Stereoselective Sequential Spirocyclopropanation/Cloke-Wilson Rearrangement Reactions for Synthesis of trans-β,γ-Disubstituted γ-Butyrolactones Using Alkylidene Meldrum's Acid and Benzyl Halides

Intramolecular 1,2-Aroyl Migration in Spiro Donor-Acceptor Cyclopropanes: Formation of 1,4-Naphthoquinones and 1-Naphthols as Ring-Expansion Products

Most of the known rearrangement reactions of donor-acceptor cyclopropanes (DACs) involve the migration of cationic carbon atom to anionic carbon or heteroatoms in 1,3- or 1,4-positions. In the present work, we observed that spiro DACs based on 1,3-indanedione or 1-indanone moiety undergo intramolecular 1,2-aroyl migration when treated with titanium(IV) chloride to afford 1,4-naphthoquinones and α-naphthols readily. The reactions take place through the formation of putative 1,3-dipolar intermediates, followed by cleavage and migration of the aroyl group to the adjacent carbon to afford the ring-expansion products.

Formation of bis-spiropyrrolidines from isatin, secondary amines, and alkylidene Meldrum's acids

A catalyst-free synthesis of stable bis-spiropyrrolidines from isatin, secondary amines, and alkylidene Meldrum's acids in MeCN in 75-95% yield is described. The antioxidant and antimicrobial properties of the synthesized compounds are investigated. For this purpose, the radical scavenging activities of four derivatives were studied by radical trapping of diphenylpicrylhydrazine and ferric reduction power experiments. Disk diffusion test on Gram-positive and Gram-negative bacteria was employed to investigate antibacterial activities of five derivatives.

Regio- and Stereospecific Hydrative Cloke-Wilson Rearrangement

The Cloke-Wilson rearrangement of unsymmetrical β-diketone-derived cyclopropanes inevitably yields a mixture of two 4-acylated 2,3-dihydrofuran regiomers. By using alkynes as masked acyls, Tf2NH-promoted Cloke-Wilson rearrangement of polysubstituted 1-(1-alkynyl)cyclopropyl ketones followed by alkyne hydration is described, regioselectively affording 2,3-dihydrofurans bearing 4-acyls nonequivalent to that involved in the Cloke-Wilson rearrangement. The 2,3-dihydrofuran rings with cis 2,3-diaryls are unexpectedly more stable than their trans diastereomers under the reaction conditions, guaranteeing the regiospecificity of this hydrative Cloke-Wilson rearrangement with high fidelity.

Organocatalytic Cloke-Wilson Rearrangement: Carbocation-Initiated Tandem Ring Opening/Cyclization of Cyclopropanes under Neutral Conditions

We report a metal-, acid-, and base-free 2-(bromomethyl)naphthalene (2-BMN)-promoted organocatalytic Cloke-Wilson rearrangement of chain doubly activated cyclopropanes for the construction of 2,3-dihydrofurans via a carbocation-initiated tandem intramolecular ring-opening/recyclization process. The strategy is especially suitable for the construction of furan units in complex molecules, providing a solution to the problem of heavy-metal residues in dihydrofuran-containing drugs synthesized by traditional metal-based protocols. Thus, it is of potential interest in synthetic and medicinal chemistry.

The Cloke-Wilson rearrangement of aroyl-substituted donor-acceptor cylopropanes containing arylethyl donors

The chemistry of donor-acceptor (D-A) cyclopropanes containing alkyl donors has been scantily investigated. In the present work, we have synthesized new D-A cyclopropanes containing arylethyl donors and explored their reactivity in the presence of Lewis acids. Upon treatment with SnCl₄, these cyclopropanes underwent the Cloke-Wilson rearrangement to yield 3,4,5-trisubstituted γ-butyrolactones in good yields with high diastereoselectivity.

Inherent Reactivity of Spiro-Activated Electrophilic Cyclopropanes

The kinetics of the ring‐opening reactions of thiophenolates with geminal bis(acceptor)‐substituted cyclopropanes in DMSO at 20 °C was monitored by photometric methods. The determined second‐order rate constants of the S_N2 reactions followed linear relationships with Mayr nucleophilicity parameters (N/s_N) and Brønsted basicities (pK_aH) of the thiophenolates as well as with Hammett substituent parameters (σ) for groups attached to the thiophenolates. Phenyl‐substituted cyclopropanes reacted by up to a factor of 15 faster than their unsubstituted analogues, in accord with the known activating effect of adjacent π‐systems in S_N2 reactions. Variation of the electronic properties of substituents at the phenyl groups of the cyclopropanes gave rise to parabolic Hammett relationships. Thus, the inherent S_N2 reactivity of electrophilic cyclopropanes is activated by electron‐rich π‐systems because of the more advanced C1−C2 bond polarization in the transition state. On the other hand, electron‐poor π‐systems also lower the energetic barriers for the attack of anionic nucleophiles owing to attractive electrostatic interactions.

