Ni(ClO₄)₂‐Catalyzed Friedel–Crafts Reaction of Coumarin‐Fused Donor–Acceptor Cyclopropanes with Indoles: Stereoselective Synthesis of trans‐3,4‐Disubstituted‐3,4‐dihydrocoumarins

Abstract

A stereoselective synthesis of trans‐4‐(3‐indolylmethyl)‐3,4‐dihydrocoumarins has been developed. The method proceeds through a catalytic Friedel–Crafts reaction of coumarin‐fused donor–acceptor cyclopropanes with indoles. The use of Ni(ClO4)2 as a Lewis acid catalyst leads to the formation of a variety of 4‐(3‐indolylmethyl)‐3,4‐dihydrocoumarins in good yields with high diastereoselectivities.

Ni(ClO4)2‐Catalyzed Friedel–Crafts Reaction of Coumarin‐Fused Donor–Acceptor Cyclopropanes with Indoles: Stereoselective Synthesis of trans‐3,4‐Disubstituted‐3,4‐dihydrocoumarins

“…Fur-thermore, the reaction between 2-methylindole and coumarin fused cyclopropane 1a proceeded smoothly to furnish the desired 3,4-dihydrocoumarin 3j in 98 % yield with excellent diastereoselectivity. The relative stereochemistry of 3a was confirmed by X-ray crystallographic analysis, [15] in which the indolylmethylene group was found to be trans to the ester group (Figure 2). The configurations of the other products were assigned by analogy.…”

Convenient Synthesis of Functionalized Cyclopropa[c]coumarin-1a-carboxylates

“…Moreover, this cyclopropacoumarin remains intact under heating with both tin(II) triflate in dichloromethane and trimethylsilyl triflate in chlorobenzene for several hours. The absence of benzoxepane derivative in the reaction mixtures together with the aforementioned literature data on the nucleophilic attack of cyclopa[ c ]coumarins at the CH 2 atom [34] demonstrate the decelerating effect of annulation on the reactivity of the C-C bond between atoms connected to donor and acceptor substituents. The effect of the annulated ring nature on the reactivity of donor-acceptor cyclopropanes deserve, evidently, a careful study.…”

“…The obtained cyclopropa[ c ]coumarins 2 are potent substrates for the synthesis of diverse coumarin derivatives (Scheme 5). Thus, it was recently shown that such cyclopropacoumarins undergo Ni(ClO 4 ) 2 -catalyzed nucleophilic ring opening under treatment with indole affording 4-(indolylmethyl)chroman-2-one-3-carboxylates [34].…”

“…Yield 75 mg (72%); colorless solid; mp = 98–100 °C (lit. 100–102 °C [34,35]; oil [37]). Spectral data are consistent with the reported ones [34,35].…”

“…The target cyclopropa[ c ]coumarins were obtained in low-to-moderate yields when the reaction was performed at 0 °C (for 3-ethoxycarbonyl-, 3-pivaloyl- and 3-cyanocoumarins) or at −40 °C (for 3-acetyl and 3-benzoyl derivatives) (Scheme 1f) [31]. Very recently it was proposed to perform this cyclopropanation using very slow addition of the Corey ylide solution (5 mL for 6 h) to the corresponding coumarin derivative, however, product yields were not given [34,35]. Therefore, the development of efficient procedure for the synthesis of 1-unsubstituted cyclopropa[ c ]coumarins remains a challenging task for organic chemists.…”