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-alpha-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-alpha-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.