A study of aromatic three membered rings

The resonance energies (REs) of neutral three membered ring analogs of the cyclopropenyl cation, computed using block localized wave function (BLW) methods, reveal considerable variations. The RE's of cyclopropenes substituted with exocyclic double bonded groups C?X, (X = O, NH, CH₂, S, PH, SiH₂) increase with the electronegativity of X in the same row (SiH₂ < PH < S and CH₂ < NH < O). The extra cyclic resonance energies (ECREs) (an energetic measure of aromaticity based on comparisons with the RE's of acyclic models) of these derivatives range from +10.5 kcal/mol for cyclopropenone (X = O) (somewhat aromatic; the benzene ECRE is 29.3 kcal/mol) to ?2.4 kcal/mol (slightly antiaromatic) for X = SiH₂. Additional disubstitution of the C?C double bond by X′ groups (X′ = CH₃, NH₂, OH, SiH₃, PH₂, SH) increases the REs considerably, but has only small effects on the ECREs. Even the ECRE of deltic acid (X = O, X′ = OH) is only increased to +13.3 kcal/mol. The conclusion based on ECRE's, that all 12 of the three membered rings are only marginally aromatic/antiaromatic, is supported by the satisfactorily plot (R² = 0.92) of ECRE against values of NICS(0)_πzz (a superior nucleus chemical independent shift magnetic index of aromaticity), which range only from ?6.1 ppm (diatropic) for deltic acid (cf., ?35.5 ppm for benzene and ?14.2 ppm for the parent cyclopropenium ion) to +8.9 ppm (paratropic) for the silicon derivative, X = SiH₂, X′ = SiH₃. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Cycloreversion reactions for five to seven membered rings

The cycloreversion of the following ring systems was studied: (1) Cyclopentyl anion → allyl anion + ethylene, (2) Cyclohexene → butadiene + ethylene, (3) Cyclohept-4-enyl cation → butadiene + allyl cation. The potential energy surfaces of these reactions were investigated by the semiempirical MO method SINDO1 under inclusion of a 4 × 4 CI. All internal coordinates were varied. We found major differences in the nature and geometry of transition states and intermediates of these reactions. Diradical character is involved only in the fragmentation of the six membered ring.     

Copper catalyzed synthesis of seven and higher membered heterocycles

Nitrogen, oxygen, and sulfur-containing heterocycles have a wide range of biological activities. Metal catalysts are used in organic reactions with high activity. New methods have been explored for the synthesis of heterocyclic compounds in the past few years. The preparation of heterocycles with the assistance of metal catalyst is becoming a highly rewarding strategy in organic synthesis. In this review article, the synthesis of seven and higher-membered heterocycles is presented using copper as a catalyst.     

The cycloaddition strategy for the synthesis of natural products containing carbocyclic seven-membered rings

Highly substituted carbocyclic seven-membered rings are frequently found in natural products and their synthesis represents a significant challenge to the synthetic chemist. Direct intramolecular cyclization of these systems often proves difficult and this fact has catalyzed the development of a variety of strategies based on a convergent intermolecular cycloaddition strategy. This concept article discusses the major cycloaddition approaches utilized to access these types of structures and primarily focuses on examples employed in the synthesis of natural products.     

On the configuration of five-membered rings: a spin-spin coupling constant approach

Five-membered rings are clearly among the most common structural motifs found in chemistry and biology. Nevertheless, the configuration of conformationally mobile five-membered rings is often difficult to assign from nuclear magnetic resonance (NMR) data. A simple, reliable, and efficient approach for the stereochemical analysis of five-membered rings based on the measurement of NMR coupling constants is presented. Density functional theory calculations using representative conformations of the full conformational space available to rings with different substitution patterns were used to identify differences between the accessible coupling constant values for cis and trans relative orientations of the substituents. The calculations were assessed experimentally using NMR data obtained from a number of models. This approach can be easily used to analyze different five-membered rings, such as oxolanes, cyclopentanes, furanosides and pyrrolidines, and their relative configuration can be determined without the need for making further conformational considerations.     

