Ring contraction in synthesis of functionalized carbocycles

Carbocycles are a key and widely present structural motif in organic compounds. The construction of structurally intriguing carbocycles, such as highly-strained fused rings, spirocycles or highly-functionalized carbocycles with congested stereocenters, remains challenging in organic chemistry. Cyclopropanes, cyclobutanes and cyclopentanes within such carbocycles can be synthesized through ring contraction. These ring contractions involve re-arrangement of and/or small molecule extrusion from a parental ring, which is either a carbocycle or a heterocycle of larger size. This review provides an overview of synthetic methods for ring contractions to form cyclopropanes, cyclobutanes and cyclopentanes en route to structurally intriguing carbocycles.

A facile and efficient route to one-pot synthesis of new cyclophanes using vinamidinium salts

In this study, an efficient method for the synthesis of new cyclophanes (5a-f, 6a-g) through the condensation of 1,4-phenylenedimethanamine (3) or 2,3,5,6-tetramethylbenzene-1,4-diamine (4) with 2-substituted vinamidiniums (2a-g) is described. The cyclophane derivatives are obtained in good to excellent yields in the presence of acetic acid in refluxing acetonitrile after 15 h. The structure of new compounds was validated based on their spectral data (1H NMR, 13C NMR, IR) and elemental analysis.

"Shadow" Synthesis, Structure, and Electronic Properties of [2.2](1,6)(1,8)Pyrenophane-1-monoene

An unexpected side product of a McMurry reaction was found to be a new [2.2]pyrenophane consisting of two pyrene units with different substitution patterns as well as different types and degrees of distortion from planarity. The new pyrenophane exhibits both monomer and intramolecular excimer fluorescence. Natural bond orbital (NBO) analysis revealed that there is an intramolecular charge-transfer interaction from the more distorted pyrene system to the less distorted one. The origin of the new pyrenophane was traced back to an impurity that was present a full five steps prior to the McMurry reaction from which it was isolated. The pathway to the pyrenophane shadowed that of the main synthetic route.

Design and Synthesis of Polycycles, Heterocycles, and Macrocycles via Strategic Utilization of Ring-Closing Metathesis

In this perspective, we summarize new synthetic approaches for the construction of various polycyclic compounds involving ring-closing metathesis as a key step. In this regard, we used ring-closing metathesis in combination with other popular reactions like Suzuki-Miyaura coupling, Claisen rearrangement, Fischer indolization, Grignard addition, Diels-Alder reaction, and [2+2] cycloaddition reaction etc. To this end, a variety of functional molecules such as α-amino acids, cyclophanes, heterocycles, propellanes, spirocycles, and macrocycles have been prepared. The strategies developed and the molecules prepared here play a key role in designing new materials and also act as lead compounds in drug design. The strategies and tactics developed here are useful to design polycycles, macrocycles, and heterocycles of diverse ring systems.

Synthesis of a Cyclophane Bearing Two Benz[a]anthracene Units Connected at the 5 and 7 Positions with Two Naphth-1,4-diyl Groups

A synthetic pathway to a cyclophane bearing two benz[a]anthracene units connected at the 5 and 7 positions through two naphth-1,4-diyl groups was developed, and its structure was confirmed by X-ray structure analysis. Because of structural constraints, the two naphthyl groups are distorted from planarity and the bonds connecting them to the benz[a]anthracene units are bent significantly. The UV-vis and fluorescence spectra of the cyclophane are red-shifted from those of 7-(1-naphthalenyl)benz[a]anthracene, which is the corresponding monomeric polycyclic aromatic hydrocarbon.

Generation of an extremely bent pyrene system using kinetic stabilization

A series of pyrenophanes was designed with the goal of providing kinetic stabilization to the bent pyrene system and, therefore, providing a means to generate more highly bent pyrene systems than ever before. Four members of this series were synthesized, isolated, and characterized. The lowest homologue was calculated to have a pyrene system that is just slightly less bent (θcalc = 106.8°) than the previous record holder for an isolated compound (θexpt = 109.2°). The next smaller member of the series was obtained as a minor component of a mixture with its direct synthetic precursor and a follow-on product after a 60 d reaction. Compelling MS and 1H NMR evidence was obtained for the formation of this compound; the pyrene system of which was calculated be extremely bent (θcalc = 130.4°).

