Lewis Base Activation of Lewis Acids: Catalytic, Enantioselective Addition of Glycolate-Derived Silyl Ketene Acetals to Aldehydes
作者:Scott E. Denmark、Won-jin Chung
DOI:10.1021/jo8006539
日期:2008.6.1
A catalytic system involving silicon tetrachloride and a chiral, Lewis basic bisphosphoramide catalyst is effective for the addition of glycolate-derived silyl ketene acetals to aldehydes. It was found that the sense of diastereoselectivity could be modulated by changing the size of the substituents on the silyl ketene acetals. In general, the trimethylsilyl ketene acetals derived from methyl glycolates
(Z)-β-Aryloxyalkenylzincs are synthesized stereoselectively via anti-carbozincation among alkynyl ethers, silyl ketene acetals, and ZnBr2. X-ray analysis revealed the structure of the zinc species is a mononuclear two-coordinate dialkenylzinc that is transformed into functionalized enol ethers as a single isomer in the reaction of various electrophiles.
A convergent synthesis of one isomer of the C14–C29 fragment of mirabalin is disclosed. The key steps include a Marshall allenylation, a Mukaiyama aldol reaction and a Crimmins aldolization, which allow the control of 10 out of 25 stereogenic centers present in the molecule.
The mixture of lithium diisopropylamide and potassium tert-butoxide (“LIDA-KOR reagent”) is a powerful base which can be advantageously employed for the regioselective deprotonation of fluoropyridines and, in particular, 3-fluoroquinoline. A novel cyclization process was elaborated for the preparation of the latter product.