1,t-Diol esters react faster with SbCls to give an acyloxonium salt containing a five-membered ring than do 1,3-diol esters to give such a salt having a six-membered ring. The reaction of 1,2,4-butanetriol esters with SbCl5 permits direct comparison of the stabilities of dioxolanylium and dioxanylium salts and shows that the five-membered ring is the more stable since a salt of ( I ) is formed preferentially (90%).Valence isomerization should, however, occur in the case of acyloxonium salts of 1,3,5-trioIs. Since the above findings indicate that this conversion should have a higher AG* value than in the case of 1,2,3-triols [1J we have directed our attention to the more readily investigatable pivalic esters. On heating with SbCls in CHzC12 (20 h), 1,3,5-pentanetriol tripivalate affords the salt of (2) in 61 "/, yield. At room temperature, the NMR spectrum of (2) in CD3CN contains two sharp tert-butyl signals, that at lower field (T = 8.64) being assigned to the group attached to the dioxanylium ring and the other (T = 8.81) to the normal pivaloyl group. Coalescence of the two signals occurs at llO°C (solvent CsHsNOz), apparently due to a fast reversible 1,3-neighboring group reaction (2) +(3). The AG* value of 19.3 kcall mole is 0.9-1.0 kcal/mole higher than the value found for the valence isomerization of the glycerolpivaloxonium salt 111.As with cyclopentanepentol[*l, a system can also be found for the dioxanylium ion in which a cyclorearrangement with total valence isomerization is observed. Pentaerythritol tetrapivalate (4) affords the monopivaloxonium salt (5) (2-rertbutyl-5,5-bis(pivaloyloxymethyl)-1,3-dioxan-2-y~ium hexachloroantimonate) in 35% yield on heating with one mol. equiv. of SbCl5 in CHzC12. As shown in the reaction scheme, (5) can undergo a 1,3-neighboring group reaction to give an ion which is structurally identical with the starting ion. Four rearrangement steps suffice to regenerate the startingmaterial. Rearrangement takes place in a spiral manner about the central tetrahedral carbon atom. The NMR spectrum of the pivaloxonium salt (5) at room temperature in CD3NOz shows a 9-proton signal (r = 8.51)for the tert-butyl group attached to the dioxanylium ring and an 18-proton signal (T = 8.77) for the two pivaloyl groups. In addition, a downfield singlet ( z = 4.82,4H) is observed for the methylene protons of the dioxanylium ring, and another singlet at 7 = 5.72 for the remaining methylene protons. The signals due to the tert-butyl groups and those of the methylene groups coalesce when the solution is heated to llO°C and 122OC respectively. This observation can be regarded as proof that (5) does undergo rearrangement in the manner indicated above. The coalescence temperature of 110 "C (Av = 20.0 Hz) for the tert-butyl groups gives a value of AG* = 19.3 kcal/mole for the rearrangements.
