Cyclic ring-closure reactions

Exo- and endo-cyclic ring closure reactions using 0-nucleophiles transform oxocarbenium ions into cyclic acetals. As an example of an endocyclic cyclization, epoxide ring opening of (96) with lithium dimethyl cuprate, and subsequent treatment of the resulting alcohol with acid, smoothly gives the bicyclic acetal (97), ° a key intermediate in the total synthesis of tirandamycic acid (Scheme 47). ... 

As for the corresponding iminium and oxocarbenium ions, thiocarbenium salts can undergo endo- or exo-cyclic ring closure reactions. Again, carbon-carbon bond formation by exocyclic-type reactions lead to heterocyclic systems only by chance , namely when an additional heteroatom is located in the chain that will form the ring, e.g. the thiol-assisted annulation of (119) to give (120 Scheme 58). ... 

An important method of preparing cyclic siloxanes and aminosiloxanes involves metallation of functionalized silanols, aminosilanols, or siloxanols followed by treatment with appropriate halosilanes. Intra- and intermolecular ring closure reactions are possible [10]. 

The cyclic stage of the reaction with sodamide in liquid ammonia corresponds to a Friedel-Crafts ring closure reaction in the carbonium ion series, a reaction that is often an alternative to the carbonium ion rearrangement. 

To the extent that cyclic transition-state strains parallel ring-product strains, the above considerations provide a proper guideline for prediction and interpretation of the oxygen-atom effect on rates of ring closure. This is clearly shown by a comparison of EM data for the two closely related ring-closure reactions (43) and (53a) in 75% aqueous ethanol (Illuminati et... 

Concerning the transformation of substituents, a special note should be made on a series of ring-closure reactions carried out on the side chain of some [l,2,4]triazolo[4,3-r]benzo[l,2,3]triazines published by Moustafa <2001SC97>. The results are summarized in Scheme 18. This scheme shows that by transformation of the R group attached to the sulfur atom of derivative 97 a fairly large set of cyclic products - involving thiazolidone 97a, [l,2,4]triazole 97b, coumarone and its imine 97c and 97d, respectively, benzoxazylpyrane, 97e, thiophene 97f, and cyclopenta- or cyclohexa-fused protected pyrone 97g substituents - have been obtained. 

These ring closure reactions, combined with the removal of tellurium by well-established methods, constitute useful procedures for the synthesis of cyclic structures, often present in natural products. 

The formation of alicyclics by electrocyclic and cycloaddition reactions (Section 9.4) proceeds by one-step cyclic transition states having little or no ionic or free-radical character. Such pericyclic (ring closure) reactions are interpreted by the Woodward-Hoffmann rules in the reactions, the new a bonds of the ring are formed from the head-to-head overlap of p orbitals of the unsaturated reactants. 

Although some interesting synthetic processes of cyclic polymers have been addressed, the cyclization principle can be classified into two main methods as indicated in Fig. 2. One is the utilization of the ring-chain equilibrium that occurs in many polycondensation and ring opening polymerization. Another is the end-to-end cyclization method that can be used for synthesizing cyclic polymers from a/o-difunclional linear precursors. The ring-closure reaction by the end-to-end cyclization is further divided into intermolecular reaction and intramolecular reaction, i.e., bimolecular process and unimolecular process, respectively. 

By application of the dimethylsilyl-bridged bisfluorenyl neodymium chloride complex [Me2Si(Ci3H8)2]NdCl (cocatalysts Buli, DIBAH, BuMgCl, BuMgOct) a complete new class of polymers is accessible by the copolymerization of ethylene and BD [307]. By reaction of 1 eq. BD and 2 eq. of ethylene 1,2-cyclohexane units are formed along the polymer chain. More than half of the inserted BD units (53-58 mol %) are involved in the ring closure reaction. The residual non-cyclicized BD units exhibit a nearly identical ratio of trans-1,4- and 1,2-units. 

In principle, any compound prepared by a ring-closure reaction in Section 14.11.5.3 and substituted with a removable substituent on ring nitrogen atoms can serve as a protected cycle. This is particularly true of a full range of the cyclic amides and sulfonamides (mostly tosylamides). Examples of the approach are 172 and 173 prepared by the tosylate method from tosylated and benzylated precursors <1995T1197>. Protected 16-membered tram 1,9-dibenzyl-l,5,9,13-tetraazacyclohexadecane was also obtained in this way <1998SC285>. 

Recent advances in the synthesis of trans-iused polycyclic ethers by hydroxy epoxide cyclization reactions via monocyclic epoxonium ion intermediates and ether ring expansion reactions via bicyclic epoxonium ion intermediates are described in a review by Fujiwara and Murai. Natural trans-iu eA polycyclic ethers (e.g., brevetoxin A and ciguatoxin), produced by marine sources such as dinoflagellates, are hypothesized to be constructed from the corresponding polyepoxide precursors by a cascade of ring-closure reactions, which has prompted much work in the development of new methods for the construction of cyclic ethers from epoxides <2004BCJ2129>. 

A completely new cyclization reaction is the PhSeBr promoted conversion of alkenyl nitrones into 1,2-oxazines, which was recently reported by Tiecco [79]. This are the first examples to use alkenyl nitrones to effect ring closure reactions induced by electrophilic reagents. As indicated in Scheme 20, the nitrones 130 and 134 react with PhSeBr to give the cyclic iminium salts 131 and 135, respectively. These intermediates suffer attack by nucleophilic reagents at the partially positive carbon atom. The nature of the final products depends on the structure of the starting nitrone and of the nucleophile employed. Thus, the intermediate... 

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