.gamma.-Substitution of allyl ylides in the Wittig reaction

PBu3-Mediated Vinylogous Wittig Reaction of α-Methyl Allenoates with Aldehydes and Mechanistic Investigations

“…A retro-Diels–Alder (rDA) pathway, first proposed by Corey, was often adopted to interpret the formation of vinylogous olefination products as well as stereo outcomes (Scheme ). , In such a mechanism, an allylic phosphorus ylide initially adds to an aldehyde at the γ-carbanion, followed by a H-shift to form a six-membered 1,2-oxaphosphonine which subsequently undergoes a retro-Diels–Alder (rDA) reaction to fulfill a vinylogous Wittig transformation. This mechanism benefits from the ability of allylic phosphorus ylide to undertake γ-addition on aldehydes. − Yet, theoretical or experimental evidence for it remains elusive. To account for formation of dienes 3 in this work, we found that the retro-Diels–Alder pathway is, however, quite contradictory to the observed distinct stereoselectivities of the di- and trisubstituted double bonds of product 3 .…”

“…In contrast, slow addition of allenoate 1a to the reaction mixture to maintain a low concentration of 1a improved E/Z selectivity of 3a (6:1) (entry 17). Addition of Lewis acid BF 3 or LiCl did not improve either E/Z selectivity or yield of product 3a (entries 18,19). Additional protic additives such as water, acetic acid, and phenol also failed to improve the reaction efficiency (entries 20À22).…”