Reduction of o-acylphenols through ethyl o-acylphenylcarbonates to o-alkylphenols with sodium borohydride.

Minami, Norio; Kijima, Shizumasa

doi:10.1248/cpb.27.1490

Cited by 21 publications

(8 citation statements)

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“…Alternatively, when 11 was subjected to treatment with ethyl chloroformate in the presence of Et 3 N in THF at 0 °C, the corresponding ethyl carbamate intermediate was formed and reacted with NaBH 4 in aqueous media after removal of the salts. After workup and purification, 12 was furnished in 66% yield [ 40 ].…”

Section: Resultsmentioning

confidence: 99%

Enantioselctive Syntheses of Sulfur Analogues of Flavan-3-Ols

Sharma

Jurayj

et al. 2010

Molecules

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The first enantioselective syntheses of sulfur flavan-3-ol analogues 1–8 have been accomplished, whereby the oxygen atom of the pyran ring has been replaced by a sulfur atom. The key steps were: (a) Pd(0) catalyzed introduction of –S t-butyl group, (b) Sharpless enantioselective dihydroxylation of the alkene, (c) acid catalyzed ring closure to produce the thiopyran ring, and (d) removal of benzyl groups using N,N-dimethylaniline and AlCl3. The compounds were isolated in high chemical and optical purity.

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Section: Resultsmentioning

confidence: 99%

Enantioselctive Syntheses of Sulfur Analogues of Flavan-3-Ols

Sharma

Jurayj

et al. 2010

Molecules

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“…The preparation procedure and characteristics for the most ortho ‐, meta ‐, and para ‐substituted phenyl tosylates (X‐phenyl 4‐methylbenzenesulfonates), 4‐CH 3 ‐C 6 H 4 SO 2 OC 6 H 4 ‐X, have been previously described . The detailed preparation procedure and characteristics of synthesized substituted phenyl tosylates, 4‐CH 3 ‐C 6 H 4 SO 2 OC 6 H 4 ‐X (X=2‐Br, 3‐N(CH 3 ) 2 , 4‐CF 3 , 2‐COCH 3 , 2‐CO 2 CH 3 , 2,6‐(NO 2 ) 2 , 2‐CH(CH 3 ) 2 , 2‐C(CH 3 ) 3 , 2‐CF 3 , 2‐COC 6 H 5 ) and phenyl 2,4‐dinitrobenzenesulfonate are available in the Supplementary Material.…”

Section: Methodsmentioning

confidence: 99%

¹⁷O NMR studies of ortho‐substituent effects in substituted phenyl tosylates

Nummert

Mäemets

Piirsalu

et al. 2012

Magnetic Reson in Chemistry

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(17)O NMR spectra for 35 ortho-, para-, and meta-substituted phenyl tosylates (phenyl 4-methylbenzenesulfonates), 4-CH(3)-C(6)H(4) SO(2)OC(6)H(4)-X, at natural abundance in acetonitrile at 50 °C were recorded. The (17)O NMR chemical shifts, δ((17)O), of the sulfonyl (SO(2)) and the single-bonded phenoxy (OPh) oxygens for para and meta derivatives correlated well with dual substituent parameter treatment using the Taft inductive, σ(I), and resonance, σº(R), constants. The influence of ortho substituents on the sulfonyl oxygen and the single-bonded phenoxy oxygen chemical shifts, δ((17)O), was found to be nicely described by the Charton equation: δ((17)O)(ortho) = δ((17)O)(H) + ρ(I)σ(I) + ρ(R)σ°(R) + δE(s)(B) when the data treatment was performed separately for electron-donating +R substituents and electron-attracting -R substituents. Electron-attracting meta and para substituents in the phenyl moiety caused deshielding while the electron-donating meta, para and ortho +R substituents produce shielding effects on the sulfonyl (SO(2)) and single-bonded phenoxy (OPh) oxygens. The influence of ortho inductive and resonance effects in the case of +R substituents was found to be approximately twice higher than the corresponding influence from the para position. Due to the steric effect of ortho substituents a decrease in shielding of the oxygens at the sulfonyl group (δE(s)(B) > 0, E(s)(B) < 0) was detected.

