Quinoline ring reduction rearrangement

Quinoline 1-oxides can be rearranged photochemically into indoles (103 — 104 — 105) or N-hydroxyindoles (106 — 107). Cinnolines are reductively ring-contracted into indoles (Section 3.2.1.6.9.ii). 

Although also a special case of a MPV reduction (Section 1.3.3.3) some unusual reactions of practical import will be considered here because of the heterocyclic characteristics. Donation in an intramolecular reaction of a hydride from a tertiary aniline derivative to generate an iminium ion initiates the reaction. An example is given in Scheme 7. Tertiary aniline derivative (99) rearranges thermally apparently via intermediate (100) to pyrrolo[ 1,2-a]quinoline (101). ° The highly electron-deficient double bond is clearly the hydride acceptor in this case exclusively the most substituted iminium salt is formed. Subsequent ring closure provides the end product (101). Formation of the more substituted iminium ion is not always the case. ° ° ... 

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