Cycloaddition reactions of vinyl oxocarbenium ions

Formal Enantioselective [4+3] Cycloaddition by a Tandem Diels–Alder Reaction/Ring Expansion

“…This mechanistic interpretation is counter to the mechanism generally described for the related [4 + 3] cycloadditions of allyl cations or allyl cation equivalent with dienes, where the allyl cation is considered to act as a 2p system. [5] Most similar to our system is the Lewis acid-catalyzed [4 + 3] cycloaddition of 2-siloxyacroleins with dienes, which has been proposed to involve a stepwise process, in which the zwitterionic intermediate goes directly to the [4 + 3] cycloadduct. [21][22][23] Even though it has been shown that the Diels-Alder cycloadduct derived from cyclopentadiene and a 2-siloxyacrolein rearranges cleanly to the formal [4 + 3] cycloadduct, [18] computational studies suggest that this process occurs by a retro DielsAlder reaction followed by a [4 + 3] cycloaddition.…”

“…In order to fully exploit this chemistry, several methods for the stereoselective synthesis of this valuable bicyclic nucleus have been developed. [5] Among the most notable methods are the [4 + 3] cycloaddition of allyl cations with dienes [Eq.…”

Chiral Brønsted acid-catalyzed enantioselective ionic [2+4] cycloadditions

“…4,5 It has been determined that vinyl oxocarbenium ion pairs (6, Scheme 1) have similar reactivity patterns to and often are more reactive than 2e4. 6 Both a,b-unsaturated ketals and acetals are known to provide 6 with equal propensity. Vinyl oxocarbenium ions (6) can be generated by a reaction of catalytic or stoichiometric quantities of Lewis or Brønsted acids with a,b-unsaturated acetals 5.…”

“…6 Both a,b-unsaturated ketals and acetals are known to provide 6 with equal propensity. Vinyl oxocarbenium ions (6) can be generated by a reaction of catalytic or stoichiometric quantities of Lewis or Brønsted acids with a,b-unsaturated acetals 5. Acetals 5 can be obtained by various methods including alkene cross-metathesis, Wittig olefination or direct acetalization of unsaturated aldehydes or ketones (Scheme 2).…”

“…Acetals 5 can be obtained by various methods including alkene cross-metathesis, Wittig olefination or direct acetalization of unsaturated aldehydes or ketones (Scheme 2). Once generated, the vinyl oxocarbenium intermediates (6) can undergo a variety of important transformations including conjugate additions, 7 [2þ4], [3þ4], and [5þ2] cycloadditions. 6 The first application of a,b-unsaturated acetals, such as 7 in DielseAlder reaction was reported by Gassman and co-workers in 1987 (Scheme 2).…”