Cyclisation reactions

Irradiation of substituted o-carbonylstyrenes promotes either cyclisation or dimer formation depending upon the substitution pattern of the substrate. Solvent and substituent effects on the photocyclisation of a-(2-acylphenoxy)-toluenes and ethyl 2-acylphenoxyacetates in the synthesis of dihydrobenzofuranols 

R = H) are reported to be obtained when alkoxynaphthoquinones (93 same R, R ) are irradiated, and the stability of the radical or carbocation intermediate is thought to be important for the efficiency of the reaction.  

CH CHR , where R = (un)substituted Ph) under time-resolved conditions indicates the presence of transients in the ps and ms domains. These are thought to possess slightly tilted educt or product-like geometries, and an energy diagram has been proposed for reaction of (108) to give (110). 

This observation is significant for the study of rapid electron-transfer events in ubiquinol oxidising enzymes. 

Photodecarboxylation of N-acylisoxazol-5-ones affords iminocarbenes which give oxazoles following intramolecular cyclisation through the oxygen of the acyl 

Potassium and rubidium perchlorates have been observed to decrease the photocyclisation quantum yield of l,2-bis(2,4-dimethylthien-3-yl)perfluorocy-clopentene having two benzo-15-crown-5 ethers (101), and this has been ascribed to increases in the ratio of the photoinactive parallel conformational analogue of (101) by intramolecular interaction of the two crown ether moieties with a metal ion.  

Irradiation of asymmetrically substituted A -benzyl-A -isopropyl-aP-unsatu-rated thioamides (102) in benzene solution induces hydrogen abstraction by the alkenyl carbon from the benzyl and isopropyl groups to give a P-thiolactam (103) and 1,3,5-dithiazinane (104) as products. In the solid state, however, photolysis causes hydrogen abstraction from only the isopropyl group to give the isomeric P-thiolactam (105). 

R = Ph R = R = H, R R = (CH CH)2, R = H) promotes photocyclisation from the naphtholic singlet excited states with formation of the five- and six-membered ring products (116) and (117). By contrast, formation of (117) occurs by a proton transfer mechanism. 

Flash photolysis studies of 2-pyridyl phenyl ketone in sodium dodecyl sulfate submicellar and micellar solutions have revealed that a fast intramolecular photocyclisation occurs followed by addition of water, and that hydrogen abstraction from the surfactants by the triplet ketone is absent. Addition of the water takes place predominantly from the intramicellar water molecules. 

The ( )-isomers of N-acyl-a-dehydro(l-naphthyl)alanines (125 n = 2,3 R = Me, Ph R = Me, Et, Pr) produce l,2-dihydrobenzo[f]quinolines (126) by an electron-transfer reaction, and the )- and (Z)-isomers produce minor amounts of the benzo[f]isoquinoline (127) and 1-azetidine derivatives respectively by an intramolecular photoaddition. Evidence suggests that the bulky diisopropylamino donor and the N-benzoyl group enhance the relative yield of (127) to (126). 

Photocyclisation of 8-alkoxy-l,2,3,4-tetrahydro-l-naphthalenones and 4-alkoxy-6,7,8,9-tetrahydro-5H-benzocyclohepten-5-ones gives naphtho[l, 8-bc]furans and cyclohepta[cd]benzofurans respectively, and conformational and substituent effects of 1,5-biradicals in the cyclisation process are discussed. The same authors also describe substituent effects on the photocyclisation of ethyl 2-(8-oxo-5,6,7,8-tetrahydro-l-naphthyloxy)acetates and ethyl 2-(5-oxo-6,7,8,9-tet-rahydro-5H-benzocyclohepten-4-yloxy)acetates to give naphtho[l,8-bc]furans and cyclohepta[c,d]benzofurans respectively. Also reported are cyclisations involving photogenerated radical cations of unsaturated silyl enol ethers, fragmentation cyclisations of unsaturated ot-cyclopropyl ketones which occur by photoelectron transfer and give polycyclics, and kinetic and theoretical studies of [2+3] cycloadditions of nitrile ylids. These reactions have been studied mechanistically and their synthetic potential investigated. 

The cyclopropanation of alkenes by ethyl diazoacetate is catalysed by W C0)s complexes derived from (C0)5W=C(0M6)Ph.Fumarate esters of the chiral alcohols (-)-dimenthol and (-)-borneol are cyclopropanated by CH2X2/Zn in the presence of Co(0) catalysts in acetonitrile in up to 1 % optical yield. 

The product selectivity in the reaction of methylene cyclopropanes with alkynes is strongly substituent-dependent the reaction is catalysed by Ni(0)/tris(o-tolyl Iphosphite (eqn.47). The intramolecular hydroacylation of 4-pentenal is catalysed by low valent Co/PPh3 complexes and produces mainly cyclopentanone.Cyclopentenones and cyclohexenones are the products of the intramolecular carbonylation of vinyl iodides (eqn.48). A large variety of heterocycles are accessible the intramolecular Pd catalysed cyclisation of unsaturated a-haloamides. Treatment with base and PdfOAclj/PPhj converts 2,6-dibromo-l,6-dienes into cyclopentanes with exocyclic double 

1 Molecular sieves as promoters for the catalytic Pauson-Khand reaction 

Jaime BiANCo-URGom, Gem A Dominguez and Javier Perez-Castells. 182 

2 Palladiumceo-catalysed cyclization of alkynes with aldehydes, ketones or nitriles 

3 Rhodiumcq-catalysed INTRAMOLECULAR ALDER-ENE reaction and syntheses of functionalised a-METHYLENE-y-BUTYROLACTONES AND CYCLOPENTANONES 

