N-alkoxyacrylamides as substrates for enantioselective Diels-Alder reactions

[reaction: see text] The use of N-alkoxyacrylamides as substrates for Lewis acid catalyzed Diels-Alder reactions has been examined. Enantioselectivities up to 92% ee have been achieved using very simple chiral Lewis acids prepared from triisobutylaluminum and 2,2-dimethyl-alpha,alpha,alpha',alpha'-tetra-1-naphthalenyl-TADDOL (1-NaphtTADDOL). The use of Yamamoto's Zn-BINOL, easily prepared from Et2Zn and 1,1'-bi-2-naphthol (BINOL), proved to be even more efficient, and enantioselectivities up to 96% ee were achieved.     

Direct comparison of 2,5-di-tert-butyl-1,4-dimethoybenzene and 4-tert-butyl-1,2-dimethoxybenzene as redox shuttles in LiFePO4-based Li-ion cells

2,5-di-tert-butyl-1,4-dimethoxybenzene (DDB) and 4-tert-butyl-1,2-dimethoxybenzene (TDB) have recently been proposed by different research groups as effective redox shuttles for overcharge protection of LiFePO₄-based Li-ion cells. Different test methods used in the published accounts make direct comparison of the merits of DDB and TDB difficult. Here DDB and TDB are tested under the same conditions in Li/LiFePO₄, graphite/LiFePO₄ and Li_4/3Ti_5/3O₄/LiFePO₄ coin-type cells under conditions that approximate those found in practical cells. The results confirm that DDB can support over 200 shuttle-protected overcharge cycles each of 100% cell capacity for all three cell types while TDB can only support between 3 and 15 overcharge cycles. This highlights the importance of testing redox shuttles under conditions that mimic those found in commercial cells.     

Low-melting sugar-urea-salt mixtures as solvents for Diels-Alder reactions

Sweet solutions are obtained upon heating mixtures of simple carbohydrates, urea and inorganic salts to moderate temperatures, to give new chiral media for organic reactions.     

Use of bis(oxazoline)-metal complexes as chiral catalysts for asymmetric Diels-Alder reactions using 2-acetyl-1,4-naphthoquinone as a dienophile

Asymmetric Diels-Alder reactions of 1,4-naphthoquinone and 2-acetyl-1,4-naphthoquinone with cyclopentadiene catalyzed by bis(oxazoline)-metal complexes afforded the corresponding Diels-Alder adducts. Moderate levels of enantiomeric excess were obtained and a number of different reaction conditions evaluated.     

Simultaneously electrogenerated cycloaddition partners for regiospecific inverse-electron-demand Diels-Alder reactions: a route for polyfunctionalized 1,4-benzoxazine derivatives

A multistep one-pot electrochemical synthesis of a variety of complex 2-alkylamino-1,4-benzoxazine derivatives is described. The reactions are regiospecific and diastereospecific in the case of heterocyclic annulation. This cascade sequence, wherein both cycloaddition partners are generated in situ, at room temperature, under metal-free conditions, allows the inverse-electron-demand Diels-Alder reaction of an o-iminoquinone diene and a secondary alkylenamine dienophile, two chemically nonaccessible unstable entities. To increase the molecular diversity, a variant in which the enamine is separately prepared completes the aforementioned procedure. The extension of this reaction should be useful to generate libraries of heterocycles.     

Molecular structures and packing arrangements of five 2,5-bis(aryl-2-vinyl)-1,4-dimethoxybenzene derivatives

The crystal and molecular structures of five 2,5-bis(aryl-2-vinyl)-1,4-dimethoxybenzene derivatives1a-1e were determined by X-ray diffraction with respect to topochemical aspects. We found three types of packing arrangements: (1)-type packing with 7 Å stacking axes, (2)-type packing with 4 Å stacking axes and (3) a third intermediate packing type. The intermolecular distances between the vinylic double bonds of all derivatives exceed the limits of 4 Å in the crystals. Therefore photochemical [2+2]cycloadditions were not observed in the crystals of these compounds. A correlation between the inclination angle of the molecular plane to the stacking axis and the separation between potentially reactive double bonds was detected.     