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“…Deoxygenation of the ketones 6a-d to the olefins 7a-d was performed by employing ethyl chloroformate and sodium borohydride in a two-step sequence. 12,17 This procedure is shorter than the two-step hydrogenation-elimination procedure used by van Rensburg et al 15 However, the flavenes 8b-d and, in the case of 6d, the saturated phenol 9d, were formed as side products in this reduction. To improve the yield of the deoxygenation reaction, the temperature dependency of the reduction was studied carefully.…”

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confidence: 94%

“…15 However, not unexpectedly, acidic treatment resulted in formation of the racemic flavan-3-ols. Next, the standard S N 2-type Mitsunobu reaction conditions 17 were employed in an attempt to convert the triols 12a-d in one step into the enantiomerically pure flavan-3-ols 13a-d. In the first experiment, the flavanol 13b was not only formed in excellent yield (77%) and in the expected 2,3-trans-configuration, but remarkably, no five-membered cyclization products were observed.…”

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confidence: 99%

Enantioselective Synthesis of Flavan-3-ols Using a Mitsunobu Cyclization

Krohn¹,

Ahmed²,

John³

2009

Synthesis

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The synthesis of four flavan-3-ols with different substitution patterns and electron densities has been achieved in high stereo-and regioselectivity by a one-step Mitsunobu reaction from the corresponding diols, which were prepared by enantioselective Sharpless dihydroxylation of suitable olefins. The six-membered flavan-3-ols were the only cyclization products and the theoretically possible formation of five-membered rings during the Mitsunobu cyclization was not observed. The flavanols are important starting materials for the synthesis of dimers such as the procyanidins or other coupling products such as the flavan part of the potent DNA polymerase b inhibitor myristinin A. The enantioselectivities of both the Sharpless dihydroxylation and the Mitsunobu cyclization steps were monitored by chiral HPLC.Polyphenols are ingredients of agricultural products such as wine, tea, fruit, vegetables and herbal medicines, and play an important role due to their multitude of biological functions. 1 The specific interactions of polyphenols with biomolecules such as proteins have stimulated the search for new pharmaceutical entities derived from polyphenolic compounds. 2 Among them, flavanoids are considered to be particularly important secondary metabolites due to their antibacterial, 3 anticancer, 4 antioxidant 5 and antiviral 6 properties. Flavan-3-ols such as (+)-catechin (1) or their dimers such as procyanidin B3 (2; Figure 1) occur in fresh fruits, tea, cocoa and chocolate, 7 and have been shown to have beneficial effects on health. 8 Other kinds of bioactivity were recently discovered in the naturally occurring flavanoid myristinin A (3; Figure 1), which is a DNA polymerase b inhibitor capable of blocking the repair of DNA damage induced by clinically used damaging agents and thereby potentiates their cytotoxicity. [9][10][11][12][13] The key step in many syntheses of flavan-3-ols such as 13 is the Sharpless dihydroxylation 14 of diaryl propenes 10 to the diols 11. 12,15 In connection with an improvement of the myristinin A synthesis, 12 we now report on a new and shorter variation of the flavan-3-ol synthesis, using a Mitsunobu reaction for the one-step cyclization of phenolic diol precursors 12, as outlined in Scheme 1. In order to probe the generality of this one-step Mitsunobu cyclization, we conducted the reaction with four olefins 10a-d with different substitution patterns and electron densities, ranging from one to five oxygen substituents (Scheme 1). The flavan-3-ols thus prepared generally serve as precursors in procyanidin synthesis. 16 We started with the formation of chalcones 6a-d by Claisen-Schmidt condensation of 2-hydroxyacetophenones 4a-d with benzaldehydes 5a-d to afford chalcones 6a-d in nearly quantitative yields (Scheme 1). Deoxygenation of the ketones 6a-d to the olefins 7a-d was performed by employing ethyl chloroformate and sodium borohydride in a two-step sequence. 12,17 This procedure is shorter than the two-step hydrogenation-elimination procedure used by van Rensburg et al. 15 However, the ...

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Reduction of o-acylphenols through ethyl o-acylphenylcarbonates to o-alkylphenols with sodium borohydride.

Cited by 21 publications

References 0 publications

Enantioselctive Syntheses of Sulfur Analogues of Flavan-3-Ols

Enantioselctive Syntheses of Sulfur Analogues of Flavan-3-Ols

¹⁷O NMR studies of ortho‐substituent effects in substituted phenyl tosylates

Enantioselective Synthesis of Flavan-3-ols Using a Mitsunobu Cyclization

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Reduction of o-acylphenols through ethyl o-acylphenylcarbonates to o-alkylphenols with sodium borohydride.

Cited by 21 publications

References 0 publications

Enantioselctive Syntheses of Sulfur Analogues of Flavan-3-Ols

Enantioselctive Syntheses of Sulfur Analogues of Flavan-3-Ols

17O NMR studies of ortho‐substituent effects in substituted phenyl tosylates

Enantioselective Synthesis of Flavan-3-ols Using a Mitsunobu Cyclization

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¹⁷O NMR studies of ortho‐substituent effects in substituted phenyl tosylates