Carbinolamine configurations

At least two pathways have been proposed for the Nenitzescu reaction. The mechanism outlined below is generally accepted." Illustrated here is the indolization of the 1,4-benzoquinone (4) with ethyl 3-aminocrotonate (5). The mechanism consists of four stages (I) Michael addition of the carbon terminal of the enamine 5 to quinone 4 (II) Oxidation of the resulting hydroquinone 10 to the quinone 11 either by the starting quinone 4 or the quinonimmonium intermediate 13, which is generated at a later stage (HI) Cyclization of the quinone adduct 11, if in the cw-configuration, to the carbinolamine 12 or quinonimmonium intermediate 13 (IV) Reduction of the intermediates 12 or 13 to the 5-hydroxyindole 6 by the initial hydroquinone adduct 7 (or 8, 9,10). 

The second compound (accedine, C20H24N202, MP 148-149°C, [a]D +72°) showed no carbonyl absorption in the IR, and H-NMR and mass spectra were in full agreement with the almost total existence of ring-closed carbinolamine structure 23. The configuration at C-16 was determined by correlation with 32. Reductive methylation (CH20-H2/Pd) of 23 afforded 29, which on LiAlH4 reduction gave the diol 257 identical in all respects to the product obtained from... 

The absolute configuration of dichomine was determined by chemical correlation. Reduction with LiAlH4 in THF gave a product identical to (145, 20/ )-velbanamine (160) through sequential carbinolamine C—O reductive cleavage... 

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