Lewis Acid-Catalyzed Carbofunctionalization of Uncommon C,N-Diacyliminium Ions: Controlling Regio- and Enantioselectivity

Abstract

The tricyclic azepino­[1,2-a]­indole acetates 7, readily accessible by visible-light-driven catalytic photooxygenation of cyclohepta­[b]­indoles 1, are convenient precursors to novel and uncommon cyclic C,N-diacyliminium ions 3. We report here the first Lewis acid-catalyzed C–C bond forming reactions of these species with TMSCN and silyl enol ethers as nucleophiles and utilizing TIPSOTf as well as Sc­(OTf)3 as catalysts. Employing Sc­(OTf)3/pybox complexes as a chiral catalyst system, regio- and enantioselective asymmetric alkylations with silyl enol ethers were achieved.

Photoredox‐Catalyzed Carbon Radical Generation from α‐Keto‐N,O‐acetals: Synthesis of Functionalized Azepino[1,2‐a]indoles and Azepino[1,2‐a]furo[3,2‐b]indoles

“…[12,13] Acetates 2 previously served us as precursors to C,N-diacylamini-nium ions 3, and we demonstrated their Lewis acidcatalyzed nucleophilic addition reactions with several types of nucleophiles. [11,12] On the other hand, one may perceive that the C10a-O single bond in compounds 2 is sufficiently labile and thus susceptible to activation by photoredox catalysis. Specifically, azepinoindoles 2 possess an α-keto-O-acyl-N,O-acetal substructure that upon SET reduction and mesolytic fragmentation would liberate an α-amino-α-carbonyl radical 4.…”

“…Our laboratory previously reported a visible light‐mediated sensitized photooxygenation reaction that converts cyclohepta[ b ]indoles 1 into tricyclic 10a‐hydroxytetrahydroazepino[1,2‐ a ]indole diones [11] in a single step, and after O ‐acetylation, the C10a‐ O ‐acetates 2 are obtained (Scheme 1b) [12,13] . Acetates 2 previously served us as precursors to C , N ‐diacylamininium ions 3 , and we demonstrated their Lewis acid‐catalyzed nucleophilic addition reactions with several types of nucleophiles [11,12] . On the other hand, one may perceive that the C10a‐O single bond in compounds 2 is sufficiently labile and thus susceptible to activation by photoredox catalysis.…”

Synthetic Applications of Ambiphilic C‐acylimines in Organic Synthesis

“…Brasholz and coworkers developed a Lewis acid‐catalyzed carbofunctionalization of the unique polycyclic C,N‐diacyliminium ions 105 (Scheme 38). [50a] Its azepinoindole precursors are readily accessible by visible‐light‐driven photooxygenation of cyclohepta[b]indole as reported by the same group . [50b] The use of Sc(OTf) 3 /pybox complexes as a chiral catalyst system resulted in the asymmetric version of alkylations with silyl enol ethers which was a drawback of their earlier work using Bronsted acid catalysis.…”

AgNTf2 catalyzed cycloaddition of N-acyliminium ions with alkynes for the synthesis of the 3,4-dihydro-1,3-oxazin-2-one skeleton

“…5 Three general approaches were often used for the preparation of NAIs. The most popular method is to utilize in situ generation, under the promotion of Brønsted acids or Lewis acids, from N , X -acetals including N , O -acetals, 6 N , S -acetals 7 and N , N -acetals 8 (Fig. 1).…”