Structural parameters for the chair or twist-boat conformations of 1,3-dioxa-5,6-benzocycloheptene and its 2-methyl and 2,2-dimethyl derivatives

Abstract

. Can. J. Chem. 64, 500 (1986). Low temperature X-ray studies of 1,3-dioxa-5,6-benzocycloheptene ( 1 ) and its 2-methyl ( 2 ) and 2,2-dimethyl derivatives (3) show that 1 and 2 exist in the crystal state exclusively in the chair ( C ) form while 3 exists exclusively in the twist-boat (TB) form.The final values of the agreement index, R,,, are 0.048, 0.059, and 0.046 for 1, 2 , and 3 , respectively. Significant structural differences between the g+g+ arrangement of the acetal moiety with the TB form and the g+g-disposition in the C form are revealed. These differences can be interpreted as arising from a less efficient back-donation of an antiperiplanar lone pair on the oxygen atom in the g+g+ relative to the g'g-arrangement. Furthermore, the C-0 bond lengths shed some light on the hybridization state of the lone pairs of electrons on oxygen. Finally the results are used to rationalize nrnr observations in terms of molecular conformations in solution.ROBERT ST-AMOUR, MARC J. OLIVIER, MAURICE ST-JACQUES et FRANGOIS BRISSE. Can. J. Chem. 64, 500 (1986).L'Ctude par diffraction des rayons X A basse temperature du dioxa-1,3 benzo-5,6 cycloheptene (1) et de ses derives methyl-2 ( 2 ) et dimethyl-2,2 ( 3 ) montre que 1 et 2 existent A 1'Ctat solide seulement sous la forme chaise ( C ) alors que 3 n'existe que sous la forme bateau-croisk (TB). Les valeurs finales du facteur d'accord R,,, sont 0,048,0,059 et 0,046 pour 1, 2 , et 3 respectivement. Des diffkrences structurales significatives entre l'arrangement g+g+ du groupe acetal dans la forme TB et la disposition g+g-dans la forme C sont mises en Cvidence. On interprkte ces differences par une retrodonation moins efficace du doublet antiperiplanaire de I'oxygkne dans I'arrangement g+g+ relativement i l'arrangement g+g-. De plus, les longueurs des liaisons C--0 clarifient le type d'hybridation des doublets de l'oxygkne. Finalement, ces resultats permettent de rationaliser les observations de rmn sur les conformations des molCcules en solution. IntroductionIn recent years, we have studied the conformational properties of 1,3-dioxa-5,6-benzocycloheptene 1 and several derivatives in solution (1, 2) by means of 'H and 13C dnrnr (dynamic nuclear magnetic resonance) methods. They reveal the existence of a mixture of the chair form 1A and the twist-boat form, l B , in a ratio of 79:21 (in CHF2C1, at -130°C) (1).

Stereochemistry of Seven-membered Heterocycles: XLIV. Spatial Structure of 4-R-3,5-Dioxabicyclo[5.1.0]octanes

“…The alternative isomers were prepared by epimerization (Scheme 2). 13 C NMR spectroscopy proved to be informative in the stereochemical investigation of seven-membered acetals [1,5,6]. The chemical shift of carbon atoms of acetals IIIaIIIc are presented in Table 1.…”