Lithium ions base-catalysed

The use of DMSO in recent studies has been largely upon the premise that the problem of base association would be avoided. Some doubt as to the validity of this assumption arises when the results of a conductometric study are considered (Exner and Steiner, 1974). Ion-pairing constants for lithium, sodium, potassium and cesium t-butoxide in DMSO have been evaluated as 108, 106, 270 and 200 M-1 respectively. Not only do these results suggest that there is base association in DMSO but they also imply that base-catalysed reactions involving alkali metal alkoxides in DMSO should be affected by the nature of the cation. If these conclusions are valid and if the possible involvement of the dimsyl anion in these reactions is also taken into consideration, then the choice of DMSO to remove the problem of base association can be a poor one, especially if the base is a lithium or sodium salt of a hindered alkoxide. It is far better to avoid association effects by the use of crown ethers (Bartsch et al., 1973, 1974, 1975). On the other hand, the use of lithium and potassium t-butoxide in DMSO solvent might aid in distinguishing reactivities of free ions and of ion pairs in certain processes. 

Synthetic work in this area includes a new, stereospecific synthesis of sparteine and a synthesis of isosophoramine. The reduction of quinolizidone (16) by means of lithium aluminium hydride affords, as noted previously, a dimeric product (17), presumably via condensation of the initially formed enamine (18) with the corresponding immonium cation (19). With di-isobutyl aluminium hydride the yield of (17) can be increased to 85%. Analogous reduction of the dilactam (20), prepared by base-catalysed condensation of the bromo-lactam (21) with a-piperidone, gave an intermediate (22), in which the enamine and immonium ion units are present within the same molecule. Intramolecular cyclization of (22), via a transition state derived from the conformation (23), spontaneously gave the... 

The base-catalysed cyclization of oximinonitriles in the presence of lithium ions to form 4-aminoisoxazoles and 5-aminooxazoles proceeds via the anion of one of the oximinonitrile lithium complexes. ... 

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