Immonium ions Diels-Alder reactions

Immonium ion based synthetic methodology a novel method for the N-methylation of dipeptides and amino acid derivatives via retro aza-Diels-Alder reactions [93]... 

Quinolizidine synthesis via intramolecular immonium ion based Diels-Alder reactions total synthesis of ( )-lupinine, ( )-epilupinine, ( )-criptopleurine and ( )-julandine [97]... 

IMMONIUM ION-BASED DIELS-ALDER REACTIONS N-BENZYL-2-AZANORBORNENE... 

The Diels-Alder reaction of a-phellandrene with acrylic acid derivatives has been examined, apparently superficially in contrast to the re-investigation of the reaction with benzalbisurethane, catalysed by BF3,Et20-CuBr. The 1,4-adducts (161 endo exo/31 63) may result effertively from the concerted [4 -I-2 ] addition involving the immonium ion (PhCH=NHC02Et) however, with the corresponding unsubstituted immonium ion, formal 1,3-addition to a-phellandrene occurs essentially via the isomerized a-terpinene and isoterpinolene to yield [(162) (163)/84 16]. ... 

Dicyclopentadiene, 66, 99 Dieckmann cyclization, 66, 52 DIELS-ALDER REACTION domino, 68, 198 immonium ion-based, 68, 206 INVERSE ELECTRON DEMAND, 66, 142,147, 148 of triethyl 1,2,4-triazine-3,5,6-tricarboxylate, 66, 150 Diels-Alder reactions, 66, 40... 

Since the initial report in 1962 by Merten and Muller of the cycloaddition of an -acyl imine with a conjugated diene,a number of examples of this type of reaction have appeared. In general, IV-acyl im> ines are highly reactive, unstable species which are rarely isolated, but rather are generated in situ from stable precursors. Depending upon the method of formation of the particular dienophile and the reaction conditions, a neutral A -acyl imine or a protonated (or Lewis acid complexed) IV-acyl immonium ion may be involved in the Diels-Alder reaction. 

In another recent study, Giieco et al. have found that C-acyl immonium ions formed in situ under aqueous conditions undergo [4 + 2] cycloadditions as shown in equation (14). This group has utilized similar methodology for other immonium Diels-Alder reactions (see Sections 4.2.2.4 and 4.2.2.8). 

Grieco has used his aqueous imnicmium Diels-Alder procedure to effect a number of intramolecular reactions. In one case, diene aldehyde (83) was treated with ammonium chloride to afford a 2.2 1 mixture of isomeric Diels-Alder adducts (87) and (85) (Scheme 10). Since intermediate immonium ions (84)/(86) cannot participate in secondary orbital effects as is the case with A-acyl imines (c/. 80), these results are probably due to steric factors. It was suggested that adduct (85) derives from conformation (84) and adduct (87) comes from (86). Conformer (84) is favored since there is a severe eclipsing of Ha b in (86). A more detailed account of the stereochemical aspects of intramolecular Diels-Alder reactions can be found in Chapter 4.4. 

Immonium Ion Based Diels-Alder Reactions N-BenzyI-2-azanorbornene... 

Water has been shown to be the polar solvent of choice for the intramolecular Diels-Alder cycloaddition of immonium ions, e.g. (77), to produce the single tricyclic amine (78) (Scheme 28). The activation free energy and transmission coefficient for the Diels-Alder reaction between methyl vinyl ketone and cyclopentadiene in water have been calculated using molecular-dynamics and reaction-flux simulations. Water has been shown to enhance significantly the Ni +- and Co -catalysed Diels-Alder reaction between 3-phenyl-l-(2-pyridyl)prop-2-en-l-ones and... 

As a rule, cycloadditions with acyclic -acyl imino dienophiles are useful synthetic reactions which demonstrate excellent regio- and stereo-selectivity. A commonly used source of IV-acyl immonium dienophiles are bis-urethanes such as (13) (equation S), which upon treatment with a Lewis acid and a diene afford Diels-Alder adducts. In this case, (13) reacts with boron trifluoride etherate to afford immonium ion (14) which adds to diene (15), yielding tetrahydfopyridine (16). ... 

Edwards and cowoikers have investigated the cycloa ition chemistry of cyclic immonium ion (39) produced from ethoxy lactam (38) (Scheme 7). It was found that under a variety of acidic reaction conditions (38) usually produced complex mixtures of Diels-Alder adducts along with one bond solvolysis products. Some representative examples are shown in Scheme 7. It was concluded that these results are more consistent with a stepwise ionic mechanism than a concerted one. 

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