Organocatalytic Cloke–Wilson Rearrangement: DABCO-Catalyzed Ring Expansion of Cyclopropyl Ketones to 2,3-Dihydrofurans

Abstract

An organocatalytic Cloke-Wilson rearrangement of cyclopropyl ketones to 2,3-dihydrofurans is exploited utilizing the homoconjugate addition process. With 1,4-diazabicyclo[2.2.2]octane as the catalyst, the rearrangement in DMSO at 120 °C proceeded in generally high yields, exclusive regioselectivity, and a broad substrate scope. An examination of the mechanism including stereochemical analysis and intermediate isolation supports an S1-type ring opening of the mechanism.

Utilizing Vinylcyclopropane Reactivity: Palladium‐Catalyzed Asymmetric [5+2] Dipolar Cycloadditions

“…To implement this idea, we examined the reaction of vinylcyclopropane 1 a′ and α‐diazoketone 2 a under irradiation by 6 W blue LEDs and with a combination of Pd 2 dba 3 ⋅CHCl 3 and our chiral P,S ligand L1 as the chiral Pd catalyst. To our disappointment, it failed to give the desired cycloadduct except the intramolecular cyclization product (Table , entry 1) . Inspired by an elegant work from Xu and co‐workers on the organocatalytic rearrangements of VCPs to cycloheptenones, we guessed that the dipolar intermediate formatted from substrate 1 a and the Pd 0 catalyst would be relatively stable, thus preventing the unfavorable intramolecular cyclization process.…”

The Bonding and Reactivity of α-Carbonyl Cyclopropanes

“…Described by Xu and co-workers in 2017, an alternative to the organocatalytic synthesis of dihydrofurans by Liang, Zhang, and co-workers (Scheme 21) 49 is the treatment of cyclopropanes 129 with DABCO at high temperature to form product dihydrofurans 131 in good to excellent yields (Scheme 32). 62 Mechanistically it is proposed to undergo attack at the cyclopropane, leading to the formation of zwitterionic intermediate 130, before attack of the resulting enolate oxygen upon the ammonium adjacent carbon releases the catalytic DABCO back into solution. This contrasts with the proposed mechanism of Liang, Zhang, and coworkers in which activation by organocatalyst is proposed to occur through promotion of the enolate without ring opening.…”

A straightforward route to alkyl 5‐arylthiophene‐2‐thiocarboxylates from alkyl 2‐aroyl‐1‐chlorocyclopropanecarboxylates

“…On the other hand, the strained cyclopropane chemistry has focused much attention in past decade [10,11]. The organocatalytic and enantioselective Cloke–Wilson rearrangement of cyclopropyl ketones to 2,3‐dihydrofurans was recently reported [12,13]. Ye's group found the nickel‐catalyzed enantioselective cycloaddition reaction of unreactive cyclopropyl carboxamide with alkyne [14].…”