Cyclialkylation of Arylalkyl Epoxides with Solid Acid Catalysts

A recombinant Escherichia coli expressing P450 pyr monooxygenase of Sphingomonas sp. HXN-200 was developed as a useful biocatalyst for regioand stereoselective hydroxylations, with no side reaction and easy cell growth. The resting E. coli cells showed an activity of 4.1 U/g cdw and 9.9 U/g cdw for the hydroxylation of N-benzylpyrrolidin-2-one 1 and N-benzyloxycarbonylpyrrolidine 3, respectively, being as active as the wide-type strain. Biohydroxylation of N-benzylpyrrolidin-2-one 1 with the resting cells gave (S)-N-benzyl-4-hydroxypyrrolidin-2-one 2 in > 99% ee and 10.8 mM, a 2.6 times increase of product concentration in comparison with the wildtype strain. Biohydroxylation of N-tert-butoxycarbonylpiperidin-2-one 5, N-benzylpiperidine 7 and Ntert-butoxycarbonylazetidine 9 with the E. coli cells afforded the corresponding 4-hydroxypiperidin-2-one 6, 4-hydroxypiperidine 8, and 3-hydroxyazetidine 10 in 14 mM, 17 mM, and 21 mM, respectively. Moreover, hydroxylation of (À)-b-pinene 11 with the recombinant E. coli cells showed excellent regio-and stereoselectivity and gave (1R)-trans-pinocarveol 12 in 82% yield and 4.1 mM, which is over 200 times higher than that obtained with the best biocatalytic system known thus far. The recombinant strain was also able to hydroxylate other types of substrates with unique selectivity: biohydroxylation of norbornane 13 gave exo-norbornaeol 14, with exo/endo selectivity of 95%; tetralin 15 and 6-methoxytetralin 17 were hydroxylated at the non-activated 2-position, for the first time, with regioselectivities of 83-84%.

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Regio‐ and Stereoselective Biohydroxylations with a Recombinant Escherichia coli Expressing P450_pyr Monooxygenase of Sphingomonas Sp. HXN‐200

Zhang

Tang

Wang

et al. 2010

Adv Synth Catal

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S_N1‐Type Substitution Reactions of N‐Protected β‐Hydroxytyrosine Esters: Stereoselective Synthesis of β‐Aryl and β‐Alkyltyrosines

Wilcke

Herdtweck

Bach

2012

Chemistry An Asian Journal

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The title compounds were prepared by aldol reaction of anisaldehyde and the respective N,N-dibenzyl glycinates. Deprotection of the nitrogen atom with Pearlman's catalyst delivered the unprotected β-hydroxytyrosine esters, which were further N-protected as N,N-phthaloyl (Phth) and N-fluorenylmethylcarbonyloxy (Fmoc) derivatives. The Friedel-Crafts reaction with various arenes was studied employing these alcohols as electrophiles. It turned out that the facial diastereoselectivitiy depends on the nitrogen protecting group and on the ester group. The unprotected substrates (NH(2)) gave preferentially syn-products but the anti-selectivity increased when going from NHFmoc over NPhth to NBn(2). If the ester substituent was varied the syn-preference increased in the order Me < Et < iPr. The reactions were shown to be fully stereoconvergent and proceeded under kinetic product control. A model is suggested to explain the facial diastereoselectivity based on a conformationally locked benzylic cation intermediate. The reactions are preparatively useful for the N-unprotected isopropyl ester, which gave Friedel-Crafts alkylation products with good syn-selectivity (anti/syn=21:79 to 7:93), and for the N,N-dibenzyl-protected methyl ester, which led preferentially to anti-products (anti/syn=80:20 to >95:5). Upon acetylation of the latter compound to the respective acetate, Bi(OTf)(3) -catalyzed alkylation reactions became possible, in which silyl enol ethers served as nucleophiles. The respective alkylation products were obtained in high yield and with excellent anti-selectivitiy (anti/syn≥95:5).

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Intramolecular Photoarylation of Alkenes by Phenyl Cations

Dichiarante¹,

Fagnoni²,

Mella³

et al. 2006

Chemistry A European J

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Acetone-sensitized irradiation of various o-chlorophenyl allyl ethers in polar solvents led to either (dihydro)benzofurans or chromanes. The reaction appeared to involve photoheterolysis of the aryl-Cl bond followed by phenyl cation addition onto the tethered double bond either in 5-exo or 6-endo modes. The adduct cation gave the end products by deprotonation; addition of chloride anion or of the solvent, depending on the structure; and the conditions used. Preference for the 5-exo mode increased in passing from medium polarity (methylene chloride, ethyl acetate) to high polarity solvents (aqueous acetonitrile, methanol, 2,2,2-trifluoroethanol), for which this was often the exclusive path. The same compounds underwent photohomolysis when irradiated in cyclohexane, and radical cyclization was one of the process occurring. Substitution of a methylene group for the ether oxygen atom made 6-endo cyclization by far the main path in a related o-chlorophenylbutene. Again, the selectivity was higher in polar protic solvents. The results are discussed in terms of in cage ion pair versus free phenyl cation reactions.

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Cyclialkylation of Arylalkyl Epoxides with Solid Acid Catalysts

Cited by 16 publications

References 18 publications

Regio‐ and Stereoselective Biohydroxylations with a Recombinant Escherichia coli Expressing P450_pyr Monooxygenase of Sphingomonas Sp. HXN‐200

Regio‐ and Stereoselective Biohydroxylations with a Recombinant Escherichia coli Expressing P450_pyr Monooxygenase of Sphingomonas Sp. HXN‐200

S_N1‐Type Substitution Reactions of N‐Protected β‐Hydroxytyrosine Esters: Stereoselective Synthesis of β‐Aryl and β‐Alkyltyrosines

Intramolecular Photoarylation of Alkenes by Phenyl Cations

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Cyclialkylation of Arylalkyl Epoxides with Solid Acid Catalysts

Cited by 16 publications

References 18 publications

Regio‐ and Stereoselective Biohydroxylations with a Recombinant Escherichia coli Expressing P450pyr Monooxygenase of Sphingomonas Sp. HXN‐200

Regio‐ and Stereoselective Biohydroxylations with a Recombinant Escherichia coli Expressing P450pyr Monooxygenase of Sphingomonas Sp. HXN‐200

SN1‐Type Substitution Reactions of N‐Protected β‐Hydroxytyrosine Esters: Stereoselective Synthesis of β‐Aryl and β‐Alkyltyrosines

Intramolecular Photoarylation of Alkenes by Phenyl Cations

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Product

Resources

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Regio‐ and Stereoselective Biohydroxylations with a Recombinant Escherichia coli Expressing P450_pyr Monooxygenase of Sphingomonas Sp. HXN‐200

Regio‐ and Stereoselective Biohydroxylations with a Recombinant Escherichia coli Expressing P450_pyr Monooxygenase of Sphingomonas Sp. HXN‐200

S_N1‐Type Substitution Reactions of N‐Protected β‐Hydroxytyrosine Esters: Stereoselective Synthesis of β‐Aryl and β‐Alkyltyrosines