Heterodienes cationic

Initially, a complex of nitroalkene (42) with LA (A) is reversibly formed. The efficient concentration of the latter is determined by the reaction conditions and the nature of heterodiene (42) and LA. This complex acts as a Michael substrate and adds alkene (43) to give bipolar adduct B, which undergoes cycliza-tion to give cationic intermediate C. The latter eliminates LA to yield target nitronate (35). In the case of nonconcerted cycloaddition, ionic intermediate B can undergo different isomerization reactions, some of which are considered below. The stereoselectivity of the process depends on the reactive conformation... 

Ghosez and co-workers (223) illustrated that azadienes could be efficiently used as heterodienes with acryloylimide dienophiles in the presence of chiral cationic Cu(II) catalysts, Eq. 183. The reaction was found to be highly exo- and enantiose-lective, provided the azadiene contains an aryl or vinyl group in the 1-position. Acryloylimide as dienophile affords equal enantioselectivities albeit reduced dias-tereoselectivities (typically 6 1). 

Guertler et al. (1996) described a wide range of cycloaddition reaction between 2-vinyl indoles acting as heterodienes and cyclic or acyclic enamines bearing acceptor groups in (3 positions. The reaction was induced by the formation of 2-vinylindole cation-radicals through anodic oxidation. The synthesis of 4a-carbomethoxy-6-cyano-5,7-dimethylindolo[l,2-a]-l,2,3,4,4a,12a-hexahydro-1,8-naphthyridine can serve as an example (Scheme 7.24). 

A [4-f2] cycloaddition was the key step in the synthesis of substituted dihydrothiopyrans from 2-alkenyl-l,3-oxathianes and an alkene. The reaction was mediated by a Lewis acid. It is assumed that the Lewis acid attacks the oxygen of the heterocycle which upon ring opening gives the highly reactive cationic heterodiene which reacts with alkenes to the thiopyrans in 31-88% yield (Scheme 75) <2000TL371>. 

The conversion under acidic conditions of 2-benzopyrylium salts into naphthols is not observed for isoquinolinium or monocyclic pyrylium salts, which become converted into phenols only in alkaline medium. Moreover, the ring opening of the heterocycle in 211, determined by its protonation, may be considered as the specific type of 1,4-addition of R2OH to the heterodiene fragment of the 2-benzopyrylium cation. This is a new version of the corresponding step in the ANRORC scheme. 

Owing to the annelated benzenoid ring the double bonds in the heterocyclic ring of 2-benzopyrylium are considerably fixed, and this fact, correlated with the high electronegativity of the oxonium atom, causes these cations to react as heterodienes. Thus, interaction of 2-benzopyrylium... 

The use of a range of modified reaction conditions including the use of protic acids or conventional and nonconventional Lewis acid catalysts, pressure-promoted reaction conditions, - cation-radical catalysts, and dry-state adsorption reaction conditions has been employed to accelerate the 4it participation of sensitive heterodienes in thermally slow or problematic Diels-Alder reactions. The former two techniques have proven useful for promoting the typically poor reactions of simple, unactivated 1-oxa-1,3-butadienes or acyclic azadienes. 

Cycloadditions between 2-vinylindoles as heterodienes and acceptor substituted enamines can be induced by formation of 2-vinylindole radical cations either via anodic oxidation or photoelectron transfer. In this way pyridoindoles and indolo-naphthyridines are formed in one step under complete regio- and stereochemical control [Eq. (24b)] [248b]. 

The past classification of the [4 + 2] cycloaddition reactions involving die 4it participation of a cationic heterodiene among the polar cycloadditions is derived not from an implied stepwise addition-cy-clization reaction mechanism but was terminology introduced to distinguish cycloadditions employing cationic or anionic components from those employing dipolar or uncharged components. Herein the reactions are simply referred to as [4 + 2] cycloaddition reactions. 

In the inverse electron-demand (lED) HDA reaction systems, a,p-unsaturated ketones, thiones, nitroalkenes, and related compounds often serve as heterodiene units and the electron-rich olefins are usually used as dienophiles for the reaction [158]. A concerted mechanism has been suggested for the HDA reaction between an a,p-unsaturated ketone with a vinyl ether mediated by a titanium complex (Scheme 14.66), although the possibility of a stepwise, cationic pathway, particularly in the presence of a Lewis acid, cannot be completely excluded [160]. 

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