Chemical cyclisation

Formation of lactones can occur though chemical cyclisation, although enzymatic formation is generally more frequent in nature. Glutamic acid is the presumed precursor for y-lactones formation during fermentation (Wurz et al. 1988). The mechanisms regulating the formation of lactones in wine are not known. 

Nucleoside Cyclic Phosphates. - Two novel fluorescent analogues of the secondary messenger molecule cyclic ADP-ribose (cADPR) have been pre-pared. Reaction of e-NAD (73) with sodium bromide in DMF in the presence of triethylamine gave the 9-cyclic ADP-ribose 74 whilst the enzyme catalysed cyclisation using Aplysia californica gives the N1-alkylated (equivalent to N7 in adenine) product. The novel cyclic etheno-CDP-ribose 75 was obtained by the enzymatic route and by the chemical cyclisation, albeit in poor yield in the latter case. A P NMR characterisation of cyclic ADP-ribose (cADPR) and its 2 -phospho-cyclic ADP-ribose has been published. ... 

Meyer, J. (1993) Gamma-decalactone microbial production from alkyl ricinoleate by hydrolysis, beta-oxidation, and chemical cyclisation of 4-hydroxydecanoic acid produced for flavoring and perfume. Patent DE 4126997. 

Even if organocatalysis is a common activation process in biological transformations, this concept has only recently been developed for chemical applications. During the last decade, achiral ureas and thioureas have been used in allylation reactions [146], the Bayhs-Hillman reaction [147] and the Claisen rearrangement [148]. Chiral organocatalysis can be achieved with optically active ureas and thioureas for asymmetric C - C bond-forming reactions such as the Strecker reaction (Sect. 5.1), Mannich reactions (Sect. 5.2), phosphorylation reactions (Sect. 5.3), Michael reactions (Sect. 5.4) and Diels-Alder cyclisations (Sect. 5.6). Finally, deprotonated chiral thioureas were used as chiral bases (Sect. 5.7). 

Many chemical derivatives of mbber, produced in the course of attempts to elucidate the structure of mbber, became of practical commercial value but have been superseded by the diversity of synthetic polymers. See Cyclised Rubber, Chlorinated Rubber, Rubber Hydrochloride, Rubbone. 

Finally, there is a large body of experimental and theoretical contributions from investigators who are mainly interested in the dynamic and conformational properties of chain molecules. The basic idea is that the cyclisation probability of a chain is related to the mean separation of the chain ends (Morawetz, 1975). Up to date comprehensive review articles are available on the subject (Semiyen, 1976 Winnik, 1977, 1981a Imanishi, 1979). Rates and equilibria of the chemical reactions occurring between functional groups attached to the ends or to the interior of a flexible chain molecule are believed to provide a convenient testing ground for theories of chain conformations and chain dynamics in solution. 

It is remarkable that in the same year, 1934, two independent approaches, those of Stoll et al. and of Kuhn, led to the definition of two quantities which are conceptually quite similar and can be practically identical in many actual cases. In either case the intramolecular reaction is compared to a corresponding intermolecular process. This is the dimerisation reaction of the bifunctional reactant in the definition of the cyclisation constant C in the case of the effective concentration Crff Winter must be determined with the aid of an inter-molecular model reaction, the choice of which is not always obvious and can possibly lead to conceptual as well as experimental difficulties. It is also worth noting that although these early workers established a firm basis for interpretation of physical as well as of preparative aspects of intramolecular reactions, no extensive use of quantities C and Qff appears to have been made in the chemical literature over more than three decades after their definition. This is in spite of the enormous development of studies in the field of... 

Our studies have confirmed that the oligomerisation of styrene by perchloric acid is both chemically and kinetically simple. However, the reactions which follow completion of the polymerisation, and during which carbonium ions are formed and destroyed, are complicated. They can be rationalised in terms of equilibria involving ions, acid, double bonds, and esters cyclisation of olefinic oligomers and formation of polyenes by way of allylic ions add further complications. We have thus shown in some detail just why such systems must be treated with the greatest circumspection if they are to yield valid information. 

The indirect cyclisation of bromoacetals via cobaloxime(I) complexes was first reported in 1985 [67], At that time the reactions were conducted in a divided cell in the presence of a base (40yo aqeous NaOH) and about 50% of chloropyridine cobaloximeflll) as catalyst precursor. It was recently found that the amount of catalyst can be reduced to 5% (turnover of ca. 50) and that the base is no longer necessary when the reactions are conducted in an undivided cell in the presence of a zinc anode [68, 69]. The method has now been applied with cobaloxime or Co[C2(DOXDOH)p ] to a variety of ethylenic and acetylenic compounds to prepare fused bicyclic derivatives (Table 7, entry 1). The cyclic product can be either saturated or unsaturated depending on the amount of catalyst used, the cathode potential, and the presence of a hydrogen donor, e.g., RSH (Table 7, entry 2). The electrochemical method was found with some model reactions to be more selective and more efficient than the chemical route using Zn as reductant [70]. 

A detailed mechanistic study of acid-catalysed monocyclization of 5,6-unsaturated epoxides, such as (66), has now provided compelling evidence for a pathway in which the oxirane C—O cleavage and the C—C bond formation are concerted. These experimental results are now further supported by theoretical evidence for a concerted mechanism of the oxirane cleavage and A-ring formation in epoxysqualene cyclisation, obtained at the RHF/6-31G and B3LYP/6-31 + G levels. The chemical pathway thus parallels the mechanism of the enzymatic cyclization that plays a role in the biosynthesis of isoprenoids. 

