Superacidic media, cyclization

Thus, in the superacidic medium, the direction and character of N-vinylpyrroles protonation change, that is, it results in the formation of stable dications, products of simultaneous protonation of the a-position of the pyrrole ring and p-position of the vinyl group, which are prone (in the case of phenyl substituent) to intramolecular cyclization, and kinetic cations having p-protonated pyrrole undergoing rearrangement via 1,2-hydride shift. 

Cyclization of IV-allyl anilines (64) in superacidic medium has been reported to afford anti-Markovnikov products (65). Investigation by the in situ NMR spectroscopy, DFT calculations, and reactions with labelled substrates suggest that new ammonium-carbenium super-electrophiles are involved as intermediates. ... 

The Pictet-Spengler reaction is an acid-catalyzed intramolecular cyclization of an intermediate imine of 2-arylethylamine, formed by condensation with a carbonyl compound, to give 1,2,3,4-tetrahydroisoquinoline derivatives. This condensation reaction has been studied under acid-catalyzed and superacid-catalyzed conditions, and a linear correlation had been found between the rate of the reaction and the acidity of the reaction medium. Substrates with electron-donating substituents on the aromatic ring cyclize faster than the corresponding unsubstituted compounds, supporting the idea that the cyclization process is involved in the rate-determining step of the reaction. 

In a series of papers, Jacquesy and Jouannetaud have demonstrated the efficacy of aromatic cyclization reactions in HF SbF5 medium.784 l,3-Bis(methoxyphenyl) propanes and other substituted phenylpropanes 233 give substituted indenes (234) and tricyclic spiro enones (235) in the superacid medium844 (Scheme 5.84). 

Diphenyl-l-propanone (19) is found to give the cyclization product 22 in good yield when the reaction is carried out in superacid.19 This acid-catalyzed cyclization also shows a dramatic dependence on the acidity of the reaction medium a yield of 72% in 100% CF3SO3H (Ho —14.1) a yield of 7% in 6% w/w CF3SO3H 94% CF3C02H (H0 -8.7) a yield... 

Superelectrophilic intermediates were also proposed in the cyclization reaction of Q -(methoxycarbonyl)diphenylmethanol (37) with superacid (eq 24).25 The yield of cyclized product is found to increase dramatically with increasing acidity of the reaction medium. The conversion is believed... 

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