The synthetic utility of oxazolines in aromatic substitution

Further Studies on the [1,2]-Wittig Rearrangement of 2-(2-Benzyloxy)aryloxazolines

The behaviour of 14 ortho-functionalised 2-aryloxazolines (11 of them prepared and characterised for the first time) with butyllithium has been examined. Significant limitations to the Wittig rearrangement of such systems are revealed. In terms of asymmetric Wittig rearrangement, good diastereoselectivity is obtained with a valine-derived 4-isopropyl oxazoline, but this is compromised by racemisation upon hydrolysis. More encouraging selectivity is achieved in the Wittig rearrangement of an acyclic phenylalanine-derived ortho-benzyloxy benzamide.

Access to Diarylmethanols by Wittig Rearrangement of ortho-, meta-, and para-Benzyloxy-N-Butylbenzamides

The N-butyl amide group, CONHBu, has been found to be an effective promoter of the [1,2]-Wittig rearrangement of aryl benzyl ethers and thus allow the two-step synthesis of isomerically pure substituted diarylmethanols starting from simple hydroxybenzoic acid derivatives. The method is compatible with a wide range of functional groups including methyl, methoxy, and fluoro, although not with nitro and, unexpectedly, is applicable to meta as well as ortho and para isomeric series.

One-pot access to 2-oxazolines via a Castro-Stephens coupling and intramolecular cyclization

Aim:

Here, we have reported easy one-pot access to a series of oxazolines using a modified Castro-Stephens coupling protocol.

Background:

2-oxazolines have been shown to have significant biological activity and wide-ranging applications in organic chemistry. These properties make oxazolines as heterocyclic compounds of immense importance.

Objective:

The objective of this study is to synthesize oxazoline derivatives via an economical and one-pot protocol.

Method:

2-oxazoline has been synthesized through Cu-powder mediated Castro-Stephens coupling and intramolecular cyclization route. The mechanism involves a rearrangement in which one of the oxygen from the N-acylamino alcohol group is liberated as water and then transferred to alkyne functionality to form 2-oxazoline derivatives. The oxazolines were characterized by NMR, mass, and XRD studies.

Result:

The protocol is economically viable and uses readily available Cu-powder along with DMF for cross-coupling and cyclization steps.

Substitution Effect on 2-(Oxazolinyl)-phenols and 1,2,5-Chalcogenadiazole-Annulated Derivatives: Emission-Color-Tunable, Minimalistic Excited-State Intramolecular Proton Transfer (ESIPT)-Based Luminophores

Minimalistic 2-(oxazolinyl)-phenols substituted with different electron-donating and -withdrawing groups as well as 1,2,5-chalcogenadiazole-annulated derivatives thereof were synthesized and investigated in regard to their emission behavior in solution as well as in the solid state. Depending on the nature of the incorporated substituent and its position, emission efficiencies were increased or diminished, resulting in AIE or ACQ characteristics. Single-crystal analysis revealed J- and H-type packing motifs and a so-far undescribed isolation of ESIPT-based fluorophores in the keto form.

Asymmetric synthesis of oxazolines bearing α-stereocenters through radical addition–enantioselective protonation enabled by cooperative catalysis

An efficient radical conjugate addition/enantioselective protonation process was developed for the asymmetric synthesis of chiral oxazolines bearing an α-stereocenter through cooperative photoredox catalysis and asymmetric organocatalysis.

Nucleophilic Addition Reaction with Dearomatization of Naphthalene Ring

Various aromatic lactones have been synthesized and their regioselectivity (1,2-addition vs. 1,4- or 1,6-addition) investigated in reactions with organolithium species, particularly n-BuLi and sec-BuLi. The regioselectivity varied greatly depending on various factors, such as the bulkiness of both substrates and organolithium species, and types of solvent and cosolvent. In particular, 1,4-addition with dearomatization occurred preferentially using sec-BuLi as the nucleophile in tetrahydrofuran (THF) with hexamethylphosphoramide (HMPA) or N,N'-dimethylpropyleneurea (DMPU) as cosolvent. For sec-BuLi, the reaction was estimated to proceed through a single-electron transfer mechanism.

Preparation of oxazolines and oxazoles via a PhI(OAc)2-promoted cyclization of N-propargylamides

A metal-free cyclization of N-propargylamides for the synthesis of various oxazolines and oxazoles via a 5-exo-dig process is presented. Using (diacetoxyiodo)benzene (PIDA) as a reaction promoter and lithium iodide (LiI) as an iodine source, intramolecular iodooxygenation of N-propargylamides proceeded readily, leading to the corresponding (E)-5-iodomethylene-2-oxazolines in good to excellent isolated yields. In addition, using the PhI(OAc)2/LiI system, N-propargylamides can be converted to the corresponding oxazole-5-carbaldehydes in the presence of oxygen under visible light irradiation. The resulting products can be further converted into various oxazoline and oxazole derivatives after simple derivatizations, and this method ultimately offers an efficient route to a variety of biologically active structures.

A catalytic highly enantioselective allene approach to oxazolines

A completely different approach for the asymmetric construction of oxazoline derivatives with an excellent enantioselectivity via the palladium-catalyzed coupling-cyclization of N-(buta-2,3-dienyl)amides with aryl iodides has been reported.

Oxazolines are a very important class of heterocyclic compounds. However, catalytic enantioselective syntheses are very limited. Here, a highly enantioselective palladium-catalyzed coupling-cyclization of readily available N-(buta-2,3-dienyl) amides with aryl or 1-alkenyl iodides has been developed for the asymmetric construction of oxazoline derivatives. Many synthetically useful functional groups are tolerated in this reaction. The absolute configuration of the chiral center in the products has been established by X-ray diffraction study. A model for prediction of the absolute configuration of the chiral center in the products from this cyclic enantioselective nucleophilic allylation has been proposed. The synthetic potentials based on the unique structure of the products formed have also been demonstrated.

