Mechanistic Studies and Improvement of Coinage Metal-Catalyzed Transformation of Alkynyloxiranes to Furans: An Alcohol Addition−Cyclization−Elimination Cascade

Abstract

In the presence of alcohol Ag or Au salts or complexes catalyze the conversion of alkynyloxiranes to substituted furans. Both catalysts are effective, and a large furan diversity can be obtained in high yield with one or the other catalyst. Mechanistic studies revealed that a cascade pathway and not the sometimes reported direct intramolecular nucleophilic addition of oxirane oxygen atom to intermediate acetylene-metal pi-complex occurs. Under the defined conditions, the intermediate formation of epoxide opening products has been identified. Depending on the catalyst, one or both of the latter cyclized to dihydrofurans, and further elimination of the alcohol led to the corresponding furans. These results highlight the duality between oxophilicity and alkynophilicity of Ag or Au salts.

Synthesis of Cyclic Alkenyl Triflates by a Cationic Cyclization Reaction and its Application in Biomimetic Polycyclizations and Synthesis of Terpenes

“…[13] Following this simple sequence,( AE)-pallescensin A( 16)w as synthesized in just 3s teps from triflate 10 a (4 steps from dienyne derivative 9a)i n4 0% global yield. Again, the entire sequence could be performed on ag ram-scale (1.1 go fp allescensin Ai no ne batch) and without any purification of the intermediates (just af inal column chromatography).…”

Transition Metal-Mediated Synthesis of Monocyclic Aromatic Heterocycles

“…It was shown that the reaction proceeds via a ring opening to form products 2-399a and 2-399b , both of which cyclize under the reaction conditions in the presence of the Au(I)-catalyst to provide furans 2-398 (Scheme 110). 202 …”

“…A single example of this reaction catalyzed by the AgOTf/TsOH system was later reported by the same group. 202 …”

Gold‐Catalyzed Migrations and Ring Expansions

“…The generally accepted mechanism involves the activation of the alkyne group by coordination to the metal followed by attack of the epoxide at the distal Csp center, forming a Csp 2 -gold σ-bond. After the loss of a proton the furan product would be generated as shown in Scheme 82 151 However, mechanistic studies later on showed that the reaction might proceed through a cascade process involving epoxide opening either by adventitious water or alcohol used in the reaction, followed by cyclization and elimination of alcohol as depicted in Scheme 83 152 .…”