Catalytic asymmetric synthesis of 2,5-dihydrofurans using synergistic bifunctional Ag catalysis

Abstract

We report a bifunctional Ag catalyst promoted intramolecular capture of oxonium ylides with alkynes for the enantioselective synthesis of 2,5-dihydrofurans.

Aluminium‐Catalyzed C(sp)−H Borylation of Alkynes

“…The reaction is second order in the pre-catalyst 1 b and first order in the alkyne substrate. [19] These measurements indicate that the rate-limiting step is deprotonation of the alkyne C À H and that this is effected a dimeric aluminium species, for example, [1 a] 2 (see SI 9.5). The small k H /k D (CÀH) value, at 1.4, is typical for asynchronous deprotonation during the rate-limiting step.…”

“…Using 19 F NMR spectroscopy, we monitored the reaction of 4-fluorophenylacetylene 2 k with HBpin in the presence of 10 mol % pre-catalyst 1 b at 80 8C (Scheme 2). In the initial stages of the reaction, alkyne consumption and product formation were rapid.…”

“…We thus examined in turn the stoichiometric reactivity of the dimethylalane pre-catalyst 1 b with alkyne and HBpin. Yields determined by 19 F NMR spectroscopy using fluorobenzene as an internal standard 1,5-COD = 1,5-cyclooctadiene.…”

Kinetic Resolution of Propargylic Ethers via [2,3]-Wittig Rearrangement to Synthesize Chiral α-Hydroxyallenes

“…Recently, our group described an asymmetric propargyl [2,3]-Wittig rearrangement of oxindole derivatives using a chiral N , N ′-dioxide/Ni II complex, and the corresponding 3-hydroxy 3-substituted oxindoles bearing an allenyl group were obtained in good results (Scheme a) . Moreover, chiral dihydrofuran derivatives represent important structural motifs existing in natural products, such as diplobifuranylone B, (+)-furanomycin, and cryptoresinol (Scheme c), and intramolecular hydroalkoxylation of chiral α-hydroxyallenes is usually one of the most powerful strategies to construct these scaffolds. In consequence, development of new and efficient catalytic asymmetric approaches for their rapid access is in high demand.…”

Brønsted Acid Catalyzed Enantioselective Assembly of Spirochroman-3,3-oxindoles

“…For instance, 3a upon NaBH 4 reduction followed by treatment with PTSA under reflux conditions afforded the olefin 6a as single geometrical isomer (E/Z > 95:5), 16 whereas the carbonyl reduction and cyclization in the presence of FeCl 3 selectively furnished a pentacyclic product 7a in fair yield with 94% ee. 17 The oxidation 18 of 3a offered access to lactone derivative 8a in 86% yield and with retention of the enantioselectivity (Scheme 5).…”