Isoquinolinium salts, Bradsher reaction

The formation of isoquinolinium salts 238 as trans-isomers is in good agreement with the concept of maximal charge separation in 237 by Coulomb interaction, which was accepted as the explanation of stereoselectivity in the Bradsher-Falk reaction (78JOC822). 

Heterocyclic systems with one heteroatom. Condensed heteroaromatic cations are reactive in [2 + 4] cycloaddition reactions with inverse electron demand. For instance, 2-benzopyrylium salts 403 react with vinyl ethyl ether to afford naphthalene derivatives 405 in good yields via initial adducts 404 <1990KGS315>. Similar transformations (Bradsher reaction) are also known for isoquinolinium salts <1984CC761>. 

Initially, the Bradsher cycloaddition reaction was used to gain greater understanding of the Diels-Alder reaction. More recently it has found utility in the constuction of carbon frameworks that provided novel entries into the synthesis of natural products. For example, the Bradsher cycloaddition reaction has been reported on the use of isoquinolinium salts for the stereocontrolled synthesis of substituted tetralins. Reaction of... 

Sammes and co-workers reported studies on an intramolecular variation on the Bradsher cycloaddition reaction. However, the Franck group,publishing on the construction of benz[c,c/]indole frameworks, reported an alternate structure for the product of their reaction. Intramolecular cycloaddition of the isoquinolinium salt 43 initially afforded adduct 44. This intermediate rapidly converted to 45, which gave 48 after acetylation. The Sammes group had assigned 47 to the final compound but the Franck group, based on NMR and crystal structure data from a related system, proposed the alternate structure 48. Thus this chemistry provided... 

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