The carbon-13 nuclear magnetic resonance spectra of some 1,3-dioxans. Part II. A demonstration of non-chair conformations

Abstract

The n.m.r. spectra of 36 1.3-dioxans are reported. Substituent effects on the chemical shifts of the ring carbon atoms are derived for a set of compounds known to exist in chair conformations. Using these effects to predict the chemical shifts in compounds that would have 4.6-or 2.4-syn-diaxial interactions in chair conformations results in large discrepancies between observation and prediction. It is concluded that such compounds exist to some extent in non-chair conformations.IN the continued speculation about the chair-boat equilibrium in 1,3-dioxan, the original estimate of the free energy difference between the boat and chair forms of 2.2 kcal/moll has been continually revised upwards, to over 3 kcal/mol,2 and more recently to 7.4 kcal/mol or more.3 One serious difficulty in forming estimates of this parameter is to be certain whether a given 1,3-dioxan exists to any appreciable extent in a non-chair conformation. We have solved this problem by examination of the 13C spectra of a large number of 1,3-dioxans having F. G. Riddell, J . Chetn. SOC. ( B ) , 1970, 331, is considered to be Part I.

Selective radical-chain epimerisation at electron-rich chiral tertiary C–H centres using thiols as protic polarity-reversal catalysts

“…This was prepared from (R,R)-pentane-2,4-diol according to the published method, 28 bp 135 ЊC/755 mmHg (lit., 28 29 The published method 29b was modified by the use of iodomethane as the methylating agent, instead of dimethyl sulfate, in order to facilitate isolation of the product. trans-Cyclopentane-1,2-diol (5.00 g, 49 mmol) in dry dimethylformamide (DMF, 20 cm 3 ) was added dropwise to a vigorously stirred suspension of sodium hydride (60% in mineral oil, 1.80 g, 125 mmol) in DMF (80 cm 3 ) cooled in an ice-water-bath.…”

Contribution à l'étude du comportement de la catéchine en milieu alcalin

“…Un pic de S = 96,6 ppm est apparu. 11 peut correspondre a un atome de carbone sur lequel sont fixis deux atomes de carbone et deux d'oxyg&ne, comme par exemple le C(2) du dimCthyl-2,2-dioxane-l, 3 dont le dCplacement chimique vaut 96,3 [12]. Dans notre cas, il doit s'agir du C(9) dont la fonction carbonyle est hydratte.…”

Carbon-13 nuclear magnetic resonance spectra of saturated heterocycles. IV. trans-Decahydroquinolines

“…Chemical Shifts* of Perhydrobenzo [h ]quinolines»(11)(12)(13), Perhydroacridine*(14), and Their -Methyl In parts per million, from internal Me4Si in CDC13. Parentheses indicate that assignments are not unambiguous.…”