藉由保護基策略探討三萜之 3-O-葡萄糖醛酸化反應的 優化研究

Abstract

三萜類皂苷廣泛分布於植物界中，其中數個三萜類葡萄醣醛酸苷衍生物，被發現具有藥物活性。例如：疫苗佐劑QS-21、和玻尿酸抑制劑Chikusetusaponin IV。而為了研究這類結構的構效關係，化學合成為一有效途徑來獲得結構多元的衍生物。然而化學合成進行葡萄醣醛酸化反應經常受限於低產率，其困難點在於葡萄醣醛酸的低反應性，同時因為三萜類化合物的立體障礙導致低親核性，使三萜類化合物的葡萄醣醛酸化反應更具有挑戰性。為了促進三萜類葡萄醣醛酸化反應，我們在葡萄醣醛酸上使用了不同的保護基，包括乙醯基（Ac），苯甲醯基（Bz），三甲基乙醯基（Piv）和叔丁基二甲基矽烷基（TBS），建構了arm與disarm的醣予體，探討其與三萜類齊墩果酸和皂皮酸在不同活化劑、溫度和溶劑下的反應性及選擇性。 實驗結果發現，在室溫下以B(PhF5)3為活化劑具有最佳的反應性及β選擇性，在O-2位置使用的保護基中，Ac保護基由於立體障礙較小而容易有副反應的發生。Bz保護基的醣給予者與齊墩果酸有60%的產率；Piv保護基的醣給予者與皂皮酸也有60%的產率。在選擇性方面，Piv保護基因為立障效應，能抑制原酯的副產物生成。另一方面，TBS保護基具有更少的副產物，較高的產物收率（50％至60％），具有更好的發展潛力。 而在以本研究中所獲得的葡萄醣醛酸衍生物進行玻尿酸分解酶抑制劑之活性測定，不幸的是，發現在1 mM下仍無明顯抑制活性。儘管如此，本研究提供了優化的葡萄醣醛酸化反應條件，未來有助於進一步合成更多葡萄醣醛酸衍生物。

Triterpenoid saponins are widely distributed in plants, and several of these compounds are found as glucuronides which possess biologically activities, such as vaccine adjuvant- QS-21 and hyaluronidase inhibitor- Chikusetusaponin IV. In order to study their structural-activity relationship, chemical synthesis is an effective way to obtain structure-relative and diversified derivatives. However, chemical synthesis for glucuronidation is often limited to low yields. The difficulty lies on the low reactivity of glucuronic acid, and also the low nucleophilicity of steric-hindrance triterpenoids, which cause the glucuronidation of triterpenes and glucuronic acid more challenging. To promote the reactivity of glucuronidation, we explored the usage of various protecting groups, including acetyl (Ac), benzoyl (Bz), trimethylacetyl (Piv) and tert-butyldimethyl silyl (TBS) groups, to construct the “armed” and “disarmed” glucuronyl donors. Using these compounds, we can investigate their reactivity and selectivity with oleanolic acid and quillaic acid under different promoters, temperature and solvent. The results showed that using B(PhF5)3 as a promotor at room temperature provided the best reactivity and β-selective products. Among these protective groups at O-2 position of donor, Ac group was easy to have side reactions due to it is less hindrance. Glucuronic acid bearing Bz groups had 60% yield with oleanolic acid and Piv groups had 60% yield with quillaic acid. In terms of selectivity, Piv protecting group can inhibit the formation of orthoester by-products due to the steric effect. TBS protecting group had the less side products, higher yield (in a range of 50% to 60%) at room termperatur which had potential for future development. The glucuronide derivative 92 obtained in this study was subjected to the hyaluronidase inhibitor assay. Unfortunately, it was found to no significant inhibitory activity at 1 mM. Nonetheless, this study provides an optimized glucuronidation conditions that will help to synthesis more glucuronic acid derivatives in future.