The MARDi cascade: a Michael-initiated domino-multicomponent approach for the stereoselective synthesis of seven-membered rings

The MARDi cascade is a recently invented three-component Michael-initiated condensation involving 1,3-dicarbonyl derivatives. It allows regio- and stereocontrolled access to a variety of functionalised and substituted seven-membered rings. The substitution array can be diastereoselectively modulated by appropriate choice of the reaction partners, and the reaction allows the control of up to five newly created stereocentres and a complete chiral induction in the case of an optically active ketone precursor. The high level of diastereoselectivity observed has been attributed to total thermodynamic control of the reaction. The attractiveness of the present domino three-component approach to seven-membered rings resides in the diversity of carbo- and heterocyclic structures that can be accessed with total regiocontrol and high stereocontrol by starting from simple substrates, under user and environmentally friendly conditions, as now required in modern organic chemistry.     

Non‐empirical calculations of NMR indirect carbon–carbon coupling constants: 1. Three‐membered rings

The carbon–carbon indirect nuclear spin–spin coupling constants in cyclopropane, aziridine and oxirane were investigated by means of ab initio calculations at the RPA, SOPPA and DFT/B3LYP levels. We found that the carbon–carbon couplings are by far dominated by the Fermi contact term. Our best SOPPA and DFT results are in a very good agreement with each other and with the experimental values, whereas calculations at the RPA level of theory strongly overestimate the carbon–carbon couplings. Significant differences in the basis set dependence of the calculated carbon–carbon coupling constants obtained with either wavefunction method, RPA or SOPPA, or the density functional method, DFT/B3LYP, are observed. The SOPPA results depend much more strongly on the quality of the basis set than the results of DFT/B3LYP calculations. The medium‐sized core‐valence basis sets cc‐pCVTZ and even cc‐pCVDZ were found to perform fairly well at the SOPPA level for the one‐bond carbon–carbon couplings investigated here. Copyright © 2002 John Wiley & Sons, Ltd.     

Selective construction of carbocyclic eight-membered rings by ring-closing metathesis of acyclic precursors

Although the efficient preparation of cyclooctanoids has remained a long-standing objective, a real breakthrough was possible only relatively recently with the development of more specific reagents and by the development of new reactions. The direct construction of carbocyclic eight-membered rings from acyclic precursors is still a challenge, mainly because of unfavorable entropic and enthalpic factors that preclude ring formation. This Minireview describes the utilization of ring-closing metathesis (RCM) as a new approach to synthesize cyclooctanoids.     

Azides in the Synthesis of Various Heterocycles

In this review, we focus on some interesting and recent examples of various applications of organic azides such as their intermolecular or intramolecular, under thermal, catalyzed, or noncatalyzed reaction conditions. The aforementioned reactions in the aim to prepare basic five-, six-, organometallic heterocyclic-membered systems and/or their fused analogs. This review article also provides a report on the developed methods describing the synthesis of various heterocycles from organic azides, especially those reported in recent papers (till 2020). At the outset, this review groups the synthetic methods of organic azides into different categories. Secondly, the review deals with the functionality of the azido group in chemical reactions. This is followed by a major section on the following: (1) the synthetic tools of various heterocycles from the corresponding organic azides by one-pot domino reaction; (2) the utility of the chosen catalysts in the chemoselectivity favoring C−H and C-N bonds; (3) one-pot procedures (i.e., Ugi four-component reaction); (4) nucleophilic addition, such as Aza-Michael addition; (5) cycloaddition reactions, such as [3+2] cycloaddition; (6) mixed addition/cyclization/oxygen; and (7) insertion reaction of C-H amination. The review also includes the synthetic procedures of fused heterocycles, such as quinazoline derivatives and organometal heterocycles (i.e., phosphorus-, boron- and aluminum-containing heterocycles). Due to many references that have dealt with the reactions of azides in heterocyclic synthesis (currently more than 32,000), we selected according to generality and timeliness. This is considered a recent review that focuses on selected interesting examples of various heterocycles from the mechanistic aspects of organic azides.     