Synthesis of Indolophanes by Photochemical Macrocyclization

Herein, we report the synthetically practical, short, and general access to novel indolophane architectures by means of a photochemical C-H activation process-the Witkop cyclization. These highly strained scaffolds were obtained by photoinduced ring closure and feature atropisomerism as well as aromatic ring current effects, which both have been investigated. The prevailing regioselectivity of theWitkop cyclization reaction was completely reversed by the presence of a quaternary carbon center, exerting a strong Thorpe-Ingold effect on the system for which experimental-evidence is provided.

A facile route to old and new cyclophanes via self-assembly and capture

Cyclophanes are a venerable class of macrocyclic and/or cage compounds that often feature high strain, unusual conformations and quite surprising properties, many of which are legendary in physical organic chemistry. However, the discovery of new, diverse cyclophanes and derivatives has been hindered by syntheses that are traditionally low-yielding, requiring long reaction times, laborious purification steps and often extreme conditions. Herein, we demonstrate a new self-assembly route to a variety of discrete cyclic and caged disulfide structures, which can then be kinetically captured upon sulfur extrusion at room temperature to give a diversity of new thioether (hetera)cyclophanes in high yield. In addition to the synthesis of novel macrocycles (dimers through hexamers), this process provides an improved route to a known macrobicyclic trithiacyclophane. This technique also enables the facile isolation of a tetrahedral macrotricyclic tetrathiacyclophane in two steps at an ambient temperature.

Exploiting ortho-substitution effect on formation of oxygen-containing [10]paracyclophane through ring-closing metathesis

The synthesis of strained paracyclophanes is facilitated byortho-substitution effect on phenyl ring.

Selected synthetic strategies to cyclophanes

In this review we cover various approaches to meta- and paracyclophanes involving popular reactions. Generally, we have included a strategy where the reaction was used for assembling the cyclophane skeleton for further functionalization. In several instances, after the cyclophane is made several popular reactions are used and these are not covered here. We included various natural products related to cyclophanes. To keep the length of the review at a manageable level the literature related to orthocyclophanes was not included.

Cyclophanes containing large polycyclic aromatic hydrocarbons

Cyclophanes have been firmly entrenched as a distinct class of compounds for well over half a century. The two main factors that have kept this field of chemistry going so strongly for such a long time are tremendous structural diversity and the interesting behaviour that is often observed. Although a very large number cyclophanes has been reported, only a very small proportion of them contain polycyclic aromatic systems that can be thought of as "large", i.e. with ≥4 rings. This Review puts the spotlight on such cyclophanes, illuminating both the chemistry that was used to synthesize them and what was learned from studying them. Context for the main body is provided by the careful consideration of the anatomy of a cyclophane and the classification of general synthetic approaches. The subsequent sections cover eleven different PAHs and are organized primarily according to increasing size of the aromatic system, starting with pyrene (C16, the only large polycyclic aromatic system to have been incorporated into numerous cyclophanes) and ending with hexabenzo[bc,ef,hi,kl,no,qr]coronene (C42).

1,1,n,n-Tetramethyl[n](2,11)teropyrenophanes (n = 7-9): a series of armchair SWCNT segments

A new iterative bridge formation strategy has been employed in the synthesis of a series of [n](2,11)teropyrenophanes (n = 7-9). The generation of the nonplanar teropyrene system, which is calculated to be bent through 178.7° for the smallest homologue (n = 7), is accomplished using a VID reaction of a cyclophanemonoene precursor for the first time.

Diversity oriented approach to crownophanes by enyne metathesis and Diels-Alder reaction as key steps

Various crownophanes are assembled starting with simple phenol derivatives such as catechol, resorcinol, and hydroquinone. Here, cross-enyne metathesis (CEM) and Diels-Alder (DA) reaction have been used as key steps. This strategy has embedded six diversity points.