Cyclobutane formation in the 2π + 2π cycloaddition of allyl and related cations to unactivated olefins. Evidence for the second step in the proposed mechanism of the ionic diels-alder reaction.

Bouncing off walls – widths of exit channels from shallow minima can dominate selectivity control

“…Reactions between butadienes and allyl cations have been studied extensively from both mechanistic and synthetic perspectives. Reactions of these two types of components can lead to 4-, 5-, 6-or 7-membered rings via concerted or stepwise (formal) (2 + 2)/[ p 2 + p 2], 1,2 (2 + 3)/[ p 2 + p 2], 3,4 (4 + 2)/[ p 4 + p 2] [5][6][7][8][9] (the so-called "ionic Diels-Alder reaction") or (4 + 3)/[ p 4 + p 2] 3,4,10-12 cycloaddition reactions, respectively (Scheme 1). Which products are observed can be inuenced by which substituents are attached to each component and the environment in which the reaction is run.…”

One-Pot Total Synthesis of Cannabinol via Iodine-Mediated Deconstructive Annulation

“…The three compounds formed in the pyrolytic study had already been described from the treatment of CBC and other cannabinoids with acids and/or by heating or irradiation . The formal [2π + 2π] thermal cyclization of CBC ( 3 ) to CBL ( 8 ) was, nevertheless, mechanistically intriguing, and a similar transformation is documented also for other homoisoprenylchromenes. − The reaction has been considered a Gassman-type cationic [2π + 2π] cyclization, but the anti-Markovnikow regiochemistry is at odds with the considerations of carbocation stability that governs this type of cycloaddition. , An alternative, and more plausible mechanism, is presented in Figure . The reaction might involve a cyclopropane intermediate ( 9 ), the result of a concerted process triggered by electrophilic (acidic) activation of the carbonyl tautomer of the resorcinyl moiety, and terminated by electrophilic Markovnikov addition to the electron-rich homoisoprenyl terminal double bond (Figure ).…”

“…5 The formal [2π + 2π] thermal cyclization of CBC (3) to CBL (8) was, nevertheless, mechanistically intriguing, and a similar transformation is documented also for other homoisoprenylchromenes. 15−17 The reaction has been considered a Gassman-type cationic [2π + 2π] cyclization, 18 but the anti-Markovnikow regiochemistry is at odds with the considerations of carbocation stability that governs this type of cycloaddition. 18,19 An alternative, and more plausible mechanism, is presented in Figure 3.…”

“…15−17 The reaction has been considered a Gassman-type cationic [2π + 2π] cyclization, 18 but the anti-Markovnikow regiochemistry is at odds with the considerations of carbocation stability that governs this type of cycloaddition. 18,19 An alternative, and more plausible mechanism, is presented in Figure 3. The reaction might involve a cyclopropane intermediate (9), the result of a concerted process triggered by electrophilic (acidic) activation of the carbonyl tautomer of the resorcinyl moiety, and terminated by electrophilic Markovnikov addition to the electron-rich homoisoprenyl terminal double bond (Figure 3).…”

Thermal Intramolecular [2+2] Cycloaddition: Synthesis of 3‐Azabicyclo[3.1.1]heptanes from Morita–Baylis–Hillman Adduct‐Derived 4,4‐Diaryl‐1,3‐dienes

“…Electron‐rich alkenes include ketene dithioacetals,4a–c enol ethers/silyl enol ethers,4d–h,5a and enamines/dienamines 4i–o. The second approach uses a combination of simple alkenes and electron‐deficient alkenes such as alkenyl oxocarbenium ions,4p,q,5b–f N ‐alkenyliminium ions,4r,s allyl cation,4t quinones,4u,5i and alkylidenemalonates 4v,5g…”