European Journal of Organic Chemistry

Many fascinating new ketenes and valuable processes involving ketene chemistry have appeared recently, and selected examples of these are briefly reviewed. They include stereoselective reactions, ketenes and polymers, ketenes and carbenes, ketenes in synthesis, ketenes from photochemical processes, and bis-β-lactam formation.

Chiral tripodal imidazoline–pyridine L6-Cu(BF₄)₂ complex catalyzed the asymmetric p-(tert-butyl)benzoylation of meso-hydrobenzoin to give the adduct in up to 85 % ee. After completion of the asymmetric benzoylation reaction, the self-supported tripodal imidazoline–pyridine–Cu catalyst was easily recovered and reused several times while maintaining the catalyst activity and selectivity.

A useful and versatile Pd-catalyzed P–C cross-coupling reaction was applied with success to a range of secondary phosphane–borane or phosphane oxide derivatives and α-amido enol phosphate reagents. The reaction afforded new chiral or achiral tertiary α-enamido phosphane derivatives.

The absolute configuration of three complex marine polypropionates was established by X-ray diffraction analysis and solid-state time-dependent density functional theory electronic circular dichroism.

A mild method for the synthesis of asymmetrical 6H-benzo[c]chromenes was developed. The process involved the palladium-catalyzed tandem reaction of β-(2-bromophenyl)-α, β-unsaturated carbonyl compounds with 2-hydroxyphenylboronic acid.

6,6′-Bis(2,4,6-triisopropylphenyl)SPINOL-based phosphoric acid was found to be a general, highly enantioselective catalyst for the asymmetric allylboration of pinacol allylboronates with different types of aldehydes.

A catalytic enantioselective Michael addition–cyclization reaction of nitroalkanes with o-hydroxycinnamaldehydes has been established by using a diphenylprolinol TMS ether as an organocatalyst. The reaction afforded the corresponding 4-substituted chroman-2-ols in excellent yields with high levels of enantioselectivities (95 to >99 % ee).

The reactions of the thiazole derivative 6 with DNBF and DNTP afforded the corresponding new Wheland–Meisenheimer complexes. Their unusually high stability allowed the first X-ray diffraction analyses of these types of complexes. The reactions are diastereoselective, and the complex derived from DNBF unexpectedly evolves to a neutral substitution product, a furazan derivative.

Dipolar ionic dendrimers that consist of dendronized pyridinium units surrounded by a shell of carboxylic acids are a new family of dendritic architectures in which opposite charges are organized within specific areas of one macromolecule. They can be considered as molecular architectures that contain a positively charged nucleus covalently incorporated within a negatively charged shell.

A series of tetrakis(tetrathiafulvalene-tetrathiacrown ether)porphyrazine triads with long and flexible aliphatic side-chains at the periphery have been synthesized and their electronic absorption, intermolecular charge transfer, electrochemistry, recognition of transition-metal ions, and structure in the bulk state have been investigated.

Biomimetic models towards a better understanding of the biosynthesis of manadomanzamine A were carried out and permitted the construction of the central pentacyclic core of this complex alkaloid.

A new BODIPY-based fluorescence chemosensor FS1 shows high selectivity for Hg²⁺ ions over other transition-metal ions. The fluorescence of FS1 was significantly enhanced in the presence of Hg²⁺, with green emission, and the addition of other metal ions barely affected the fluorescence. This FS1 chemosensor is an effectivemethod for Hg²⁺ sensing in living cellimaging.

A mild and efficient enzymatic method for the aza-Michael addition of mono- and bifunctional amines to acrylates was developed. The high substrate specificity showed by Rhizomucor miehei lipase as the catalyst for this reaction was a key feature for obtaining various N-substituted β-amino esters in high yield and purity.

Hunter/Vögtle-type tetralactam macrocycles are equipped with metal coordination sites through Suzuki cross-coupling. The same bromo-substituted macrocycle can be used irrespective of the individual binding site, which provides versatile access to different complexes. The complexes represent multivalent hosts that are potentially useful for the formation of multiply interlocked molecules.

The difficult task of employing aryl-Grignard reagents in Cu-catalyzed enantioselective 1,4 addition reactions was achieved with the assistance of readily accessible chiral modular P,P ligands. High enantioselectivities were obtained in a number of synthetically relevant cases.

Structurally similar plane-nonsymmetric dienes and dienophiles have been found to have similar facial selectivities in their Diels–Alder reactions. This indicates steric rather than electronic stereochemical control. The computed facial selectivities for these reactions are in accord with experiment and computational data for simple model compounds corroborated steric control.

We have synthesized azido-1,2,4-triazoles 1–5 from triaminoguanidine hydrochloride and a carboxylic acid by a three-step route. The procedure has been optimized from a “green agreeability” point of view. The thermal decomposition of 1–5 has been investigated theoretically by using predictive software, and experimentally by using differential scanning calorimetry.

Chiral perhydrobenzoxazines derived from (–)-8-aminomenthol and containing a 1,6-enyne moiety participate in intramolecular diastereoselective Pauson–Khand reactions leading to cyclization products with moderate to good diastereoselectivity. The diastereoselection depends on both the substitution pattern of the double bond and the regiochemistry of the starting perhydrobenzoxazines

Four diastereoisomers of mycaperoxide B methyl ester (1a) have been synthesised by employing a Michael addition across an α,β-unsaturated ester precursor 2 as the key step. The stereochemistry observed suggests stereocontrol in this step and discloses the importance of choosing the correct geometry of the α,β-unsaturated double bond when attempting to synthesise mycaperoxide B (1).

A new approach to a tetrahydropyransubunit found in pederin, psymberin, and onnamides D–F is presented. The key sequence involves a ring expansion from a γ-lactone to a tetrahydropyran ring. Conditions for achieving both the kinetic and thermodynamic cyclization reactions have been developed, and models to explain the course of each are presented.

The fluorescence intensity of receptors R-1 and S-1 increased on addition of saccharides due to the formation of intramolecular 1:1 cyclic complexes. In general, R-1 has higher binding constants and produces a higher fluorescent response with saccharides than S-1.

An efficient approach to optically pure α-aryl-3-indolylnitroethanes through rhodium-catalyzed asymmetric 1,4-addition of arylboronic acids to indolylnitroalkenes was developed. Excellent yields (up to 99 %) and enantiomeric excesses (up to 99 %) were achieved under mild conditions.

An efficient asymmetric dihydroxylation reaction of alkenyl β-D-hexopyranoside derivatives is described. New glycosylglycerol and glycoglycerolipid analogues have been synthesised by this methodology. Preliminary cytotoxic activity assays are presented.

Putaminoxin E, a natural nonanolide, and its C-9 epimer were synthesized for the first time starting from pentane-1,5-diol and butyraldehyde.