http://sulflower.com Sulflower: At the Cutting Edge of Organic Chemistry Wed, 08 Sep 2010 10:05:24 +0000 http://wordpress.org/?v=2.5.1 en http://sulflower.com/?p=30924 http://sulflower.com/?p=30924#comments Wed, 08 Sep 2010 10:05:24 +0000 Tetrahedron Fresh Issues tag:feeds.feedburner.com://9955f3ffc645168357648001912f7f02 A novel [4+2] annulation between activated terminal alkynes and aza-dienes or oxo-dienes has been developed with the use of triphenylphosphine catalyst (20 mol %), which provides a facile method for synthesis of the corresponding highly functionalized dihydropyridines or dihydropyrans in good to excellent yields. The reaction mechanism has also been established, consisting formal hetero-Diels–Alder reaction catalyzed by PPh₃ and [1,3]-proton transfer, which exhibits a large isotopic effect.

http://dx.doi.org/10.1016/j.tet.2010.07.043]]> A novel [4+2] annulation between activated terminal alkynes and aza-dienes or oxo-dienes has been developed with the use of triphenylphosphine catalyst (20 mol %), which provides a facile method for synthesis of the corresponding highly functionalized dihydropyridines or dihydropyrans in good to excellent yields. The reaction mechanism has also been established, consisting formal hetero-Diels–Alder reaction catalyzed by PPh₃ and [1,3]-proton transfer, which exhibits a large isotopic effect.

http://dx.doi.org/10.1016/j.tet.2010.07.043]]> http://sulflower.com/?feed=rss2&p=30924 http://sulflower.com/?p=30921 http://sulflower.com/?p=30921#comments Wed, 08 Sep 2010 10:05:22 +0000 Tetrahedron Fresh Issues tag:feeds.feedburner.com://b0d3fc2c537603b4ba06cb0cf3342c1f The natural antioxidant (−)-gloeosporiol, isolated as a peracetylated derivative from a culture of the fungus Colletotrichum gloeosporioides, has been enantioselectively prepared from 3,4-dihydroxybenzaldehyde by means of a chemoenzymatic synthesis. The key intermediate was obtained by resolution with a lipase from Pseudomonas cepacia. Its stereochemistry, initially assigned as R, according to the Kazlaukas empirical rule for secondary alcohols, was independently confirmed by NMR and chirooptic methods. This, in turn, allowed the assignment of compound (−)-1 as (−)-(2S,3R,4R)-2-(3′,4′dihydroxyphenyl)tetrahydrofuran-3,4-diol.

http://dx.doi.org/10.1016/j.tet.2010.07.074]]> The natural antioxidant (−)-gloeosporiol, isolated as a peracetylated derivative from a culture of the fungus Colletotrichum gloeosporioides, has been enantioselectively prepared from 3,4-dihydroxybenzaldehyde by means of a chemoenzymatic synthesis. The key intermediate was obtained by resolution with a lipase from Pseudomonas cepacia. Its stereochemistry, initially assigned as R, according to the Kazlaukas empirical rule for secondary alcohols, was independently confirmed by NMR and chirooptic methods. This, in turn, allowed the assignment of compound (−)-1 as (−)-(2S,3R,4R)-2-(3′,4′dihydroxyphenyl)tetrahydrofuran-3,4-diol.

http://dx.doi.org/10.1016/j.tet.2010.07.074]]> http://sulflower.com/?feed=rss2&p=30921 http://sulflower.com/?p=30919 http://sulflower.com/?p=30919#comments Wed, 08 Sep 2010 10:05:19 +0000 Tetrahedron Fresh Issues tag:feeds.feedburner.com://a86d5820c2baaf538c08756116bf6ce6 The arylation of thiazoles in 4- and 5-position was investigated in detail. Suzuki–Miyaura and Stille cross-coupling reactions were tested using thiazoles either as halide or organometal species. The obtained results were critically compared to develop helpful guidelines for selection of the cross-coupling methodology with the best potential for good and reliable results for a given synthetic problem without the need for tedious optimization of reaction parameters.

http://dx.doi.org/10.1016/j.tet.2010.07.081]]> The arylation of thiazoles in 4- and 5-position was investigated in detail. Suzuki–Miyaura and Stille cross-coupling reactions were tested using thiazoles either as halide or organometal species. The obtained results were critically compared to develop helpful guidelines for selection of the cross-coupling methodology with the best potential for good and reliable results for a given synthetic problem without the need for tedious optimization of reaction parameters.

http://dx.doi.org/10.1016/j.tet.2010.07.081]]> http://sulflower.com/?feed=rss2&p=30919 http://sulflower.com/?p=30918 http://sulflower.com/?p=30918#comments Wed, 08 Sep 2010 10:05:16 +0000 Tetrahedron Fresh Issues tag:feeds.feedburner.com://efc98a6cb02a9438b09c74cf36c1a285 Although originally designed for anion recognition, bis(N′-p-nitrophenylureido)calix[4]arenes immobilised in the 1,3-alternate conformation can act as receptors for small neutral molecules, such as sulfoxides and ketones. Their binding ability was studied both in solution and in the solid state using the combination of UV/vis, NMR, and X-ray crystallography.

