Aluminosilicate ester

By using 10 mol% of 51, MS4A, and t-BuSH, the desired product 52 was obtained in up to 98% ee in 80% yield. A complementary role by two metals (Ga and Li) in activating and positioning both of the substrates has been proposed. The MS4A (sodium aluminosilicate) accelerated the reaction however, the actual role of this additive was not clearly defined, although the possibilty that MS4A delivers Na ions was pointed out. Tomioka et al. reported the asymmetric Michael addition of an aromatic thiol to a,P-unsaturated esters in the presence of 8 mol% of 53 to provide 54 in up to 97% ee in 99% yield (Eq. 7.40) [47]. 

Other apparent support for the solubility theory by silicosis came from several sources. The less soluble types of silica were less harmful. Thus the deposition of soluble aluminum ions on the silica surface reduces solubility as well as toxicity (337-340). Also, aluminosilicate minerals such as clays, which are even less soluble than quartz, do not cause silicosis. The report that silicotic lung tissues contained esters of silicic acid, for example, cholesterol (341), seemed also to support the idea that soluble silica must be involved. 

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