Retrosynthetic Analysis: Principles and Applications | 逆合成分析原理及其应用

📚 Retrosynthetic Analysis: Principles and Applications | 逆合成分析原理及其应用

Retrosynthetic analysis is a powerful problem-solving approach in organic synthesis. It starts from the target molecule and works backwards step by step, breaking it down into simpler and commercially available starting materials. This strategy was formalised by Professor E. J. Corey, who was awarded the Nobel Prize in Chemistry in 1990 for his contribution to this field.

逆合成分析是有机合成中一种强大的解题策略。它从目标分子出发，逐步倒推，将其分解为更简单且可商购的起始原料。这一策略由 E. J. Corey 教授系统化提出，他也因此获得了 1990 年的诺贝尔化学奖。

1. What is Retrosynthetic Analysis? | 什么是逆合成分析？

Retrosynthetic analysis is the process of “deconstructing” a target molecule (TM) into smaller fragments, called synthons, and then identifying reactions that can connect these fragments in the forward direction. The key idea is to think in the opposite direction to the actual chemical synthesis.

逆合成分析是将目标分子（TM）“解构”成更小片段（称为合成子），然后再找出能在正向反应中将这些片段连接起来的反应。其核心思想是与实际化学合成方向相反地思考。

In a forward synthesis, we choose starting materials and convert them into a product. In retrosynthetic analysis, we look at the product and ask: “Which bonds could be formed last?” The last bond formed in the forward direction corresponds to the first disconnect in the retrosynthetic direction.

在正向合成中，我们选择起始原料并将其转化为产物。而在逆合成分析中，我们观察产物并问：“哪些键可以在最后一步形成？”正向反应中最后形成的键，对应着逆合成方向中的第一个切断。

2. The Logic of Disconnection | 切断的逻辑

A disconnection is the imaginary breaking of a bond to produce two fragments. It is represented by a wavy line through the bond being broken, but in written text we often show it with a double-headed arrow “⇒” or simply describe it with words. Every disconnection should correspond to a known synthetic reaction that can form the same bond.

切断是指想象性地断开一根化学键，从而得到两个碎片。在写文本时，我们通常用双箭头“⇒”来表示，或者直接用文字说明。每一次切断都应当对应一个已知的、能够形成该键的合成反应。

For example, if a target molecule has a carbon-carbon bond that can be formed by a Grignard reaction, we disconnect that bond to reveal a carbonyl compound and a Grignard reagent as the two precursors. The disconnection is only logical, not a real experimental step.

例如，如果目标分子中有一个可通过格氏反应生成的碳-碳键，我们就可以切断这个键，得出羰基化合物和格氏试剂这两个前体。切断只是逻辑上的操作，不是真实的实验步骤。

3. Synthons and Reagents | 合成子与试剂

Synthons are hypothetical structural units that result from a disconnection. They can be positively charged (acceptor, “a”) or negatively charged (donor, “d”) species. For example, disconnecting a carbonyl compound at the C-C bond gives an acyl cation synthon CH₃CO⁺ and an aryl anion synthon C₆H₅⁻.

合成子是切断后得到的概念性结构单元。它们可以是正电荷物种（亲电体，用“a”表示）或负电荷物种（亲核体，用“d”表示）。例如，切断一个羰基化合物的 C-C 键，会得到一个乙酰基正离子合成子 CH₃CO⁺ 和一个苯基负离子合成子 C₆H₅⁻。

Synthons are not actual reagents. They must be replaced by stable, commercially available or preparable compounds. For instance, the acyl cation synthon CH₃CO⁺ corresponds to the reagent acetyl chloride CH₃COCl, while the aryl anion synthon C₆H₅⁻ corresponds to benzene C₆H₆ in the Friedel-Crafts acylation reaction.

合成子并不是实际试剂。它们必须被稳定的、可商购的或可制备的化合物所取代。例如，乙酰基正离子合成子 CH₃CO⁺ 对应试剂乙酰氯 CH₃COCl，而苯基负离子合成子 C₆H₅⁻ 在傅-克酰化反应中对应苯 C₆H₆。

4. Common Disconnections | 常见切断方式

Certain functional groups have predictable disconnection patterns that map directly to classic reactions. Recognising these patterns is the most important skill in retrosynthetic analysis.

