Ring interconversions

A somewhat similar type of ring interconversion involving attack on sulfur has been postulated in the 1,2,4-thiadiazole series, e.g. (337) (338). Such reactions are common in the 1,2,4-dithiazolium series, e.g. (339) —> (340). 

HETEROCYCLIC RING INTERCONVERSIONS 4.03.7.1 Small Rings as Diehophiles or Dipolarophiles... 

This type of ring interconversion is represented by the general expression shown in Scheme 15. Analogous rearrangements occur in benzo-fused systems. The known conversions are limited to D = O in the azole system, i.e. cleavage of the weak N—O bond occurs readily. Under the reaction conditions, Z needs to be a good nucleophile in its own right or by experimental enhancement (base catalysis, solvent, etc.) and Z is usually O, S, N or C. 

Another interesting reaction of the pyrylium salt (396) has been reported (73TL2195). With nitrous acid in alcohol, (396) gave an intermediate (402) which on heating in acetic acid gave the diacylisoxazole (403). The structure of (402) was determined by X-ray crystallography. These ring interconversions are shown in Scheme 96. 

There are no methods for the preparation of benz- and dibenzazepines by ring transformation of other seven-membered rings. Interconversions between tautomeric forms of these systems are dealt with in Section 3.2.1.5.8. 

Molecular Packing and Ring Interconversion by Solid State and Solntion State... 

Ring interconversion is often of minor importance in total reactivity therefore, some related reactions should also be tackled. 

The chair conformation is the preferred conformer for the oxane ring and substituted derivatives. In the case of poly-substitution (e.g., 1,3-diaxial groups), twist conformers can also participate in the equilibrium. Substituents can adopt the axial and equatorial positions ring interconversion between the chair conformers is fast on the NMR timescale at ambient temperature but becomes slow at low temperature (AG = 10.3 kcal mol- ) (73JA4634). 

A number of l,3-dioxan-2-ylium ions have been studied with respect to the preferred conformation. From a detailed H and C NMR study (790MR616) and the X-ray crystal structural analysis (94CJC2084), it was concluded that the 1,3-dioxan-2-ylium ions adopt the envelope conformation with the C —O —C —O—moiety in one plane (due to mesomerism) and C —C —forming the flap (Scheme 16). Ring interconversion 19a 19b and a 1 1 conformational equilibrium was assumed. 

Barriers to Ring Interconversion [AG (Real mol )] of Oxane Derivatives... 

The ring interconversion of 1,2-, 1,3-, l,4-ox-4-enes, and ej o-methylene-1,3-dioxanes has been reviewed by Oki (85MI3). 

Table 5 Syntheses of five-membered heterocycles containing three oxygen or sulfur atoms by ring interconversion.

The geometries of both 3,4-dihydro- and 3,6-dihydro-l,2-dioxin have been calculated at the Hartree-Fock (HF) and MP2 ab initio levels of theory. In each case, half-chair conformers were found to be the most stable structures followed by boat conformers which represent the transition states for ring interconversion <2000JST(503)145, 1997JGC1392,... 

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