Chapter 28 — Key Takeaways

What you should leave Chapter 28 with

1. Aldol = enolate + carbonyl → β-hydroxy carbonyl. This is the foundational C-C bond-forming reaction in synthesis. The α-C of one carbonyl (after deprotonation, Ch 27) attacks the C=O of another carbonyl (Family I addition, Ch 25). The result is a 4-bond β-hydroxy compound.

2. Aldol condensation = aldol + dehydration → α,β-unsaturated carbonyl (enone). Under warm conditions, the β-hydroxy intermediate loses water (E1cb mechanism), forming a conjugated enone. The conjugation provides thermodynamic stability and irreversibility.

3. Aldol mechanism (base-catalyzed): - Step 1: Hydroxide (or other base) removes the α-H of one carbonyl → enolate. - Step 2: Enolate attacks the C=O of another carbonyl → tetrahedral alkoxide. - Step 3: Protonation of alkoxide → β-hydroxy carbonyl. - Step 4 (condensation): Base removes α-H between the new C=O and the β-OH; water leaves; conjugated C=C-C=O forms.

4. Crossed aldol (two different carbonyls) is best directed by: - Using a carbonyl with no α-H (e.g., benzaldehyde, formaldehyde, pivaldehyde) as the electrophile. - Pre-forming the enolate with LDA at low temperature, then adding the other carbonyl. - Using a silyl enol ether + Lewis acid (Mukaiyama aldol).

5. Mukaiyama aldol: silyl enol ether + Lewis acid (TiCl₄, BF₃) + carbonyl → β-hydroxy compound. Modern, stereocontrolled, often used in asymmetric synthesis.

6. Claisen condensation = ester enolate + ester → β-keto ester. The mechanism is similar to aldol but with the ester's OR' as the leaving group (Family II acyl substitution from Ch 26). The α-H of the resulting β-keto ester (pKa 11) is fully deprotonated by NaOR, driving the equilibrium forward.

7. Claisen mechanism: - Step A: NaOR removes α-H of an ester → ester enolate. - Step B: Enolate attacks another ester's C=O → tetrahedral intermediate. - Step C: OR' leaves (acyl substitution) → β-keto ester. - Step D: NaOR deprotonates the β-keto ester (now very acidic) → stable enolate.

8. Why the Claisen works (and the aldol doesn't always): the final deprotonation in step D pulls the equilibrium to product. The β-keto ester is dramatically more acidic (pKa 11 vs 25 for the starting ester) than the starting ester, so the deprotonation is essentially complete. This is the "aufbau principle" for Claisen.

9. Match the base to the ester's OR'! Use NaOEt for ethyl esters; NaOMe for methyl esters. Mismatch causes transesterification side reactions.

10. Crossed Claisen uses a non-enolizable ester (e.g., ethyl benzoate, ethyl formate, diethyl carbonate) as the electrophile. The other ester (with α-H) becomes the enolate.

11. Dieckmann cyclization = intramolecular Claisen on a diester → cyclic β-keto ester. Best for 5- and 6-membered rings. After hydrolysis + decarboxylation, gives cyclic ketones (cyclopentanone, cyclohexanone).

12. Knoevenagel condensation: 1,3-dicarbonyl + aldehyde + base → α,β-unsaturated dicarbonyl + water. Used to make α,β-unsaturated dicarbonyls for Michael (Ch 29) addition.

13. Mannich reaction: aldehyde + amine + 1,3-dicarbonyl → β-amino dicarbonyl + water. Used for β-amino ketones in alkaloid synthesis.

14. Stereochemistry: Zimmerman-Traxler transition state. The 6-membered chair-like TS with both substituents in equatorial positions controls syn vs anti aldol selectivity. Z-enolate → syn aldol; E-enolate → anti aldol.

15. Asymmetric aldol (with chiral auxiliary, Lewis acid catalyst, or organocatalyst) gives one enantiomer in high yield. Evans's chiral oxazolidinones are the most-used auxiliary; the Mukaiyama aldol with chiral Lewis acid is also widely used.

16. Biology runs on aldol chemistry:

    • Glycolytic aldolase: retro-aldol of fructose-1,6-bisphosphate → DHAP + G3P (step 4 of glycolysis).
    • Citrate synthase: aldol condensation of acetyl-CoA enolate + oxaloacetate → citrate (step 1 of TCA cycle).
    • Fatty acid synthase: iterative decarboxylative Claisen for chain extension (Ch 34 in detail).
    • Polyketide synthases: PKS modules build natural products by iterated Claisen (lovastatin, erythromycin, doxorubicin).
    • Transketolase / transaldolase: aldol-style sugar interconversions in pentose phosphate pathway.

17. The aldol-Claisen-Michael progression: from Chapter 28 (aldol and Claisen) to Chapter 29 (Michael addition) is a logical sequence. The α,β-unsaturated carbonyl (product of aldol condensation) is the substrate for Michael. Ch 29 builds on Ch 28.

18. Crucial mechanism: The aldol attacks at the carbonyl C of the second molecule, not the α-C. Don't confuse the two roles. The α-C of molecule A is the nucleophile; the C=O of molecule B is the electrophile.

19. Computational tools can predict the relative energies of syn vs anti aldol products. Modern DFT calculations get the diastereoselectivity within a few percent of experimental values.

20. Mastery of Chapter 28 is the foundation for retrosynthetic analysis (Ch 31). When you see a β-hydroxy compound or a β-keto ester, ask: "Where did this come from? An aldol or Claisen reaction?" The retrosynthetic disconnection is to break the bond between α-C and the C=O of the would-be electrophile.

Cross-references

• Chapter 24 — Carbonyl group; reactivity ordering. Foundation.
• Chapter 25 — Nucleophilic addition (Family I). The C=O electrophile.
• Chapter 26 — Nucleophilic acyl substitution (Family II). The ester leaving group (Claisen).
• Chapter 27 — α-Carbon chemistry (Family III). The α-C nucleophile.
• Chapter 29 — Conjugate (Michael) addition; uses the enone product of aldol condensation.
• Chapter 31 — Synthesis Workshop 2; retrosynthetic disconnections involving aldol/Claisen.
• Chapter 34 — Fatty acid metabolism; iterative Claisen.
• Chapter 36 — Cholesterol biosynthesis and statins; Claisen-derived steroids.
• Appendix B — pKa table.
• Appendix F — Named reactions: Mannich, Knoevenagel, Mukaiyama, Dieckmann, etc.

Study tip

For each aldol or Claisen problem, identify three things: 1. Which carbonyl is the nucleophile (enolate)? It must have an α-H. 2. Which carbonyl is the electrophile? It must have an electrophilic C=O. 3. What is the new C-C bond? It is between the α-C of the nucleophile and the C=O carbon of the electrophile.

If you can identify these three elements for any aldol or Claisen, you can predict the product and write the mechanism. Practice with 10 different combinations until it's automatic.