Iridium‐Catalyzed Asymmetric Allylic Etherification and Ring‐Closing Metathesis Reaction for Enantioselective Synthesis of Chromene and 2,5‐Dihydrobenzo[b]oxepine Derivatives

Abstract

Iridium‐catalyzed asymmetric etherifications of allylic carbonates with 2‐vinylphenols and 2‐allylphenols were realized. With a catalyst generated from 2 mol% of [Ir(cod)Cl]2 (cod=cycloocta‐1,5‐diene) and 4 mol% of the phosphoramidite ligand L2, the etherification products were obtained in excellent ees and then subjected to the ring‐closing metathesis reaction providing an efficient synthesis of enantioenriched 2H‐chromene and 2,5‐dihydrobenzo[b]oxepine derivatives.

Enantioselective Synthesis of Chromenes via a Palladium‐Catalyzed Asymmetric Redox‐Relay Heck Reaction

“…The main strategies are based on the enantioselective addition of aryl boronates to in situ generated pyrylium ions, the intramolecular asymmetric Brønsted acid‐catalyzed allylic etherification, and the 6‐ endo ‐trig Pd‐catalyzed asymmetric allylic substitution . You's group reported an iridium‐catalyzed asymmetric allylic etherification and the subsequent Ru‐catalyzed ring‐closing metathesis reaction for enantioselective synthesis of chromene . In spite of these significant advancement, some problems exit such as the limitation of intramolecular approach or the need of multiple step reactions .…”

“…You's group reported an iridium‐catalyzed asymmetric allylic etherification and the subsequent Ru‐catalyzed ring‐closing metathesis reaction for enantioselective synthesis of chromene . In spite of these significant advancement, some problems exit such as the limitation of intramolecular approach or the need of multiple step reactions . Therefore, the development of new approaches to this important motifs is still in great demand.…”

“…A halide atom as a substituent at the 6‐position of 4 H ‐chromene 1 exerted limited effects on the enantioselectivity (entries 11–13). Notably, the reaction of 4 H ‐chromene 1 d with boronic acid 2 k provided 2 H ‐chromene 3 m with 93 % ee , which has been reported to be transformed into BW683C by the Pt‐catalyzed hydrogenation . A methyl groups as a substituent at the 6‐ and 7‐ positions of 4 H ‐chromene 1 have negative effects on the enantioselectivity (entries 14 and 15).…”

“…However, substantial amounts of unknown side product were observed for the arylboronic acids with an electron‐withdrawing group on the aromatic ring, thus resulting in a poorer yield of the Heck product, although with high ee value (entries 7, 16–18). The absolute configuration of product 3 f was determined to be ( R ) by comparison of optical rotation to that of literature …”

A Diastereoselective Route to trans‐2‐Aryl‐2,3‐dihydrobenzofurans through Sequential Cross‐Metathesis/Isomerization/Allylboration Reactions: Synthesis of Bioactive Neolignans

“…For compounds 9dd and 9ed , each of which also contains an allylic alcohol moiety in the carbon skeleton, the formation of the desired five‐membered‐ring compound was accompanied in an unforeseen manner by another product that was unfortunately inseparable by silica gel chromatography. Its structure was assigned in each case as a seven‐membered‐ring system, the formation of which could be the result of an intramolecular S N 2′ substitution in an unconventional Mitsunobu reaction (Scheme ) 28,29. The relative configuration of the stereocenters in the 2,5‐dihydrobenzo[ b ]oxepine derivative with R = Ph was not unambiguously assigned, although analysis of 1 H NMR spectrum seems to indicate the presence of only one diastereoisomer.…”