The enantioselective syntheses of each enantiomer of ethyl
2,6-dideoxy-6,6,6-trifluoro-β-arabino-hexopyranoside
(3) and of 2,6-dideoxy-6,6,6-trifluoro-arabino-hexose
(4) are described. The key step in the approach is the inverse electron demand
Lewis acid catalysed Diels–Alder reaction of the heterodiene
(E)-1,1,1-trifluoro-4-[(1R)-1-
phenylethoxy]but-3-en-2-one (5) and either ethyl vinyl ether (7a) or
benzyl vinyl ether (7b). The titanium(IV) chloride catalysed cycloadditions at
low temperature displayed high endo-selectivity and
modest diastereofacial selectivity. Hydroboration of the mixture of the
cis-cycloadducts (8a) afforded the separable
diastereoisomers (9a) and (10a). Hydrogenolysis of (9a) gave ethyl
2,6-dideoxy-6,6,6-trifluoro-β-L-arabino-hexopyranoside
(+)-(3), and of (10a) gave the corresponding D-glycoside (−)-(3).
Similarly, hydroboration of the cis-cycloadducts (8b)
gave the protected benzyl glycosides (9b) and (10b). Hydrogenolysis of each
gave 2,6-dideoxy-6,6,6-trifluoro-L-arabino-hexopyranose
(−)-(4) and the corresponding D-sugar (+)-(4) respectively. The
absolute configurations of fluorinated carbohydrates (+)- and
(−)-(3) were determined by comparison of their molar rotations with
those of the parent glycosides. These assignments were confirmed by an X-ray
crystallographic structure determination of the glycoside (9a).
乙基-2,6-二脱氧-6,6,6-三氟-β-阿拉伯-六吡喃糖苷各对映体的对映选择性合成
2,6-二脱氧-6,6,6-三氟-β-阿拉伯并六吡喃糖苷 (3) 和
(3) 和 2,6-二脱氧-6,6,6-三氟-阿拉伯吡喃己糖 (4) 的各对映体的合成。
(4) 进行了描述。该方法的关键步骤是反电子需求
路易斯酸催化的杂二烯 Diels-Alder 反应
(E)-1,1,1-trifluoro-4-[(1R)-1-
苯乙氧基]丁-3-烯-2-酮 (5) 和乙烯基乙醚 (7a) 或
乙烯基乙基醚 (7a) 或乙烯基苄基醚 (7b)。氯化钛(IV)催化的环加成反应在低温条件下
在低温下催化的环加成反应显示出较高的内向选择性和适度的非对映选择性。
的非对映选择性。氢硼化合
顺式环加成产物(8a)的混合物进行氢硼化合,可得到可分离的
非对映异构体 (9a) 和 (10a)。氢解 (9a) 得到乙基
2,6-二脱氧-6,6,6-三氟-β-L-阿拉伯-吡喃己糖苷
(+)-(3),而 (10a) 则得到相应的 D-糖苷 (-)-(3)。
同样,顺式氰基加载产物(8b)的氢硼化反应
得到了受保护的苄基糖苷 (9b) 和 (10b)。氢解每种
得到 2,6-二脱氧-6,6,6-三氟-L-阿拉伯并六吡喃糖
(4)和相应的 D-糖 (+)-(4)。氟化碳水化合物的
和(-)-(3)的绝对构型。
(-)-(3)的绝对构型是通过将它们的摩尔旋转与母苷的摩尔旋转进行比较而确定的。
母苷的摩尔旋转进行比较,从而确定了氟化碳水化合物 (+)- 和 (-)-(3) 的绝对构型。通过 X 射线
苷 (9a) 的晶体学结构测定确认了这些分配。