Fmoc是有機合成中用于氨基、羥基臨時保護的經典基團，廣泛用于肽、寡糖的液相與固相合成，具備引入簡便、溫和選擇性脫除的優勢，還可用于功能材料設計與熒光探針。

傳統Fmoc脫除方法需過量堿（如哌啶、DBU）極性溶劑（如 DMF），部分需硫醇類清除劑捕獲二苯并富烯（DBF）副產物；且無溶劑 Fmoc 脫除此前無報道。

2024年， Serena Traboni等人 首次報道了一種無溶劑催化方法用于從胺和醇官能團上脫除芴甲氧羰基（Fmoc）保護基。多種糖類、肽類及糖氨基酸底物， 僅用20 mol% 4-二甲氨基吡啶處理，即可高效脫保護，無需任何溶劑或化學計量添加劑。通過一鍋法、堿催化、無溶劑多步反應序列，將 Fmoc 脫除與酯化、酰胺化、糖基化步驟結合，最終完成了小型模型結構的構建。【Org. Lett.2024， https://doi.org/10.1021/acs.orglett.4c00918】

反應條件優化N-Fmoc-L - 丙氨酸酯 1為模型底物，以無溶劑、催化量堿為核心條件，篩選多種堿的脫保護效果。

DMAP（20 mol%）：100 ℃、30 min，N-Fmoc 定量脫除，分離收率100%；克級規模下仍保持 100% 收率；催化劑用量降至 5 mol%，100 ℃、1 h，收率仍達83%。

DBU（20 mol%）：100 ℃、30 min，收率 100%；50 ℃、45 min，收率98%，低溫仍高效。

N-乙基嗎啉（20 mol%）：梯度升溫（100→110→120 ℃），總時長 2 h，收率99%。

TMEDA（20/10 mol%）：100 ℃、1 h，收率分別為76%、78%，效率略低但仍有效。

吡啶、2,4,6 - 三甲基吡啶：無脫保護效果。

底物適用范圍

在最優條件（ 20 mol% DMAP，無溶劑，100 ℃，30 min）下， N-Fmoc 與 O-Fmoc雙類型底物，糖類、肽類、糖氨基酸等多功能底物皆可高效脫保護。

N-Fmoc 脫除：甘露糖基-L-丙氨酸綴合物、二肽 4，高效脫除得到游離氨基產物。N-Fmoc - 葡萄糖胺，脫除后原位 N-乙酰化，得到 N-乙酰化產物 5。

O-Fmoc 脫除：甘露糖苷6、7，蘇氨酸甘露糖苷8，α-1,6 - 二甘露糖苷 9：均高效脫除 O-Fmoc 得到對應醇產物。

所有底物均無需清除劑，無游離氨基與 DBF 副反應，收率優異。

一鍋法多步串聯應用

將無溶劑Fmoc脫除與酯化、酰胺化、糖基化結合，單催化劑 DMAP/DIPEA 催化多步反應。

脫保護-酰胺化：酯 10 先脫 N-Fmoc，再與 Fmoc-L - 丙氨酸無溶劑酰胺化，總收率45%。

酯化-脫保護-肽偶聯：單鍋DMAP催化三步反應，從單糖前體 12 合成糖二肽酯 13，總收率27%。

脫保護-糖基化：DIPEA 先脫 O-Fmoc，再一鍋法糖基化合成 α-1,2 - 二甘露糖苷 16，總收率47%。

反應機理

Fmoc為堿敏感保護基，脫除遵循芐位去質子化→β- 消除→釋放 CO?→游離胺 / 醇的經典機理；無溶劑、催化量堿條件下，無 DBF 副產物與游離氨基的加成副反應，無需清除劑。

實驗操作

Gram-scale procedure for the N-Fmoc deprotection of model compound 1 (gram-scale synthesis of compound 2)promoted by 20 mol %of DMAP ：To a round-bottom flask containing compound 1 (1.0 g,2.1 mmol)was added 4-dimethylaminopyridine(DMAP)(51 mg,0.42 mmol).The flask was placed in an oil bath under magnetic stirring at 100°C.Upon heating,the heterogeneous solvent-free mixture was observed to gradually turn into a fluid,nearly homogeneous slurry,allowing the initially clustered DMAP particles to spread over the flask and mix with the substrate.In order to ensure the best performance of the reaction,it is important to make sure in the first few minutes that the substrate and the catalyst are allowed to be homogeneously exposed to each other (e.g.no residues should be left on the neck or other parts of the flask where the mixing could be hampered or fall out of the stirring area).When the TLC analysis showed quantitative consumption of the starting material and the successful removal of the Fmoc group (30 minutes),the flask was cooled down to room temperature and the crude mixture was directly subjected to silica-gel chromatography (from ethyl acetate to ethyl acetate/methanol9:1 v/v)to isolate pure compound 2 (506 mg,100%) as a colorless oil.

General procedure for the removal of the Fmoc group (synthesis of compounds 3,4,5,6,7,8,9)：To a small round-bottom flask containing the N-or O-Fmoc protected substrate (0.2 mmol,1 equiv.),4-dimethylaminopyridine (DMAP)(4.9 mg,0.04 mmol,0.2 equiv.)was added.The flask was placed in an oil bath under magnetic stirring at 100°C.Upon heating,the heterogeneous solvent-free mixture was observed togradually turn into a fluid,nearly homogeneous slurry,allowing the initially clustered DMAP particles to spread over the flask and mix with the substrate.In order to ensure the best performance of the reaction,it is important to make sure in the first few minutes that the substrate and the catalyst are allowed to be homogeneously exposed to each other (e.g.no residues should be left on the neck or other parts of the flask where the mixing could be hampered or fall out of the stirring area).After 30 minutes,TLC and/or NMR analysis evidenced the consumption of the starting material and the successful removal of the Fmoc group. The flask was cooled down to room temperature and the crude mixture was directly subjected to silica-gel chromatography to isolate the pure deprotected alcohol (or amine)product in the yield indicated in Table 2 in the manuscript.

本文首次證明，Fmoc 保護基可在無任何溶劑的條件下，經多種有機堿催化實現脫除。以20 mol%無溶劑DMAP、100 ℃為標準條件，對 N-Fmoc 和 O-Fmoc 底物（含糖類、肽類、糖氨基酸）均表現出高效脫保護性能。該無溶劑脫除方法可整合至多步合成中，單一有機堿可在一鍋無溶劑體系中同時催化 Fmoc 脫除、酯鍵、肽鍵、糖苷鍵的構建。

參考資料：Catalytic Cleavage of the 9?Fluorenylmethoxycarbonyl (Fmoc) Protecting Group under Neat Conditions：Serena Traboni,* Fabiana Esposito, Marcello Ziaco, Noemi De Cesare, Emiliano Bedini, and Alfonso Iadonisi； Org. Lett. 2024， https://doi.org/10.1021/acs.orglett.4c00918