2‐Vinylcyclobutanones by Cycloaddition of Vinylketenes to Simple Olefins

Abstract

SummarySelected [2 + 21-cycloadditions of three alkylvinylketenes 2 to one mono-and seven dialkyl-olefins 3 yielded eleven 2-alkyl-2-vinylcyclobutanones 4 (Tables 1 and 2).Three methods were compared, all involving in situ generation of the ketenes 2 by HCI-elimination from a,P-unsaturated acid chlorides 1 ; the most effective employed a large excess of olefin 3 and a high reaction temperature. The [2 + 21-cycloadditions were fully regio-and stereoselective with respect to the olefin 3, but less so with respect to thc ketene 2, so that -where possible -two stereoisomers of 4 resulted, namely A and B, whose configurations were determined from their 'H-NMR, spectra, mechanistic considerations and, in one case, 4f, by chemical correlation with a previously known cycloadduct 8.

Total Synthesis and Biological Evaluation of Zealactone 1a/b

“…With ketene precursor 16 in hand, we investigated the key [2 + 2]-cycloaddition, to synthesize the highly functionalized strained cyclobutanone. Inspired by results from Dreiding and coworkers [16] using α-vinyl ketene species and olefins in [2 + 2]-cycloadditions, we attempted the cycloaddition of unprecedented α,α'-vinylacetylenyl ketene 19 with isobutene, a moderately reactive olefinic substrate. 3 Unfortunately, the preliminary results obtained on this reaction led to very low yield of the desired cyclobutanone 25 (Scheme 3).…”

“…To optimize the reaction conditions, a liquid and more reactive substrate, methylenecyclopentane, was used as a model coupling partner ( Table 1). [16] Carboxylic acid 16 was converted to the corresponding acid chloride in situ using oxalyl chloride and a catalytic amount of N,N-dimethylformamide (DMF), then cycloaddition with methylenecyclopentane was performed by slow addition of the base. The base could trigger the formation of the reactive ketene 19 as well as ammonium enolate 21 depending on the base used (Scheme 3).…”

“…Since ammonium enolate 21 might react with ketene species 19 to form the undesired dimer products 22 and 23, we investigated the effect of the base on the [2 + 2]-cycloaddition. [16] At first, to avoid the formation of ammonium enolate, we attempted less nucleophilic bases such as proton sponge, collidine derivative and Hünig's base. Whereas proton sponge and collidine gave the undesired dimer product and no conversion of starting material, respectively, Hünig's base provided the desired product in a moderate yield ( Table 1, .…”

Cycloaddition and Electrocyclic Reactions of Vinylketenes, Allenylketenes, and Alkynylketenes

“…96,97 Stereoselectivity is lost and the yield is low with a 1,1-disubstituted alkene (Scheme 60). 96,97 The reaction of methyl(vinyl)ketene with the electron-rich ethyl vinyl ether is more facile and also generates a mixture of stereoisomers (Scheme 61 The preference for formation of the more crowded products is also seen in reactions of 2-substituted vinylketenes with 1,3-cyclopentadiene and 1,3-cyclohexadiene, as exemplified by the ratio of products 44 and 45 in Scheme 62. 95 processes, as well as other reactions that compete with these cyclizations, and further transformations of initial products under the reaction conditions.…”

Transition metal-stabilised vinylketenes

“…[11][12][13][14][15] and the frequent postulation of metalbound vinylketenes as reaction intermediates (ref. [16][17][18], we decided that a study of the reactivity of our (viny1ketene)tricarbonyliron complexes would prove interesting and rewarding.…”