Asymmetric α‐Hydroxylation of Tetralone‐Derived β‐Ketoesters by Using a Guanidine–Urea Bifunctional Organocatalyst in the Presence of Cumene Hydroperoxide

Odagi, Minami; Furukori, Kota; Watanabe, Tatsuya; Nagasawa, Kazuo

doi:10.1002/chem.201303006

Cited by 49 publications

(16 citation statements)

References 48 publications

(46 reference statements)

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“…[1][2][3][4][5] With high thermal stability, the ease of charge delocalization and coordination properties as well as the possibility to attach up to six different substituents on the guanidine moiety has driven research activities in diverse directions, resulting in their use as superbases, [6,7] ligands for coordination complexes, [8][9][10][11] organocatalysts, [12][13][14][15][16] stimuli-responsive materials, [17] hydrogels, [18] anion exchange polymer electrolytes for fuel cells, [19] and biologically active compounds [20][21][22][23][24][25][26][27][28][29] for drug development. Furthermore, guanidinium salts have also entered the field of ionic liquid crystals (ILCs) [30][31][32][33][34][35][36][37][38][39][40][41][42][43][44][45] as an alternative cationic head group to the imidazolium-derived ILCs, which have dominated this research area so far.…”

Section: Introductionmentioning

confidence: 99%

Cyanobiphenyl versus Alkoxybiphenyl: Which Mesogenic Unit Governs the Mesomorphic Properties of Guanidinium Ionic Liquid Crystals?

et al. 2014

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A series of phenylguanidinium salts 3 . X, which are linked via an alkoxy spacer either to a 4-decyloxy-or 4-cyanosubstituted biphenyl mesogen, was prepared and the mesomorphism studied. A decyloxybiphenyl core and a spacer of at least C6 chain length were required for mesophase formation. Replacement of the chloride counterion by other anions like bromide or tetrafluoroborate improved the thermal stability of the mesophase. A comparison of substitution pattern (meta v. para) on the phenyl ring revealed decreased melting and clearing points for the bent cationic head group. All guanidinium ionic liquid crystals 3 displayed only smectic A (SmA) phases. A packing model is assumed where the molecules in a bilayer stack over each other in opposite direction with interdigitated terminal decyloxy groups and spacers.bromides 9 giving the tethered compounds p-10a,b and m-10b-e in 78-88 % yield. N-Boc-deprotection to derivatives p-11a,b and m-11b-e was achieved very cleanly with methane sulfonic acid in CHCl 3 . Derivative m-11a was obtained in 94 % yield by acidic hydrolysis [62] of acetamide m-7a. The aniline derivatives 11 were finally treated with chloro-N,N,N 0 ,N 0tetramethylformamidinium chloride [63] and either NEt 3 or NaHCO 3 . Workup strictly required an inert gas atmosphere to provide the neat target guanidinium salts 3 . Cl in 70-98 % yield. To further study the anion effect, decyloxybiphenyl guanidinium chloride m-3a . Cl was submitted to salt metathesis with NaBr, KI, NaOTf, NaBF 4 , KPF 6 , KOAc, and KSCN yielding the corresponding guanidinium ILCs m-3a . X (Scheme 2). Anion Effect in Guanidinium ILCsComparison of the 1 H NMR spectra of phenylguanidinium salts m-3a . X revealed a significant downfield shift of the N-H signal from ,7.5 to 12.0 ppm in the following order: PF 6 , BF 4 , OTf , I , SCN , Br ECl (Fig. 1).This correlation between the anion and the N-H proton shift indicates the presence of contact between the ion pairs that also led to small chemical shifts of the aromatic protons of the phenylguanidinium moiety (Fig. 1, grey). To estimate the effect of concentration on the N-H signal, 1 H NMR measurements of m-3b . Cl with concentrations varying from 0.83 to 2.50 mg mL À1 were carried out. The N-H signal shift of 0.1 ppm turned out to be negligible with respect to the strong dependence of the NH signal on the anion (see Supplementary Material for details). This effect rather might be due to either the interaction of the anion with the anisotropic cone of the aryl ring than due to the conjugation between the guanidinium cation and the aryl ring. Previous DFT calculations of several guanidinium salts revealed no or only very little conjugation, i.e. the guanidinium moiety behaves like an isolated cation. [39] In agreement with these studies, the N-H signal shifted upfield with increasing anion radii [64,65] with an almost linear correlation (Fig. 2).

