NMR EXPERIMENTS ON ACETALS. PART 28. 1.3-DIOXANES WITH NON-CHAIR CONFORMATIONS AND THE 2 ~( ~~~2 ~) CRITERIUM M . ANTEUNIS, G. SWAELENS, F. ANTEUNIS-DE KETELAERE and P. DIRINCK Several 1,3-dioxanes, not possessing chair conformations (Chart I), are newly prepared. The PMR data are discussed in terms of possible conformations. Typically ZJ(OCH20) coupling constants are too positive in accordance with disturbed torsional relations between a(CL-H) bonds and adjacent p ( 0 ) lobes. A semiquantitative discussion, relating 2J with presumed torsion ring angles, is given. Thus values for 'J(OCH,O) in carbon disulfide are: -5 . 5 cps for a twist conformation, with (idealized) torsion angles of 30' and 60" (form (14)); -3.4 cps for a conformation between a chair form a half-chair form and sofa form (resp. (16) and (17)); and -4.7 cps for a form close to a classical boat (19) with torsion angles of 0' and 60'.
We have previously described at least three 1,3-dioxanic derivates having either flattened chair [I] or twist [2, 31 conformations. In all these cases, a net decrease of the absolute value of 2J(OCH20) is found, in accordance with the general trend [3, 4, 51 that 2J becomes more positive (a decrease in absolute value; '5 being negative in m-dioxanesj when p lobes eclipse the a-bonds. The purpose of the present paper is to extend this "pa parallelity" effect by a series of newly synthesized rn-dioxanes having non-chair conformations, and to try to discuss the implications on 2J(OCH20) on a semi-quantitative base. Chart I collects the structures studied, and we refer to the experimental part for their preparation.
* Approximate values obtained from 5,7,7'-trideutero-derivatives