A series of anomeric nitrate esters and N-phthalimido glycosides of carbohydrates in furanose and pyranose forms have been synthesized in order to generate the corresponding alkoxy radicals and study the C1-C2 fragmentation reaction under reductive conditions. This reaction constitutes a two-step method for the transformation of carbohydrates into the corresponding alditols with one less carbon. Using this methodology, interesting four- and five-carbon building blocks for natural products synthesis possessing D-erythritol, D-threitol, D-xylitol, and D-arabinitol stereochemistry have been prepared. The synthesis of 1,2-O-isopropylidene-beta-L-threose (40) and 1-acetamido-2,4,5-tri-O-acetyl-D-arabinitol (50) have also been achieved from 1,2:5,6-di-O-isopropylidene-beta-D-glucofuranose and 2-acetamido-3,4,6-tri-O-acetyl-2-deoxy-D-glucopyranose, respectively.
A number of tertiary or hemiacetalic steroidal alcohols, 2-hydroxy-3,4-dinor-2,3-secocholestan-5-one 2,5-hemiacetal (1), 2beta,3beta-dihydro-3'H-cyclopropa[2,3]-cholestan-5alpha-ol (7), 3beta-phenyl-5alpha-hydroxycholestan-2-one (10), 5alpha-hydroxycholestan-3-one (15), 3beta,5alpha-dihydroxycholestan-7-one 3-acetate (21), and 4,4-dimethyl-19-hydroxy-5alpha-cholestan-3-one 19,3-hemiacetal (27) have been prepared in order to test a new tandem beta-fragmentation-hydrogen abstraction reaction. The alkoxy radicals generated by irradiation of these alcohols with visible light in the presence of (diacetoxyiodo)benzene, iodine, and molecular oxygen undergo a beta-fragmentation reaction followed by peroxidation of the C-radical formed. The peroxy radical reacts further with iodine to give an alkoxy radical and iodoxyl radical (IO(*)). The new alkoxy radical can produce intramolecular functionalization of suitably positioned carbons by hydrogen abstraction through a six-membered cyclic transition state to give tetrahydrofurans or can be intramolecularly trapped by carbonyl groups suitably disposed so as to generate a new alkoxy radical that can afford gamma-lactones by beta-fragmentation.
[reaction: see text] The reaction of 3-azido-2,3-dideoxy-hexopyranose compounds from the d-gluco, d-galacto, d-lacto, and l-arabino carbohydrate series, with (diacetoxyiodo)benzene and iodine, generated 2-azido-1,2-dideoxy-1-iodo-alditols with one carbon less than the starting carbohydrate. These beta-iodo azides could be transformed by dehydroiodination into vinyl azides, which in turn afforded 3-monosubstituted 2H-azirines under thermal conditions. These beta-iodo azides and 2H-azirines may be interesting chiral synthons for the preparation of more complex heterocyclic systems.
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