A unified synthesis of all stereoisomers of 2-(aminomethyl)cyclobutane-1-carboxylic acid

Photosensitized [2 + 2]-Cycloadditions of Dioxaborole: Reactivity Enabled by Boron Ring Constraint Strategy

A strategy to achieve photosensitized [2 + 2] cycloadditions by means of temporary ring constraint is reported. Specifically, a dioxaborole is prepared that undergoes [2 + 2] cycloadditions with a wide variety of alkenes. This strategy overcomes some challenges with the cycloaddition of acyclic substrates. The products can be easily transformed into cyclobutyl diols or 1,4-dicarbonyl compounds; the latter represents a formal alkene vicinal diacylation. The synthetic utility of this method is shown in the synthesis of valuable heterocycles and biatriosporin D.

Expanding the Chemical Space of 1,2-Difunctionalized Cyclobutanes

An efficient approach to the synthesis of previously unavailable or hardly accessible 1,2-difunctionalized cyclobutanes (mostly with NH₂/NHBoc, OH, SH, or SO₂F groups attached to the carbocycle either directly or via a CH₂ unit) relying on the divergent strategy is described. This class of compounds provides sp³-enriched and conformationally restricted building blocks that are of special demand for medicinal chemistry. The target compounds were prepared not only as pure racemic (±)-cis- and (±)-trans-diastereomers but in some cases also as single enantiomers. The developed procedures are readily scaled up and allow obtaining the target compounds on an up to hundred-gram scale. On the basis of the results of 20 X-ray diffraction experiments, structural characterization of the 1,2-difunctionalized cyclobutane core was performed using the extended Cremer-Pople puckering parameters and exit vector (EVP) plots.

Acetylene and Ethylene: Universal C2 Molecular Units in Cycloaddition Reactions

Acetylene and ethylene are the smallest molecules that contain an unsaturated carbon–carbon bond and can be efficiently utilized in a large variety of cycloaddition reactions. In this review, we summarize the application of these C2 molecular units in cycloaddition chemistry and highlight their amazing synthetic opportunities.

1 Introduction

2 Fundamental Features and Differences of Cycloaddition Reactions Involving Acetylene and Ethylene

3 (2+1) Cycloaddition

4 [2+2] Cycloaddition

5 (3+2) Cycloaddition

6 [4+2] Cycloaddition

7 (2+2+1) Cycloaddition

8 [2+2+2] Cycloaddition

9 The Use of Acetylene and Ethylene Cycloaddition for Deuterium and 13C Labeling

10 Conclusions

Carbocycle-Based Organogelators: Influence of Chirality and Structural Features on Their Supramolecular Arrangements and Properties

The rational design and engineer of organogel-based smart materials and stimuli-responsive materials with tuned properties requires the control of the non-covalent forces driving the hierarchical self-assembly. Chirality, as well as cis/trans relative configuration, also plays a crucial role promoting the morphology and characteristics of the aggregates. Cycloalkane derivatives can provide chiral chemical platforms allowing the incorporation of functional groups and hydrophobic structural units able for a convenient molecular stacking leading to gels. Restriction of the conformational freedom imposed by the ring strain is also a contributing issue that can be modulated by the inclusion of flexible segments. In addition, donor/acceptor moieties can also be incorporated favoring the interactions with light or with charged species. This review offers a perspective on the abilities and properties of carbocycle-based organogelators starting from simple cycloalkane derivatives, which were the key to establish the basis for an effective self-assembling, to sophisticated polycyclic compounds with manifold properties and applications.

Photochemical [2 + 2] Cycloaddition of Alkenyl Boronic Derivatives: An Entry into 3-Azabicyclo[3.2.0]heptane Scaffold

The synthesis of 3-azabicyclo[3.2.0]heptyl boropinacolates and trifluoroborates via the [2 + 2] photocycloaddition of the corresponding alkenyl boronic derivatives and maleimides or maleic anhydride is described. Optimization of the reaction conditions (i.e., wavelength, concentration of the reagents, photosensitizer) was carried out, and the scope and limitations of the method were studied. Alkenyl boronic acid pinacolates were found to be more suitable for the [2 + 2] cycloaddition, providing better reaction outcomes compared to the trifluoroborates. The utility of this approach was shown by the preparation of bi- and trifunctional building blocks (21 examples), which could be easily synthesized on up to 60 g scale. These cycloadducts provide a convenient entry into the 3-azabicyclo[3.2.0]heptane scaffold through the C-C coupling or oxidative deborylation reactions.

Cyclobutane-Containing Scaffolds as Useful Intermediates in the Stereoselective Synthesis of Suitable Candidates for Biomedical Purposes: Surfactants, Gelators and Metal Cation Ligands

Efficient and versatile synthetic methodologies are reported for the preparation of products that are suitable candidates to be used as surfactants, gelators for hydroxylic solvents or metal cation ligands, with potential use in several fields including biomedical applications. The common structural feature of all the synthesized products is the presence of a cis or trans-1,2- or cis-1,3-difunctionalized cyclobutane ring. In the two first cases, the key intermediates including enantiomerically pure 1,3-diamines and 1,3-amino alcohols have been prepared from β-amino acid derivatives obtained, in turn, from a chiral half-ester. This compound is also precursor of γ-amino esters. Furthermore, two kind of polydentate ligands have also been synthesized from a symmetric 1,5-diamine obtained from norpinic acid, which was easily prepared from commercial verbenone.

Conformational Effects through Hydrogen Bonding in a Constrained γ-Peptide Template: From Intraresidue Seven-Membered Rings to a Gel-Forming Sheet Structure

A series of three short oligomers (di-, tri-, and tetramers) of cis-2-(aminomethyl)cyclobutane carboxylic acid, a γ-amino acid featuring a cyclobutane ring constraint, were prepared, and their conformational behavior was examined spectroscopically and by molecular modeling. In dilute solutions, these peptides showed a number of low-energy conformers, including ribbonlike structures pleated around a rarely observed series of intramolecular seven-membered hydrogen bonds. In more concentrated solutions, these interactions defer to an organized supramolecular assembly, leading to thermoreversible organogel formation notably for the tripeptide, which produced fibrillar xerogels. In the solid state, the dipeptide adopted a fully extended conformation featuring a one-dimensional network of intermolecularly H-bonded molecules stacked in an antiparallel sheet alignment. This work provides unique insight into the interplay between inter- and intramolecular H-bonded conformer topologies for the same peptide template.

Recent Advances in the Synthesis of Cyclobutanes by Olefin [2 + 2] Photocycloaddition Reactions

The [2 + 2] photocycloaddition is undisputedly the most important and most frequently used photochemical reaction. In this review, it is attempted to cover all recent aspects of [2 + 2] photocycloaddition chemistry with an emphasis on synthetically relevant, regio-, and stereoselective reactions. The review aims to comprehensively discuss relevant work, which was done in the field in the last 20 years (i.e., from 1995 to 2015). Organization of the data follows a subdivision according to mechanism and substrate classes. Cu(I) and PET (photoinduced electron transfer) catalysis are treated separately in sections 2 and 4 , whereas the vast majority of photocycloaddition reactions which occur by direct excitation or sensitization are divided within section 3 into individual subsections according to the photochemically excited olefin.