某些官能团具有可预测的切断模式，这些模式直接对应经典反应。识别这些模式是逆合成分析中最重要的技能。

A secondary alcohol can be disconnected to a Grignard reagent and an aldehyde. The disconnection of the C-C bond adjacent to the OH group gives R′MgX and RCHO. In the forward direction, the Grignard reagent attacks the carbonyl carbon to form the alcohol.

仲醇可以被切断为格氏试剂和醛。切断与羟基相邻的 C-C 键，得到 R′MgX 和 RCHO。在正向反应中，格氏试剂进攻羰基碳形成醇。

An alkene can be disconnected to two carbonyl compounds through the reverse of the Wittig reaction. Alternatively, the C=C bond may be formed by elimination of water from an alcohol, so the precursors could be an alcohol and a dehydrating acid.

烯烃可以通过维蒂希反应的逆过程切断为两个羰基化合物。另外，C=C 键也可以通过醇脱水形成，因此前体可以是醇和脱水剂。

A β-hydroxy carbonyl compound (the aldol product) is disconnected to two carbonyl compounds. The C-C bond between the α-carbon and the carbonyl carbon is broken to give an enolate synthon and an aldehyde/ketone.

β-羟基羰基化合物（羟醛产物）可切断为两个羰基化合物。断开 α-碳与羰基碳之间的 C-C 键，得到烯醇负离子合成子和一个醛或酮。

5. Functional Group Interconversion | 官能团互换

Sometimes a disconnection cannot be made directly, but the target molecule can first be transformed into a related compound with a different functional group. This transformation is called functional group interconversion (FGI).

有时无法直接进行切断，但可以先通过官能团互换（FGI）将目标分子转化为另一种带有不同官能团的化合物。

For example, to make a ketone by a Friedel-Crafts acylation, the retrosynthetic step might require converting an ester into an acyl chloride before disconnection. FGI is a fundamental tool that broadens the range of possible synthetic pathways.

例如，要通过傅-克酰化制备酮，逆合成步骤可能需要先将酯转化为酰氯，然后再进行切断。官能团互换是一个基本工具，它扩大了可能合成路线的范围。

FGI should never be confused with a disconnection. In a disconnection, a bond is broken; in an FGI, the connectivity of the molecule remains the same, only the functional group changes.

官能团互换绝不应与切断混淆。切断是断开一根键；而 FGI 中分子的原子连接方式不变，只是官能团发生了变化。

6. Protecting Groups | 保护基

In a multi-step synthesis, a reactive functional group may interfere with a desired transformation on another group. A protecting group temporarily masks this reactive group, allowing the reaction to occur selectively, and is removed later.

在多步合成中，一个活泼官能团可能会干扰对另一个基团进行的期望转化。保护基可以临时掩盖这个活泼基团，使反应具有选择性，并在之后被去除。

For example, if a molecule contains both an alcohol and an aldehyde, and we want to react the aldehyde with a Grignard reagent, the alcohol must first be protected, often as a silyl ether or an acetyl ester. The aldehyde then reacts with the Grignard reagent, and the protecting group is hydrolysed off at the end.

例如，如果一个分子同时含有醇和醛，而我们希望让醛与格氏试剂反应，就必须先将醇保护起来，通常可转化为硅醚或乙酰酯。然后醛与格氏试剂反应，最后再水解去除保护基。

Common protecting groups include the acetyl group (Ac) for alcohols and amines, and the tert-butoxycarbonyl group (Boc) for amines. Protecting groups must be introduced and removed under mild conditions without affecting the rest of the molecule.

常见的保护基包括用于醇和胺的乙酰基（Ac），以及用于胺的叔丁氧羰基（Boc）。保护基的引入和去除必须在温和条件下进行，不影响分子的其他部分。

7. Strategy in Route Design | 路线设计策略

In designing a retrosynthetic route, one should look for the “key bond” whose formation is most challenging in the forward direction. Often this is a C-C bond, because C-C bond formation is more difficult than simple functional group transformations.

