Novel rearrangement of twisted 2,4-di-alkyl 2-trans-4-trans pentadienals: An approach to cyclopentenones based on molecular mechanical consideration

One-step and non-catalytic intramolecular redox reactions of conjugated all E-dienals to non-conjugated Z-enoic acids in subcritical water

“…The researchers specifically used 2Z,4E-isomer as a substrate in dichloromethane in the presence of strong Lewis acids as catalysts, while we used 2E,4E-isomer and only water without any catalyst. Similar cyclization was reported for the congested dienal with plural sterically bulky groups via a ketene intermediate as a special case [35]; however, non-substituted simple dienal 1a was used in our reaction.…”

The iso-Nazarov reaction

“…The same year, the group of Ogawa reported the isomerization of conjugated all-trans-dienals 18 using concentrated hydrochloric acid in THF at 40 °C (Sch eme 4). 19 Consistent with the mechanism depicted in Scheme 1B, the authors stated that non-conjugated cyclopentenones 19 were the initially obtained products of these cycloisomerizations. Instead, if temperature was raised above 45 °C, a ca.…”

Pronounced Steric Effects of Substituents in the Nazarov Cyclization of Aryl Dienyl Ketones

“…This barrier appears to be the result of an unfavorable 1,3‐allylic interaction between the two methyl groups in 13 that is not present in 12 . The strain imparted to the AlCl 3 ‐bound reactant by this interaction (see the distorted C‐C‐C angle, 132 °, in 13 , Figure 1; note also that the Me⋅⋅⋅Me distance is significantly greater for TS‐13 than for 13 ) is reduced as the transition‐state structure is reached 18. 19 Replacement of the γ‐methyl groups of 12 and 13 with hydrogen atoms reduces the energy difference (3.2 kcal mol −1 ) between the corresponding barriers for cyclization (compare to the 6.6 kcal mol −1 difference between 12 and 13 ), additionally demonstrating the importance of the 1,3‐allylic steric interaction 14c…”