We have reported' the transformation of 2,3-unsaturated glycosides 1 into the respective 2,3-dideoxyhex-2-enono-1 ,Slactones 2 by oxidation with 30% hydrogen peroxide in the presence of molybdenum trioxide as catalyst, followed by dehydration of the resulting hydroperoxide 3. Other syntheses of 2 have since been published*. We now report on the oxidation of the 2,3-unsaturated glycosides 4-6 and on the intermediate hydroperoxides.
The oxidation of 4-6 with hydrogen peroxide, when catalysed by molybdenum trioxide (1% mol), required several days. Thus, 4 and 5 afforded the corresponding a-hydroperoxides 7 and 8; -5% of the /3-anomer 9 of 8 was also detected. Oxidation of 6 gave a 2: 1 a&mixture of the hydroperoxides 10 and 11.
The structures of the hydroperoxides 7-11 were determined on the basis of spectral data (i.r., 'H-and r3C-n.m.r.), elemental analysis, and iodometric titration (see Experimental). A characteristic spectral feature of 7-11, when compared with the 2,3-unsaturated hemiacetals 12 and 13 (ref.
3), was the down-field shift of 10 p.p.m. of the C-l signal. The configuration at C-l in 7-11 was deduced from the 51,2, 53,4, and 5.,s values, and the LY configuration of 8 was confirmed by optical rotation data4. In solution, 11 occurs almost exclusively in the 5H, conformation with the AcOCH, group axial, and AcO-4 and OOH pseudo-axial. This inference was confirmed by the relatively small 54,5 value as well as by large .I,,, and J3,4 value$. Some participation of the 5H, conformation can also be postulated for the prhreo hydroperoxide 9. The conformational hehaviour of 11 reflects the allylic effect6 of the secondary acetoxyl substituent and the anomeric effect of the hydroperoxide group. The exceptionally large anomeric effect operating in 7-11 is caused by the more effective interaction of the lone pair of the ring oxygen atom and the lowest unoccupied orbital of the glycosidic C-O bond. This effectiveness can be explained in terms of the so-called o-effect' which, for the hydroperoxide group, leads to a low-lying LUMO, and hence makes its overlapping with the p-type lonepair orbital more energy-lowering, Compounds 7-11, which are the first examples of stable hydroperoxides of potential biological importance, were stable under the conditions of flash