- Diazaphosphinyl radical-catalyzed deoxygenation of α-carboxy ketones: A new protocol for chemo-selective C-O bond scission: Via mechanism regulation
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C-O bond cleavage is often a key process in defunctionalization of organic compounds as well as in degradation of natural polymers. However, it seldom occurs regioselectively for different types of C-O bonds under metal-free mild conditions. Here we report a facile chemo-selective cleavage of the α-C-O bonds in α-carboxy ketones by commercially available pinacolborane under the catalysis of diazaphosphinane based on a mechanism switch strategy. This new reaction features high efficiency, low cost and good group-tolerance, and is also amenable to catalytic deprotection of desyl-protected carboxylic acids and amino acids. Mechanistic studies indicated an electron-transfer-initiated radical process, underlining two crucial steps: (1) the initiator azodiisobutyronitrile switches originally hydridic reduction to kinetically more accessible electron reduction; and (2) the catalytic phosphorus species upconverts weakly reducing pinacolborane into strongly reducing diazaphosphinane. This journal is
- Cheng, Jin-Pei,Yang, Jin-Dong,Zhang, Jingjing
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p. 8476 - 8481
(2020/09/07)
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- Desyl and phenacyl as versatile, photocatalytically cleavable protecting groups: A classic approach in a different (visible) light
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A highly efficient, catalytic strategy for the deprotection of classical phenacyl (Pac) as well as desyl (Dsy) protection groups has been developed using visible light photoredox catalysis. The deliberate use of a neutral two-phase acetonitrile/water mixture with K3PO4 applying catalytic amounts of [Ru(bpy)3](PF6)2 in combination with ascorbic acid is the key to this truly catalytic deprotection of Pac- and Dsy-protected carboxylic acids. Our mild yet robust protocol allows for fast and selective liberation of the free carboxylic acids in very good to quantitative yields, while only low catalyst loadings (1 mol %) are required. Both Pac and Dsy, easily introduced from commercially available precursors, can be applied for the direct protection of carboxylic acids and amino acids, offering orthogonality to a great variety of other common protecting groups. We further demonstrate the general applicability and versatility of these formerly underrated protecting groups in combination with our catalytic cleavage conditions, as underscored by the gained high functional group tolerance. Moreover, this method could successfully be adapted to the requirements of solidphase synthesis. As a proof of principle for an efficient visible light, photocatalytic linker cleavage, a Boc-protected tripeptide was split off from commercially available brominated Wang resin.
- Speckmeier, Elisabeth,Zeitler, Kirsten
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p. 6821 - 6826
(2017/11/06)
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- Ready protease-catalyzed synthesis of carbohydrate-amino acid conjugates.
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The protease-catalyzed synthesis of amino acid est-carbohydrate conjugates as glycopeptide analogues has been achieved in a highly regioselective and carbohydrate-specific manner using amino acid vinyl ester acyl donors and minimally or completely unprotected carbohydrate acyl acceptors, which together probed active sites of proteases to reveal yield efficiencies that are modulated by the carbohydrate C-2 substitutent, and that may be exploited to allow selective one-pot syntheses.
- Boyer,Stanchev,Fairbanks,Davis
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p. 1908 - 1909
(2007/10/03)
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- Preferential hydrogenolysis of NAP esters provides a new orthogonal protecting group strategy for carboxylic acids
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Selective hydrogenolysis of 2-naphthylmethyl (NAP) esters in the presence of a benzyl ester has been observed with a wide range of dicarboxylic acids. Orthogonal deprotection of NAP esters with challenging substrates can be achieved if the other carboxylic acids in the molecule are protected with 4-trifluoromethyl benzyl group instead of benzyl groups.
- Gaunt, Matthew J.,Boschetti, Carlos E.,Yu, Jinquan,Spencer, Jonathan B.
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p. 1803 - 1806
(2007/10/03)
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- A study of the delivery-targeting concept applied to antineoplastic drugs active on human osteosarcoma. I. Synthesis and biological activity in nude mice carrying human osteosarcoma xenografts of gem-bisphosphonic methotrexate analogues
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With the aim of verifying the concept of osteotic vectorisation, synthesis of three methotrexate (MTX) gem-diphosphonic analogues (compounds A, B and C) was performed. These molecules were tested on BALB/c and NIH III mice previously grafted with subcutaneous implants of OHS, TTX p7 and/or TTX p11 human osteosarcoma cell lines. Antineoplasic activity of compound B and C (active compounds) was compared to the activity for MTX alone and to activity of compound A (inactive compound). Compounds B and C exhibited an increased antineoplasic activity compared to MTX alone and to compound A. At equimolar doses, compound B was found to be 5-6-fold more active than MTX given alone. We have discussed the concept of osteotic vectorisation of compound B, which could be regarded as a prodrug.
- Sturtz,Appere,Breistol,Fodstad,Schwartsmann,Hendriks
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p. 825 - 833
(2007/10/02)
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- SYNTHESIS OF AMINO ACID ESTERS BY PAPAIN
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A wide range of N-Boc-amino acid esters were synthesized from N-Boc-amino acids and alcohol using papain as catalyst.Suitable biphasic reaction mixtures were found for most amino acids to achieve high yield of ester synthesis.With N-Boc-L-aspartic and glutamic acids only the α carbonyl group esterified, without racemisation.
- Cantacuzene, D.,Pascal, F.,Guerreiro, C.
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p. 1823 - 1826
(2007/10/02)
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