The first ring-contraction monofluorination reaction mediated by a hypervalent iodine reagent is reported, and the use of the reaction for the synthesis of monofluorinated five-membered ring-fused oxazolines is described. By means of this reaction, a fluorine atom can be selectively introduced either on the five-membered ring or external to it, depending on whether or not the substrate has C-4 alkyl substituents. The reaction was used for the further conversion of probenecid and isoxepac.
In recent years, an explosive growth of reactivities of hypervalent iodine reagents has witnessed and these reagents, featuring facile availability and easy handleness, offer multiple advantages over establised methods as an efficient multipurpose oxidants, whose reactivities are similar to derivatives of mercury, chromium, lead, thallium and other heavy metals, but without the toxicity and environmental problems of these heavy metal agents. Thus, hypervalent iodine reagents have received much more attention from the synthetic chemists. This account mainly summarizes our recent research progress in the area of hypervalent iodine chemistry, especially focusing on the excellent performances and unique applications brought about by the new hypervalent iodine reagents and new combinations of them both developed in our group: (1) first utilization of a recyclable iodine(III) reagent iodosodilactone for the direct esterification, amidation and peptide synthesis and high efficient liquid-phase oligo-peptide synthesis mediated by a more powerful iodosodilactone-type reagent, 6-(3,5-bis-(trifluoromethyl)phenyl)-1H,4H-2aλ 3 -ioda-2,3-dioxacyclopenta[hi]indene-1,4-dione (FPID); (2)
We have developed an efficient method for direct formation of epoxide groups from carbon(sp)-carbon(sp) single bonds of β-keto esters; the reaction is mediated by the water-soluble hypervalent iodine(V) reagent AIBX (5-trimethylammonio-1,3-dioxo-1,3-dihydro-1λ-benzo[d][1,2]iodoxol-1-ol anion). On the basis of the results of density functional theory calculations and experimental studies, we propose that the reaction proceeds by a two-stage mechanism involving dehydrogenation of the β-keto ester substrates and epoxidation of the resulting enone intermediates. The rate-limiting step is abstraction of the β'-C-H (calculated free energy of activation, 24.5 kcal/mol).
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