Diastereoselective Spirocyclization ofC-(Alkyloxycarbonyl)formimines of 2-Substituted 1HIndole-3-ethanamines (= Tryptamines): Basic Studies. 5th Communication on Indoles, Indolenines, and Indolines

C-(Alkoxycarbonyl)formimines of type 15–18 were derived from the 2-substituted tryptamines 2, 9, 10, and 11 and transformed with tosyl chloride into tricyclic 3-spiroindoles of types 19–22 (Scheme 3). The influence of the homochiral alkoxy moieties A–D on the stereochemical outcome of this reaction was studied. Good-to-excellent diastereoselectivities were observed with the (−)-8-(phenylmenth-3-yl)oxy group (B) as homochiral auxiliary. The structures of the tricycles 4, (2′R,3S)-19B, and (2′S,3R)20C were established by X-ray analysis, the structures of the others by NOE and CD studies, and by chemical correlation. Possibilities to explain the steric course of the spirocyclizations are discussed.