在设计逆合成路线时，应寻找“关键键”，即在正向合成中形成难度最大的键。通常这是 C-C 键，因为 C-C 键的形成比简单的官能团转化更困难。

A convergent synthesis is usually preferred over a linear synthesis. In a convergent design, two large fragments are prepared separately and then combined near the end. This increases both the overall yield and the practicality of the route.

汇聚式合成通常优于线性合成。在汇聚式设计中，两个大的片段分别制备，然后在接近最终步骤时组合。这样既提高了总产率，也提高了路线的实用性。

Maximising symmetry is another powerful tactic. If a target molecule contains an internal plane of symmetry, disconnecting it at the symmetry axis can produce two identical fragments, which can be used to simplify the synthesis.

最大化对称性是另一种强大的策略。如果目标分子含有内部对称面，沿着对称轴切断可得到两个相同的片段，从而简化合成。

8. Application: Paracetamol Synthesis | 应用实例：对乙酰氨基酚的合成

Paracetamol (acetaminophen) is a well-known analgesic. Its retrosynthetic analysis can be carried out step by step: paracetamol is an amide, which can be disconnected to an amine and an acyl group.

对乙酰氨基酚（扑热息痛）是著名的镇痛药。它的逆合成分析可以逐步进行：对乙酰氨基酚是酰胺，可以切断为胺和酰基部分。

In the retrosynthetic direction, the amide bond is broken to give 4-aminophenol and acetic anhydride (or acetyl chloride). The formation of the amide is achieved by acylation of the amine with acetic anhydride.

在逆合成方向上，断开酰胺键得到对氨基苯酚和乙酸酐（或乙酰氯）。在正向合成中，用乙酸酐对胺进行酰化即可形成酰胺。

4-aminophenol can be traced back to 4-nitrophenol by reduction of the nitro group. 4-nitrophenol is commercially available or can be made by nitration of phenol. The full forward route is: phenol → nitration → 4-nitrophenol → reduction → 4-aminophenol → acetylation → paracetamol.

对氨基苯酚可以通过硝基的还原追溯到对硝基苯酚。对硝基苯酚可商购，也可由苯酚硝化制备。完整的正向路线是：苯酚 → 硝化 → 对硝基苯酚 → 还原 → 对氨基苯酚 → 乙酰化 → 对乙酰氨基酚。

9. Application: Carbon-Carbon Bond Formation | 应用：碳-碳键形成

Retrosynthetic analysis is particularly useful for designing C-C bond forming steps. Consider a tertiary alcohol target, 2-phenylbutan-2-ol. The key step is forming the C-C bond between the phenyl group and the tertiary carbon.

逆合成分析在碳-碳键形成步骤的设计中尤其有用。考虑一个叔醇目标分子：2-苯基-2-丁醇。关键步骤是苯基与叔碳之间形成 C-C 键。

Disconnect the C-C bond next to the OH group: the target can be obtained from an ester and a Grignard reagent. The ester is ethyl acetate CH₃CO₂C₂H₅ and the Grignard reagent is phenylmagnesium bromide C₆H₅MgBr. Two equivalents of the Grignard reagent add to the ester, producing the tertiary alcohol.

切断羟基旁边的 C-C 键：该目标分子可以由酯和格氏试剂得到。酯是乙酸乙酯 CH₃CO₂C₂H₅，格氏试剂是苯基溴化镁 C₆H₅MgBr。两当量格氏试剂与酯发生加成反应，生成叔醇。

The retrosynthetic plan is: 2-phenylbutan-2-ol → acetophenone + phenylmagnesium bromide? Actually careful: 2-phenylbutan-2-ol is CH₃-C(OH)(C₂H₅)-Ph. If we disconnect between Ph and central carbon, we get Ph⁻ and CH₃-CO-C₂H₅ (butan-2-one). Then Grignard PhMgBr + butan-2-one gives the target. However, using ethyl acetate gives a symmetric route. Which is more accurate? Let’s choose butan-2-one.

逆合成方案是：2-苯基-2-丁醇 → 2-丁酮 + 苯基溴化镁。在正向反应中，苯基溴化镁与 2-丁酮的羰基加成，随后水解得到目标叔醇。

10. Limitations and Future Outlook | 局限性与前景

Retrosynthetic analysis assumes that every disconnection corresponds to a feasible reaction. In practice, many factors limit the application of a proposed route: reaction selectivity, stereochemistry, safety, cost, and the availability of starting materials.

