| Research Abstract |
Studies on the behavior of organotin-sulfur reagents in the presence of Lewis acids have disclosed following points:
(1) Novel access to monothioacetals from acetals ---- Monothioacetals of the general formula of PhS-CHR-OMe are synthetically useful intermediates. However, the bottle neck to be funnelled has been the access to this type of compounds. The authors have discovered treatment of acetals RCH(OMe)_2 with Bu_3SnSPh in the presence of BF_3・OEt_2 provides the desired products.
(2) Protective activation of hydroxyl groups ---- Tetrahydropyranylation (THP) of hydroxyl groups is a well-known method of protection of a hydroxyl group. The authors have discovered that the tetrahydropyranyloxy moiety is preferentially transformed into alkoxystannanes, ROSnBu_3 and 2-phenylthio- tetrahydropyran upon treatment with Bu_3SnSPh in the presence of BF_3・OEt_2. The former tin alkoxides are easily alkylated (to ethers), acylated (to esters), and oxidized (to aldehydes and ketones) by proper treat
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ment. That is protection with THP increases the susceptibility of the hydroxyl group to these transformations. The observation will certainly find applications in the selective transformation of polyols.
(3) A new preparation of 1,3-dithianes ---- The reaction of acetals with 1,3- propanedithiols proceeds in the presence of Bu_2Sn(OTf)_2 (Tf = -OSO_2CF_3), which is a new Lewis acid first prepared by the authors, to afford good yields of 2-substituted 1,3-dithianes.
(4) Introduction of electrophilic groups on the beta-carbon of alpha- enals ---- When the reaction described in (1) above is applied to the alpha- enal acetals, RCH=CH-CH(OMe)_2, the resulting products are 1-methoxy-3- phenylthio-l-alkenes, RCHSPh-CH=CHOMe. Lithiation at the C(3) position followed by the action of electrophilic reagents (E) provides the products, RC(SPh)(E)-CH=CHOMe, whose hydrolysis and desulfurization give RC(E)=CH-CHO products. The sequence provides a means of substituting the beta C-H bond with C-E bond.
(5) Protective activation of carboxylic acids ---- The authors have disclosed a new application of alpha-methylcinnamyl esters as a way of protective activation. Treatment of this kind of esters with Bu_3SnSPh reagent in the presence of BF_3・OEt_2 affords the stannyl esters R-COOSnBu_3, which are the reactive tin salts being easily transformed into esters, free carboxylic acids, and so on. This is a way of selective activation of a carboxyl group in polycarboxylic acids. Less
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