A novel, manganese-catalyzed oxidative azidation of cyclobutanols is described. A wide range of primary, secondary, and tertiary alkyl azides were generated in synthetically useful yields and exclusive regioselectivity. Aside from linear alkyl azides, otherwise elusive medium-sized cyclic azides were also readily prepared. Preliminary mechanistic studies reveal that the reaction likely proceeds by a radical-mediated C-C bond cleavage/C-N3 bond formation pathway.
A conceptually new, efficient, and metal-free approach for the challenging azidocyanation of unactivated alkenes is presented. The strategy of intramolecular distal cyano migration is combined with alkene difunctionalization for the first time. A variety of useful azido-substituted alkyl nitriles are prepared in good yields and, most importantly, with exquisite regio- and stereo-selectivities.
A novel C-C bond-forming strategy employing manganese-catalyzed ring-opening of cyclobutanol substrates, followed by cyanation or ethynylation, is described. A cyano C1 unit and ethynyl C2 unit are regiospecifically introduced to the γ-position of ketones at room temperature, providing a mild yet powerful method for production of elusive aliphatic nitriles and alkynes. All transformations described are based on a common sequence: 1) oxidative ring-opening of cyclobutanol substrates by C-C bond cleavage; 2) radical addition to triple bonds bearing an arylsulfonyl group; and 3) radical-mediated C-S bond cleavage.
The asymmetric total synthesis of lagunamide A (3.0%, 20 steps longest linear sequence) and its five analogues, including the structure dehydrated at the C37 position, are detailed in this report. The key feature in this diverse synthesis includes the elaboration of four consecutive chiral centers at C37-40 and the final macrocyclization. Starting from chiral aldehyde 10, we synthesized both 1,3-anti and 1,3-syn homoallylic alcohols 20a and 20b through asymmetric aldol condensation and stereoselective allylation. The following esterification to introduce the L-N-Me-Ala unit resulted in significant epimerization. This problem was finally overcome by coupling the alcohols with the corresponding acid chloride of the L-alanine derivative. The key α,β-unsaturated carboxylic acid unit was produced by cross-metathesis (CM) of methacrylaldehyde and related olefins. Interestingly, we found that the C7 configuration dramatically affected the ring closure. Natural lagunamide A (1a), its 39-epimer (1c), and its 2-epimer (1d) were obtained through macrolactamization between alanine and isoleucine moieties.
A mild and rapid procedure to the synthesis of 2-substituted pyrimidines was developed via sequential functionalization of easily available Biginelli 3,4-dihydropyrimidine-2(1H)-ones via oxidation, esterification, followed by cross-coupling reaction of pyrimidin-2-yl sulfonates with N, S, and O nucleophiles in PEG-400 as a green reaction medium at room temperature.
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