Organic chemistry tricks that make synthesis stick

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.

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.

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.

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.

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.

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.

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.

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.

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.

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).

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.

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.