Sc(OTf)3 Catalyzed [4 + 2]-Annulation Reaction between Electron-Rich Phenols and Donor–Acceptor Cyclopropanes: Synthesis of Polysubstituted Dihydronaphthols

Sc(OTf)3 catalyzed intramolecular single-electron transfer of 2-alkyl-1,4-benzoquinones: synthesis of 6-chromanols from donor-acceptor cyclopropanes

A Sc(OTf)3 catalyzed intramolecular cyclization reaction of 2-alkyl-1,4-benzoquinone derived from D-A cyclopropane was discovered. This reaction involves single-electron transfer, proton-transfer, an aromatization driven spin center shift, and radical coupling processes, and offers an efficient method for the synthesis of 6-chromanols from D-A cyclopropanes.

Ambident reactivity of 5-aminopyrazoles towards donor-acceptor cyclopropanes

Lewis acid-catalysed reactions of donor-acceptor cyclopropanes with 1,3-disubstituted 5-aminopyrazoles were investigated. Under catalysis with gallium(III) chloride, products of the three-membered ring opening via a nucleophilic attack of the exocyclic amino group were obtained in a chemoselective manner. Oppositely, in the presence of scandium(III) triflate, products of either N-alkylation or C(4)-alkylation, or a mixture of both were formed. The products of the C(4) alkylation were transformed in one step into tetrahydropyrazolo[3,4-b]azepines that are attractive for medicinal chemistry and pharmacology.

HFIP in Organic Synthesis

1,1,1,3,3,3-Hexafluoroisopropanol (HFIP) is a polar, strongly hydrogen bond-donating solvent that has found numerous uses in organic synthesis due to its ability to stabilize ionic species, transfer protons, and engage in a range of other intermolecular interactions. The use of this solvent has exponentially increased in the past decade and has become a solvent of choice in some areas, such as C-H functionalization chemistry. In this review, following a brief history of HFIP in organic synthesis and an overview of its physical properties, literature examples of organic reactions using HFIP as a solvent or an additive are presented, emphasizing the effect of solvent of each reaction.

Direct activation of alcohols via perrhenate ester formation for an intramolecular dehydrative Friedel–Crafts reaction

A general and highly efficient intramolecular dehydrative Friedel–Crafts reactions via Re2O7 mediated hydroxyl group activation is described for the syntheses of tetrahydronaphthalene, tetrahydroquinoline, tetrahydroisoquinoline, chromane, and isochromane derivatives.

DDQ-Catalyzed Oxidative C(sp3)-H Functionalization of Aryltetralins and Subsequent Chemoselective Oxidative Demethylation to Access Dihydronaphthalenes and Dihydronaphthoquinones

A highly controlled DDQ-catalyzed oxidative C(sp³)-H functionalization of three contiguous carbon atoms in aryltetralins is reported for efficient access to diverse oxygenated dihydronaphthalene scaffolds. The first total synthesis of pachypostaudin B is realized. Further, a CAN-mediated chemoselective oxidative demethylation on the dihydronaphthalene scaffolds is demonstrated to arrive at the rarely observed dihydronaphthoquinone core in moderate to good yields. The present methodology enables quick access to a library of magnoshinin and merrilliaquinone analogs.

Recent advances in ring-opening of donor acceptor cyclopropanes using C-nucleophiles

Ring-opening transformations of donor-acceptor cyclopropanes (DAC) with carbon-centered nucleophiles is a simple, straight-forward approach to 1,3-bifunctional compounds that has witnessed remarkable progress over the past several years. To date, different reactivity patterns of DACs have been successfully exploited in racemic/stereoselective syntheses of various acyclic compounds or carbocycles with an impressive structural diversity. The thriving strategies have been successfully utilized in multistep synthesis of complex target molecules. Herein, the recent advances (2015-present) in the ring-opening of DAC involving electron rich arenes and indoles, active methylene compounds, various dipolarophiles, organoborates/boronates, vinyl ethers etc. following Friedel-Crafts alkylation, annulation/formal cycloaddition reaction, organocatalytic reaction, Nazarov cyclisation etc. are presented.

Exploiting hexafluoroisopropanol (HFIP) in Lewis and Brønsted acid-catalyzed reactions

Hexafluoroisopropanol (HFIP) is a solvent with unique properties that has recently gained attention for promoting a wide range of challenging chemical reactions. It was initially believed that HFIP was almost exclusively involved in the stabilization of cationic intermediates, owing to its high polarity and low nucleophilicity. However, in many cases, the mechanism of action of HFIP appears to be more complex. Recent findings reveal that many Lewis and Brønsted acid-catalyzed transformations conducted in HFIP additionally involve cooperation between the catalyst and HFIP hydrogen-bond clusters, akin to Lewis- or Brønsted acid-assisted-Brønsted acid catalysis. This feature article showcases the remarkable versatility of HFIP in Lewis and Brønsted acid-catalyzed reactions, with an emphasis on examples yielding mechanistic insight.

Lewis acid catalysed asymmetric cascade reaction of cyclopropyl ketones: concise synthesis of pyrrolobenzothiazoles

A concise method to synthesize pyrrolobenzothiazoles via a cascade reaction of cyclopropyl ketones was developed by using a chiral N,N′-dioxide-scandium(iii) complex catalyst.

Palladium catalysed [3 + 2]-annulation reaction of vinylcyclopropanes with pentafulvenes: synthesis of polysubstituted spiro[4,4]nona-6,8-dienes

A novel palladium catalysed [3 + 2]-annulation reaction of vinylcyclopropanes with pentafulvenes has been developed for the synthesis of spiro[4,4]nona-6,8-dienes.

Expanding the Reactivity of Donor-Acceptor Cyclopropanes: Synthesis of Benzannulated Five-Membered Heterocycles via Intramolecular Attack of a Pendant Nucleophilic Group

Lewis-acid-induced domino transformations of donor-acceptor cyclopropanes, possessing a nucleophilic center embedded in a donor group, into functionalized 2,3-dihydrobenzo[ b]furans and 2,3-dihydrobenzo[ b]thiophenes are reported herein. An unusual switch of the electrophilic center in the three-membered ring, from the atom bearing a donor substituent to an unsubstituted carbon atom, was achieved by a judicious choice of Lewis acid, which induces the isomerization of a cyclopropane to an electrophilic alkene, and the length of linker, connecting a nucleophilic moiety and the small ring.

Asymmetric ring-opening of cyclopropyl ketones with β-naphthols catalyzed by a chiral N,N′-dioxide–scandium(iii) complex

An efficient asymmetric ring-opening reaction of cyclopropyl ketones with 2-naphthols has been realized using a chiral N,N′-dioxide–Sc^III complex catalyst. A variety of chiral β-naphthol derivatives were obtained in excellent outcomes (up to 99% yield, 97% ee).

Donor–acceptor cyclopropanes as ortho-quinone methide equivalents in formal (4 + 2)-cycloaddition to alkenes

A new reactivity of donor–acceptor cyclopropanes as synthetic equivalents of o-quinone methides was revealed for 2-hydroxyarylcyclopropane diesters in their formal (4 + 2)-cycloaddition to alkenes yielding chromanes.

Lewis acid catalyzed diastereoselective [3+4]-annulation of donor–acceptor cyclopropanes with anthranils: synthesis of tetrahydro-1-benzazepine derivatives

The [3+4]-annulation between donor–acceptor cyclopropanes and anthranils has been developed for the diastereoselective synthesis of tetrahydro-1-benzazepine derivatives.