Diastereoselective conjugate addition to chiral alpha,beta-unsaturated carbonyl systems in aqueous media: an enantioselective entry to alpha- and gamma-hydroxy acids and alpha-amino acids

The stereoselectivity of the ultrasonically induced zinc--copper conjugate addition of iodides to chiral alpha,beta-unsaturated carbonyl systems under aqueous conditions was studied. Alkyl iodides add diastereoselectively to methylenedioxolanone 1 and methyleneoxazolidinone 2 to afford the 1,4-addition products in good yields (38-95 %) and with high diastereomeric excess (44-90 % de). The 1,4-addition to chiral gamma,delta-dioxolanyl-alpha,beta-unsaturated esters 3-5 also proceeds with good yields (51-99 %). The diastereoselectivity is dependent on the geometry of the olefin: the Z isomer 3 gives high diastereoselectivity, while the reactions with the E isomer 4 are nonstereoselective. The reaction proceeds with excellent chemoselectivity and allows the use of iodides bearing ester, hydroxy, and amino groups. Since the 1,4-addition products can be readily hydrolyzed, this methodology constitutes a novel entry for the enantioselective synthesis of alpha- and gamma-hydroxy acids and alpha-amino acids in aqueous media. The results obtained support the radical mechanism proposed by Luche, and represent one of the few examples of a radical stereoselective conjugate addition in aqueous medium.     

Synthesis of new 18-substituted analogues of calcitriol using a photochemical remote functionalization

A novel convergent synthetic approach to new analogues of calcitriol modified at the C-18 position is reported. The key step in the synthesis is the 20-hydroxyl-directed photochemical iodination of the 18-methyl group in the presence of (diacetoxyiodo)benzene. Using this methodology, two new analogues of calcitriol were prepared: the first contains a hydroxylated alkyl side chain attached at C-18 with the natural side chain replaced by an isopropylidene group; the second is a conformationally locked analogue due to an extra oxacycle between the C-18 and C-20 positions.     

Cross-coupling reactions of indium organometallics with 2,5-dihalopyrimidines: synthesis of hyrtinadine A

The palladium-catalyzed cross-coupling reaction of triorganoindium reagents (R3In) with 5-bromo-2-chloropyrimidine proceeds chemoselectively, in good yields, to give 5-substituted-2-chloropyrimidines or 2,5-disubstituted pyrimidines using 40 or 100 mol % of R3In, respectively. Sequential cross-couplings are also performed, in one pot, using two different R3In. This method was used to achieve the first synthesis of the alkaloid hyrtinadine A. The key step was a two-fold cross-coupling reaction between a tri(3-indolyl)indium reagent and 5-bromo-2-chloropyrimidine.     

Discontinuous first-order functional boundary value problems

In this paper we prove a new existence result for functional boundary value problems with first-order functional differential equations under weak conditions on the nonlinear part and monotonicity (but not continuity) with respect to the functional variable. Discontinuities with respect to the unknown are allowed to occur over general classes of time-dependent sets which are assumed to satisfy a kind of unviability or solubility condition.     

Palladium-catalyzed cross-coupling reactions of organogold(I) phosphanes with allylic electrophiles

Aryl and alkenylgold(I) phosphanes react regioselectively with allylic electrophiles such as cinnamyl and geranyl halides (bromide, chloride and acetates) under palladium catalysis in THF at 80 °C to afford the α-substitution product with moderate to high yields. When the reaction is performed with a chiral enantiopure secondary acetate, the α-substituted cross-coupling product is obtained with complete inversion of the stereochemistry.     

Total Synthesis of (+)‐Neomarinone

The first total synthesis of (+)‐neomarinone has been achieved by following a concise and convergent route using methyl (R)‐lactate and (R)‐3‐methylcyclohexanone as chiral building blocks. Key steps of the synthesis are the stereocontrolled formation of the two quaternary stereocenters by diastereoselective 1,4‐conjugate addition and enolate alkylation reactions, and the construction of the furanonaphthoquinone skeleton by regioselective Diels–Alder reaction between a 1,3‐bis(trimethylsilyloxy)‐1,3‐diene and a bromoquinone. The synthesis proves the relative and absolute stereochemistry of natural neomarinone.     

Palladium-catalyzed cross-coupling reaction of triorganoindium reagents with propargylic esters

[reaction: see text] Triorganoindium reagents (R(3)In) react with propargylic esters under palladium catalysis via an S(N)2' rearrangement to afford allenes in good yields and with high regioselectivity. The reaction proceeds smoothly at room temperature with a variety of R(3)In (aryl, alkenyl, alkynyl, and methyl). When chiral, nonracemic propargylic esters are employed, the reaction takes place with high anti-stereoselectivity providing allenes with high enantiomeric excess.     

Atom-efficient metal-catalyzed cross-coupling reaction of indium organometallics with organic electrophiles.

The novel metal-catalyzed cross-coupling reaction of indium organometallics with organic electrophiles is described. Triorganoindium compounds (R(3)In) containing alkyl, vinyl, aryl, and alkynyl groups are efficiently prepared from the corresponding lithium or magnesium organometallics by reaction with indium trichloride. The cross-coupling reaction of R(3)In with aryl halides and pseudohalides (iodide 2, bromide 5, and triflate 4), vinyl triflates, benzyl bromides, and acid chlorides proceeds under palladium catalysis in excellent yields and with high chemoselectivity. Indium organometallics also react with aryl chlorides as under nickel catalysis. In the cross-coupling reaction the triorganoindium compounds transfer, in a clear example of atom economy, all three of the organic groups attached to the metal, as shown by the necessity of using only 34 mol % of indium. The feasibility of using R(3)In in reactions with different electrophiles, along with the high yields and chemoselectivities obtained, reveals indium organometallics to be useful alternatives to other organometallics in cross-coupling reactions.     

Study of structural and electronic transport properties of Ce-doped LaMnO3

The structural and electronic transport properties of La_1−x Ce _x MnO₃ (x=0.0–1.0) have been studied. All the samples exhibit orthorhombic crystal symmetry and the unit cell volume decreases with Ce doping. They also make a metal-insulator transition (MIT) and transition temperature increases with increase in Ce concentration up to 50% doping. The system La_0.5Ce_0.5MnO₃ also exhibits MIT instead of charge-ordered state as observed in the hole doped systems of the same composition.