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Notes: The acid-catalyzed rearrangement of N-(1′,1′-dimethylprop-2′-ynyl)-, N-(1′-methylprop-2′-ynyl)-, and N-(1′-arylprop-2′-ynyl)-2,6-, 2,4,6-, 2,3,5,6-, and 2,3,4,5,6-substituted anilines in mixtures of 1N aqueous H2SO4 and ROH such as EtOH, PrOH, BuOH etc., or in CDCl3 or CCl4 in the presence of 4 to 9 mol-equiv. trifluoroacetic acid (TFA)has been investigated (cf. Scheme 12-25 and Tables 6 and 7). The rearrangement of N-(3′-X-1′,1′-dimethyl-prop-2′-ynyl)-2,6- and 2,4,6-trimethylanilines (X = Cl, Br, I) in CDCl3/TFA occurs already at 20° with τ1/2 of ca. 1 to 5 h to yield the corresponding 6-(1-X-3′-methylbuta-1,2′-dienyl)-2,6-dimethyl- or 2,4,6-trimethylcyclohexa-2,4-dien-1-iminium ions (cf. Scheme 13 and Footnotes 26 and 34) When the 4 position is not substituted, a consecutive [3,3]-sigmatropic rearrangement takes place to yield 2,6-dimethyl-4-(3′-X-1′,1′-dimethylprop-2′-ynyl)anilines (cf. Footnotes 26 and 34). A comparable behavior is exhibited by N-(3′-chloro-1′-phenylprop-2′-ynyl)-2,6-dimethylaniline (45., cf. Table 7). The acid-catalyzed rearrangement of the anilines with a Cl substituent at C(3′) in 1N aqueous H2SO4/ROH at 85-95°, in addition, leads to the formation of 7-chlorotricyclo[3.2.1.02,7]oct-3-en-8-ones as the result of an intramolecular Diels-Alder reaction of the primarily formed iminium ions followed by hydrolysis of the iminium function (or vice versa; cf. Schemes 13,23, and 25 as well as Table 7). When there is no X substituent at C(1′) of the iminium-ion intermediate, a [1,2]-sigmatropic shift of the allenyl moiety at C(6) occurs in competition to the [3,3]-sigmatropic rearrangement to yield the corresponding 3-allenyl-substituted anilines (cf. Schemes 12,14-18, and 20 as well as Tables 6 and 7). The rearrangement of (-)-(S)-N-(1′-phenylprop-2′-ynyl)-2,6-dimethylaniline ((-)-38; cf. Table 7) in a mixture of 1N H2SO4/PrOH at 86° leads to the formation of (-)-(R)-3-(3′-phenylpropa-1′,2′-dienyl)-2,6-dimethylaniline ((-)-91), (+)-(E)- and (-)-(Z)-6-benzylidene-1,5-dimethyltricyclo[3.2.1.02′7]oct-3-en-8-one ((+)-(E)- and (-)-(Z)-92, respectively), and (-)-(S)-2,6-dimethyl-4-( 1′-phenylprop-2′-ynyl)aniline((-)-93). Recovered starting material (10%) showed a loss of 18% of its original optical purity. On the other hand, (+)-(E)- and (-)-(Z)-92 showed the same optical purity as (minus;)-38, as expected for intramolecular concerted processes. The CD of (+)-(E)- and (-)-(Z)-92 clearly showed that their tricyclic skeletons possess enantiomorphic structures (cf. Fig. 1). Similar results were obtained from the acid-catalyzed rearrangement of (-)-(S)-N-(3′-chloro-1′phenylprop-2′-ynyl)-2,6-dimethylaniline ((-)-45; cf. Table 7). The recovered starting material exhibited in this case a loss of 48% of its original optical purity, showing that the Cl substituent favors the heterolytic cleavage of the N-C(1′) bond in (-)-45. A still higher degree (78%) of loss of optical activity of the starting aniline was observed in the acid-catalyzed rearrangement of (-)-(S)-2,6-dimethyl-N-[1′-(p-tolyl)prop-2′-ynyl]aniline ((-)-42; cf. Scheme 25). N-[1′-(p-anisyl)prop-2-ynyl]-2,4,6-trimethylaniline(43; cf. Scheme 25) underwent no acid-catalyzed [3,3]-sigmatropic rearrangement at all. The acid-catalyzed rearrangement of N-(1′,1′-dimethylprop-2′-ynyl)aniline (25; cf. Scheme 10) in 1N H2SO4/BuOH at 100° led to no product formation due to the sensitivity of the expected product 53 against the reaction conditions. On the other hand, the acid-catalyzed rearrangement of the corresponding 3′-Cl derivative at 130° in aqueous H2SO4 in ethylene glycol led to the formation of 1,2,3,4-tetrahydro-2,2-dimethylquinolin-4-on (54; cf. Scheme 10), the hydrolysis product of the expected 4-chloro-1,2-dihydro-2,2-dimethylquinoline (56). Similarly, the acid-catalyzed rearrangement of N-(3′-bromo-1′-methylprop-2′-ynyl)-2,6-diisopropylaniline (37; cf. Scheme 21) yielded, by loss of one i-Pr group, 1,2,3,4-tetrahydro-8-isopropyl-2-methylquinolin-4-one (59).