Phosphine-mediated diverse reactivity of γ-substituted allenoates with aldehydes: syntheses of highly functionalized chromans and (E,E)-1,3-dienes

Advances in Annulation Reactions Initiated by Phosphorus Ylides Generated in situ

“…On the other hand, when R is a non-conjugated group such as a hydrogen atom or a methyl group, the allylic carbanions 112a are believed to be the favorable forms of the ylides 112 and tend to undergo nucleophilic addition to the alde- This kind of substituent-induced chemoselectivity was further elaborated upon by He and co-workers in their subsequent investigation into phosphane-mediated reactivity of γ-substituted allenoates with salicylaldehydes. [50] It was found that γ-benzyl allenoates preferentially underwent olefination with salicylaldehydes to furnish the diene products 152, whereas γ-methyl allenoates instead tended to undergo [4 + 2] annulation with salicylaldehydes, affording the highly functionalized chromans 156 (Scheme 58).…”

Phosphine‐Catalyzed [3+2] and [2+4] Annulations of γ‐Methyl Allenoates with Aryl α‐Keto Esters: Stereoselective Syntheses of Functionalized Tetrahydrofurans and 4‐Chromanols

“…The structures of the [3+2] annulation products 3 and the [2+4] annulation products 5 were identified by a combination of 1 H and 13 C NMR including NOESY analysis and HRMS‐ESI measurements(see Supporting Information). The configuration of the products was further confirmed by comparison with analogues of our previous studies via 1 H NMR analysis …”

“…According to our previous closely related studies and current observation, a plausible mechanism to account for the formation of tetrahydrofurans 3 and 4‐chromanols 5 is proposed in Scheme . The nucleophilic attack of phosphine catalyst at the β‐carbon of allenoate 1 initially generates the resonance‐stabilized zwitterionic intermediates A and B .…”

“…Our group has previously reported a phosphine‐catalyzed [3+2] annulation of γ‐methyl allenoates with aromatic aldehydes for synthesis of tetrahydrofurans, in which the γ‐methyl group is directly involved in the ring‐forming event . Employment of salicylaldehydes, a phosphine‐catalyzed [4+2] annulation with γ‐methyl allenoates occurred to furnish the corresponding chromans . These two reactions provided facile syntheses of oxygenated heterocycles such as tetrahydrofurans and chromans, whereas both of them showed poor stereoselectivity.…”

Phosphine-Mediated Iterative Arene Homologation Using Allenes

“…4 In general, the union of an allenoate and an aldehyde in the presence of a phosphine yields an olefin through a Wittig-like process. 5 Interestingly, all such reports have described reactions between α- or γ -substituted allenoates and aldehydes. In contrast, Wittig reactions involving simple allenoates are rare.…”