Formation of cyclic compounds

In substrates where there is a good leaving group in the same molecule as the nucleophile, one may get an intramolecular process and create a ring system. It is usually necessary to find conditions that favour an intramolecular process over the alternative intermolecular reaction. This is typically achieved by carrying out the reaction at relatively higher dilutions, thereby minimizing the intermolecular processes. 

Simple examples shown above are the base-catalysed formation of oxygen- and nitrogen-containing ring systems. We have shown base-initiated ionization of the alcohol to an alkoxide anion in epoxide formation the anion is a better nucleophile than the alcohol. For pyrrolidine synthesis, the amino group is sufficiently nucleophilic for reaction to occur, but base is required to remove a proton from the first-formed intermediate. 

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Explain how intramolecular hydrogen abstraction in carbonyl compounds can lead either to cleavage (Norrish type 2 reaction) or to the formation of cyclic compounds (Yang cyclisation). 

Figure 6-3. The principal problem associated with the formation of cyclic compounds is in the control of intramolecular as opposed to intermolecular reactions. The open circles represent sites that can react with each other. Intramolecular reaction leads to a cyclic compound, whereas intermolecular reaction leads to oligomers and polymers.

The effect of reaction time on the major components of the reaction of cystine and DMHF in water is shown in Table IV. It is noteworthy that amounts of 2,4-hexanedione, 3,5-dimethyl-l,2,4-trithiolanes and thiophenones were found at a maximum after one hour. It was also found that the amount of 2-acetylthiazole increased with time and that acetol acetate decreased with time as expected. In the glycerol medium, the effect of reaction time on the major components is shown in Table V. Apparently, the 1,3-dioxo-lane, which is a ketal formed from glycerol and acetone, decreased over time. Also, long reaction time favors the formation of cyclic compounds, including 2,5-dimethyl-2-hydroxy-3(2H)-thiophene, cyclo-pentenones and 4,5-dimethyl-l,2-dithiolenone. 

The formation of cyclic compounds increases with the size of the organic radicals bound with silicon. For example, in the process of hydrolytic condensation methylphenyldichlorosilane and diphenyldichlorosilane form mostly cyclic products. The conditions of the hydrolysis of dior-ganodichlorosilanes, especially pFl of the medium, are essential for cycli-sation. As pFl increases, i.e. the acidity of the medium decreases, the process of ring formation can be reduced but cannot be completely avoided. 

Photochemical reactions of 1,4-phenyliodonium dipoles leading to the formation of cyclic compounds... 

Hunger M, Wang W. Formation of cyclic compounds and carbenium ions by conversion of methanol on weakly dealuminated zeolite H-ZSM-5 investigated via a novel in situ CF MAS NMR/UV-Vis technique. Chem Commun. 2004. 

The formation of cyclic compound 381, formed in 50% by reaction of 377 with thiol (1 equiv) in a 1/1 mixture of acetonitrile and sodium phosphate buffer, was explained as a result of attachment by water on the episulfonium ion 380, which is stmcturally related to the intermediate 376. 

D. Formation of Cyclic Compounds from Arsonium Ylides. 674... 

By far the most important reactions of ylides are those of the Wittig type, especially those with carbonyl compounds, and they will be dealt with first other reactions of this sort are those with other C = X functions and with nitroso-compounds. Other carbanionic reactions are then considered and a final section deals with the formation of cyclic compounds from arsonium ylides. Hydrolysis has already been discussion in Section III.B. 

The formation of cyclic compounds by intramolecular alkylations of stabilized enolates has been widely used in organic synthesis. A recent study of the kinetics of the reaction of diethyl (a)-bromoalkyl)ma-lonates in DMSO using tetramethylammonium hydroxide as the base has shown that relative rates of closure of rings of varying size follow the order 3>5>6>4>7> 12-21 > 8 > 9 > 11 > 10. These results are consistent with earlier studies in which other base-solvent combinations were employed. The high rates of closure of three-membered rings allow the formation of a variety of cyclopropane deriva-... 

From these examples it is clear that the principles of acyclic stereocontrol that govern the allylation reactions of achiral Type II allyl- and crotylmetal reagents with chiral aldehydes can be used to excellent advantage in the stereoselective synthesis of natural products. In the following section, the factors that influence the stereoselective formation of cyclic compounds in the ring-closing allylation reaction are discussed and selected synthetic applications are reviewed. 

A wealth of information exists regarding intramolecular conjugate additions in the formation of cyclic compounds. Due to space limitations, only a small sample of these reactions will be discussed here. For a more thorough treatment the reader should consult a recent review on this subject [68]. 

Variable-temperature Si H CP/MAS-NMR spectroscopy was used to study the effect of molecular oxygen on the location of sorbate molecules in highly-siliceous zeolite framework, e.g. ZSM-5 with adsorbed />-dibromoben-zene.662 13C MAS-NMR spectra were used to follow the conversion of methanol on weakly-dealuminated zeolite H-ZSM-5 - showing the formation of cyclic compounds and carbonium ions.663 There is 13C CP/MAS-NMR evidence for surface ra-alkoxyl groups formed by the modification of the proton-ated perovskite HCa2Nb3Oi0 by w-alcohols.664 13C CP/MAS-NMR spectra... 

Formation of Cyclic Compounds via Cyclic Transition States... 

Formation of cyclic compounds via cyclic transition states... 

By the intermolecular reactions between epoxy groups and the hydroxyl groups formed there are not generated new hydroxyl groups, but the molecular weight increases (dimers, trimers) and of course the hydroxyl number decreases. In principle, intramolecular reactions of the same type are possible with formation of cyclic compounds and of course, without generation of new hydroxyl groups. 

The results cited above demonstrate that the equilibrium is independent of the nature of the catalyst and involves a range of cyclic species extending to very high molecular weights. Furthermore, for cyclic species other than D3 the position of equilibrium is determined by purely entropic factors. A theory for formation of cyclic compounds in such polymer systems has been developed by Jacobson and Stockmayer6 from which equation (2) for the molar cyclisation constant Kx can be derived,... 

These observations give rise to the question wether there exists a correlation between the formation of C3 and C4 fragments inside the pore structure of erionite and the observed formation of cyclic compounds. 

Marked decreases of the primarily formed butenes and butadiene with ethylene conversion suggest that these olefins play an important role forming secondary products. In fact, subsequent experiments showed that the addition of butadiene, 3-5 mole %, to ethylene accelerates the formation of cyclopentene, cyclohexene, cyclohexadiene, and benzene (Table I). It seems reasonable, therefore, to propose the following reaction scheme for the formation of cyclic compounds from olefins. 

Formation of Cyclic Compounds When we noted that the addition of small amounts of butadiene increased the yield of cyclics formed in the thermal reaction of ethylene and propylene, an effort was made to relate directly the formation of cyclics in thermal reaction of ethylene and propylene, respectively, to the Diels-Alder reaction between feed olefins and product butadiene. Reactions between product olefins and product butadiene were neglected owing to their small concentrations. Cyclics were deferred as the sum of C rings with and without alkyl or vinyl groups. 

Photocyclization is a particularly valuable route to the formation of cyclic compounds. There is a wide variety of photocyclization reactions reported in the literature of organic photochemistry, but relatively few of these have been carried out in solid polymers. The earliest reports concern the photodimerization of cinnamic acid derivatives, leading to crosslinking in solid polymers. These polymers have important applications as commercial photoresists. The chemistry has been reviewed by Delzenne (46) and Williams (47). 

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