Expanding Stereoelectronic Limits of endo-tet Cyclizations: Synthesis of Benz[b]azepines from Donor-Acceptor Cyclopropanes

The importance of intramolecular constraints in cyclic transition-state geometries is especially pronounced in n-endo-tet cyclizations, where the usual backside approach of a nucleophile to the breaking bond is impossible for the rings containing less than eight atoms. Herein, we expand the limits of endo-tet cyclizations and show that donor-acceptor cyclopropanes can provide a seven-membered ring via a genuine 6-endo-tet process. Substrates containing a N-alkyl-N-arylcarbamoyl moiety as an acceptor group undergo Lewis acid-induced cyclization to form tetrahydrobenz[b]azepin-2-ones in high yields. The reaction proceeds with the inversion of the configuration at the electrophilic carbon. In this process, a formally six-membered transition state yields a seven-membered ring as the pre-existing cycle is merged into the forming ring. The stereochemistry of the products can be controlled by the reaction time and by the nature of Lewis acid, opening access to both diastereomers by tuning of the reaction conditions.

Selected applications of Meldrum's acid - a tutorial

Due to its unique structure and the vast array of substituents that can be attached to its core, Meldrum's acid is a molecule with exceptional chemical properties. In water, it has a remarkably low pKa value of about 4.9. Its C5 position is readily involved in electrophilic substitution reactions whereas the C4 and C6 positions are easily attacked by nucleophiles. At elevated temperatures Meldrum's acid undergoes distinctive decomposition pathways, which can be used in cycloaddition and acylation reactions. In this Tutorial Review, the authors intend to introduce the principles of the synthetic chemistry of Meldrum's acid and provide the essential knowledge for the design and preparation of compounds with desired properties. As there are many reviews focusing on a specific detail of Meldrum's acid chemistry, we would like to give a broader picture of this diverse molecule for undergraduate and graduate students as well as experienced lab leaders. For achieving this goal, some recent advances in using Meldrum's acid derivatives in synthetic scenarios are presented with the hope to further stimulate and promote research leading to additional innovative applications of this synthetically highly relevant molecule.

Reverse orientation in the ultrasound-assisted [3 + 2]-cycloaddition reaction of nitrile imines with 3-formylchromone-Meldrum's acid adducts

The [3 + 2]-cycloaddition reaction of nitrile imines with 2,2-dimethyl-5-[(4-oxo-4H-chromen-3-yl)methylene]-1,3-dioxane-4,6-dione tends to form the reverse-orientation products under ultrasound irradiation in EtOH in the presence of Et₃N. Evidence for the structure of product 5b was obtained from single-crystal X-ray analysis.

Synthesis of disubstituted γ-butyrolactones and spirocyclopropanes via a multicomponent reaction of aldehydes, Meldrum's acid and sulfoxonium ylides

An efficient synthesis of disubstituted γ-butyrolactones and spirocyclopropanes via multicomponent reaction of aldehydes, Meldrum's acid and sulfoxonium ylides has been developed.

N-Heterocyclic carbene (NHC)-catalyzed oxidative [3+2] annulation of dioxindoles and enals: mechanism, role of NHC, role of a mixture of bases with different strength, and origin of stereoselectivity

Over recent years, in-depth understanding of the mechanism of oxidative N-heterocyclic carbene (NHC) catalyzed reactions in the presence of a mild oxidant and the structure of key radical intermediates have been considered as an important challenge in organic chemistry. Furthermore, the role of using a mixture of bases with different strengths is unclear in NHC-catalyzed reactions. In this paper, the detailed competing oxidative mechanisms, origin of stereoselectivity, and role of the NHC-organocatalyst in the NHC-catalyzed reactions of dioxindoles with enals were studied using the density functional theory method. In addition, the roles of newly produced Brønsted acids of the applied bases, i.e.DBU·H+ and DABCO·H+, are examined. The computational results indicated that the oxidation of the Breslow intermediate by nitrobenzene (NB) occurs first through a hydrogen atom transfer (HAT) pathway from the Breslow intermediate, and then it is oxidized into acyl azolium by single electron transfer (SET). We found that the energy barrier of the proton transfer processes is remarkably reduced by the conjugated Brønsted acid of the weaker base in the solution. Further, the calculated results revealed that the NHC catalyst has different behavior before and after the oxidation of the Breslow intermediate in these reactions. Before oxidation, the nucleophilicity of R1 increased by adding R1 to NHC, while, after the oxidation process, the electrophilicity of R1 increases, and as a result the product of oxidation, α, β unsaturated acyl azolium, acts as an electrophile. This mechanistic study paves the way for the rational design of oxidative NHC-catalyzed reactions.

A thermal decarboxylative Cloke–Wilson rearrangement of dispirocyclopropanes derived from para-quinone methides and bromo-Meldrum's acids: an approach to synthesize spirobutyrolactone para-dienones

An unprecedented approach to synthesize spirobutyrolactone para-dienones from para-quinone methides and bromo-Meldrum's acids has been developed.