Formal High-Order Cycloadditions of Donor-Acceptor Cyclopropanes with Cycloheptatrienes

The design of various cycloaddition/annulation processes is one of the most intriguing challenges in the development of donor-acceptor (D-A) cyclopropane chemistry. In this work, a new class of formal high-order [6+n]-cycloaddition and annulation processes of D-A cyclopropanes with cycloheptatriene systems has been designed and reported, to fill a significant gap in the chemistry of D-A cyclopropanes. The reactivity of methylated cycloheptatrienes from Me1 to Me5 as well as unsubstituted cycloheptatriene was studied in detail under GaCl3 activation conditions, which makes it possible to efficiently generate gallium 1,2-zwitterionic complexes or 1,3-zwitterionic intermediates starting from D-A cyclopropanes, while other Lewis acids are ineffective and non-selective. New examples of formal [6+2]-, [6+3]-, [6+4]-, [6+1]-, and [4+2]-cycloaddition and annulation reactions with cycloheptatrienes along with more complex processes were discovered. Cycloheptatriene itself can also successfully act as a hydride anion donor, which allows the ionic hydrogenation of D-A cyclopropanes to be performed under mild conditions. As a result, a number of efficient and highly diastereoselective protocols for the synthesis of seven-membered carbocycles has been developed.

Quantum Chemical Study of the Cycloaddition Reaction of Tropone with 1,1-Diethoxyethene Catalyzed by B(C6F5)3 or BPh3

Cycloaddition reaction of tropone with 1,1-diethoxyethene catalyzed by Lewis acid (LA), B(C6F5)3 or BPh3, was examined by using ωB97X-D-level density functional theory (DFT) calculations. In the absence of LA, the reaction proceeds in a stepwise fashion to form two chemical bonds, first between the C2 atom in tropone and the C2 atom in ethene and then between the C5 atom in the former and the C1 atom in the latter. When B(C6F5)3 is attached to the O atom in tropone, the C5 atom in tropone is attacked preferentially by the C1 atom in ethene in the second stage. The attack of the O atom in tropone is shown to be less likely; thus, the [4 + 2] addition is favored in the B(C6F5)3-catalyzed reaction. In contrast, the attack of the O atom in the BPh3-attached tropone to the C1 atom in ethene is preferred over the attack of the C5 atom, indicating that the [8 + 2] cycloaddition instead of the [4 + 2] cycloaddition proceeds in the BPh3-catalyzed reaction. Whether the C1 atom in ethene is attacked by C5 or by O in the second bond formation step is shown in this study to be governed mainly by the nucleophilicity of σ-lone pair electrons of the carbonyl O atom of tropone in the presence of LA. These results are consistent with the experiments reported by Li and Yamamoto.

Enantioselective (8+3) Cycloadditions by Activation of Donor-Acceptor Cyclopropanes Employing Chiral Brønsted Base Catalysis

A novel enantioselective (8+3) cycloaddition between donor-acceptor cyclopropanes and heptafulvenoids catalysed by a chiral bifunctional Brønsted base is described. Importantly, the reaction, which leverages an anionic activation strategy, is divergent from prototypical Lewis-acid activation protocols. A series of cyclopropylketones react with tropones affording the desired (8+3) cycloadducts in high yield and enantiomeric excess. For barbiturate substituted heptafulvenes, the (8+3) cycloaddition with cyclopropylketones proceeds in good yield, excellent diastereoselectivity and high enantiomeric excess. The experimental work is supported by DFT calculations, which indicate that the bifunctional organocatalyst activates both the donor-acceptor cyclopropane and tropone; the reaction proceeds in a step-wise manner with the ring-closure being the stereodetermining step.

Tandem [5 + 1]/[8 + 2] cycloaddition reactions involving phosphiranes and tropones: facile access to 6,5,7-fused tricyclic skeletons

Tandem [5 + 1] carbonyl cyclization/[8 + 2] cycloaddition reactions of phosphiranes and tropones were developed as a straightforward method to access 6,5,7-fused tricyclic scaffolds.

Recyclable heterogeneous gold(I)-catalyzed oxidative ring expansion of alkynyl quinols: a practical access to tropone and its analogues

The heterogeneous gold(i)-catalyzed oxidative ring expansion of alkynyl quinols has been achieved by using a benzyldiphenylphosphine-modified MCM-41-immobilized gold(i) complex [MCM-41-BnPh2P-AuNTf2] as the catalyst and 8-methylquinoline N-oxide as the oxidant under mild reaction conditions, yielding a variety of functionalized tropone derivatives in good to excellent yields. Extension of this methodology allows for facile construction of other seven- or six-membered ring systems including dibenzotropones, dibenzooxepines, phenanthrenes, and quinolin-2(1H)-ones. This new heterogeneous gold(i) complex can be readily recovered through a simple filtration process and recycled at least eight times without any apparent decrease in catalytic efficiency.

Organocatalytic Enantioselective 1,3-Dipolar [6+4] Cycloadditions of Tropone

A highly stereoselective 1,3-dipolar [6+4] cycloaddition towards bridged azabicyclo[4.3.1]decane scaffolds has been developed, reacting aldehydes, 2-aminomalonates and tropone under mild conditions in the presence of a chiral phosphoric acid catalyst. The scope is demonstrated for a series of aldehydes and 2-aminomalonates, and the reaction proceeds in high yields, >95:5 d.r. and up to 99 % ee. A series of transformations, as well as a mechanistic proposal, are presented.

Donor-Acceptor Cyclopropanes in the Synthesis of Carbocycles

Donor-acceptor cyclopropanes not only participate in a broad range of ring openings with nucleophiles, electrophiles, radical and red-ox agents, but also are excellent substrates for various (3+n)-cycloaddition and (3+n)-annulation processes. Moreover, under treatment with Lewis acid donor-acceptor cyclopropanes can produce new ring systems via isomerization or cyclodimerization. Authors' contribution to the synthesis of diverse carbocycles from donor-acceptor cyclopropanes is summarized in this account.

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).

Diastereoselective Intramolecular [3 + 2]-Annulation of Donor-Acceptor Cyclopropane with Imine-Assembling Hexahydropyrrolo[3,2-c]quinolinone Scaffolds

An intramolecular [3 + 2]-annulation of amide-linked donor-acceptor cyclopropane with in situ-generated imine is described. As a result, diverse hexahydropyrrolo[3,2-c]quinolinones, as the tricyclic core of martinellines, were efficiently assembled in good to excellent yield (up to 93%) with a good diastereomeric ratio (up to 98:2).

Amide-assisted intramolecular [3+2] annulation of cyclopropane ring-opening: a facile and diastereoselective access to the tricyclic core of (±)-scandine

The amide-linked intramolecular [3+2] annulation of cyclopropane ring-opening has been developed to rapidly and diastereoselectively install the tricyclic dihydroquinolinone core of (±)-scandine.

Ag-catalyzed diastereoselective [6 + 3] cycloaddition of tropone with homoserine lactone-derived azomethine ylides: synthesis of tricyclic spiropiperidines

Ag-catalyzed [6 + 3] cycloaddition of tropone with homoserine lactone derived azomethine ylides has been developed.