- 學位論文
天然物(±)-Xiamycin A 之全合成研究
Total Synthesis of Natural Product (±)-Xiamycin A
摘要
Xiamycin A 天然物在2010年由Hertweck等人從鏈黴菌屬(Streptomyces)的菌落中分離出來，屬於吲哚倍半萜類(indolosesquiterpenoids)天然物的一種。xiamycin A 單體及雙體皆具有廣泛的生物活性，包含抗細菌及抗病毒的活性等，也使得此類分子具有成為新型態抗生素的潛力。合成挑戰包含了反式十氫萘(trans-decalin)環及咔唑(carbazole)中心的建立。在本篇研究中，為了完成xiamycin A的全合成，我們共探索了4代不同的合成路徑。xiamycin A其多環的骨架可藉由環狀插烯酯(Vinylogous ester)與溴化芳基(Aryl bromides)進行一系列的α芳基化反應，在三步內建立出來。之後我們選擇了伯奇還原羧化反應(Birch-type reductive carboxylation)引進酯類基團，並同時建立正確的反式十氫萘環骨架。 第一代路徑中，咔唑可透過芳基化反應被接在環狀插烯酯上。然而在夫里德耳-夸夫特環化(Friedel-Crafts cyclization)反應中會出現位置選擇性的問題，因而降低總產率。且咔唑在伯奇還原羧化反應也會發生被部分還原的問題，所以此路徑在這個階段被終止。 在第二代路徑中，有鑑於之前的實驗結果，咔唑被結構較簡單的苯甲醚取代，因為苯環在伯奇還原的環境中反應性較咔唑低，且不會有位置選擇性的問題，然而在最後一步水解時，我們發現乙酯的部分在neo-pentyl位置時可能因為較大的立體障礙，所以無法用傳統的方式水解，也代表著合成上需要更容易水解的酯類。 第三代路徑包含了許多針對夫里德耳-夸夫特環化以及伯奇還原反應的細微修正，乙酯被替換成甲酯以期能解決水解的問題。我們嘗試在夫里德耳-夸夫特環化反應中了保留苯甲醚氧上的甲基並觀察在接上甲酯基團後是否能被去掉。另一條路徑是在伯奇還原之前先建立起聯苯胺的結構，原因是苯基團可同時作為氮原子保護基以及生成咔唑的配體。然而實驗上發現用傳統的苯甲醚去保護試劑如: BBr3 會生成過反應去羧基的產物。至於聯苯胺的部分則是在伯奇還原反應中會分解掉。 之後我們轉向發展第4代合成路徑。咔唑基團在最後階段才被生成。立體障礙大的甲酯基團利用Krapcho去羧基化方法順利去掉。最終，我們成功的合成了天然物(±)-Xiamycin A，以及其甲基的衍生物。
並列摘要
Xiamycin A, an indolosesquiterpenoids natural product was first isolated from the Streptomyces (鏈黴菌屬) in 2010 by Hertweck. Xiamycin A and its dimer exhibit a broad spectrum of bioactivities, including antibacterial and antiviral properties, which make it a potential candidate for new type antibiotics. Synthetic challenges include trans-decalin ring and carbazole core construction. In this study, we explored four different generations of synthetic pathways in order to achieve the racemic total synthesis of xiamycin A. The polycyclic framework of xiamycin A was constructed through a 3-step sequence featuring the α-arylation reaction of a cyclic vinylogous ester with aryl bromides, and we chose Birch-type reductive carboxylation to introduce ester and correct trans-decalin ring structure simultaneously. In the 1st generation, carbazole moiety was connected with cyclic vinylogous ester through arylation, however, during Friedel-Crafts cyclization step, site-selectivity problem took place, thus reducing the total yield. And carbazole would undergo inevitable partial reduction during Birch-type reductive carboxylation step. So this route was aborted at this stage. In the 2nd generation, based on previous experiment results, carbazole was replaced by a relatively simple anisole structure, since benzene ring is less reactive compared to carbazole under Birch reduction environment and there was no site-selectivity problem for this symmetric anisole substrate. However, in the last hydrolysis step, we found that ethyl ester moiety at neo-pentyl position is hard to be hydrolyzed using traditional methods, presumably due to the large steric hindrance, which implies that an easily hydrolyzed such as methyl ester is required. The 3rd generation consisted of several minor modifications to the Friedel-Crafts cyclization and Birch reduction. Ethyl ester is replaced by methyl ester in hopes of solving the hydrolysis problem. We tried keeping O-methyl group of anisole after the Friedel-Crafts cyclization and explored whether it can be removed after installing methyl ester. Another route which construct diphenyl aniline before birch reduction step was also developed since phenyl group can act as both N-protecting group and carbazole formation partner. However, experimental results showed that conventional anisole deprotecting reagent such as BBr3 will generate further decarboxylated product. As for aniline case, such diphenyl structure will decompose under birch reduction. Then we turned to the 4th generation. Carbazole was formed at late stage. Steric hindered methyl ester was smoothly cleaved using Krapcho decarboxylation method. Finally, we successfully synthesized natural product (±)-Xiamycin A and its methyl ester derivatives.