The orthogonal arrangement of cumulated double bonds has attracted a great deal of attention from the scientific community since the first allene was prepared by Burton and von Pechmann in 1887. [1] The selection of substituents having a different electronic nature allows the ambivalent reactivity of allenes towards nucleophilic as well as electrophilic reagents with controlled regio-and stereoselectivity. [2,3] Thus, donor/acceptor-substituted allenes have emerged among the most versatile synthetic building blocks in the development of novel carbon-carbon bond-forming reactions (Scheme 1). In contrast, the reactivity behavior of [n]cumulene derivatives, compounds with three or more consecutive double bonds, has not been fully explored because of the difficulty associated with their preparation other than tetrasubstituted [3]cumulene derivatives. [4] While a limited number of acyl-substituted [3]cumulene derivatives are known to be stable [5] and subsequently utilized in the enyne cycloaddition, [6] palladium-catalyzed arylation, [7] Lewis acid catalyzed Diels-Alder reactions, [8] phosphine- [9] and silvercatalyzed [3+2] cycloaddition reactions, [10] the synthesis as well as synthetic utility of donor-substituted [3]cumulene derivatives have yet to be established. [11] Recently, we reported a facile a-vinyl enolization pathway of (E)-b-chlorovinyl ketones in which transient oxy-substituted [3]cumulene derivatives [i.e., cumulenol(ate)s] were postulated as nucleophilic species, reacting either with a protic source [12] or aldehydes [13] (Scheme 2). Drawn by the possibility of investigating the ambivalent reactivity modes of donor-substituted [3]cumulene derivatives, we envisioned a reaction of in situ generated [3]cumulene derivatives with nucleophiles. Herein, we report an electrophilic reactivity mode of oxy-substituted [3]cumulene derivatives in the syntheses of vinyl allenones and 3-methylenepyrrolidines with excellent stereoselectivities.During the course of our investigation into the nucleophilicity of b-chlorovinyl ketones, we observed the formation of the aldol product 2 a as well as the vinyl allenone 3 a under the influence of lithium salts (Table 1). [13] While 3 a was obtained with a low yield of 15 %, we were particularly encouraged by the fact that 3 a, a trisubstituted alkene, was formed stereoselectively. To optimize the formation of 3 a, we Scheme 1. Known and proposed reactivity patterns of functionalized allenes and [3]cumulenes. Scheme 2. Ambivalent reactivity modes of in situ generated oxy-substituted [3]cumulenes.
Scheme 1. Known and proposed reactivity patterns of functionalized allenes and [3]cumulenes. Scheme 2. Ambivalent reactivity modes of in situ generated oxy-substituted [3]cumulenes.
Treatment of β‐chlorovinyl ketones with NEt3 and LiClO4 or LiBr results in formation of electrophilic lithium cumulenolates which smoothly react with nucleophiles such as another cumulenolate or ketimines to afford vinyl allenones like (II) or pyrrolidines of type (IV)/(VII) in a stereoselective manner.
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