Reactivity of [(Ph2Si)2O3]4[Al(OH)]4 and [(Ph2Si)2O3]4[Al(OLi)]4 towards NaOEt and LiOH. Synthesis of New Twelve‐membered Alumopolysiloxane Rings

The reaction between [(Ph₂Si)₂O₃]₄[Al(OH)]₄ ( 1 ) or [(Ph₂Si)₂O₃]₄[Al(OLi)]₄ ( 2 ) with sodium ethoxide, or lithium hydroxide in presence of CuI·H₂O leads to the formation of new alumopolysiloxane compounds. Indeed, transformations of 1 under the partial incorporation of the reactants are found giving rise to new heteroleptic inorganic macrocycles. The molecular structure of [(Ph₂Si)₂O₃]₄[Al(ONa)]₂[Al(OH)(NaOEt)]₂·2Et₂O ( 3 ) and [(Ph₂Si)₂O₃]₄[Al(OLi)]₂[Al(OH)(LiOH)]₂·2Et₂O·2THF ( 4 ) have been determined by single‐X‐ray diffraction analysis. Both alumosiloxanes 3 and 4 are constituted by a twelve‐membered ring.     

Some studies on liquid crystals incorporating three membered linking units combined with 1,3 linked rings

From studies of molecular models, by the combination of more than one 'wrong' building unit (three membered linking units, 1,3-linked rings, 1,3-cis-saturated rings), molecules exhibiting close to linear conformations are feasible. Accordingly, a number of suitable substances have been synthesized. All of them exhibit a moderate to strong depression of clearing temperature. Surprisingly, the extent of the depression decreases with conformational mobility of the linking unit and is, in contrast to normally constructed liquid crystal molecules, not affected by the presence of lateral substituents in the terminal ring.     

Trapping of the highly strained [5](2,4)quinolinophane system

The highly strained [5](2,4)quinolinophane system can be generated as an intermediate (2b), which is extremely susceptible towards the attack of both nucleophilic and electrophilic species. Addition of water at the carbon bridgehead C2 occurs rapidly and is followed by rearrangements to give a strain free product 10. An unusual carbene addition at the N1C2 bond of 2b is proposed to explain the formation of the strained ‘anti-Bredt’ type olefin 11.     

The symmetrical porphyrazine with annulated six membered rings

Tetrakis(cyclohexylpyrazino)porphyrazinato magnesium(II) with annulated six membered rings has been prepared by two different methods. In Method A: cyclotetramerization of 2,3-dicyano-5,6-cyclohexanopyrazine (1) in the presence of Mg(OPr)₂ in n-propanol afforded the tetrakis(cyclohexylpyrazino)porphyrazinato magnesium(II) (2). In Method B: selenodiazole rings on the periphery of tetrakis(selenodiazole)porphyrazinato magnesium(II) were opened by the action of H₂S to yield the vicinal diamino functionalities. The ring closure of the vicinal diamino groups with 1,2-cyclohexanedione afforded the tetrakis(cyclohexylpyrazino) porphyrazinato magnesium(II) (2).     

Studies on the Aromatic Dihalocarbonyl Ylides and Their Deoxygenation

The deoxygenation reaction of heptanones, cycloheptanone, cycloheptatrienone or substituted cycloheptatrienone with dihalo-carbene results in carbon monoxide and corresponding halides. The yield of CO produced by 2 , 4 , 6-triphenylcycloheptatrienone is 2.6-3.5 times as high as that produced by the saturated heptanones. The structures, energies, charge distributions, bond orders, and other relative parameters of the dihalocarbonyl glides were calculated by using the SCF-MNDO method. The obtained data reveal that the ylides from cycloheptatrienone have aromatic structure and are different from those produced from saturated cycloheptanone. The reactivities of the dihalocarbonyl ylides are discussed. It is proposed that this aromatic structure should be responsible for the high yield of CO from the reaction of cycloheptatrienone with dihalocarbene.