Inter‐ and intramolecular hetero‐Diels‐Alder reactions, 37. Syntheses of the 3‐amino sugar glycosides rac‐4‐deoxydaunosaminide rac‐4‐deoxyristosaminide, and rac‐acosaminide

The hetero-Diels-Alder reaction of the phenylthio-activated N-monoacyl- and N,N-diacyl-enamino ketones 8a-c containing a methyl group at position 2 of the oxadiene with the electron-rich dienophiles 9a-c yields the exo/endo adducts 10a-d and 11a-d in 82–95% yield with the endo adducts as the main products. The enamino ketone 8d with a S-ethyl group at C-3 does not react. Hydrogenation of 11a with Raney nickel in THF leads to the desulfurized dihydropyran 12, whereas in MeOH gives stereoselectively the perhydrophthalimido derivative 13, which, after deprotection, yields the β-methyl glycoside 14 of rac-4-deoxydaunosamine. Under similar conditions 10c is converted into the β-ethyl glycoside 15 of rac-N-benzoyl-4-deoxyristosamine. Stereoselective hydroboration of 12 affords 17 and subsequent cleavage of the protecting group leads to the β-methyl glycoside 18 of rac-acosamine.