On the electronic effects of OH groups. Synthesis and investigation of tetrahydroxylated azabicycloheptanes
作者:Anette Gregersen、Christian Marcus Pedersen、Henrik Helligs� Jensen、Mikael Bols
DOI:10.1039/b419154d
日期:——
Two stereoisomeric 2,3,5,6-tetrahydroxyazabicyclo[2.2.1]heptanes were synthesised and their base strengths determined. The 2,3,5,6-exo-isomer 1 and the 2,3-exo-5,6-endo-isomer 2 were prepared from the Diels–Alder adduct of Boc-pyrrole and tosylacetylene by a route involving osmium catalyzed dihydroxylation and protection, tosyl group reduction and repeated dihydroxylation. Deprotection gave 1, while 2 was obtained by conversion of the diol into the ditriflate, followed by nucleophilic inversion with KNO2 and deprotection. Synthesis of the 2,3,5,6-endo-isomer by a similar strategy was attempted but failed. The pKa of 1 and 2 was determined to be 7.0 and 6.4 respectively. This means that the change in base strength as a result of stereoisomerism of an OH is smaller in the [2.2.1]-azabicyclic system than in the piperidines. This is explained by a difference in charge–dipole interactions in the two systems.
合成了两种立体异构体2,3,5,6-四羟基氮杂双环[2.2.1]庚烷并测定了它们的碱强度。 2,3,5,6-外型异构体 1 和 2,3-外型-5,6-内型异构体 2 由 Boc-吡咯和甲苯磺酰乙炔的 Diels-Alder 加合物通过锇催化二羟基化反应制备而成和保护、甲苯磺酰基还原和重复二羟基化。脱保护得到1,而2是通过将二醇转化为二三氟甲磺酸酯,然后用KNO 2 亲核反转并脱保护得到的。尝试通过类似的策略合成2,3,5,6-内异构体,但失败了。 1和2的pKa分别确定为7.0和6.4。这意味着在[2.2.1]-氮杂双环系统中由于OH的立体异构而导致的碱基强度的变化比在哌啶中更小。这是通过两个系统中电荷-偶极子相互作用的差异来解释的。