Sur la réduction du noyau pyridique par l'acide formique, partie III. Réduction de la β-picoline

[4,5] Anionic sigmatropic rearrangements

“…' arrangement of the salt (IIe) yielded only 5% of the [4,5] rearrangement and the enamine (IIId) (30%) in somewhat rearrangement product (IIIe) ; the alternative symmetryallowed and sterically acceptable [6, 31 rearrangement might have occurred, but was not observed. The major component isolated was the aldehyde (X, mixture of diastereomers in the ratio 1: 1) (20%).…”

“…

Allyl(pentadieny1)ammonium ylides (VII) re-ALTHOUGH the Woodward-Hoffmann rules1 have predicted arrange to the enamines (111) a t room temperature ; the that high-order sigmatropic rearrangements should be reaction is shown to be a concerted [4,5] sigmatropic feasible in suitable systems, the first examples of the rearrangement proceeding via a nine-membered transi-thermal [5, 512 and cationic [3,413 rearrangements have been tion state involving 1 0 ~ electrons. reported only r e ~e n t l y .

…”

“…the salt (IIb). In the case of the salt (IId), the major product (VIIId) (50%) was formed by a [2,3] sigmatropic lower yield by a [4,5] sigmatropic rearrangement. Re-…”

“…Three of the eight possible transition state geometries associated with suprafacial-suprafacial [4,5] sigmatropic rearrangemeizts.…”

“…The reaction (IIa --+ IIIa) certainly involves the ylide (VIIa) which undergoes a [4,5] sigmatropic rearrangement (VII, see arrows) via a bishomo-azonine 'aromatic' transition state involving Ionelectrons.6 In the transition state (see Scheme l), the pentadienyl fragment will adopt a transoid configuration for effective orbital overlap and this determines the stereochemistry of the ena ' ine (IIIa) . I n spite of the nine-membered transition state, the reaction (IIa -+ IIIa) proceeds in an impressive yield.…”