Elisa I. León

Fragmentation of Carbohydrate Anomeric Alkoxy Radicals: A New Synthesis of Chiral 1-Halo-1-iodo Compounds

One less carbon atom is found in 1-halo-1-iodo compounds obtained by C1-C2 radical fragmentation of carbohydrate 1,2-halohydrins. This fragmentation is achieved via the anomeric alkoxy radicals of the halohydrins, formed upon reaction with (diacetoxyiodo)benzene and iodine [Eq. (1); X=Cl, Br, I].

Aminoselenenylation of olefins. Syntheses of β-phenylseleno carbamates

Abstract (β-Phenylseleno carbamates have been synthesized by reaction of olefins with phenylselenenyl chloride and carbamates in presence of silver tetrafluoroborate. This reaction constitutes a good method for the conversion of olefins to β-functionalized protected amines.

Sequential Norrish Type II Photoelimination and Intramolecular Aldol Cyclization of 1,2‐Diketones in Carbohydrate Systems: Stereoselective Synthesis of Cyclopentitols

Opened and closed: Visible-light photostimulation of 2,3-diuloses I triggers an unprecedented sequential rearrangement: A Norrish type II photoelimination to give an isolable acyclic photoenol intermediate II is followed by an intramolecular enolexo aldolization. The contraction of the pyranose ring in this process leads to a new type of cyclopentitol derivative III. R=acyl, alkyl, silyl group.

Fragmentation of carbohydrate anomeric alkoxy radicals: a new synthesis of chiral 1-halo-1-bromo compounds

The reaction of 1,2-halohydrins derived from easily available carbohydrates with (diacetoxyiodo)benzene (DIB) in the presence of bromine is a mild procedure for the synthesis of 1-deoxy-1-halo-1-bromo-alditols with one carbon less than the original carbohydrate. The reaction goes through β-fragmentation of the intermediate anomeric alkoxyl radicals. These 1-halo-1-bromo polyhydroxy compounds may be valuable synthetic intermediates in organic synthesis.

Easy Access to Modified Cyclodextrins by an Intramolecular Radical Approach

A simple method to modify the primary face of cyclodextrins (CDs) is described. The 6(I)-O-yl radical of α-, β-, and γ-CDs regioselectively abstracts the H5(II), located in the adjacent D-glucose unit, by an intramolecular 1,8-hydrogen-atom-transfer reaction through a geometrically restricted nine-membered transition state to give a stable 1,3,5-trioxocane ring. The reaction has been extended to the 1,4-diols of α- and β-CD to give the corresponding bis(trioxocane)s. The C2-symmetric bis(trioxocane) corresponding to the α-CD is a stable crystalline solid whose structure was confirmed by X-ray diffraction analysis. The calculated geometric parameters confirm that the primary face is severely distorted toward a narrower elliptical shape for this rim.

2-Oxazolines (4,5-dihdro-oxazoles) by organoselenium-induced cyclisation of allylic ureas

The reaction of allylic ureas with phenylselenenyl chloride in the presence of silica gel affords 2-oxazolines in good to excellent yields; 2-oxazolines are also stereospecifically obtained from β-phenylseleno ureas by chemoselective alkylation of the selenium atom followed by basic treatment.

Intramolecular 1,8-Hydrogen Atom Transfer Reactions in Disaccharide Systems Containing Furanose Units

A previously developed 1,8-hydrogen atom transfer (HAT) reaction promoted by 6-O-yl alkoxyl radicals between the two pyranose units in Hexp-(1→4)-Hexp disaccharides has been extended to other systems containing at least a furanose ring in their structures. In Penf-(1→3)-Penf (A) and Hexp-(1→3)-Penf (B) disaccharides, the 1,8-HAT reaction and concomitant cyclization to a 1,3,5-trioxocane ring are in competition with radical β-scission of the C4-C5 bond and formation of dehomologated products. The influence of the stereoelectronic β-oxygen effect on the β-scission and consequently on the 1,8-HAT reaction has been studied using the four possible isomeric d-furanoses. d-xylo- and d-lyxo-derivatives afforded preferentially 1,8-HAT products, whereas d-arabino- and d-ribo-derivatives gave exclusively direct β-scission of the alkoxyl radical. When the 6-O-yl radical is on a pyranose ring, as occurs in Penf-(1→4)-Hexp (C), it has been shown to provide the cyclized products exclusively.

Synthesis and radical fragmentation of carbohydrate anomeric nitrates. Formation of convenient chiral synthetic intermediates

Abstract A new procedure for the C1–C2 fragmentation of cyclic carbohydrates, through the formation of anomeric alkoxy radicals from the corresponding nitrate esters, with n Bu 3 SnH/azobisisobutyronitrile (AIBN), affords acyclic alditols in good yields.

Aminoselenenylation of alkenes: syntheses of β-phenylseleno carbamates and β-phenylseleno cyanamides

β-(Phenylseleno)alkylcarbamates and β-(phenylseleno)alkylcyanamides have been synthesized in good yields by reaction of alkenes with carbamates and cyanamide, respectively, in presence of benzeneselenenyl chloride–silver tetrafluoroborate or N-phenylselenophthalimide–H+. The subsequent reductive or oxidative removal of the phenylseleno group affords alkylcarbamates and alkylcyanamides, and allylic carbamates, cyanamides, and cyanimides.

Sequential Norrish Type II Photoelimination and Intramolecular Aldol Cyclization of α‐Diketones: Synthesis of Polyhydroxylated Cyclopentitols by Ring Contraction of Hexopyranose Carbohydrate Derivatives

The excitation of the innermost carbonyl of nono-2,3-diulose derivatives by irradiation with visible-light initiates a sequential Norrish type II photoelimination and aldol cyclization process that finally gives polyfunctionalized cyclopentitols. The rearrangement has been confirmed by the isolation of stable acyclic photoenol intermediates that can be independently cyclized by a thermal 5-(enolexo)-exo-trig uncatalyzed aldol reaction with high diastereoselectivity. In this last step, the large deuterium kinetic isotope effect found for the 1,5-hydrogen atom transfer seems to indicate that the aldol reaction runs through a concerted pericyclic mechanism. Owing to the ready availability of pyranose sugars of various configurations, this protocol has been used to study the influence of pyranose ring-substituents on the diastereoselectivity of the aldol cyclization reaction. In contrast with other pyranose ring contraction methodologies no transition-metal reagents are needed and the sequential rearrangement occurs simply by using visible light and moderate heating (0 to 60 °C).