Diels-Alder reaction iminium salt

Aza-Diels-Alder reactions in aqueous soiutions cycioaddition of dienes with simpie iminium salts generated under Mannich conditions [22]... 

As depicted in Scheme 5-10, iminium salts such as 42 have been found to exhibit substantially increased reactivity in Diels-Alder reactions.9 This means... 

Platts and Tomkinson reported a detailed quantitative study of the iminium ion catalysed Diels-Alder reaction between cyclopentadiene and cinnamaldehyde catalysed by trifluoromethyl pyrrolidine salt 182-HPFg [227]. This combination of secondary amine, co-acid and dienophile allowed the isolation and structural elucidation of the reactive iminium ion intermediate. As a result it was possible to... 

Aqueous hetero Diels-Alder reaction was first described by Grieco, who reported the use of water as solvent for cyclocondensations of iminium salts (Larsen and Grieco, 1985). Known as being a very energy demanding reaction, the retro Diels-Alder process is usually not considered as a competitive pathway in most Diels-Alder... 

Carbon-carbon multiple bonds are not the only units that can participate in Diels-Alder reactions. Other double- and triple-bond compounds can be dieno-philes and they give rise to heterocyclic compounds.Among these are N=C-, -N=C-, iminium salts,-N=N-, and -C=0 com-... 

In alkoxymethyleneiminium salts the anions are exchangeable (Cl versus C104" or SbCU etc.). This procedure can be used to purify the iminium compounds for analytical purposes. Tetraphenylborates (105), which are easy to obtain from the iminium salt and sodium tetraphenylborate in d acetonitrile (equation 64), and which crystallize easily, are suitable reagents. The carbon skeleton of a,p-unsaturated alkoxymethyleneiminium compounds can be altered by cycloadditions, e.g. Diels-Alder reactions, to give compounds of type (106 equation 65). - °... 

The aza-Diels-Alder reaction combines three components (an aldehyde, an amine salt and a diene) to produce heterocyclic products which are useful synthetic intermediates. Grieco first reported that such a reaction could occur in water. Preliminary studies focused on the reaction of dienes with iminium ions generated by an amine hydrochloride and 37% aqueous formaldehyde solution [59]. When dienylamine hydrochlorides are treated with aqueous formaldehyde at 50 °C, bicyclic ring systems are formed (Scheme 9). 

Other reactions are possible with iminium salts derived from a,p-unsaturated carbonyl compounds. The Baylis-Hillman reaction (chapter 18) also uses conjugate addition as a first step and we shall deal with that immediately. Much more work has been done on the Diels-Alder reaction that will follow. The Baylis-Hillman reaction between an electrophilic a,p-unsaturatcd carbonyl compound and an... 

Aqueous aza Diels-Alder reactions were first described in cycloadditions of imini-um salts and dienes [38], Likewise, iminium salts derived from amino acids react in excellent yields in aqueous medium [39]. Such an aqueous aza Diels-Alder reaction was found to be catalyzed by lanthanide(III) trifluoromethanesulfonates [40]. 

Cycloaddition reactions. Asymmetric induction in cycloaddition of enals is based on the formation of conjugated iminium salts with bulky prolinol derivatives. Reaction partners include enamides and cyclopentadiene. The Diels-Alder reaction (catalyst 3B) is exo-... 

Cycloadditions of iminium compounds with 1,3-dienes to afford Diels-Alder adducts seems to be a reaction of general applicability.46 On the other hand, simple electron-rich imino species are less reliable dienophiles. Some electron-rich imines will undergo Diels-Alder reactions under acid catalysis and thus are probably actually reacting as iminium salts. However, inverse electron demand [4 + 2] cycloadditions of neutral imines with electron-deficient dienes have been reported, as have additions to exceptionally reactive dienes (vide infra). 

A few additional imino Diels-Alder reactions have been reported which involve iminium salts. Bohme and Ahrens found that the perchlorate salt in Eq. (11)... 

Grieco and co-workers also showed that iminium salts such as 123, formed by reaction of an amine and formaldehyde (sec. 4.4) reacted with cyclopentadiene in aqueous media to give a near-quantitative yield of the Diels-Alder adduct. An internal Diels-Alder reaction was also possible (sec. 11.8). Treatment of an aqueous solution of 124 with formaldehyde at 50°C gave a 98% yield of indolizidine 125. 

---