Retrosynthesis: work backwards from target using '=>' (retrosynthetic arrow)
Retrosynthetic Analysis
Start at the target molecule and work backwards. Identify key bond disconnections. Use '=>' retrosynthetic arrow. Each step asks: 'What two pieces could combine to make this?' Transform target into simpler precursors.
⚗️ Synthesis
Protect, React, Deprotect — the synthesis mantra
Protecting Groups Strategy
When a reagent would react with multiple functional groups, PROTECT the one you don't want to react. Common protections: alcohol → TMS ether or acetal. Amine → Boc or Cbz. Carbonyl → acetal. Then react. Then deprotect.
⚗️ Synthesis
Disconnection at C-C bonds formed by: Grignard, aldol, Wittig, Diels-Alder
Key C-C Bond Forming Reactions
Most synthesis problems hinge on C-C bond formation. Key reactions: Grignard (RMgX + carbonyl). Aldol condensation. Wittig (carbonyl → alkene). Diels-Alder (diene + dienophile → cyclohexene). Know these 4.
⚗️ Synthesis
Diels-Alder: diene must be s-cis conformation, electron-rich diene + electron-poor dienophile
Diels-Alder Reaction
Concerted [4+2] cycloaddition. Diene must adopt s-cis conformation. Best with electron-donating groups on diene, electron-withdrawing groups on dienophile. Stereospecific: syn addition, endo rule for major product.
⚗️ Synthesis
Oxidation state changes: reduction adds H or removes O, oxidation removes H or adds O
Oxidation State Tracking
Track oxidation states to plan redox steps. Reduction: add H2 (hydrogenation), add H⁻ (NaBH4/LiAlH4), or remove O. Oxidation: add O (KMnO4, OsO4) or remove H (PCC, Cr2O7²⁻). Balance oxidation states across synthesis.
⚗️ Synthesis
Acetoacetic ester synthesis: alkylation at alpha-C then decarboxylation → methyl ketone
Acetoacetic Ester Synthesis
Strategy for making methyl ketones (CH₃COCH₂R). Steps: (1) Ethyl acetoacetate + NaOEt → enolate at central C. (2) Alkylate with R-X (SN2). (3) Saponify (NaOH/H₂O) → beta-keto acid. (4) Heat → decarboxylation → methyl ketone (R-CH₂-CO-CH₃). Compare to malonic ester synthesis (→ carboxylic acids). Both use base-assisted alkylation + decarboxylation. Choose acetoacetic ester when you want a methyl ketone product.
H₂ + Pd/C (hydrogenation): reduces alkenes and alkynes to alkanes. Does NOT reduce carbonyls under normal conditions. LiAlH₄: reduces all carbonyls (COOH, ester, aldehyde, ketone, amide → alcohols/amines). Very reactive — use anhydrous ether. NaBH₄: selective — reduces only aldehydes and ketones (not esters or COOH). Safe in protic solvents. DIBAL-H: reduces esters to aldehydes (stop at aldehyde stage) at –78°C. Lindlar's catalyst: reduces alkynes to cis-alkenes only.
H₂/Pd
Alkenes, alkynes → alkanes (not carbonyls)
LiAlH₄
All C=O → alcohols/amines (use anhydrous Et₂O)
NaBH₄
Only aldehydes + ketones → alcohols (selective)
DIBAL-H
Ester → aldehyde (–78°C)
Lindlar
Alkyne → cis-alkene only
⚗️ Synthesis
Oxidation comparison: PCC stops at aldehyde; KMnO₄ goes to acid; OsO₄ gives diol
Choosing the Right Oxidizing Agent
PCC (pyridinium chlorochromate): 1° alcohol → aldehyde (STOPS). 2° alcohol → ketone. KMnO₄ (hot, conc.): 1° alcohol → COOH. Cleaves C=C. OsO₄: syn-dihydroxylation of alkene → syn-diol. mCPBA: alkene → epoxide (stereospecific). Swern oxidation (oxalyl chloride/DMSO): 1° → aldehyde (mild, works for sensitive substrates). Jones reagent (CrO₃/H₂SO₄): 1° or 2° alcohol → COOH or ketone.
PCC
1° → aldehyde; 2° → ketone
KMnO₄ hot
1° → COOH; 2° → ketone; cleaves alkene
OsO₄
Alkene → syn-diol
mCPBA
Alkene → epoxide
Jones (CrO₃/H₂SO₄)
Alcohol → COOH or ketone
⚗️ Synthesis
Umpolung: normally electrophilic C made nucleophilic using thioacetal + n-BuLi
Umpolung — Polarity Reversal
Umpolung ('polarity reversal') makes a normally electrophilic carbon into a nucleophile. Classic example: Corey-Seebach reaction. Aldehyde (electrophilic at C) + ethanedithiol → dithiane → n-BuLi deprotonates → dithiane anion (nucleophilic at what was formerly the aldehyde carbon). Reacts with electrophile. Oxidation removes thioacetal → ketone. Allows disconnections not possible with normal polarity. Key for making 1,2-diol and related synthons.
