Angeles Martín

Reactivity of Alkynyl Platinum Complexes towards PPh2H and PPh2(O)H: Unexpected Formation of Alkynyl Tetralithium Diplatinum Compounds Stabilized byμ3-(κ3P,O,O-PPh2O−) Ligands

The reactivity of “Li2[Pt(C≡CR)4]” and cis-[Pt(C≡CR)2COD] (R = tBu, Ph) towards PPh2H and PPh2(O)H has been investigated; it results in the synthesis of five types of alkynyl platinum compounds, for example, [{Pt(C≡CtBu) (PPh2O)3Li2(thf) (H2O)}2] (1). Displacement of PPh2(O)H by donor ligands in the compound [{Pt(C≡CR){(PPh2O)2H}(PPh2OH)] yields the related mononuclear complexes [Pt(C≡CR){(PPh2O)2H}L]n−

Synthesis of oxa-aza spirobicycles by intramolecular hydrogen abstraction promoted by n-radicals in carbohydrate systems

The nitrogen-centred radical generated by reaction of an N-phosphoramidate or N-cyanamide, attached to a tri- or tetramethylene tether extended from the C-1 of a carbohydrate, with (diacetoxyiodo)benzene (DIB) and iodine can undergo a regio- and stereoselective intramolecular hydrogen atom transfer (HAT) reaction to furnish four different oxa-azaspirobicyclic systems: 1-oxa-6-azaspiro[4.4]nonane, 1-oxa-6-azaspiro[4.5]decane, 6-oxa-1-azaspiro[4.5]decane and 1-oxa-7-azaspiro[5.5]undecane. A tandem 1,5- or 1,6-HAT-oxidation-nucleophilic cyclisation mechanism is proposed.

SYNTHESIS OF DISPIROACETALS FROM CARBOHYDRATES BY INTRAMOLECULAR HYDROGEN ABSTRACTIONS

Abstract The synthesis of 1,6,8-trioxadispiro[4.1.5.3]pentadecanes 15 and 16 from d -galactopyranose is described. The key steps are two intramolecular hydrogen abstraction reactions promoted by alkoxy radicals.

Synthesis of oxa-aza spirobicycles by intramolecular hydrogen atom transfer promoted by N-radicals in carbohydrate systems

The nitrogen-centred radical generated by reaction of an N-phosphoramidate or N-cyanamide, attached to a tri- or tetramethylene tether extended from the C-1 of a carbohydrate, with (diacetoxyiodo)benzene (DIB) and iodine can undergo a regio- and stereoselective intramolecular hydrogen atom transfer (HAT) reaction to furnish four different oxa-azaspirobicyclic systems: 1-oxa-6-azaspiro[4.4]nonane, 1-oxa-6-azaspiro[4.5]decane, 6-oxa-1-azaspiro[4.5]decane and 1-oxa-7-azaspiro[5.5]undecane. A tandem 1,5- or 1,6-HAT-oxidation-nucleophilic cyclisation mechanism is proposed.

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.

Synthesis of (5R, 7S, 13S)-13-methoxy-1,6,8-trioxadispiro[4.1.5.3]pentadecane

Abstract The synthesis of (5 R , 7 S , 13 S )- and (5 S , 7 S , 13 S )-13-methoxy-1,6,8-trioxadispiro[4.1.5.3] pentadecane from tri- O -acetyl- d -glucal, by a convenient homologation of C-1 and C-6 atoms which led to an appropriate dihydroxyl intermediate 7 , is reported. The key steps for the construction of the two spiroacetal systems involved a double, step by step, radical abstraction promoted by (diacetoxyiodo)benzene and iodine.

Sequential alkoxy radical fragmentation-intermolecular allylation in carbohydrate systems

The C-radical originated by β-fragmentation of carbohydrate anomeric alkoxy radicals, generated under reductive conditions by treatment of N-phthalimido glycosides with allyltri-n-butyltin/AIBN, may subsequently undergo an intermolecular allylation to give hept-1-enitol derivatives. These compounds can be useful chiral synthons for the synthesis of polyhydroxylated compounds.

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).

Synthesis of alditols by reductive radical fragmentation of N-phthalimido glycosides. Preparation of chiral synthetic intermediates

The reaction of N-phthalimido glycofuranosides and glycopyranosides with nBu3SnHAIBN produces radical β-fragmentation of the carbohydrate C1C2 bond through the formation of anomeric alkoxy radicals. This constitutes a new two-step methodology for the facile conversion of carbohydrates into the corresponding acyclic alditols with one fewer carbon.

Photochemistry of α‐Diketones in Carbohydrates: Anomalous Norrish Type II Photoelimination and Norrish–Yang Photocyclization Promoted by the Internal Carbonyl Group

A series of four α-diketones placed as 1α-pyruvoyl tethers on D-glucopyranose and D-glucopyranosiduronic acid skeletons was prepared in order to determine the influence of captodative and stereoelectronic effects on the regioselectivity of the hydrogen atom transfer (HAT) in Norrish type II photochemical processes. We observed that the 1,5-HAT regioselectivity can be switched between the two potentially abstractable syn-1,3-diaxial hydrogens at H6 and H8. Highly unusual photoproducts from Norrish type II photoelimination and Norrish-Yang photocyclization initiated by the excited internal carbonyl group were obtained, in some cases in excellent synthetic yield. The 1,5-HAT transition state in the Norrish type II photoelimination was investigated by photochemical experiments in the crystalline state.