Divergent synthesis of fused Benzo-xanthene and oxazine derivatives via Cu-catalyst

Cu-Cu₂O combination showed synergic effects in catalyzing intramolecular Ullmann coupling reaction for halo-Betti bases to afford fused benzo-xanthenes from both electron-rich and electron-deficient aromatic systems in good yield under mild reaction conditions. The sterically hindered halo-Betti bases also provided products in excellent yield. Under optimized condition, a gram scale reaction was also performed to afford the product in excellent yield. However, without ortho-halo substituted Betti-bases failed to afford fused benzo-xanthene derivatives. Exploring the reaction optimization without Cu metal, serendipitously it was produced 1,3-oxazine derivatives in excellent yield via intramolecular cross-dehydrogenative coupling (CDC) reaction. Similarly, electron-rich, electron-deficient and sterically hindered Betti-bases provided the products in good to excellent yield under mild condition.     

Stabilization of low density polyethylene with sterically hindered derivatives of 4-hydroxybenzoic acid

In comparison with other commercial light stabilizers, sterically hindered 4-hydroxybenzoates are found to possess only a weak power, if they are used for low density polyethylene (LDPE). It is known that these benzoates exhibit significant synergistic effects together with 2-hydroxybenzophenones and hindered amine light stabilizers in light stabilization of high-density and linear low-density polyethylene. The light stabilizing efficiencies of the above mentioned stabilizers and mixtures of them in different weight ratios are determined by weathering in a Xenotest 150 unit. Only small synergistic effects of about 15 −30% are found for light stabilizing LDPE. The effects can be explained by the stabilizing efficiency of sterically hindered 4-hydroxybenzoates during processing. Combining of light stabilizing structure components like 2-hydroxy-benzophenone resp. 2, 2, 6, 6-tetramethylpiperidine with sterically hindered 4-hydroxybenzoates in one stabilizer molecule only, from the first mentioned combination, results a powerful light stabilizer. Its efficiency is the sum of the powers of the single structure components.     

Nickel‐Catalyzed Amination of Silyloxyarenes through C–O Bond Activation

Silyloxyarenes were utilized as electrophilic coupling partners with amines in the synthesis of aniline derivatives. A diverse range of amine substrates were used, including cyclic or acyclic secondary amines, secondary anilines, and sterically hindered primary anilines. Additionally, a range of sterically hindered and unhindered primary aliphatic amines were employed, which have previously been challenging with other classes of aryl ether electrophiles. Orthogonal couplings of silyloxyarenes with aryl methyl ethers are illustrated, where selectivity between the two C?O electrophiles is determined by ligand control, thereby allowing complementary and selective late‐stage diversification of either electrophile. Finally, a sequential coupling displays the utility of this amination method along with the reversal in intrinsic reactivity between aryl methyl ethers and silyloxyarenes.     

Lithium hexamethyldisilazide-mediated ketone enolization: the influence of hindered dialkyl ethers and isostructural dialkylamines on reaction rates and mechanisms

Mechanistic studies of the enolization of 2-methylcyclohexanone mediated by lithium hexamethyldisilazide (LiHMDS; TMS(2)NLi) solvated by hindered dialkyl ethers (ROR') are described. Rate studies using in situ IR spectroscopy show that enolizations in the presence of i-Pr(2)O, 2,2,5,5-tetramethyltetrahydrofuran, and cineole proceed via dimer-based transition structures [(TMS(2)NLi)(2)(ROR')(ketone)]. Comparing the relative solvation energies and the corresponding solvent-dependent activation energies shows that the highly substituted ethers accelerate the enolizations by sterically destabilizing the reactants and stabilizing the transition structures. Comparisons of hindered dialkyl ethers with their isostructural dialkylamines reveal that the considerably higher rates elicited by the amines derive from an analogous relative destabilization of the reactants and relative stabilization of the transition structures.     

