The molecular and crystal structures of two diastereomeric 3,4-dihydropyrromethenones

The molecular and crystal structures of the two racemic diastereomeric 3,4-dihydropyrromethenone derivatives1 and2 of configurations (Z) and (E) were determined at 93 K (1, 2) and at room temperature (1). From an analysis of the crystallographically observed temperature coefficients a pseudorotation flexibility of the pyrrolidinone ring in the crystal is deduced. In both compounds a nearly orthogonal arrangement between the two heterocyclic ring systems is observed, which is ascribed to the steric bulk of the substituents in positions 1 and 3 of the pyrrolidinone ring.Herrn Prof.Josef Schurz zum 60. Geburtstag gewidmet.     

Enantioselective total synthesis of (−)-microcarpalide

The enantioselective total synthesis of the actin-targeting metabolite (−)-microcarpalide is described. Key steps include ring-closing metathesis (RCM) for the final construction of the 10-membered lactone framework and stereoselective homologation of boronic esters for the insertion of all stereocentres with the desired absolute configuration. In particular, the acidic fragment was prepared in seven steps from a suitable chiral bromomethane boronate by means of two sequential stereoselective homologations to install the two stereocentres with the correct final R stereochemistry, employing (−)-pinanediol as the chiral director. Subsequent elaboration to the required C₇ backbone entailed nucleophilic displacement with a vinyl Grignard reagent, oxidative cleavage of the boronic scaffold and protection-deprotection manipulations. Interestingly, when the tribenzyloxy diene ester resulting from DCC-mediated coupling of the two key synthons was subjected to RCM in the presence of Grubbs' catalyst, the reaction proceeded stereoselectively to yield the desired trans oxecin-2-one, albeit with poor conversion.     

Radical trifluoromethylation of ammonium enolates

The easy radical trifluoromethylation of a series of 1,3-dicarbonyl compounds with CF₃I is described. The reaction occurs in the presence of a nitrogen base and sodium dithionite in CH₃CN-H₂O solution. Additionally, we report a new access to ammonium triflinates.     

Synthesis of Naturally Occurring Pyrazine and Imidazole Alkaloids from Botryllus LeachiRID=”a”ID=”a”?Dedicated to Prof. G. M?rkl on the occasion of his 75th birthday

The synthesis of the naturally occurring and biologically active alkaloids 1 and 2, first isolated from the red ascidian Botryllus leachi by Duran et al. [1], is described and the structure proposed for Botryllazine B (1) is confirmed. The analytical data for 2-(p-hydroxybenzoyl)-4-(p-hydroxyphenyl)imidazole (2) are discussed and compared with the literature. With special emphasis of ¹H NMR data the tautomerism of aroylimidazolemethanones is described.     

Selective mono-and diamination of polyfluorinated benzenes and pyridines with liquid ammonia

Amination of pentafluoropyridine, 2,3,5,6-tetrafluoropyridine, 4-chlorotetrafluoropyridine, 3,5-dichlorotrifluoropyridine, octafluorotoluene, α,α,α,2,3,5,6-heptafluorotoluene, decafluoro-m-xylene, decafluorobiphenyl, hexafluorobenzene, and pentafluorobenzene with liquid ammonia was investigated. Bis-aminodefluorination temperatures for the majority of substrates were shown to exceed significantly the corresponding temperatures of monoaminodefluorination. The optimal conditions for selective preparation of mono-and diaminopolyfluoro(het)arenes were elucidated. An efficient method for isolation of particular polyfluorophenylenediamines from product mixtures formed in nonselective reactions of pentafluorobenzene and hexafluorobenzene with aqueous ammonia based on complexation with a crown ether is proposed. Dedicated to the memory of Academician N. N. Vorozhtsov on the occasion of his 100th anniversary. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 2163–2170, November, 2007.     

Rhodium-catalyzed addition of arylboron compounds to nitriles, ketones, and imines

The rhodium-catalyzed addition reactions of sodium tetraphenylborate and arylboronic acids to nitriles, ketones, and imines were examined. The reaction of nitriles could be carried out efficiently in the presence of a catalyst system of [RhCl(cod)]₂-dppp and H₂O to give the corresponding monoarylated products selectively. Although unactivated ketones and imines are known to be poor electrophiles for rhodium-catalyzed arylation, the phenylation of them with use of sodium tetraphenylborate proceeded smoothly in the presence of [RhCl(cod)]₂ and Rh(acac)(cod) as catalysts, respectively. The addition of NH₄Cl was found to be crucial to effectively conduct the reaction of ketones and imines.     

A convenient synthesis of 4-aminoaryl substituted cyclic imides

An efficient one-step synthesis of 4-nitro-N-aryl substituted glutarimides, succinimides and maleimides in polyphosphoric acid is described together with the subsequent reduction to the corresponding anilines. The scope and limitation of this cyclocondensation are presented.     

Acetylenic/cyanoacetylenic complexes: simulation of the Titan’s atmosphere chemistry

The structures and energies of the 1:1 acetylene/cyanoacetylene, acetylene/dicyanoacetylene and cyanoacetylene/dicyanoacetylene complexes in solid argon matrices have been investigated using FT-IR spectroscopy and ab initio calculations, at the B3LYP/6-31G** level of theory. For the three complexes, predicted frequency shifts for the L shaped structures, characterized by a hydrogen bond between the nitrogen of the cyano group and the acetylenic proton, were found to be in good agreement with those experimental. Only in the case of acetylene/cyanoacetylene complex, we obtained a second minimum with a T shaped structure characterized by an interaction between the proton of cyanoacetylene and the Π system of acetylene. It appears clearly that HC₃N acts as an electrophile or as a nucleophile in these complexes.     

4-Hydroxy-2-quinolones 120. Synthesis and structure of ethyl 2-hydroxy-4-oxo-4H-pyrido-[1,2-a]pyrimidine-3-carboxylate

An improved method for the preparation and purification of ethyl 2-hydroxy-4-oxo-4H-pyrido[1,2-a]-pyrimidine-3-carboxylates is proposed. According to ¹H and ¹³C NMR spectroscopic data the synthesized compounds exist in DMSO solution in the 2-hydroxy-4-oxo form while in the crystal (at least for the case of the unsubstituted derivative) X-ray analysis shows that it occurs in the bipolar 2,4-dioxo form. __________ Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 864–876, June, 2007.