Pyran conformational analysis

Conformational analysis of proanthocyanidins is, in principle, concerned with the conformation of the pyran heterocycle and with the phenomenon of conformational isomerism due to... 

Recently, only the conformational analysis of 4-hydroxy-tetrahydro-pyran 38 and of 2-(hydroxymethyl)-tetrahydropyran 39 was published (98JCS(P2)1751). The low temperature i I NMR study of 38 gave 8.5%... 

Conformational analysis of proanthocyanidins involves the conformational flexibility of the pyran heterocycle and conformational isomerism due to restricted rotation about the interflavanyl bond axis or axes. 

Two n.m.r. studies of 5,6-dihydro-a-pyrans (1) have been presented. The Polish group used the sum of the vicinal coupling constants on H-6 for a conformational analysis. The Canadian group adopted a different approach, using variable temperatures, spin-spin decoupling, and shift... 

X-Ray analysis of hexahydrofuro[2,3- ]pyran derivative 39 shows that the tetrahydropyran ring assumes a chair conformation with the C(7a)-0(1) bond found in the axial position <1998T8753>. This conformation agrees with that obtained by analysis of the NMR spectrum of 39. 

Analysis of the far IR-spectra of 3,4-dihydro-2//- pyran (13) (72JCP(57)2572> and 5,6-dihydro-2/f- pyran (14) (81JST(71)97> indicates that for both molecules the most stable conformation is a half-chair form. The barrier to planarity is greater for the former compound. These preferred structures are in accord with the half-chair conformation established for cyclohexene and its derivatives. The conformational mobility of cyclohexene is greater than that of the 3,4-dihydropyran. The increased stabilization of the pyran has been attributed to delocalization of the v- electrons of the alkenic carbon atoms and the oxygen lone-pairs (69TL4713). 

The dimethyl 2-(ferf-butylamino)-7,7-dimethyl-5-oxo-5,6,7,8-tetrahy-dro-4H-chromene-3,4-dicarboxylate was synthesized by the reaction of f erf-butyl isocyanide and dimethyl acetylenedicarboxylate with dime-done in cold CH2C12. Its structure was established by X-ray analysis which revealed that it was stabilized by one intramolecular hydrogen bond and the pyran ring had a distorted boat conformation (05MI545) Condensation of 1 with 2,3-disubstitutedamino-propenoates 454 or N-(2,2-dicyanoethenyl)-2,2-dimethylaminoethyl)aminoacetonitrile in acetic... 

Solid-state C NMR analysis of two fiusemide crystal forms revealed differences in chemical shifts [39]. Marked differences between solution and solid state appear for the carboxyl group and for pyrane ring carbons however, most likely the conformation in solution is not the same as those present in the solid-state crystal forms. 

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