⚗️ Synthesis
Sharpless epoxidation: asymmetric — chiral tartrate + Ti(OiPr)₄ controls face
Asymmetric Synthesis: Sharpless Epoxidation
Sharpless asymmetric epoxidation converts allylic alcohols to epoxy alcohols with high enantioselectivity. Reagents: Ti(OiPr)₄ + TBHP (oxidant) + tartrate ester (chiral). (+)-tartrate: oxygen delivered to bottom face (by convention). (–)-tartrate: oxygen delivered to top face. Predictable by mnemonic drawing. The alcohol group anchors to Ti, which controls face selectivity. This was Nobel Prize-winning chemistry (2001). Routinely achieves >90% ee.
⚗️ Synthesis
Robinson annulation: Michael addition + aldol = 6-membered ring with enone
Robinson Annulation
Robinson annulation = Michael addition + aldol condensation → forms a 6-membered ring with an enone. Steps: (1) Michael acceptor (enone) + 1,3-dicarbonyl → Michael addition at beta-C. (2) Intramolecular aldol condensation → 6-membered ring. (3) Dehydration → cyclohexenone. Used to build 6-membered rings in steroid synthesis and complex molecules. Example: methyl vinyl ketone + cyclohexanone → Hajos-Parrish ketone (in proline-catalyzed version).
⚗️ Synthesis
Functional group interconversion (FGI): change one group to another to enable the retrosynthesis
Functional Group Interconversion (FGI)
In retrosynthesis, when a direct disconnection doesn't work, use FGI — convert the target functional group to a different one that allows a simpler disconnection. Examples: ketone → alcohol (reduce) → ether (protect). Amine → amide (protect) → carbamate. Alkene → epoxide → alcohol. Alkene → diol. COOH → acid chloride → ester/amide. Terminal alkyne → acetylide (nucleophile). FGI and disconnection are the two main tools of retrosynthetic analysis.
⚗️ Synthesis
Phase-transfer catalysis: moves ionic reagent from water to organic phase using quaternary ammonium salt
Phase-Transfer Catalysis
Phase-transfer catalysts (PTC) transfer ions from aqueous phase to organic phase to enable reactions between otherwise incompatible reagents. Common PTCs: quaternary ammonium salts (Bu₄N⁺X⁻), crown ethers. Mechanism: PTC extracts anion into organic phase as an ion pair → anion reacts with organic substrate → PTC returns to water phase. Applications: alkylations, substitutions, oxidations, carbene reactions. Crown ethers complex metal cation, leaving anion 'naked' and reactive in organic phase.
🎓 Common Exam Questions
Q: What is retrosynthetic analysis and how do you use it?
A: Retrosynthesis: work backwards from the target molecule using the retrosynthetic arrow (⟹). Identify key bond disconnections — which C–C bonds were formed? What simpler precursors could combine to make this? Transform the target into simpler pieces at each step. The most useful disconnections are at C–C bonds formed by Grignard, aldol, Wittig, or Diels-Alder reactions.
Q: What is the protecting group strategy in synthesis?
A: When a reagent would react with multiple functional groups, protect the one you don't want to react. Three steps: PROTECT (block the functional group with a protecting group), REACT (perform the desired transformation), DEPROTECT (remove the protecting group under mild conditions). Common protections: alcohol → TMS ether (TMSCI/Et₃N) or acetal. Amine → Boc or Cbz. Carbonyl → acetal (1,3-dioxolane).
Q: What are the four key C–C bond-forming reactions for synthesis?
A: (1) Grignard: RMgX + carbonyl → alcohol. (2) Aldol condensation: enolate + carbonyl → beta-hydroxy carbonyl → enone. (3) Wittig: Ph₃P=CHR + carbonyl → alkene + Ph₃P=O. (4) Diels-Alder: diene + dienophile → cyclohexene (concerted [4+2]). Knowing which reaction forms which bond type is the key to retrosynthesis.
Q: What are the requirements and products of the Diels-Alder reaction?
A: Diene must be in the s-cis conformation (locked or freely rotating). Best results: electron-rich diene + electron-poor dienophile (EWG on dienophile). Concerted [4+2] cycloaddition → six-membered ring (cyclohexene). Stereospecific: substituents on dienophile maintain relative stereochemistry (syn addition). Endo rule: endo transition state is kinetically preferred → endo product is major.
Q: How do you track oxidation state changes in synthesis planning?
A: Reduction: add H (hydrogenation, NaBH₄, LiAlH₄) or remove O. Oxidation: add O (KMnO₄, OsO₄, PCC) or remove H. Count oxidation state changes to plan redox steps. Common oxidation levels: alkane < alcohol < aldehyde/ketone < carboxylic acid. Reduce: go left. Oxidize: go right. Multi-step synthesis often requires redox steps to reach the target oxidation level.
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