Abstract
The efficient catalytic activation of donor–acceptor aminocyclopropanes lacking the commonly used diester acceptor is reported here in a (3 + 2) dearomative annulation with indoles. Bench-stable tosyl-protected aminocyclopropyl esters were converted into cycloadducts in 46–95% yields and up to 95 : 5 diastereomeric ratio using catalytic amounts of triethylsilyl triflimide. Tricyclic indoline frameworks containing four stereogenic centers including all-carbon quaternary centers were obtained.
Highlights
Vicinal donor–acceptor (D–A) cyclopropanes are useful threecarbon 1,3-zwitterion synthetic equivalents for the synthesis of carbocyclic scaffolds.[1]
Preliminary experiments using TMS tri ate led to no reaction, such conditions have been successful for alkoxycyclopropanes.8b Compared to silyl tri ates, silyl tri imides have shown superior catalytic activity.[17]
TMS tri imide was formed through the protodesilylation of trimethylsilyl ketene acetal 3a with bistri imide.[18]
Summary
Vicinal donor–acceptor (D–A) cyclopropanes are useful threecarbon 1,3-zwitterion synthetic equivalents for the synthesis of carbocyclic scaffolds.[1]. D–A cyclopropanes with a single carbonyl acceptor have been less studied (Scheme 1b) Such substrates lead to the formation of one more stereocenter and do not require a decarboxylation step to remove the diester group.[5] activation and control over diastereoselectivity is challenging for these less reactive cyclopropanes. Indoline-fused cyclopentylamines are present in the core of alkaloid natural products, such as vindolinine, pleiomutinine or huncaniterine A and B.13 3-Methylindoles were used by Kerr and co-workers in a ytterbium tri ate catalyzed (3 + 2) annulation with cyclopropane diesters (Scheme 2a).[14] in the absence of substituent at the C-3 position, ring-opening products were obtained. Scheme 4 Scope of the catalytic (3 + 2) annulation of tosyl-protected aminocyclopropane 1 with indoles 2 (reaction on 0.1 to 0.3 mmol scale, yields are given for the mixture of both isomers). The method is unique for its tolerance towards substitution patterns, as it works for both C2- and C3substituted indoles and can be used for the rst time to introduce a non-symmetrical all carbon quaternary center at the acceptor position in good yield and high diastereoselectivity
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