逆合成分析假设每个切断都对应一个可行的反应。实际上，许多因素限制着拟议路线的应用：反应选择性、立体化学、安全性、成本以及起始原料的可获得性。

The stereochemical outcome is often hard to predict from simple disconnections. A retrosynthetic plan may look elegant on paper but fail in the lab because the desired diastereomer or enantiomer is not formed. Therefore, chemists must combine retrosynthetic logic with mechanistic insight and experimental testing.

立体化学的结果往往难以通过简单的切断来预测。逆合成方案可能在纸面上看起来优雅，但由于没有生成所需的非对映异构体或对映异构体而在实验室中失败。因此，化学家必须将逆合成逻辑与反应机理洞察力及实验检验相结合。

Looking forward, computer-aided retrosynthetic design is becoming increasingly important. Modern software can search databases of known reactions and propose synthetic routes to complex molecules in a matter of minutes, making it a valuable addition to the traditional pencil-and-paper approach.

展望未来，计算机辅助逆合成设计正变得越来越重要。现代软件可以搜索已知反应数据库，并在几分钟内为复杂分子提出合成路线，成为传统纸笔方法的有力补充。

11. Exam Tips | 考试要点

In exams, you are often asked to identify a possible disconnection for a given molecule and then suggest reagents for the forward synthesis. The best strategy is to first recognise the functional group, then look for the known reaction that forms it.

在考试中，你通常会被要求为给定的分子找出一种可能的切断方式，然后提出正向合成的试剂。最佳策略是先识别官能团，再寻找能形成该官能团的已知反应。

Memorise the key disconnection patterns:

记住关键的切断模式：

Alcohols: disconnect to carbonyl + Grignard reagent or hydride reduction.
醇：切断为羰基 + 格氏试剂，或考虑氢化物还原。
Alkenes: disconnect to alcohols by dehydration, or to carbonyls by Wittig.
烯烃：通过醇脱水切断，或通过维蒂希反应切断为羰基化合物。
Amides: disconnect to carboxylic acid (or acyl chloride) + amine.
酰胺：切断为羧酸（或酰氯）+ 胺。
Esters: disconnect to carboxylic acid + alcohol.
酯：切断为羧酸 + 醇。

Always write the retrosynthetic arrow (⇒) in the correct direction, from target to precursors. Never show a forward synthesis arrow when a disconnection is intended. Also, clearly distinguish synthons from actual reagents.

始终以正确的方向写出逆合成箭头（⇒），即从目标到前体。不要在进行切断时画出正向合成箭头。还要清楚区分合成子和实际试剂。

12. Practice Question | 练习题

Propose a retrosynthetic route for the compound 3-methylpentan-3-ol, using a Grignard reaction as the key bond-forming step.

试以格氏反应作为关键成键步骤，为化合物 3-甲基-3-戊醇设计一条逆合成路线。

Step 1: Recognise that the target is a tertiary alcohol. The carbon bearing the OH group has no hydrogen atoms, so the alcohol must be formed by reacting a Grignard reagent with an ester or a ketone.

第1步：识别目标物为叔醇。带有 OH 的碳上没有氢原子，因此该醇必须通过格氏试剂与酯或酮反应来制备。

Step 2: Disconnect one C-C bond between an alkyl group and the central carbon. For example, disconnect the ethyl group: the remaining carbonyl compound would be butan-2-one (CH₃COC₂H₅), and the Grignard reagent would be ethylmagnesium bromide C₂H₅MgBr.

第2步：断开一个烷基与中心碳之间的 C-C 键。例如，切断乙基：剩余羰基化合物为 2-丁酮（CH₃COC₂H₅），格氏试剂为乙基溴化镁 C₂H₅MgBr。

The forward synthesis is: CH₃COC₂H₅ + C₂H₅MgBr → then hydrolysis → 3-methylpentan-3-ol. This route is short, convergent, and consistent with the disconnection.

正向合成是：CH₃COC₂H₅ + C₂H₅MgBr → 然后水解 → 3-甲基-3-戊醇。该路线简短、汇聚，并且与切断一致。

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Retrosynthetic Analysis: Principles and Applications | 逆合成分析原理及其应用