Ring closures five-membered

An important application of the Friedel-Crafts acylation is the intramolecular reaction, leading to ring closure. This variant is especially useful for the closure of six-membered rings, but five-membered ring products and larger rings, as well as heterocycles, are also accessible ... 

Metathetic ring closure. Five-, six-, and seven-membered carbocycles as well as N and O heterocycles are constructed from dienes in the presence of a Ru-carbene complex. For unhindered amines, prior protonation is necessary. Fused bicyclic structures are accessible from dienynes. ... 

Numerous examples of the ring closure of a binucleophilic system with a 1,1 -bielectrophile leading to five-membered heterocycles with three or more heteroatoms have been described, the popularity of this route no doubt reflecting the comparative ease with which the penultimate product may be obtained. 

Oxidative procedures have been utilized for the synthesis of both monocyclic five-membered heterocycles and their ring-fused analogs, although the ease of synthesis of the precursors for the latter ring closures results in wider application of this procedure. A variety of oxidizing agents have been used and the conversion of the benzylidene hydrazidines (221) into the 4-arylamino-l,2,4-triazole (222) was effected with mercury(II) oxide (77BCJ953). 

Although some of the oxidative ring closures described above, e.g. reactions with lead tetraacetate (Section 4.03.4.1.2), may actually involve radical intermediates, little use has been made of this reaction type in the synthesis of five-membered rings with two or more heteroatoms. Radical intermediates involved in photochemical transformations are described in Section 4.03.9. Free radical substitutions are described in the various monograph chapters. 

Table 3.11 gives rate data for ring closure of a series of diethyl (malonate anions for ring sizes 4-13, 17, and 21. The rates range fhm a maximum of 6 X 10 s for the five-membered ring to 2.9 x 10 s for the 11-membered ring. ... 

Let us focus attention on the unfavorable ring closures. Why, for example, should formation of a five-membered ring by an endo-trig process be difficult The answer is provided by a consideration of the trajectory of approach of the nucleophile." If Z is an electron-attracting conjugating group of the type necessary to activate the double bond to nucleophilic attack, the reaction would involve the LUMO of the conjugated system, a 7t ... 

An illustration of the tendency of alkane 1,2- and alkane-1,3-dicarboxylie acids to a ring closure during the reaction with sulfur tetrafluoride is the reaction of propane-1,2,3-tricarboxylie acid The corresponding six- and five-membered cyclic ethers are formed in a 1 4 ratio [211] (equation 106)... 

Finally, examine the transition states for closure of the Ci enolate to the 7-membered ring product, and of the C3 enolate to the 5-membered ring product. Calculate activation energy barriers from their respective enolates. Which ring closure (to the five or the seven-membered ring) occurs more readily ... 

This type of reaction is represented by 1,3-dipolar cycloadditions, corresponding to one of the Huisgen categories (69MI1). The 1,3-dipolar cycloaddition corresponds to the interaction between a 1,3-dipole and a multiple system [Pg.3]

This tendency is especially significant in compounds containing functional groups capable of addition with the formation of both five- and six-membered rings. It has been shown that for amides and hydrazides of azolecarboxylic acids, selectively, and for the acids with any arrangement of a function and triple bond, heterocyclization always leads to the closure of the six-membered ring. Similar reactions in the benzoic series mainly lead to the formation of five-membered rings. 

This method has been used to close rings of from three n = 0) to seven members, although five-membered ring closures proceed in highest yields. Another ringclosing method involves internal alkylation. ... 

These reactions are most successful for the preparation of six-membered rings, though five- and seven-membered rings have also been closed in this manner. For other Friedel-Crafts ring-closure reactions, see 11-13, 11-14, and 11-23. 

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