Sigmatropic Rearrangement of Uronic Acid Derived Saccharide Allyl Ketene Acetals. A Study of Stereochemistry and Structural Limitations
作者:Barbara Werschkun、Joachim Thiem
DOI:10.1055/s-1999-3686
日期:——
Allyl ketene acetal Claisen rearrangement of urinoc acid derivatives with furanoside structure was studied. Reproducibility of the observed stereodiscrimination was confirmed in the conversion of enantiomeric starting materials. From X-ray structural data of the major reaction product obtained from a cis-pentenyl uronate, the transition state was proven to preferentially adopt a chair-like geometry. Alterations in the protecting group pattern led to a significant decrease in total yields. Use of β-deoxygenated starting material allowed a greater extent of variation, but was accompanied by the total loss of stereodiscrimination. For the first time, the reaction was applied to pyranoid starting material structures. The conversion of a gluco-configurated compound remaining unsuccessful, with the corresponding galacto and 4-deoxy derivatives an interesting stereoselectivity could be observed. This was lost by further structural simplification to a 1,4-dideoxypyranoid ring. This way, however, a substantial improvement in yield could be achieved.
研究了具有呋喃糖苷结构的尿囊酸衍生物的烯丙基乙缩醛克莱森重排。在对映体起始材料的转化过程中,观察到的立体区分的可重复性得到了证实。从顺式戊烯基尿苷酸的主要反应产物的 X 射线结构数据来看,过渡态被证明优先采用椅子状几何结构。保护基团模式的改变导致总产率显著下降。使用δ-脱氧起始原料可以实现更大范围的变化,但同时也完全丧失了立体区分性。该反应首次应用于吡喃起始原料结构。葡萄糖构型化合物的转化仍未成功,但相应的半乳糖和 4-脱氧衍生物却出现了有趣的立体选择性。如果进一步将结构简化为 1,4-二脱氧吡喃环,这种立体选择性就会消失。不过,通过这种方法,可以大大提高产率。