RESUMO
An efficient, visible light Ru(bpy)3Cl2-catalyzed method for the preparation of 2,3-dihydrofurans is reported. This approach employs 2-bromoketoesters as radical precursors and alkyl enol ethers as acceptors. The photoredox cycle furnishes an oxonium ion that is captured by an internal nucleophile to render the corresponding dihydrofurans. Moreover, the obtained products contain a versatile acetal moiety at C-2, allowing its transformation into a diverse variety of heteroaromatic and nonaromatic compounds. This method could serve as an important tool in the synthesis of complex tetrahydro- and dihydrofurans as well as heteroaromatic structures.
Assuntos
Luz , Processos Fotoquímicos , Catálise , Estrutura MolecularRESUMO
A stereoselective total synthesis of natural product aspergillide A is reported. The adopted strategy relies on the direct access to the key tetrahydropyran core through a visible light-mediated photoredox reaction from an allylic alcohol and iodoacetic acid. In a single manipulation, a γ-iodo-δ-valerolactone is obtained through an atom transfer radical addition followed by in situ acid-catalyzed lactonization. The obtained lactone possesses three functionalized sites, which were seized to link the required substituents in the final product and thus completing the total synthesis of aspergillide A.
Assuntos
Técnicas de Química Sintética/métodos , Macrolídeos/síntese química , Piranos/química , Ciclização , Luz , Macrolídeos/química , Estrutura Molecular , Oxirredução , EstereoisomerismoRESUMO
A Ru2+-photocatalyzed, visible-light-mediated ATRA reaction for the straightforward preparation of 1,4-ketoaldehydes, 1,4-diketones, and 1,4-ketoesters, which are of difficult access by other means, is reported herein. This method employs readily accessible α-bromoketones and alkyl vinyl ethers as starting materials, allowing the construction of secondary, tertiary, and challenging quaternary centers. In addition, the synthetic usefulness of this method is illustrated by applying it to the construction of substituted pyrroles.
RESUMO
Stereoselectivity in the C-glycosidation of lactones derived from D-fucose by following Kishi's method, which involves the addition of a nucleophile onto a carbohydrate-derived lactone and subsequent reduction of the lactol, was found to be reliant on the nature of the C2 and C3 protective groups. Lactones bearing TBDMS protecting groups selectively afford 1,3-trans products (α anomer), in which the stereoselective outcome is in apparent concordance with Woerpel's model. On the other hand, their benzylated congeners produce the 1,3-cis products (ß anomer) as the major diastereoisomers. The latter results suggest an abnormal behavior during the stereoselective nucleophilic substitution at the anomeric position of the benzylated lactones.