4 Rhodium-catalysed [2+2 + 2] cyclotrimerization in an aqueous-organic biphasic system 

In 2005, Hilt and co-workers disclosed the facile intermolecular ring expansion of styrene oxide with several dienes, acrylates, enynes or styrenes under iron catalysis to afford polysubstituted tetrahydrofuran derivatives in a highly chemo- and regioselective fashion. The authors used FeCl2 mixed with a 

Recently, Okamoto et al. reported that NHC-Fe species derived from FeCl2 or FeCla by in situ reduction with zinc powder in the presence of IMes or IPr were efficient catalysts for the intramolecular cyclotrimerisation of triynes into annulated benzenes (Equation (7.3)). Although no well-defined complex could be isolated from this mixture, its activity was preserved for a few days when kept under inert atmosphere. 

2-(naphthalen-l-yl)-3-(thien-2-yl)propenoic acid containing iodine give 

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Tetramethoxyisoflavone (5) was the target of the cyclisation reaction of 3,4-dimethoxy-phenol (7) with formyl-(3,4-dimethoxyphenyl)acetic acid (2) in the presence of polyphosphoric acid. 

In other instances the reactions appear to occur in sequence down the chain, for example in the depolymerisation reaction of polyformaldehyde (polyacetal) and polymethyl methacrylate which are referred to as zippering or sometimes unzippering reactions. In other cases cyclisation reactions can occur such as on heating polyacrylonitrile ... 

Despite the fact that transition metal complexes have found wide application in the synthesis of carbo- and heterocycles, [3+3] cyclisation reactions mediated or assisted by transition metals remain almost unexplored [3, 86]. However, a few examples involving Fischer carbene complexes have been reported. In all cases, this complex is a,/J-unsaturated in order to act as a C3-synthon and it reacts with different types of substrates acting as C3-synthons as well. 

Scheme 37 Transmetalation of chromium to nickel in a metal carbene-mediated cyclisation reaction (L=cod, MeCN, alkyne)...

Similar results are obtained from incineration of polymeric materials with octabromo- and pentabromodiphenyl ether (refs. 11,12). The temperature with the maximum PBDF-yield depends on the kind of polymeric matrix. All three bromo ethers 1-2 give the same isomer distribution pattern with preference for tetrabrominated dibenzofiirans. The overall yield of PBDF is lower for incineration of pentabromobiphenyl ether 2, 4 % at 700°C compared to 29 % for ether 1 at 500 °C (ref. 12). The preferred formation of tetrabrominated fiirans observed at all temperatures cannot be a result of thermodynamic control of the cyclisation reaction it is likely due to the special geometry of the furnaces. One explanation is that a spontaneous reaction occurs at approximately 400°C while the pyrolysis products are transferred to the cooler zones of the reactor details can be found elsewhere (ref. 12). 

J. Falbe Cyclisation reactions with carbon monoxide, pp. 181-210 (102). 

Strategy III Reversal of Polarity, Cyclisation Reactions, Summary of Strategy... 

A further study on cyclisation reactions needed unsaturated diol (22). Can you suggest a synthesis ... 

Frohlem Chemists studying cyclisation reactions wanted a series of nitrogen-containing tertiary alkyl chlorides of general structure (51). How might they be made ... 

A central focus in modem organic synthesis has been the development of highly efficient catalytic processes for the syntheses of natural and unnatural compounds of medicinal interest or intermediates useful for functional materials. A particularly attractive approach is to apply transition metal catalysed cyclisation reactions for the transformation of simple starting materials into monocyclic, bicyclic and polycyclic scaffolds that can be further elaborated into specific targets. 

Due to its marked atom economy, the intramolecular hydroamination of alkenes represents an attractive process for the catalytic synthesis of nitrogen-containing organic compounds. Moreover, the nitrogen heterocycles obtained by hydroamination/cyclisation processes are frequently found in numerous pharmacologically active products. The pioneering work in this area was reported by Marks et al. who have used lanthanocenes to perform hydroamination/cyclisation reactions in 1992. These reactions can be performed in an intermolecular fashion and transition metals are by far the more efficient catalysts for promotion of these transformations via activation of the... 

Occurring Acetylene Compounds. XXVII. The Synthesis of a Mixture of Trideca-1.3,1l-triene-5,7,9-triyne (II) and trans-l-Phenylhepta-l,3-diyn-5-ene (V). A Novel Cyclisation Reaction. Acta Chem. Scand. 13, 2101 (1959). 

Cyclisation reactions may also be attended by a decrease in entropy,... 

Cyclisation of polymer chains. Theory and experiment 64 Entropy changes for cyclisation reactions in solution 74... 

This chapter is concerned with reaction rates, equilibria, and mechanisms of cyclisation reactions of chain molecules. A detailed analysis of the historical development of experimental approaches and theories concerning the intramolecular interactions of chain molecules and the processes of ring closure is outside the scope of this chapter. It must be borne in mind, however, that the present state of the art in the field is the result of investigations which have been approached with a variety of lines of thought, methods, and objectives. 

Equilibrium studies in ring formation, being much less numerous than kinetic studies, have received but limited attention. Nevertheless, for any reversible cyclisation reaction (12) of a bifunctional chain molecule for which... 

Hypothetical gas-phase cyclisation reactions. Effective molarities and thermodynamic parameters ... 

Fig. 5 Entropy effect on simple gas-phase cyclisation reactions. Symmetry cor rected 0 quantities plotted against the number of single bonds in the open-chain reac tants. Data from Table 5...

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