Influence of Alkyl Substitution on the Intramolecular Ionic Diels-Alder Reaction of Tetraenes

Abstract

Fifteen tetraenes, comprised primarily of methylated analogs of (3E,8E)-1,3,8,10-undecatetraene, were synthesized and treated with acid to study the influence of alkyl substitution on the intramolecular ionic Diels-Alder reaction. Depending on methyl substitution patterns of these tetraenes, bicyclo[4.3.0] nonyl, bicyclo[4.4.0]decyl, and bicyclo[5.4.0]undecyl ring systems were produced. For two tetraenes, the ring preference between two different Diels-Alder-derived ring skeletons could be controlled using different temperatures and acid catalysts. Mechanistically, a stepwise process was postulated for at least some of these Diels-Alder reactions. The irreversible and stereoselective formation of the same Diels-Alder product from two noninterconverting tetraenes, which differed only by the cis-trans relationship of a terminal methyl group, was best explained by a stepwise process. A stepwise process was best demonstrated when intermediate triene products isolated from a reaction mixture irreversibly cyclized to Diels-Alder products when resubmitted to the reaction conditions. Although several of the tetraenes failed to give significant amounts of Diels-Alder products, certain alkyl substitution patterns were identified which generally led to high yields of cyclized products.

Superacid‐Catalyzed Reactions

“…This was interpreted as indicating that the intermediate dication 273 is transformed to dication 274 through 1,2-hydride shifts and the presence of deuterium hinders the hydride shifts. 905 Recently, Shin and co-workers 907 have studied the cyclization of the trienol 279 and isolated two compounds (280 and 281) (Scheme 5.94). Ring contraction of 1hydroxycycloheptanecarboxylic acid, in turn, did take place to yield 1-methylcyclohexanecarboxylic acid 1,4-lactone as the major product.…”

The Intramolecular Radical Cation Induced Diels-Alder Reaction in the Diene - Diene Format

“…For our investigation of the radical cation induced Diels-Alder reaction in the diene -diene format, (3) was selected because of its optimal placement of a substituted electron rich diene together with an unhindered diene. In addition, Gorman and Gassman [22] have shown that this compound does not give Diels-Alder products under protic conditions. Compound (4), which lacks stabilizing methyl substituents, was included for comparison.…”

“…In selecting substrates with which to study the radical cation induced Diels-Alder reaction, the intramolecular arrangement of diene and dienophile reactants is attractive because of the facility with which bicyclic systems can be constructed. Gassman and coworkers [21,22] have previously observed protic acid catalyzed formation of Diels-Alder adducts from tetraenes such as (2). Treatment of (2) with the radical cation initiator (1) also induced the formation of Diels-Alder adducts, but this outcome was completely prevented by the inclusion of the hindered base, 2,6-di-t-butyl pyridine, in the reaction mixture.…”

Computational Examination of (4 + 3) versus (3 + 2) Cycloaddition in the Interception of Nazarov Reactions of Allenyl Vinyl Ketones by Dienes

“…(4 + 3) Cycloadditions involving 2 and dienes may be considered to be [4π + 2π] cycloadditions, just like classical Diels–Alder reactions, which are concerted. However, the reactions of allyl cations with dienes, which Gassman termed ionic Diels–Alder reactions, were deemed “so asynchronous as to be stepwise” based on experimental results . A stepwise mechanism for the ionic Diels–Alder reaction has been supported by a computational study, but more recent experiments have indicated that minor variations in the substrates can tip the reaction pathway toward a mechanism that was more clearly described as concerted …”