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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.

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The l H n.m.r. spectra of several 1.3-dioxans known to exist in non-chair conformation have been recorded in carbon tetrachloride, methylene chloride, benzene, or nitrobenzene solutions a t 60 and 220 MHz. In all cases the values of the vicinal coupling constants are almost independent of temperature, and in several cases large ( > l o Hz) vicinal coupling constants, indicative of anancomeric twisted boat conformations are observed. The results lead to conclusions concerning the preferred boat conformations of the 1.3-dioxan ring, and the preferred orientations of substituents under this circumstance.IT has recently become well established that 1,3-dioxans that would have a 2,ksyn-diaxial interaction between two alkyl groups in a chair conformation assume nonchair, probably twisted boat, conformations. Heat of formation data 2-4 has shown that travts-2,2,4,6tetramethyl-1,3-dioxan has a non-chair conformation. l H N.m.r. studies have confirmed this and also led to the same conclusion for 2,2-~-4-~-5-t-6-pentamethyl-1,3dioxan.l~5~6 Two of us have recently investigated the l3C n.m.r. spectra of some l,3-dioxans7 A study of substituent effects upon ring carbon chemical shifts showed

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