Stereoselectivity reversal of a photochemical reaction in the solid state

The stereoselectivity of Norrish type II cyclobutanol formation resulting from photolysis of α-adamantyl-p-methoxyacetophenone is altered in favor of the more hindered cis isomer as the reaction medium is changed from isotropic liquid phases (benzene or acetonitrile) to the pure crystal. Based on the reactant X-ray crystal structure, it is suggested that the intermediate 1,4-biradical in the solid state is born in, and restricted to, a conformation which is ideal for direct closure to the more hindered product. In the relatively unrestricted solution environment, however, conformational isomerism of the biradical is faster than closure, thus leading to a predominance of the less hindered trans cyclobutanol.     

A simple approximation for the vibrational partition function of a hindered internal rotation

A simple formula is presented for calculating the approximate partition function of a hindered internal rotational mode of a polyatomic molecule. The formula gives useful accuracy over the whole range from harmonic oscillator to hindered rotator to free rotator.     

Synthesis of highly substituted cyclohexenes via mixed Lewis acid-catalyzed Diels-Alder reactions of highly substituted dienes and dienophiles

[reaction: see text] A high-yielding method is described for the rapid synthesis of very hindered cyclohexenes by the Diels-Alder reaction of hindered silyloxy dienes and dienophiles using the mixed Lewis acid catalyst system (AlBr(3)/AlMe(3)). Thus, reaction of the hindered diene 4 with various substituted enones gave good yields of the expected cycloadducts even though both partners are quite sterically hindered.     

Interaction between coloured pigments and hindered piperidine/antioxidant combinations in the photo-stabilisation of polypropylene

The complex interaction between coloured pigments, a primary antioxidant and a hindered piperidine compound in the photo-stabilisation of polypropylene has been examined using infra-red absorption spectros-copy. Alone, the pigments offered little photo-protection, their order of efficiency being copper phthalocyananine > chromic oxide > cadmium yellow. In the presence of an antioxidant only an additive effect was observed whereas, in the presence of a hindered piperidine stabiliser, there was powerful synergism. However, in the presence of both antioxidant and hindered piperidine compound the synergistic effects were considerably reduced with two of the pigments, cadmium yellow and chrome green, whereas, with copper phthalocyanine, the synergism was enhanced even further. The results indicate that additive adsorption is important with the first two inorganic pigments whereas, with the latter, the bulky organic ligands inhibit any such interaction. In fact, copper phthalocyanine and a hindered piperidine stabiliser exhibit a highly favourable interaction for photo-stabilisation.     

Position of coordination of the lithium ion determines the regioselectivity of demethylations of 3,4-dimethoxymorphinans with L-selectride

[reaction: see text] L-Selectride is an efficient agent for the 3-O-demethylation of opioids and is known to cleave the least hindered methoxyl group in a molecule. The treatment of a 3,4-dimethoxymorphinan containing a 6-ketal with L-Selectride gave selective 4-O-demethylation, rather than cleavage of the less hindered 3-methoxyl. In contrast, a 3,4-dimethoxymorphinan lacking a 6-ketal gave selective 3-O-demethylation, suggesting that the regiochemistry of L-Selectride-mediated O-demethylation can be manipulated through altering the position of coordination of the lithium ion.     

NMR studies of the dynamic behavior of 2-anilino-6-chloro-4-methoxy-1,3,5-triazine in solution

2-Anilino-6-chloro-4-methoxy-1,3,5-triazine was synthesized and studied by dynamic NMR. The activation parameters of hindered internal rotation in unsymmetrically substituted arylamino-sym-triazine were determined for the first time. It was found that a sterically more hindered rotational isomer is thermodynamically more stable in this compound (slow-exchange NOE data).     

Microwave-assisted, zinc-mediated peptide coupling of N-benzyl-α,α-disubstituted amino acids

Incorporation of the extremely hindered amino acid N-benzylaminoisobutyric acid into dipeptides under microwave irradiation with commercial zinc dust is described. A comparative survey of various methodologies for hindered peptide couplings was undertaken. The optimised coupling conditions were free from racemisation and applicable to other hindered amino acids.     