Iron(II) complexes of sterically bulky alpha-ketocarboxylates. structural models for alpha-ketoacid-dependent nonheme iron halogenases

Reaction of the sterically hindered alpha-ketocarboxylate 2,6-di(mesityl)benzoylformate (MesBF) with the iron(II) complexes LFeCl 2 [L = N, N, N', N'-tetramethylpropylenediamine (Me 4pda) or 6,6'-dimethyl-2,2'-bipyridine (dmby)] yielded LFe(Cl)(MesBF) ( 1 or 2). X-ray crystal structures of these complexes showed that they closely model the active site structure of the nonheme iron halogenase enzyme SyrB2. A similar synthetic procedure using benzoylformate with L = dmby yielded (dmby)Fe[(O 2CC(O)Ph)] 2 ( 3) instead, demonstrating the need for the sterically hindered alpha-ketocarboxylate to assemble the halogenase model compounds. In order to make reactivity comparisons among the structurally related iron(II) complexes of benzoylformates of varying steric properties, the complexes [LFe(O 2CC(O)Ar)] n ( 4- 6) were prepared, where L' = tris(pyridylmethyl)amine (tpa) and Ar = 2,6-dimesitylphenyl, 2,6-di p-tolylphenyl, or 2,4,6-trimethylphenyl, respectively. X-ray structures for the latter two cases ( 5 and 6) revealed dinuclear topologies ( n = 2), but UV-vis and (1)H NMR spectroscopy indicated that all three complexes dissociated in varying degrees to monomers in CH 2Cl 2 solution. Although compounds 1- 6 were oxidized by O 2, oxidative decarboxylation of the alpha-ketocarboxylate ligand(s) only occurred for 3. These results indicate that the steric hindrance useful for structural modeling of the halogenase active site prohibits functional mimicry of the enzyme.     

Synthesis of Sterically Hindered Heteroaromatic Ketones under Phase-transfer and Metal-complex Catalysis Conditions. (Review)

We have correlated data on methods for obtaining sterically hindered ketones, derivatives of furan, thiophene, pyrrole, and pyridine. We examine routes to arylation of chroman-4-ones and synthesis of sterically hindered unsaturated and silicon-containing heteroaromatic ketones.     

Synthesis of novel “hybrid” structures based on phosphorylacetic acid hydrazides and sterically hindered phenols

Abstract

The insertion of fragments of sterically hindered phenols into the structure of phosphorylacetic acids hydrazides in their reactions with 3,5-di-tert-butyl-4-hydroxybenzylacetates and sterically hindered 2- and 4-hydroxybenzaldehydes has been carried out. The synthesis of 3,5-di-tert-butyl-4-benzyl derivatives of 5-[(diphenylphosphoryl) methyl]-2,4-dihydro-3H-1,2,4-triazol-3-thiones was also carried out.     

Synthesis of acylhydrazones of sterically hindered hydroxybenzaldehydes based on phloroglucinol

New acylhydrazones containing sterically hindered phenolic fragments with different degrees of steric shielding were synthesized by reactions of 2,2′,2″-[benzene-1,3,5-triyltris(oxy)]triacetic acid trishydrazide with hydroxybenzaldehydes. According to the NMR data, the products in solution exist as relatively stable conformational cis/trans isomers with respect to the C(O)-NH bond.     

A distal pocket Leu residue inhibits the binding of O2 and NO at the distal heme site of cytochrome c'

Cytochromes c' are pentacoordinate heme proteins with sterically hindered distal sites that bind NO and CO but do not form stable complexes with O(2). Removal of distal pocket steric hindrance via a Leu→Ala mutation yields favorable O(2) binding (K(d) ~49 nM) without apparent H-bond stabilization of the Fe-O(2) moiety, as well as an extremely high distal heme-NO affinity (K(d) ~70 fM). The native Leu residue inhibits distal coordination of diatomic ligands by decreasing k(on) as well as increasing k(off). The connection between distal steric constraints, k(off) values, and distal to proximal heme-NO conversion is discussed.     

Mukaiyama Aldol Reactions Catalyzed by a Trimeric Organo Aluminum(III) Alkoxide

Abstract

Mukaiyama aldol reactions of enol ethers with a variety of aldehydes and ketones are efficiently catalyzed at 0–25 °C by the sterically bulky trimeric organo aluminum(III) alkoxide 1 synthesized via the reaction of 3 equiv of AlMe₃ with tripodal tris(2-hydroxy-3-tert-butyl-5-methylphenyl) methane and the elimination of 3 equiv of methane. Comparisons of its catalytic properties with the less sterically hindered analogue 2, the more sterically hindered analogue 3, a monomeric aluminum near-analogue 4, and a dimeric alumatrane 5 revealed that 1 possesses superior activity.     

Synthesis,structure, and properties of 2-[(4,6-di-tert-butyl-2,3-dihydroxyphenyl)thio]acetic acid amides

Russian Chemical Bulletin - New potential biologically active derivatives, in which the sterically hindered pyrocatechol moiety is linked through a 2-thioacetyl covalent bridge to a number of...     

Structural characterization and electrochemical properties of the 3,3'-5,5'-tetra-tert-butyl-4,4'-diphenoquinone

Crystals of the 3,3'-5,5'-tetra-tert-butyl-4,4'-diphenoquinone (TTBDQ) in the reaction mixture DCM/MeOH (1:1, v/v) were obtained as a result of CC coupling reaction of the sterically hindered phenol (2,6-di-tert-butylphenol, DTBP) using the binuclear Co(II) complexes. The oxidation product (TTBDQ), C(28)H(40)O(2), crystallizes in the space group P1 with one-half molecule in the asymmetric unit and the other half generated by an inversion centre. The diphenoquinone moiety is planar within +/-0.016(3)A. The crystal structure is stabilized by intramolecular C-H...O hydrogen bonds. The spectroscopic and electrochemical properties of the TTBDQ also have been studied.     

Solvation properties of N-substituted cis and trans amides are not identical: significant enthalpy and entropy changes are revealed by the use of variable temperature 1H NMR in aqueous and chloroform solutions and ab initio calculations

The cis/trans conformational equilibrium of N-methyl formamide (NMF) and the sterically hindered tert-butylformamide (TBF) was investigated by the use of variable temperature gradient 1H NMR in aqueous solution and in the low dielectric constant and solvation ability solvent CDCl3 and various levels of first principles calculations. The trans isomer of NMF in aqueous solution is enthalpically favored relative to the cis (deltaH(o) = -5.79 +/- 0.18 kJ mol(-1)) with entropy differences at 298 K (298 x deltaS(o) = -0.23 +/- 0.17 kJ mol(-1)) playing a minor role. The experimental value of the enthalpy difference strongly decreases (deltaH(o) = -1.72 +/- 0.06 kJ mol(-1)), and the contribution of entropy at 298 K (298 x deltaS(o) = -1.87 +/- 0.06 kJ mol(-1)) increases in the case of the sterically hindered tert-butylformamide. The trans isomer of NMF in CDCl3 solution is enthalpically favored relative to the cis (deltaH(o) = -3.71 +/- 0.17 kJ mol(-1)) with entropy differences at 298 K (298 x deltaS(o) = 1.02 +/- 0.19 kJ mol(-1)) playing a minor role. In the sterically hindered tert-butylformamide, the trans isomer is enthalpically disfavored (deltaH(o) = 1.60 +/- 0.09 kJ mol(-1)) but is entropically favored (298 x deltaS(o) = 1.71 +/- 0.10 kJ mol(-1)). The results are compared with literature data of model peptides. It is concluded that, in amide bonds at 298 K and in the absence of strongly stabilizing sequence-specific inter-residue interactions involving side chains, the free energy difference of the cis/trans isomers and both the enthalpy and entropy contributions are strongly dependent on the N-alkyl substitution and the solvent. The significant decreasing enthalpic benefit of the trans isomer in CDCl3 compared to that in H2O, in the case of NMF and TBF, is partially offset by an adverse entropy contribution. This is in agreement with the general phenomenon of enthalpy versus entropy compensation. B3LY/6-311++G** and MP2/6-311++G** quantum chemical calculations confirm the stability orders of isomers and the deltaG decrease in going from water to CHCl3 as solvent. However, the absolute calculated values, especially for TBF, deviate significantly from the experimental values. Consideration of the solvent effects via the PCM approach on NMF x H2O and TBF x H2O supermolecules improves the agreement with the experimental results for TBF isomers, but not for NMF.     

Cobalt Triarylcorroles with Sterically Hindered Haloginated Phenyl Rings:Synthesis,Crystal Structure and Catalytic Activity for Electroreduction of Dioxygen

Three new cobalt triarylcorroles with sterically hindered haloginated phenyl rings were synthesized and characterized by UV-vis, ~1H NMR spectroscopy, mass spectrometry and electrochemistry. The compounds are represented as(Ar)3Cor Co(PPh3), where Cor is a trianion of the corrole macrocycle and Ar is a 2-Cl Ph(1), 2,6-diC l Ph(2) or 2,6-diF Ph(3) group on each of the three meso-positions. The structures of 1 and 3 were characterized in the solid state by single-crystal X-ray analysis. Rotating-disk electrode was utilized to examine the electrocatalytic activity of the corroles for reduction of O_2 in 1.0 MHClO_4. Effect of the sterically hindered meso-substituents on UV-vis spectra and redox potentials as well as the electrocatalytic activity for reduction of dioxygen was discussed.     

Phosphorus pentoxide in organic synthesis. XIX. Mixtures of phosphorus pentoxide and ortho-substituted arylamines as reagents in the synthesis of 9-aryl-9H-purin-6-amines

9-Aryl-9H-purin-6-amines 2 were readily obtained by heating 5-acetamido-4-amino-2-methylpyrimidin-6(1H)-one 1 in mixtures of phosphorus pentoxide, triethylamine hydrochloride and sterically hindered arylamines. Nonsterically hindered arylamines resulted in formation of 6-arylaminopurines 3. The mechanism for rearrangement of 2 into 3 is discussed.     

Ligand-Controlled Site- and Enantioselective Carbene Insertion into Carbon-Silicon Bonds of Benzosilacyclobutanes

We report herein a highly efficient palladium-catalyzed carbene insertion into strained Si−C bonds of benzosilacyclobutanes, which provides an efficient method to access α-chiral silanes. With a sterically hindered ligand, carbene insertion into the C(sp³)−Si bond of benzosilacyclobutanes occurred in excellent site- and enantioselectivity, while C(sp²)−Si bond insertion occurred selectively with less sterically hindered ligands. Reaction mechanism, in particular the roles of the chiral ligands in controlling the site-selectivity of the insertion reactions, are elucidated by using hybrid density functional theory.     

Combining aspects of the platinum anticancer drugs picoplatin and BBR3464 to synthesize a new family of sterically hindered dinuclear complexes; their synthesis, binding kinetics and cytotoxicity

Picoplatin is a sterically hindered mononuclear platinum drug undergoing clinical trials. The 2-methylpyridine ring provides steric hindrance to the drug, preventing attack from biological nucleophiles. BBR3464 is a trinuclear platinum drug which was recently in Phase II clinical trials, and is highly cytotoxic both in vitro and in vivo; it derives this activity through the flexible adducts it forms with DNA. In this work we sought to combine the properties of both drugs to synthesise a family of sterically hindered, dinuclear platinum complexes as potential anticancer agents. The bis-pyridyl-based ligands were synthesised through a peptide coupling reaction using diaminoalkanes of differing lengths (n = 2, 4 or 8) and 4-carboxypyridine or 2-methyl-4-carboxypyridine. The resultant dinuclear platinum complexes were synthesised by reacting two equivalents of transplatin or mono-aquated transplatin to each ligand, followed by purification by precipitation with acetone. The unprotected complexes react faster with 5'-guanosine monophosphate (drug to nucleotide ratio 1?:?2; t(1/2) = 2 h), glutathione (1?:?10, t(1/2) = 55 min) and human serum albumin (HSA) (1?:?1, t(1/2) = 24 h) compared to their hindered, protected equivalents (5'-guanosine monophosphate, t(1/2) = 3.5 h; glutathione = 1.7 h; HSA, t(1/2) = 110 h). The complexes were tested for in vitro cytotoxicity in the A2780 and A2780/cp70 ovarian cancer cell line. The unprotected platinum complexes were more cytotoxic than their protected derivatives, but none of the complexes were able to overcome resistance. The results provide important proof-of-concept for the development of a larger family of sterically hindered multinuclear-based platinum complexes.     

Ruthenium-Catalyzed Direct Asymmetric Reductive Amination of Diaryl and Sterically Hindered Ketones with Ammonium Salts and H2

A Ru-catalyzed direct asymmetric reductive amination of ortho-OH-substituted diaryl and sterically hindered ketones with ammonium salts is reported. This method represents a straightforward route toward the synthesis of synthetically useful chiral primary diarylmethylamines and sterically hindered benzylamines (up to 97 % yield, 93–>99 % ee). Elaborations of the chiral amine products into bioactive compounds and a chiral ligand were demonstrated through manipulation of the removable and convertible -OH group.     

Reaction of sterically hindered o-benzoquinones with diazomethane

Conclusions In contrast to certain o-benzoquinones, the sterically hindered o-quinones react with CH₂N₂ both at the oxygen atoms to give cyclic ethers, and at the C=C and C=O bonds of the quinoid ring; the predominant direction of the reaction depends on the ratio of the reactants.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2360–2362, October, 1981.