Rhodium(II)-catalyzed decomposition of 3-O-(2-diazo-2-phenylacetyl)-1,2;5,6-di-O-isopropylidene-alpha-D-allofuranose: diastereoselective ether formation

Standard diazo transfer to 3- O -(2-phenylacetyl)-1,2;5,6-di- O -isopropylidene-α- d -allofuranose ( 2 ), using p -acetamidobenzenesulfonyl azide ( p -ABSA, 3 ) and DBU as base, provides the expected 3- O -(2-diazo-2-phenylacetyl)-1,2;5,6-di- O -isopropylidene-α- d -allofuranose ( 4 ) as an orange syrup in 49% isolated yield. Subsequent decomposition of 4 using Rh 2 (OAc) 4 yields ether 5 in a highly diastereoselective manner and in 58% isolated yield. The X-ray crystal structure of 5 proves that both newly produced stereocenters have the ( S ) configuration; the conformation of the ester group at O-3 of the furanose ring of 5 is used to discuss the possible cause of the observed stereoselectivity.