoselective hydrolysis of the furanosides to the corresponding hemiacetals and on their conversion to compounds with a disaccharide structure.
First experiments with racemic compounds showed that the hydrolysis could be performed by heating the acetals with 0.1 ๏ฎ H 2 SO 4 in tetrahydrofuran. The 5-methoxy ester rac-1 [3] [4] furnished the 5-hydroxy derivative rac-2 in 69% yield as the only isolated product. In this case an additional treatment with diazomethane was necessary to convert the free acid, formed by partial hydrolysis, to the methyl ester 2. The 1 H-NMR spectrum of 2 showed no 3 J coupling between 4-H and 5-H, thus indicating the same all-trans configuration in the tetrahydrofuran ring of 1 [3] [4] and 2 as well.
This assignment was finally established by an X-ray structural analysis of suitable crystals of 2 by starting with a sample of 1 that was enriched in (ฯฉ)-1 obtained by our reported resolution procedure [4] . Figure shows that the hydroxy group at C-5 is located on the site opposite to the heptano bridge.
It should be noted that we have no indications so far for equilibria mixtures of anomers in the case of 2 and of the [แญ] Part XLV: