An Iron(III) Catalyst with Unusually Broad Substrate Scope in Regioselective Alkylation of Diols and Polyols
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Abstract In this study, [Fe(dibm) 3 ] (dibm=diisobutyrylmethane) is shown to have unusually broad scope as a catalyst for the selective monoalkylation of a diverse set of 1,2‐ and 1,3‐diol‐containing structures. The mechanism is proposed to proceed via a cyclic dioxolane‐type intermediate, formed between the iron(III) species and two adjacent hydroxyl groups. This approach represents the first transition‐metal catalysts that are able to replace stoichiometric amounts of organotin reagents in regioselective alkylation. The reactions generally lead to very high regioselectivities and high yields, on par with, or better than, previous methods used for regioselective alkylation.β-Aminoenones react with monoalkyl hydrazines to give regioselectively 1,3,5-trisubstituted pyrazoles. The mechanism and level of regioselectivity depend on both the substitution pattern of the substrates and the reaction conditions. When the least bulky substituent is attached at the β-position of the enone, a high regioselectivity is obtained, usually higher than that from equivalent β-diketones. If the β-substituent is the bulkiest group, the regioselectivity decreases. Nevertheless, in the conditions reported in this paper, it is possible to prepare pyrazoles not obtainable from β-diketones.
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Abstract The alkynes (IIa) or (IIb) react almost regioselectively or with poor regioselectivity with the pyrones (I), (V) and (VIII), while the reaction of both with the pyrone (XI) is regioselective.
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Pyrazole is the ubiquitous sub-structure of many natural products and biologically active compounds. In this respect, its regioselective synthesis drew a lot of attention from synthetic organic chemists, leading to the development of various regioselective modifications of the classical Knorr cyclocondensation reaction and alternative methods. This review covers the period of 2003 to the middle of 2009 on the regioselective synthesis of pyrazoles, which are categorized into four reaction types: (i) modified Knorr condensation reaction using 1,3-dicarbonyl surrogates; (ii) 1,3-dipolar cycloadditions approach; (iii) regioselective direct substitution reaction of the pyrazole ring system; and (iv) other methods. Keywords: Pyrazole, regioselective dipolar cycloaddition, regioselective condensation, regioselective cross-coupling
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Abstract A perfectly regioselective and sequential method for the preparation of orthogonally protected glucopyranosides has been developed. An acyl group was introduced at C(4)‐OH by organocatalysis with >99 % regioselectivity. TBDPS, Boc, and BOM groups were sequentially introduced into the 4‐ O ‐acyl‐glucopyranoside at C(6)‐OH, C(2)‐OH, and C(3)‐OH, respectively, with ca. 100 % regioselectivity in each step.
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The regioselectivity of the Suzuki couplings of several 4,5- and 3,4-dibromopyrrole-2-carboxylate esters has been studied. In general, regioselectivity can be achieved for initial coupling at the more electron-deficient site (C5 and C3, respectively). At the same time, conversions are often modest (40-60%) and attempts to force the reactions to higher conversions often lead to competitive dicoupling.
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To gain insights into 2-indolylmethanol-involved reactions and to understand the origins of regioselectivity and enantioselectivity, theoretical investigations on the reaction mechanisms of three representative cycloadditions of 2-indolylmethanols have been carried out. In Ir-catalyzed regioselective (3 + 3) cycloaddition, it was found that the great difference between the energy barriers of the first initiating steps of the two pathways played a key role in determining the observed high regioselectivity and the C3-nucleophilicity of 2-indolylmethanol in this reaction. In chiral phosphoric acid (CPA)-catalyzed regioselective and enantioselective (3 + 3) and (3 + 4) cycloadditions, it was discovered that the great difference between the energy barriers of the transition states corresponding to the (R)- and (S)-configurations led to the observed high enantioselectivity of the products. In the presence of CPA, the C3-nucleophilicity of 2-indolylmethanol increased, resulting in exclusive regioselectivity. It was discovered that the electronic nature is not a decisive factor for the observed C3-regioselectivity in the delocalized cation of 2-indolylmethanol, and the steric factor should play a crucial role in the observed C3-regioselectivity. This study offers insights into the mechanisms of 2-indolylmethanol-involved reactions, which will give an in-depth understanding of the chemistry of 2-indolylmethanols and advance the development of this research field.
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