2,5,-Dimethoxy- and 2,5-di-n-butoxy-1,4-benzoquinone reactions and polymerization with 1,6-hexanediamine

The serendipitous formation of 2,5-dimethoxy- 1,4-benzoquinone is reported from the reaction of 1,4-benzoquinone with methanol, DABCO, and paraformaldehyde. This monomer, and its di-n-butoxy analog, are also available from 2,5-dihydroxy-1,4-benzoquinone. These materials are capable of novel polycondensation reactions with diamines such as 1,6-hex-anediamine. Use of m-crexsol as polymerization solvent gave a dark, insoluble product while various amide solvents lead to orange or pink polymers that had average degrees of polymerization from 5 up to >30. These polymers, Plus model compounds obtained from 1-aminopropane and N,N'- dimethyl-1,6-hexanediamine, were characterized by FTIR, solution, and solid-state NMR.     

2-Nitrofurans as dienophiles in Diels-Alder reactions

α-Nitrofuran derivatives are studied in Diels-Alder reactions under thermal conditions. In contrast to α-acylfurans, they proved to be efficient dienophiles.     

Substrate dependence in aqueous Diels-Alder reactions of cyclohexadiene derivatives with 1,4-benzoquinone

A reactivity difference based on the position of substituents on cyclohexa-1,3- diene was observed for the title reaction. The effect of water as solvent was more distinct for 1-methyl-4-isopropylcyclohexa-1,3-diene than for 2-methyl-5-isopropylcyclohexa- 1,3-diene or non-substituted cyclohexa-1,3-diene. The effect of NaCl (salting-out) and guanidium chloride (salting-in) was also large for 1-methyl-4-isopropylcyclohexa-1,3- diene.     

Intramolecular Diels-Alder reactions using alpha-methylene lactones as dienophile

Several dienals were prepared and reacted in the presence of zinc metal with ethyl 2-bromomethylacrylate to provide in a Reformatsky-like reaction alpha-methylene lactones carrying a dienyl side chain. Thermolysis of these compounds (11, 21, 29, and 37) gave in an intramolecular Diels-Alder reaction the corresponding tricyclic cycloadducts (51, 52, 53, and 54). The cycloadditions took place with good diastereoselectivity and yields. The stereochemistry of the major isomer is in accordance with an endo transition state for cycloadducts 51, 52, and 54. In one instance (compound 58), the structure was supported by X-ray crystallography. In contrast to the other substrates, the nonatriene 29 cyclized to the exo product 53exo. The stereochemical situation could also be proven by NOESY NMR. However, the intramolecular Diels-Alder reaction did not work with furan as dienophile (compound 41) and substrate 50 featuring a densely functionalized tether connecting diene and alpha-methylene lactone.     

Ruthenium-catalysed double trans-hydrosilylation of 1,4-diarylbuta-1,3-diynes leading to 2,5-diarylsiloles

Dihydrosilanes undergo double trans-hydrosilylation with 1,4-diarylbuta-1,3-diynes in the presence of a cationic ruthenium catalyst to afford 2,5-diarylsiloles: in particular, 9-silafluorene is a good hydrosilylating agent to produce spiro-type siloles in good yield.     

Alpha'-hydroxy enones as achiral templates for Lewis acid-catalyzed enantioselective Diels-Alder reactions

alpha'-Hydroxy enones react with dienes in the presence of (S,S)-[Cu(tBu-box)](OTf)2 or (S,S)-[Cu(tBu-box)](SbF6)2 (2 to 10 mol %) to afford the corresponding Diels-Alder adducts in high yield and selectivity. Isomeric ratios (regioselectivity, endo/exo or cis/trans) of up to >99:1 and ee values of up to >99% are obtained. Significantly, difficult dienes such as isoprene, 2,3-dimethyl butadiene and piperylene behave satisfactorily. Subsequent oxidative cleavage of the ketol in the resulting cycloadducts by treatment with cerium ammonium nitrate (CAN) yields the corresponding enantiopure carboxylic acids. Alternatively, carbonyl addition and subsequent diol cleavage with CAN produces the corresponding ketone adducts.     

