Can the progress of Fischer's indole synthesis be stopped?

1,3-cycloadditions of pyridinium N-arylimides

“…Dimethyl (rel-2b,4ab,9ab)-(‫4,2-)ע‬a,9,9a-Tetrahydro-1H-2, 9-ethanopyrido[2,3-b] is visible in the 100 MHz spectrum. 13 C NMR (100 MHz, CDCl 3 , DEPT) d 41.4, 43.9 (C-4a, C-11), 47.9 (C-2), 52.1, 52.6 (2 OCH 3 ), 60.3 (C-10), 71.3 (C-9a), 114.3 (C-8), 121.8 (C-10), 125.0, 128.0, 128.1, 130.5 (C-3, C-4, C-5, C-7), 134.1 (C-4b), 149.9 (C-8a), 172.0, 173.1 (2 C‫ס‬O); the assignments followed those of 9 and a two-dimensional correlation [14]. Anal.…”

“…The C12-C13 tether between positions 5 and 7 prevents 9 from indole formation. When this tether is missing, the aminal flips open, and N7 becomes the indole nitrogen [14]. The hydrazo rearrangement is a [3.3]-sigmatropic reaction that leaves untouched the configuration at C-1 and C-2 of 12; an X-ray analysis confirmed the structure of 9 [18].…”

“…The 3,4-double bond of 22 deshields the allylic 4a-H (d 3.51) less than the benzo ring in 9, where 11b-H is found at d 4.33. The aminal hydrogen 9a-H of 22 comes at lowest field (d 5.22) vs. d(6a-H) 5.62 in 9 [14]. The vinylic hydrogens 3-H (d 6.06) and 4-H (d 5.74) show J cis ‫ס‬ 9.9 Hz, and J 3,4a ‫ס‬ 1.4 Hz indicates the allylic coupling.…”

“…The cycloadducts of 2a with olefinic or acetylenic dipolarophiles undergo an acid-catalyzed Fischer indole synthesis that stops at the aminal stage as illustrated by the rearranged structure 9 that is formed from 12, the adduct of dimethyl fumarate [14]. Some cycloadducts of 5a are so sensitive to acid that procedure A already provided the products of the hydrazo rearrangement.…”

“…This structure element is incorporated in the cycloadducts of 2a to ethylenic dipolarophiles, for example, the dimethyl fumarate adduct 12. Picric acid or hydrochloric acid converted 12 to the cyclic aminal 9, that is, one step before the aromatic indole derivative is reached [14]. The C12-C13 tether between positions 5 and 7 prevents 9 from indole formation.…”