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Section: Introductionmentioning

confidence: 99%

Cyanobiphenyl versus Alkoxybiphenyl: Which Mesogenic Unit Governs the Mesomorphic Properties of Guanidinium Ionic Liquid Crystals?

et al. 2014

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“…Hereby both, approaches relying on either asymmetric metal- or organocatalysis, have been well-investigated already [ 105 – 122 ]. In the field of non-covalent asymmetric organocatalysis, chiral H-bonding catalysis [ 37 , 121 – 122 ] and chiral phase-transfer ion-pairing catalysis [ 110 – 119 ] turned out to be extremely powerful.…”

Section: Reviewmentioning

confidence: 99%

“…Alternative approaches by utilizing chiral backbones like, e.g., tartaric acid, biphenyls, or tricyclic ammonium salts were also heavily investigated [ 31 – 35 ], thus leading to an enormous recent progress in the field with respect to catalyst design and the development of new asymmetric applications. Besides quaternary ammonium salts, also chiral phosphonium salts [ 21 , 36 ], chiral (bis)guanidinium systems [ 22 , 27 , 37 ], chiral crown ethers [ 38 – 39 ], bifunctional onium salts [ 17 , 40 – 46 ], or even sulfonium salts [ 47 – 48 ] have been systematically developed very recently. Thus, it can be said without exaggeration that chiral phase-transfer or ion-pairing catalysis has become one of the major catalytic principles, which allows for a broad variety of powerful asymmetric applications that are only very difficult to achieve with other catalytic principles.…”

Section: Introductionmentioning

confidence: 99%

Chiral phase-transfer catalysis in the asymmetric α-heterofunctionalization of prochiral nucleophiles

Schörgenhumer¹,

Tiffner²,

Waser³

2017

Beilstein J. Org. Chem.

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Chiral phase-transfer catalysis is one of the major catalytic principles in asymmetric catalysis. A broad variety of different catalysts and their use for challenging applications have been reported over the last decades. Besides asymmetric C–C bond forming reactions the use of chiral phase-transfer catalysts for enantioselective α-heterofunctionalization reactions of prochiral nucleophiles became one of the most important field of application of this catalytic principle. Based on several highly spectacular recent reports, we thus wish to discuss some of the most important achievements in this field within the context of this review.

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“…Thus, stereoselective construction of this tert-hydoxyl group is an important issue in the synthesis of CPTs. Recently, we have reported α-hydroxylation of tetralone-derived β-ketoesters 4 using cumene hydroperoxide (CHP) in the presence of guanidine-bisurea bifunctional organocatalyst 6 (Scheme 1a) [13]. In this reaction, the guanidine group and urea group were suggested to interact with the enolate of tetralone and the oxidant CHP, respectively.…”

Section: Open Accessmentioning

confidence: 99%

Development of Guanidine-Bisurea Bifunctional Organocatalyst Bearing Chirality at the Inner and Outer Sides of the Urea Groups, and Application to Enantioselective α-Hydroxylation of Pyranoindolizine Intermediate for Camptothecin Synthesis

Odagi¹,

Watanabe²,

Nagasawa³

2015

Symmetry

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Pyranoindolizine is a tricyclic structure found in various biologically active compounds, such as camptothecin (CPT) and its derivatives. In the case of CPTs, the chirality at the α-position in the α-hydroxyl lactone moiety of pyranoindolizine is important for the antitumor activity. This paper deals with enantioselective oxidation of the α-position in pyranoindolizine lactone, which corresponds at C20 in CPT, with cumene hydroperoxide (CHP) in the presence of newly synthesized guanidine-bisurea bifunctional organocatalysts bearing chirality on both the inner and outer sides of the urea groups.

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Asymmetric α‐Hydroxylation of Tetralone‐Derived β‐Ketoesters by Using a Guanidine–Urea Bifunctional Organocatalyst in the Presence of Cumene Hydroperoxide

Cited by 49 publications

References 48 publications

Cyanobiphenyl versus Alkoxybiphenyl: Which Mesogenic Unit Governs the Mesomorphic Properties of Guanidinium Ionic Liquid Crystals?

Cyanobiphenyl versus Alkoxybiphenyl: Which Mesogenic Unit Governs the Mesomorphic Properties of Guanidinium Ionic Liquid Crystals?

Chiral phase-transfer catalysis in the asymmetric α-heterofunctionalization of prochiral nucleophiles

Development of Guanidine-Bisurea Bifunctional Organocatalyst Bearing Chirality at the Inner and Outer Sides of the Urea Groups, and Application to Enantioselective α-Hydroxylation of Pyranoindolizine Intermediate for Camptothecin Synthesis

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