Priebe et al. [79] investigated the chemical stabiHty of iodixanol under accelerating cleavage of the central bridge under ultraviolet irradiation by a Norrish Type-II reaction. Basic conditions (pH 14) combined with heat (60 °C) initiated a cyclisation reaction. On the other hand, less than 1 % iodixanol decomposed in solution heated to 140 °C for 2 days or under both basic conditions (pH 11,20°C, 5 days) and acidic conditions (pH 0.4,80 °C, 5 days) or under an oxygen atmosphere (100°C,3 days). 

In the chemical and petroleum industries, the term cracking is used to describe a chemically complex process in which the decomposition of larger hydrocarbon molecules into smaller fragments plays a dominant role but is accompanied by a number of other reactions (isomerisation, cyclisation, polymerisation, disproportionation etc.). In this section, under catalytic cracking, only the primary fission of a C—C bond, which yields an alkene and a fragment with a C—H bond in the place of the former C—C bond... 

In spite of the unsaturation of rubber it is said to be resistant to chemical attack since a few chemical derivatives like chlorinated rubbers, cyclised rubbers and chlorides are formed as protective layers retarding further attack by the chemicals. Properly compounded vulcanizates satisfactorily resist the action of most inorganic acids other than oxidizing acids such as nitric, hydrochloric and sulphuric acids. Rubber linings which are to resist swelling to a lower degree should be "tightly" vulcanized. 

The formation of macrocyclic ligands by template reactions frequently involves the reaction of two difunctionalised precursors, and we have tacitly assumed that they react in a 1 1 stoichiometry to form cyclic products, or other stoichiometries to yield polymeric open-chain products. This is certainly the case in the reactions that we have presented in Figs 6-8, 6-9, 6-10, 6-12 and 6-13. However, it is also possible for the difunctionalised species to react in other stoichiometries to yield discrete cyclic products, and it is not necessary to limit the cyclisation to the formal reaction of just one or two components. This is represented schematically in Fig. 6-19 and we have already observed chemical examples in Figs 6-4, 6-11 and 6-18. We have already noted the condensation of two molecules of 1,2-diaminoethane with four molecules of acetone in the presence of nickel(n) to give a tetraaza-macrocycle. Why does this particular combination of reagents work Again, why are cyclic products obtained in relatively good yield from these multi-component reactions, rather than the (perhaps) expected acyclic complexes We will try to answer these questions shortly. 

Chemical modification of rubber such as hydrogenation, chlorination, epoxidation, cyclisation, addition of maleic anhydride, is a popular technique to give a desirable... 

Pandey, G., Devi Reddy, G., and Chakrabarti, D. (1996) Stereoselectivity in the photoinduced electron transfer (PET) promoted intramolecular cyclisations of l-alkenyl-2-silyl-piperidines and -pyrrolidines rapid construction of 1-azabicyclo [m.n.o] alkanes and stereoselective synthesis of ( )-isoretronecanol and ( )-epilupinine. Journal of the Chemical Society, Perkin Transactions 1, 219-224. 

Benzophenones have been described as useful sensitisers for PET catalysed conjugate addition reactions of a-amino alkyl radicals to enones (Bertrand et al. 2000). We tried to modify this reaction and synthesised the pyrrolidinylethyl-substituted quinolone 35 from the known bromide (Bauer et al. 2005). Upon electron transfer from the pyrrolidine to a given acceptor, a radical cyclisation occurs (Scheme 15), which after electron and proton transfer generates a pyrrolizidine. We found 4,4/-dimethoxybenzophenone to be a suitable catalyst for this reaction. Remarkably, the reaction proceeded with excellent simple diastereos-electivity and a single diastereoisomeric product rac-36 was obtained. With 10 mol% of the catalyst, a chemical yield of 71% was achieved. 

Through this study we suggest a useful synthetic procedure for good candidate compounds of a new cage-cluster. The ort/io-carboranes are very stable chemicals in this research. Carboranyl acetic ester can be easily introduced to the ortho- carborane skeleton and then many types of cyclisation of carboranyl cluster can be made to occur. The reactivity and reaction mechanism for cyclisation of carborane cluster have been discussed. The mechanisms for boron cluster expansion reaction and cyclisation at carboranyl edge are summarized as follows. 

An elegant example of rearrangement followed by cyclisation has been recently reported by Kim. In a study directed towards the development of di-hydrobenzoxathiin as antagonist for osteoporosis, the MNK rearrangement was utilised to achieve the chemical synthesis of its major metabolite, confirming its structure (Scheme 34) [87]. 

The action of heat on calciferol (Vitamin D2 10), or on pre-calciferol (2), with which it is in chemical equilibrium (see below), gives two further isomers, pyrocalciferol (ii) and isopyrocalciferol (12), both of which have sy i-9,io-configura-tions [ii]. Although these reactions were incomprehensible before the orbital-symmetry principles were developed, we can now see that the formation of sy i-isomers is the consequence of a preference for overlap of C<9) and C io) orbital envelopes on the same side of the molecule. The necessary conformations of the open-chain triene systems for these reactions are considerably strained, but no doubt the elevated temperature (180-200 ) at which cyclisation occurs makes the transition states accessible. 

If the starting material is decomposed under the reaction conditions, then consider carefully any possible chemical reasons which might be responsible. For example, basic conditions might be causing deprotonation and/or elimination at an undesired site or sites in the molecule, and acidic conditions might be responsible for unexpected cyclisation or protecting group loss. There are often several possible sources of instability towards a particular set of reaction conditions, and it will be necessary to examine all possibilities. 

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