Acetyl heterocycles, conformation

Studies of the conformations of 2-formyl and 2-acetyl derivatives of the five-membered heterocycles have attracted great interest in recent years, but some of the results have been contradictory. At low temperatures the interconversion of the two conformers, trans-SS and cis-86, is slow, and can be studied by NMR. Thus, the 1H-NMR spectra of some selenophene deriva-... 

Acetyl five-membered heterocycles are characterized by a conformational equilibrium with prevailing X,0-trans form (81RCR336). Their benzocondensed forms contain high amounts of the X,0-trans conformer (71 Mil 73KGS139 83JCS(P2)911 84JCS(P2)1479). 

C7) varied with the C3 substituent. The order of the 13C chemical shifts of the 6-(2,3,5-tri-C>-acetyl-/TD-ribofuranosyl)-l,2,4-triazolo[4,3-c]pyrimidin-5-one C-nucleoside 130 (95JOC7066) conforms with the same order for the parent heterocycle 127. 

The structure of penta-O-acetyl-catechin is an interesting example of problems encountered in definition of the conformation of these compounds. In the crystal state, the heterocyclic ring is in a reverse half-chair conformation with both the 5-ring and 3-acetoxy substituents in axial positions (120). However, the heterocyclic ring proton coupling constants 2,3 = h,4a - and... 

Acetylation of the isonicotinoylhydrazone of sodium D-glucuronate (which exists in the -pyranose form) gave the 6,1-lactam derivative (31)) the conformation of which was examined in detail. A variety of N-6-D-glucopyranosylated heterocycles have been synthesized by condensation of 2,3,, 6-tetra-O-benzyl-D-glucose hydrazone (which exists as a mixture of ring and chain isomers) with acetyl-acetone [leading to the synthesis of the 3,5-dimethylpyrazole derivative (32)], (ethoxymethylene)malondinitrile, (ethoxymethylene)... 

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