Photostabilizing activity of tertiary hindered amines

The influence of the N-alkyl group of tertiary hindered amines on the photostabilization of polymers was studied. The photostabilizing effects of the tertiary amine derivatives of 4-benzoyloxy-2,2,6,6-tetramethylpiperidine ( 1a ) in polypropylene were compared. All tertiary amine derivatives having α-H to hindered N showed higher effectiveness than 1a . Model liquid phase photoxidations were carried out by irradiating (UV-lamp) the solutions of tertiary hindered amines containing tert-butyl hydroperoxide as a photoinitiator. The tertiary hindered amines were oxidized more easily than corresponding parent hindered amine and converted to the parent amine, which was identified as its salt, resulting from the carboxylic acid produced from the N-alkyl group by oxidation. The thermal reaction of the tertiary hindered amines with tert-butyl hydroperoxide was also studied in the liquid phase. The tertiary hindered amines decomposed tert-butyl hydroperoxide more rapidly than the parent secondary hindered amine, and generated the parent amine. It was also found that the photostabilizing effects of tertiary hindered amines for polyolefins were higher than that of the parent secondary hindered amine.     

An expedient synthesis of spiroketals: model studies for the calyculin C16-C25 fragment

A new short strategy to prepare the spiroketal fragment of calyculins is presented. A novel Seyferth-Gilbert type homologation of hindered lactols to the corresponding alkynes has been achieved for the first time. The spirocyclization was achieved efficiently via a DIHMA (double intramolecular hetero-Michael addition) process of this hindered ynone. The spirocyclization rate is not dependent on the stereochemistry of the alkoxy substituent in the oxolane ring.     

Photo-stabilisation of polypropylene by a hindered piperidine compound: Influence of hydroperoxide and carbonyl groups

The influence of prior thermally generated hydroperoxides and added benzophenone on the photo-stabilising action of a hindered piperidine compound, bis[2,2,6,6-tetramethyl-4-piperidinyl]-sebacate, in polypropylene film has been examined. The hindered piperidine compound was found to effectively inhibit both the hydroperoxide and carbonyl group sensitised photo-oxidation of the polymer. Using ESR spectroscopy, evidence is presented to show that the hindered piperidine compound reacts stoichiometrically with the hydroperoxide groups generated by thermal oxidation to give a stable nitroxyl radical. The photo-stabilising effects observed are discussed in relation to our understanding of the ultraviolet anti-oxidant action of the hindered piperidine systems.     

Effect of hindered piperidine compounds on the thermal reduction of p-benzoquinone in polypropylene and its effect on photo-oxidation

The effect of a hindered piperidine compound and stable nitroxyl radical on the thermal reduction of p-benzoquinone in polypropylene has been examined using ESR, uv-visible and fluorescence spectroscopy. Thermal reduction to hydroquinone was inhibited by both compounds. With the hindered amine the nitroxyl radical was regenerated through the formation of the hydroxylamine whereas, with the nitroxyl radical, there was quantitative conversion to the hydroxylamine and no nitroxyl radical regeneration. Both inhibition processes were found to antagonise the photo-stabilising action of the hindered piperidine compounds due to the regeneration of the quinone.     

Steric hindrance in synthesis : Dealkylation of hindered carboxylic esters via the action of organolithium reagents. synthetic and mechanistic aspects

The reaction of n-propyllithium with a number of hindered aliphatic carboxylic esters in ether at 0° produces cleanly the corresponding carboxylic acid in satisfactory yields. This reaction represents a facile dealkylation of hindered carboxylic esters. The mechanistic aspects are briefly examined.     

An improved aldehyde linker for the solid phase synthesis of hindered amides

A novel aldehyde dual-linker system has been developed for the solid phase synthesis of sterically hindered amides. The linker [5-(4-formyl-3-hydroxyphenoxy)pentanoic acid] exploits an intramolecular oxygen-nitrogen acyl transfer mechanism to prepare compounds that are unattainable with current commercially available linkers. A dual linker system, exploiting the hyper-acid labile Sieber amide linker as part of the construct, enabled the initial reductive alkylation reactions of hindered amines and their subsequent acylation with a range of carboxylic acids with varying stereoelectronic properties to be monitored. Simple acylation conditions (HBTU/HOBt/NMM) sufficed to provide near quantitative reaction of test acids with support-bound hindered amines, reaction conditions which failed when commercial linkers were used.     

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.     

Synthesis of the western half of breviones C, D, F and G

The enantioselective synthesis of the diterpenic moiety in the abeo-breviano skeleton is reported. The synthesis is carried out starting from 2-methyl-1,3-cyclohexanedione and EVK in eleven steps following a ring-expansion strategy once the tricyclic perhydrophenantrene skeleton has been obtained. A new Tiffeneau-Demjanov rearrangement under dark conditions is reported in which the insertion of the new methylene group is directed towards the most hindered side of the carbonyl group. This result is new and opposite to those reported in the literature under light (filtered or un-filtered wavelengths) conditions where insertion in the less hindered side is usually preferred.     

