Spiroindolenine intermediate

The mechanism of this reaction is still not clear. It has always been thought that the Pictet-Spengler reaction proceeds via a spiroindolenine intermediate 20 followed by a Wagner-Meenvein rearrangement to 21 (way A). On the other hand in the case of very reactive electrophiles it was shown that the cyclization can occur by direct attack at the C-2 position of the indole (way B). However, after deprotonation of 21 compound 7 is formed.10... 

Taylor, M. S., Jacobsen, E. N. Highly enantioselective catalytic acyl-Pictet-Spengler reactions. J. Am. Chem. Soc. 2004, 126, 10558-10559. Kowalski, P., Mokrosz, J. L. Structure and spectral properties of P-carbolines. Part 9. New arguments against direct rearrangement of the spiroindolenine intermediate into P-carboline system in the Pictet-Spengler cyclization. An MNDO approach. Bull. Soc. Chim. Belg. 1997, 106, 147-149. 

Table 6.16 Enantioselective synthesis of polycyclic indole derivatives 190 via the in situ formation and migration of spiroindolenine intermediates reported by You.

Bischler-Napieralski reactions of N-acyl tryptamine derivatives 16 are believed to proceed via a related mechanism involving the initial formation of intermediate spiroindolenines (17) that rearrange to the observed 2-carboline products (18). The presence of these intermediates has been inferred by the observation of dimerized products that are presumably formed by the intermolecular trapping of the spiroindolenine by unreacted indole present in the reaction mixture." ... 

Reactions of indoles with inorganic radical ions 79UK1968. Spiroindolenines as intermediates 78H(9)1O89. 

A similar reaction affords a simple route to cyclopent[b]indoles. Indole-3-carbinol (68) reacts with j8-methylstyrene in the presence of titanium tetrachloride to give compound (70) <93TL8527>. The presumed four-membered spiroindolenine (69) in this sequence receives support from the observation that the conversion of the D-tryptophan derived compound (71) to its mesylate leads to the formation of racemic cyclopent[b]indole (73) via the achiral intermediate (72) (Scheme 17) <93TL439>. 

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