http://dx.doi.org/10.1016/j.tet.2010.08.006]]> Although originally designed for anion recognition, bis(N′-p-nitrophenylureido)calix[4]arenes immobilised in the 1,3-alternate conformation can act as receptors for small neutral molecules, such as sulfoxides and ketones. Their binding ability was studied both in solution and in the solid state using the combination of UV/vis, NMR, and X-ray crystallography.

http://dx.doi.org/10.1016/j.tet.2010.08.006]]> http://sulflower.com/?feed=rss2&p=30918 http://sulflower.com/?p=30917 http://sulflower.com/?p=30917#comments Wed, 08 Sep 2010 10:05:13 +0000 Tetrahedron Fresh Issues tag:feeds.feedburner.com://dd3906f5389e63ae0e070014ee8d3af1 The synthesis of the putative prion-associated heparan sulfate tetrasaccharide containing two d-glucuronic acid units is reported. Key to the synthesis were the differentiation of the N-acetylated and N-unsubstituted glucosamine units, the high-yielding glucosylation at O-4 of an N-acetyl-d-glucosamine derivative and the α-selective glycosylation of the 4′-OH group of a β-d-GlcpA-(1→4)-d-GlcpNAc disaccharide building block with a disaccharide thioglycoside donor.

http://dx.doi.org/10.1016/j.tet.2010.08.004]]> The synthesis of the putative prion-associated heparan sulfate tetrasaccharide containing two d-glucuronic acid units is reported. Key to the synthesis were the differentiation of the N-acetylated and N-unsubstituted glucosamine units, the high-yielding glucosylation at O-4 of an N-acetyl-d-glucosamine derivative and the α-selective glycosylation of the 4′-OH group of a β-d-GlcpA-(1→4)-d-GlcpNAc disaccharide building block with a disaccharide thioglycoside donor.

http://dx.doi.org/10.1016/j.tet.2010.08.004]]> http://sulflower.com/?feed=rss2&p=30917 http://sulflower.com/?p=30916 http://sulflower.com/?p=30916#comments Wed, 08 Sep 2010 10:05:11 +0000 Tetrahedron Fresh Issues tag:feeds.feedburner.com://ca7a6b1e764c90c5b2461b5ed7408ade Two approaches to the synthesis of the plant-derived biaryl neolignan honokiol are described. The second approach provided the natural product in either four steps with 34% overall yield or five steps and 55% overall yield.

http://dx.doi.org/10.1016/j.tet.2010.08.005]]> Two approaches to the synthesis of the plant-derived biaryl neolignan honokiol are described. The second approach provided the natural product in either four steps with 34% overall yield or five steps and 55% overall yield.

http://dx.doi.org/10.1016/j.tet.2010.08.005]]> http://sulflower.com/?feed=rss2&p=30916 http://sulflower.com/?p=30915 http://sulflower.com/?p=30915#comments Wed, 08 Sep 2010 10:05:05 +0000 Tetrahedron Fresh Issues tag:feeds.feedburner.com://3a57fa4fea7d609322292878cccea886 Proline-catalysed aldol reactions are central to the development of organocatalysis, and have been the subjects of many studies. List’s results on the effect of fluorine substitution on proline catalysts for an intramolecular aldol condensation provide a perfect test set for computational analysis, as subtle changes in the catalyst structure lead to clear changes in the product ratios. The results show that the carbon–carbon bond forming transition states for the Hajos–Parrish–Wiechart reaction do not account for the observed selectivity in all cases. However, if an analysis of post transition-state epimerization pathways is also included, together with the effect of water, it is possible to account for all of the experimental data.

http://dx.doi.org/10.1016/j.tet.2010.08.003]]> Proline-catalysed aldol reactions are central to the development of organocatalysis, and have been the subjects of many studies. List’s results on the effect of fluorine substitution on proline catalysts for an intramolecular aldol condensation provide a perfect test set for computational analysis, as subtle changes in the catalyst structure lead to clear changes in the product ratios. The results show that the carbon–carbon bond forming transition states for the Hajos–Parrish–Wiechart reaction do not account for the observed selectivity in all cases. However, if an analysis of post transition-state epimerization pathways is also included, together with the effect of water, it is possible to account for all of the experimental data.

http://dx.doi.org/10.1016/j.tet.2010.08.003]]> http://sulflower.com/?feed=rss2&p=30915 http://sulflower.com/?p=30913 http://sulflower.com/?p=30913#comments Wed, 08 Sep 2010 10:04:59 +0000 Tetrahedron Fresh Issues tag:feeds.feedburner.com://0a2e30aacfaebe33bca19325a334c29f The optical and electronic properties of novel siloxane-bridged cyclic dimers with naphthylene (CD1) or pyrenylene (CD2) moieties are described. CD1 and CD2 were obtained by the cyclic dimerization of 1,4-bis(dimethylhydroxysilyl)naphthalene (M1) and 1,6-bis(dimethylhydroxysilyl)pyrene (M2), respectively. CD1 and CD2 mainly exhibited the emission from their excimers owing to their short distances between aryl moieties in CD1 and CD2, which were determined to be 3.44 Å and 3.41 Å by the X-ray crystallographic analysis, respectively. The absorption spectra of CD2 in the presence of 7,7,8,8-tetracyanoquinodimethane (TCNQ) revealed that CD2 has the ability to form 1:1 charge-transfer complex with TCNQ, indicating the existence of the transannular π–π interaction between closely located pyrene components in CD2.

