Ring formation reaction

Keehn and Mashraqui, in their studies on cyclophanes, used this ring-formation reaction to prepare the oxazole 21, which was then elaborated to give the [2,2]-(2,5)oxazolophanes 22, via a Hofmann elimination (Scheme 12).<82JA4461>... 

The true significance of ring opening and ring formation reactions in metabolism is not always recognized and, to the best of our knowledge, these two types of reaction have never been reviewed per se in a systematic manner. Here, we offer a preliminary classification of these reactions in an attempt to clarify this complex field. 

Moreover, interesting ring-formation reactions to give 7 have also been reported for substituted mclhoxyallenyl(cyclopropyl)methanols using 70% hydrogen fluoride/pyridine in combination with sodium fluoride or potassium hydrogen fluoride.82... 

Section III contains a discussion of the electrolytic reactions in which a heterocyclic system is formed the reactions will be treated under the following headings ring formation reactions, ring contractions, and ring expansions. 

The pKa of a nitrogen is a convenient measure of its nucleophilicity in proton addition steric effects are unimportant. All other types of electrophilic attack at nitrogen are sensitive in varying degrees to steric effects from a-substituents. (Exception certain ring formation reactions as in metal chelation.)... 

The thermal rearrangement of cyclobutenones to naphthols represents a ring enlargement reaction or, depending on the reaction conditions, a ring formation reaction. While the latter type is not a subject of this review, the former is. 

Additional examples of selenophene ring-formation reaction that result from the treatment of 1,4-dienes with selenium dioxide have been reported. For example, treatment of verbenone derivative 116 with selenium dioxide in the presence of pyridine gave fused selenophene 117 in 92% yield (Equation 19) <20010L3161, 2002JOC6553>. Additional applications of this reaction include taxol analog 150 <1997BML1941> and a [l]benzopyrano[3,2-/ ]-selenophene-9-one derivative <2001SAA1427>. 

These occur between a-bromo esters and carbonyl compounds and are often used in ring-formation reactions. 

Since methyl pyrldlnes are readily available from ring formation reactions, the methylation reaction is useful, primarily, as a means of preparing polymethyl pyridines especially 2,1 -, 2,5-and 2,6-lutidine and 2,l, 6-collldine. 

Three basic approaches have been developed for the formation of these unsaturated systems. Two of these involve direct ring-formation reactions and generally give only one of the isomers. The third method involves reaction of a preformed ring system and yields both of the possible isomers. 

Figure 5. (A) Proposed mechanism for the formation of aromatic polyketides by OKS and its mutants. The enzymes catalyze chain initiation/elongation, and possibly initiating the first aromatic ring formation reaction at the methyl end of the polyketide intermediate. The partially cyclized intermediate are then releasedfrom the active-site and undergo spontaneous cyclizations, which leads to formation of the fused ring systems. (B) Distribution pattern of aromatic polyketides produced by OKS and its mutants. (Reproduced from reference 7b. Copyright 2005 American Chemical Society.)...

Six-membered ring formation. Reactions of orf/io-quinones with l-(dimethylamino)-but-3-ene-l-)me (80MI29) and ethyl diethoxyphosphor-ylacetate (92PS241) have been reported to give benzo[6]pyran derivatives. 

Seven-membered ring formation. Reaction of 3,5-di-7-butyl-l,2-benzoquinone with 1-phenylcyclopropylamine gives a mixture of the... 

The oxathiocins remain extremely rare members of the class of 1,4-diheterocins. Ring-formation reactions, through either unimolecular cyclization or condensation, have been scarcely examined, and those cases reported suffer from low yields. The thiophene ylide rearrangement route provides ready access to the fully unsaturated oxathiocin ring system, but demands the use of 2,5-dihalo-thiophenes. This may not represent an insurmountable problem, and in fact the halides may permit functionalization of the oxathiocins through, for example, nucleophilic displacement, but apparently no such studies have yet been carried out. 

The ring formation reaction is monomolecular, and so, is given by the concentration of growing molecules ... 

Cyclopolymerization is an intramolecular ring formation reaction. For example, as well as double bonds attached to the main chain [Equation (16-54a), rings in the main chain (Equations (16-54b) and (16-54c)] can also be formed in the polymerization of the acrylic anhydride, a 1-6 diene ... 

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