1,4-Dithiane-2,5-diol as an efficient synthon for a straightforward synthesis of functionalized tetrahydrothiophenes via sulfa-Michael/aldol-type reactions with electrophilic alkenes

‘One-pot’ tandem reactions of commercially available 1,4-dithiane-2,5-diol (the dimer of mercaptoacetaldehyde) with electrophilic alkenes resulted in the facile formation of substituted tetrahydrothiophene derivatives. Thus, sulfa-Michael/Henry and sulfa-Michael/aldol sequences provided polysubstituted tetrahydrothiophenes using in situ generated nitroalkenes and α,β-unsaturated carbonyl compounds as the electrophilic partners of mercaptoacetaldehyde dimer, respectively.     

2,5-Bis-(butyltelluro) thiophene as a convenient precursor for the synthesis of 2,5-bis-(acetylenic) thiophenes

Both symmetrically and unsymmetrically substituted 2,5-bis-(acetylenic) thiophene derivatives were obtained in good yields under mild conditions through palladium-catalyzed cross coupling reaction of 2,5-bis-(butyltelluro) thiophene 2 and terminal alkynes. The methodology represents a general and efficient protocol for carrying out the synthesis of thiophene derivatives with potential biological activities.     

Vinyl benzenes as dienes in mild solid-phase Diels-Alder reactions

Wang resin-bound intermediates derived from Fmoc-L-phenylalaninal and Fmoc-L-valinal, and a resin supported Horner-Wadsworth-Emmons reaction, were treated with cinnamaldehyde derivatives, acetic acid, and borohydride to give secondary amines which were subsequently benzoylated to afford various derivatives of 3. Heating 3 at 95 degrees C induced cycloaddition reactions and produced 4 as the major product. Compounds 3 which were derived from 4-methoxycinnamaldehyde were more reactive, but did not give 4 and 4-7g. The direct cleavage of 3b using TFA led to the isolation of cycloaddition-demethylation product 10. The derivative of 3, which contained an electron-withdrawing nitro group on the phenyl ring, produced a single diastereomer of 4. The Diels-Alder cycloaddition between two electron-deficient counterparts showed similar reactivity to that of the reactions which have a normal complementary electron-demand.     

Tunnelling effects in the double proton transfer reaction of 2,5-dihydroxy-1,4-benzoquinone

Two dynamic models were used to investigate the double proton transfer reaction of 2,5-dihydroxy-1, 4-benzoquinone. The stationary points of the potential energy surface were located using the AM1 method. A model two-dimensional potential surface based on the Cartesian coordinates of the protons is constructed. According to the first model, the synchronous and asynchronous mechanisms of the reaction are treated separately. In the second model the two-dimensional vibrational problem is solved on the corresponding potential energy surface. The unimolecular rate constants are calculated in terms of the RRKM theory and the one- and two-dimensional approximations are compared. The second model predicts the dominant role of tunnelling and the synchronous and asynchronous processes can hardly be separated.     

Siloxacyclopentenes as dienophile-linked directing groups in intramolecular Diels-Alder reactions

The synthesis and intramolecular Diels-Alder reactions of trienes 5 and 6 with a siloxacyclopentene-constrained dienophile are described. These reactions provide the primary cycloadducts 7 and 8 with high diastereoselectivity. These cycloadducts possess trans-relationships between the ring fusion proton and the adjacent C(1) alkoxy group and can be further elaborated to alcohols 9 and 11 (via protiodesilylation) or to 10 and 12 (via Fleming-Tamao oxidation) depending on the substitutent R.     

Concise assembly of highly substituted furan-fused 1,4-thiazepines and their Diels-Alder reactions with benzynes

A facile and highly efficient three-component reaction of thiazole or benzothiazole carbenes, disubstituted ketenes, and activated alkynes is disclosed. With this methodology, a polysubstituted ring system containing furo[2,3-c]thiazepine core can be constructed from simple and readily accessible starting materials in good yields. The scope and limitation of this transformation were investigated in detail by using various thiazole carbene, ketene, and alkyne components. Furthermore, the synthetic utilities of these unique polyheterocyclic compounds were demonstrated via their Diels-Alder reactions with benzynes to furnish thiazepine-fused 7-oxanorbornadiene derivatives in excellent yields.