Synthesis, Structure, and Unusual Reactivity of a Stable 3-(Oxazolidin-2-ylidene)thiophen-2-one

Treatment of 2- and 3-thienyloxazolines with butyllithium and bis(trimethylsilyl) peroxide results in ring hydroxylation to give products that exist mainly as the oxazolidinylidenethiophenones. The 3-oxazolidinylidenethiophen-2-one is a rare example of a stable heterocyclic o-quinone methide analogue that shows a varied pattern of reactivity, including both C- and O-alkylation, Michael addition via C-5 to an acetylenic ester, tetrachlorobenzannulation across positions 4 and 5, and formation of a hexacyclic fused-ring product with N-phenyltriazolinedione. Crystal structures of the products are dominated by inter- and intramolecular NH to CO hydrogen bonding.

Intramolecular oxyacetoxylation of N-allylamides: an expeditious synthesis of oxazolines and oxazines by using a PhI(OAc)2/hydrogen fluoride-pyridine system

The synthesis of oxazolines and oxazines from N-allylamides was accomplished via an unprecedented reaction set. This reaction involved an intramolecular cyclization of N-allylamides resulting from nucleophilic attack of the allylamine on the benzoxazin-4-one. Unlike the previous literature, the isolable N-allylamide could readily be subjected to exo and endo fashion ring-closure under conditions to afford the title compounds.

Copper N-Heterocyclic Carbene Complexes As Active Catalysts for the Synthesis of 2-Substituted Oxazolines from Nitriles and Aminoalcohols

The reaction between nitriles and aminoalcohols to access 2-substituted oxazolines was investigated. Using copper-NHC complexes, various nitriles were successfully converted into the corresponding oxazolines, under milder and less wasteful conditions than those of previously reported methods.

Telmisartan

Telmisartan is an angiotensin-II receptor antagonist (ARB) used in the treatment of hypertension. Generally, angiotensin-II receptor blockers such as telmisartan bind to the angiotensin-II type 1 receptors with high affinity, causing inhibition of the action of angiotensin II on vascular smooth muscle, ultimately leading to a reduction in arterial blood pressure. The present study gives a comprehensive profile of telmisartan, including detailed nomenclature, formulae, elemental analysis, and appearance of the drug are mentioned. The uses and applications and the several methods described for its preparation of the drug are outlined. The profile contains the physicochemical properties including: pKa value, solubility, X-ray powder diffraction, melting point, and methods of analysis (including compendial, electrochemical, spectroscopic, and chromatographic methods of analysis). Developed validated stability-indicating (HPLC and biodiffusion assay methods under accelerated acidic, alkaline, and oxidative conditions, in addition to effect of different types of light, temperature, and pH. Detailed Pharmacology also presented (Pharmacological actions, Therapeutic uses and Dosing, Interactions, and adverse effects and precautions). More than 80 references were given as a proof of the above-mentioned studies.

Zincation of 4,4-dimethyloxazoline using TMPZnCl·LiCl. A new preparation of 2-aryloxazolines

4,4-Dimethyloxazoline is directly zincated using TMPZnCl·LiCl and further functionalized by Pd-catalyzed cross-couplings, as well as Cu-mediated acylation and allylation reactions.

Catalytic constructive deoxygenation of lignin-derived phenols: new C–C bond formation processes from imidazole-sulfonates and ether cleavage reactions

Anticipating that 2-methoxyphenol could become an important renewable feedstock derived from lignin, various catalytic constructive deoxygenation reactions are reported (see scheme).

Pre-inverse-crowns: synthetic, structural and reactivity studies of alkali metal magnesiates primed for inverse crown formation

The synthesis, structure and reactivity of donor-free sodium and potassium magnesiates are assessed culminating in the unique 1,4-dimetallation of naphthalene.

Antimicrobial activity of synthetic bornyl benzoates against Trypanosoma cruzi

We report here for the first time the in vitro effects of (1S,2R,4S)-1,7,7-trimethyl-bicyclo[2·2·1]heptan-2-yl-3',4',5'-trimethoxy benzoate (1) and (1S,2R,4S)-1,7,7-trimethyl-bicyclo[2·2·1]heptan-2-yl benzoate (2) on the growth and ultrastructure of Trypanosoma cruzi. These two synthetic compounds exerted an antiproliferative effect on the epimastigote forms of the parasite. The ICs(50/72h) of two synthetic L-bornyl benzoates, 1 and 2, was 10·1 and 12·8 μg/ml, respectively. Both compounds were more selective against epimastigotes than HEp-2 cells. Ultrastructural analysis revealed intense cytoplasmic vacuolization and the appearance of cytoplasmic materials surrounded by membranes. The treatment of peritoneal macrophages with compounds 1 and 2 caused a significant decrease in the number of T. cruzi-infected cells. L-Bornyl benzoate derivatives may serve as a potential source for the development of more effective and safer chemotherapeutic agents against T. cruzi infections.

Pincer oxazolines: emerging tools in coordination chemistry and catalysis--where to next?

A brief overview of the coordination chemistry aspects of pincer ligands and their complexes containing at least one oxazoline (i.e., 4,5-dihydro-2-oxazole) unit is presented. This historical perspective is placed into a context of the possible future direction(s) in this particular arena of pincer chemistry. These ideas are compared and contrasted to the overall direction of pincer chemistry since the first such complexes were reported in the 1970s.