Interaction of pentafluoropyridine with 4-nitrophenol and pentafluorophenol in the presence of potassium fluoride and 18-crown-6-ether

Total Synthesis of the Natural Chalcone Lophirone E, Synthetic Studies toward Benzofuran and Indole-Based Analogues, and Investigation of Anti-Leishmanial Activity

The potential of natural and synthetic chalcones as therapeutic leads against different pathological conditions has been investigated for several years, and this class of compounds emerged as a privileged chemotype due to its interesting anti-inflammatory, antimicrobial, antiviral, and anticancer properties. The objective of our study was to contribute to the investigation of this class of natural products as anti-leishmanial agents. We aimed at investigating the structure–activity relationships of the natural chalcone lophirone E, characterized by the presence of benzofuran B-ring, and analogues on anti-leishmania activity. Here we describe an effective synthetic strategy for the preparation of the natural chalcone lophirone E and its application to the synthesis of a small set of chalcones bearing different substitution patterns at both the A and heterocyclic B rings. The resulting compounds were investigated for their activity against Leishmania infantum promastigotes disclosing derivatives 1 and 28a,b as those endowed with the most interesting activities (IC₅₀ = 15.3, 27.2, 15.9 μM, respectively). The synthetic approaches here described and the early SAR investigations highlighted the potential of this class of compounds as antiparasitic hits, making this study worthy of further investigation.

Concerted Nucleophilic Aromatic Substitution Reactions

Recent developments in experimental and computational chemistry have identified a rapidly growing class of nucleophilic aromatic substitutions that proceed by concerted (cSN Ar) rather than classical, two-step, SN Ar mechanisms. Whereas traditional SN Ar reactions require substantial activation of the aromatic ring by electron-withdrawing substituents, such activating groups are not mandatory in the concerted pathways.

Theoretical Explanation of Reaction Site Selectivity in the Addition of a Phenoxy Group to Perfluoropyridine

Pentafluoropyridine, a potentially useful precursor in organofluorine methodology, undergoes selective substitution of a fluorine with a phenoxide at the site para to the nitrogen. Subsequent aryloxide substitutions can be accomplished at the ortho-positions with aryloxide groups containing various functional groups para to the phenoxide oxygen. During this phase of the reaction, "reverse reactions" involving substitutions of the original para substituent with a free fluoride or with another aryloxide moiety are observed with a frequency that depends on the functional group para to the oxygen on the aryloxide. Herein, we provide a theoretical explanation of these observations through use of density functional theory.

Tetrafluoropyridyl (TFP): a general phenol protecting group readily cleaved under mild conditions

Herein we introduce tetrafluoropyridyl (TFP) as a new general protecting group for phenols. The TFP protecting group is readily cleaved under mild conditions.

Phenols are extremely valuable building blocks in the areas of pharmaceuticals, natural products, materials and catalysts. In order to carry out modifications on phenols, the phenolic oxygen is routinely protected to prevent unwanted side reactions. Presently many of the protecting groups available can require harsh conditions, specialist equipment, expensive or air/moisture-sensitive reagents to install and remove. Here we introduce the use of the tetrafluoropyridyl (TFP) group as a general protecting group for phenols. TFP can be installed in one step with no sensitivity to water or air, and it is stable under a range of commonly employed reaction conditions including acid and base. The TFP protecting group is readily cleaved under mild conditions with quantitative conversion to the parent phenol, observed in many cases in less than 1 hour.

Borylation of fluorinated arenes using the boron-centred nucleophile B(CN)32- - a unique entry to aryltricyanoborates

The potassium salt of the boron-centred nucleophile B(CN)32- (1) readily reacts with perfluorinated arenes, such as hexafluorobenzene, decafluorobiphenyl, octafluoronaphthalene and pentafluoropyridine, which results in KF and the K+ salts of the respective borate anions with one {B(CN)3} unit bonded to the (hetero)arene. An excess of K21 leads to the successive reaction of two or, in the case of perfluoropyridine, even three C-F moieties and the formation of di- and trianions, respectively. Moreover, all of the 11 partially fluorinated benzene derivatives, C6F6-n H n (n = 1-5), generally react with K21 to give new tricyano(phenyl)borate anions with high chemo- and regioselectivity. A decreasing number of fluorine substituents on benzene results in a decrease in the reaction rate. In the cases of partially fluorinated benzenes, the addition of LiCl is advantageous or even necessary to facilitate the reaction. Also, pentafluorobenzenes R-C6F5 (R = -CN, -OMe, -Me, or -CF3) react via C-F/C-B exchange that mostly occurs in the para position and to a lesser extent in the meta or ortho positions. Most of the reactions proceed via an SNAr mechanism. The reaction of 1,4-F2C6H4 with K21 shows that an aryne mechanism has to be considered in some cases as well. In summary, a wealth of new stable tricyano(aryl)borates have been synthesised and fully characterized using multi-NMR spectroscopy and most of them were characterised using single-crystal X-ray diffraction.

Pentakis(trifluoromethyl)phenol from Nitrobenzene

Pentakis(trifluoromethyl)phenol, a long requested compound, is now available within a two-step foolproof method from commercially available starting materials with a reasonably high yield (58%).

Regiochemically Flexible Substitutions of Di-, Tri-, and Tetrahalopyridines: The Trialkylsilyl Trick

[reactions: see text] 2,4-Difluoropyridine, 2,4-dichloropyridine, 2,4,6-trifluoropyridine, 2,4,6-trichloropyridine and 2,3,4,6-tetrafluoropyridine react with standard nucleophiles exclusively at the 4-position under halogen displacement. However, the regioselectivity can be completely reversed if a trialkylsilyl group is introduced in the 5-position of the 2,4-dihalopyridines or in the 3-position of the 2,4,6-trihalopyridines or 2,3,4,6-tetrahalopyridine. Then only the halogen most remote from the bulky silyl unit (at the 2-position in the case of the 2,4-halopyridines, at the 6-position with the other substrates) gets involved in the exchange process. After removal of the silyl protective group the nucleophile is invariably found to occupy the nitrogen-neighboring position.

Rerouting Nucleophilic Substitution from the 4-Position to the 2- or 6-Position of 2,4-Dihalopyridines and 2,4,6-Trihalopyridines: The Solution to a Long-Standing Problem

2,4-Difluoro-, 2,4,6-trifluoro-, and 2,3,4,6-tetrafluoropyridine undergo nucleophilic substitution preferentially if not exclusively at the 4-position. However, after the introduction of a trialkylsilyl group at C-3 or C-5, the halogen at the 6-(2-)position is displaced selectively. This synthetically valuable regiocontrol can also be realized with other halopyridines such as 2,4-dichloro- and 2,4,6-trichloropyridine.