http://dx.doi.org/10.1016/j.tet.2010.08.010]]> The optical and electronic properties of novel siloxane-bridged cyclic dimers with naphthylene (CD1) or pyrenylene (CD2) moieties are described. CD1 and CD2 were obtained by the cyclic dimerization of 1,4-bis(dimethylhydroxysilyl)naphthalene (M1) and 1,6-bis(dimethylhydroxysilyl)pyrene (M2), respectively. CD1 and CD2 mainly exhibited the emission from their excimers owing to their short distances between aryl moieties in CD1 and CD2, which were determined to be 3.44 Å and 3.41 Å by the X-ray crystallographic analysis, respectively. The absorption spectra of CD2 in the presence of 7,7,8,8-tetracyanoquinodimethane (TCNQ) revealed that CD2 has the ability to form 1:1 charge-transfer complex with TCNQ, indicating the existence of the transannular π–π interaction between closely located pyrene components in CD2.

http://dx.doi.org/10.1016/j.tet.2010.08.010]]> http://sulflower.com/?feed=rss2&p=30913 http://sulflower.com/?p=30912 http://sulflower.com/?p=30912#comments Wed, 08 Sep 2010 10:04:57 +0000 Tetrahedron Fresh Issues tag:feeds.feedburner.com://7c78df5df9667fe1713730541573222e The complexation of the potent olive oil antioxidant hydroxytyrosol (HT) with commercial β-cyclodextrin (β-CD) and hydroxypropyl-β-cyclodextrin (HP-β-CD) in aqueous solutions has been studied for the first time. The 1:1 stoichiometries of the complexes were evidenced by Job’s plot and the association constants were determined by linear regressions from Scott’s method. The geometry of the HT/β-CD complex was elucidated by 2D-ROESY experiments, suggesting insertion of the catechol moiety with the hydroxyalkyl chain directed to the primary rim. The antioxidant activity of encapsulated HT was also evaluated by scavenging of the stable DPPH radical, observing an improvement of the scavenging activity upon increase of the cyclodextrin concentration. Complexation of HT with β-CD also provided a strong photoprotection of the polyphenolic compound upon irradiation with UV (λ=254 nm).

http://dx.doi.org/10.1016/j.tet.2010.08.009]]> The complexation of the potent olive oil antioxidant hydroxytyrosol (HT) with commercial β-cyclodextrin (β-CD) and hydroxypropyl-β-cyclodextrin (HP-β-CD) in aqueous solutions has been studied for the first time. The 1:1 stoichiometries of the complexes were evidenced by Job’s plot and the association constants were determined by linear regressions from Scott’s method. The geometry of the HT/β-CD complex was elucidated by 2D-ROESY experiments, suggesting insertion of the catechol moiety with the hydroxyalkyl chain directed to the primary rim. The antioxidant activity of encapsulated HT was also evaluated by scavenging of the stable DPPH radical, observing an improvement of the scavenging activity upon increase of the cyclodextrin concentration. Complexation of HT with β-CD also provided a strong photoprotection of the polyphenolic compound upon irradiation with UV (λ=254 nm).

http://dx.doi.org/10.1016/j.tet.2010.08.009]]> http://sulflower.com/?feed=rss2&p=30912 http://sulflower.com/?p=30911 http://sulflower.com/?p=30911#comments Wed, 08 Sep 2010 10:04:56 +0000 Tetrahedron Fresh Issues tag:feeds.feedburner.com://3370a536ae38033cd17548dc33e1d6f6 One-pot reaction for the synthesis of novel phenylpyridyl derivatives and mixed quater phenylpyridyl compounds is described by using the Garlanding approach. The reactions proceed with moderate to good yields in mild conditions and good reaction times. This work represents a second application of the simplicity and versatility of Garlanding concept for the construction of new phenylpyridyl scaffolds, which can be considered as non-peptidic foldamer α-helix mimetics.

http://dx.doi.org/10.1016/j.tet.2010.08.008]]> One-pot reaction for the synthesis of novel phenylpyridyl derivatives and mixed quater phenylpyridyl compounds is described by using the Garlanding approach. The reactions proceed with moderate to good yields in mild conditions and good reaction times. This work represents a second application of the simplicity and versatility of Garlanding concept for the construction of new phenylpyridyl scaffolds, which can be considered as non-peptidic foldamer α-helix mimetics.

http://dx.doi.org/10.1016/j.tet.2010.08.008]]> http://sulflower.com/?feed=rss2&p=30911