NBS-Promoted reactions of symmetrically hindered methylphenols via p-benzoquinone methide

Symmetrically hindered methylphenols 1 react smoothly with NBS to form transient intermediates, p-benzoquinone methides (BM), which can be further processed to give hydroxybenzaldehydes in the presence of DMSO. This reaction is initiated by the formation of the phenoxy radical, followed by disproportionation to afford BM. None of the side-chain-brominated product is observed. The existence of BM is supported by the following observations: the formation of BM in solution can be monitored by GC and GC-MS; the electrophilic methine part participates in electrophilic aromatic substitution with anisoles to give hydroxybenzylated products 15; and the double bond character of the exocyclic methine plays a role in [4 + 2] cycloaddition with diene to afford Diels-Alder adducts. In contrast, unsymmetrically hindered or simple methylphenol (p-cresol) with NBS gives the nuclear brominated products, as usual. The energies of symmetrically hindered BMs, unsymmetrically hindered BM, and simple BM were calculated using density functional theories. Relative stabilization energies calculated at the B3LYP/6-31G//B3LYP/6-31G level by an isodesmic equation are enhanced 3-6 kcal/mol for symmetrically hindered BMs.     

Novel Procedure for the Coupling of Sterically Hindered Electron-Deficient Anilines to the 6-Position of the Purine Core

Sterically hindered electron-deficient anilines are coupled to the 6-position of the purine core only when activated as their corresponding TFA-amide. The free anilines did not react under all conditions tested. After aqueous work-up, the TFA-group is lost.

This procedure provides a new tool in the construction of purines functionalized with a sterically hindered electron-deficient aniline in the 6-position.      

An NMR investigation on the phase structure and molecular mobility of the novel exfoliated polyethylene/palygorskite nanocomposites

Novel exfoliated polyethylene (PE)/palygorskite nanocomposites prepared by in situ polymerization are characterized by solid‐state nuclear magnetic resonance (NMR). The phase structure and molecular mobility are investigated by a combination of proton and carbon NMR. The results showed that incorporation of small amounts of palygorskite had great influence on the phase structure and molecular mobility. The incorporated palygorskite hindered the crystallization process and introduced motion‐hindered chains in the NMR crystalline and amorphous phase. ¹³C cross‐polarization and magic‐angle spinning NMR revealed two orthorhombic crystalline phase with different line‐width. The chain mobility of orthorhombic crystalline phase with broad resonance line is obviously hindered compared with the phase with narrow resonance line when the filler is introduced. Additionally, the results of pulsed field gradient NMR technique show those the tortuosities in the nanocomposites are much higher than that in the bulk PE. The self‐diffusion process of probe molecules is also influenced by the palygorksite load. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 1363–1371, 2010     

Refined NMR analysis of the phase structure of solution-crystallized linear polyethylene

Nuclear magnetic resonance (NMR) spectroscopy reveals that solution-grown polyethylene samples have a unique phase structure independent of molecular weight. The lamellar crystallites are composed of about 85% crystalline material with the noncrystalline overlayer as large as 15%. The molecular motion in the overlayer is comparatively hindered and the liquid-like component, which is generally recognized in melt-grown crystals, cannot be produced appreciably, even at 60°C. Such hindered molecular mobility can be understood in terms of a rather restricted conformation of the molecular chains in the noncrystalline overlayer, arising from the special mode of crystallization from dilute solution.     

Synthesis of chiral binaphthalenes using the asymmetric Suzuki reaction

The synthesis of atropisomeric 1,1′-binaphthalenes can be achieved using an asymmetric Suzuki cross-coupling reaction. The Suzuki reaction leading to such hindered compounds is challenging and competing hydrolytic deboronation frequently dominates unless carefully chosen conditions are employed. The simple, standard mechanism is inadequate when describing the Suzuki coupling of hindered partners. Evidence suggests that the key step leading to asymmetry is transmetallation (delivery of the organometallic by the asymmetric ligand) and the reactions operate under kinetic control. Reductive elimination (itself likely to be triggered by oxidative addition of another molecule of halide) is fast compared with equilibration (epimerisation and/or cis-trans isomerisation).