INTRODUCTIONWhen optically active methyl hydrogen P-methylglutarate is converted into the acid chloride using thionyl chloride and then reconverted into the half-acid by hydrolysis, racemization occurs.' Similar rearrangements of the half-esters derived from other alkyl substituted succinic (butanedioic) and glutaric (pentanedioic) acids have been and intermediates such as 1,2 2 3,4 and 3 3,4 have been suggested. None has been identified. The involvement of oxonium ions related to 1 provides an entry to the simplest sequence of reactions that would explain the rearrangements, but such ions would be expected to be thermodynamically less accessible than the delocalized carbenium ions 2. The large chemical shift range in 13C NMR, compared with 'H NMR spectroscopy, allows the symmetry differences between the ions of type 1 and 2 to be established easily. Further, 13C NMR shifts are more affected than are 'H shifts by changes in electron densities at carbon atoms. Although 13C NMR data are available for a wide range of protonated carbonyl compounds little information is available for alkoxycarbenium ions5 or oxonium i0ns.6.~ As far as we are aware, no convincing evidence has been presented that proves the existence of solutions of dialkylacyloxonium, alkyldiacyloxonium or triacyloxonium ions, although each has been claimed.8-" Few I3C NMR data are available even for trialkyloxonium ions, and except where a reference is given the data given in this paper have not been reported previously. We have recently demonstrated that o-methoxycarbonylbenzoyl chloride can be converted, in solution, into an acyloxyalkoxycarbenium ion 4,12 * Author to whom correspondence should be addressed.and we now present results that show that solutions of ions of type 2 can be prepared and can be characterized by 13C NMR spectroscopy. On the other hand, replacement of aluminium chloride by antimony pentafluoride resulted in the formation of the ions 14 and 15. Chemical shift data for these species and also for 6-valerolactone (pentanolactone) (16) and the related hydroxy-(17) and methoxy-(18) carbenium ions were also obtained.It is noteworthy that although y-butyrolactone (butanolactone) reacts smoothly with trimethyloxonium fluoroborate to give the ion 7, the same procedure gave an unassigned mixture with 6-valerolactone. The salt 18 was obtained using dimethoxycarbenium fluoroborate.A number of features allow the carbenium ions to be distinguished from the donor-acceptor complexes. In the donor-acceptor complexes downfield shifts are observed for the methyl, carbonyl and methylene groups a-to the ester carbonyl, while the methylene group ato the chlorocarbonyl group experiences a small upfield shift. The carbenium ions 14 and 15 show significant differences; the residual carbonyl groups and their adjacent methylene groups both experience considerable shielding. This initially surprising effect has been reported previously in the 13C N M R spectra of a-acylcarbenium ions17 and the ion 4.'* The most likely explanation is that since the neighb...
