Premed · Premed · Organic Chemistry 2
Lecture 23: Multistep Synthesis Strategy
Organic Chemistry II
Learning Objectives
By the end of this lecture, students will be able to:
- Apply retrosynthetic analysis to break down a target molecule into simpler precursors
- Identify strategic disconnections at key bonds (C-C, C-heteroatom)
- Recognize synthon-reagent pairs for major bond-forming reactions
- Plan multi-step syntheses that control regiochemistry, stereochemistry, and functional group compatibility
- Use protecting group strategies to manage functional group reactivity
- Evaluate and compare synthetic routes for efficiency and selectivity
Lecture Content
I. Retrosynthetic Analysis: Thinking Backwards
Retrosynthetic analysis, formalized by E.J. Corey, is the discipline of working backwards from a target molecule to identify viable starting materials and reaction sequences. The retrosynthetic arrow (=>) means "can be made from." Each step in retrosynthesis involves a disconnection, the mental cleavage of a bond in the target to reveal simpler precursors, and a transform, which is the reverse of a known synthetic reaction.
A disconnection generates synthons -- idealized fragments that can be either nucleophilic (carbanion equivalents) or electrophilic (carbocation equivalents). Each synthon maps to a real reagent that serves as its synthetic equivalent. The retrosynthetic strategy follows a systematic procedure: identify the target and its key functional groups, look for bonds that can be formed by known reactions (strategic bonds), disconnect those bonds to generate synthons, map the synthons to real reagents, and continue disconnecting until commercially available starting materials are reached. The final step is to write the synthesis in the forward direction and verify that each step gives the desired product with correct regiochemistry and stereochemistry.
II. Key Carbon-Carbon Bond-Forming Reactions
Carbon-carbon bond formation is the central challenge of organic synthesis, and the reactions mastered across two semesters of organic chemistry provide a diverse toolkit for meeting it.
From Organic Chemistry I, acetylide alkylation uses an acetylide anion (formed from a terminal alkyne with NaNH2) in an SN2 reaction with a primary or methyl alkyl halide to form a new C-C bond with a triple bond. Grignard and organolithium reagents attack carbonyls to form alcohols: formaldehyde gives primary alcohols, aldehydes give secondary alcohols, ketones give tertiary alcohols, esters give tertiary alcohols (with two equivalents of the organometallic), and CO2 gives carboxylic acids. Gilman reagents (R2CuLi) perform conjugate 1,4-addition to alpha,beta-unsaturated carbonyls.
From Organic Chemistry II, the aldol reaction and aldol condensation connect an enolate to a carbonyl, producing beta-hydroxy carbonyls or alpha,beta-unsaturated carbonyls. The Claisen condensation connects an ester enolate to another ester, forming beta-keto esters. The malonic ester synthesis and acetoacetic ester synthesis use sequential alkylation, hydrolysis, and decarboxylation to produce substituted carboxylic acids and methyl ketones, respectively. The Michael reaction adds a stabilized enolate to the beta position of an alpha,beta-unsaturated carbonyl. The Diels-Alder reaction forms two C-C bonds and one ring in a single step. The Wittig reaction replaces a C=O with a C=C, providing unambiguous alkene placement.
III. Functional Group Interconversions (FGIs)
Many syntheses require converting one functional group to another before or after a key bond-forming step. The oxidation ladder progresses from alcohol to aldehyde (using PCC or Swern oxidation) to carboxylic acid (using Jones reagent or KMnO4), while secondary alcohols oxidize to ketones. Reductions run in the other direction: carboxylic acids are reduced to primary alcohols (LiAlH4), esters to primary alcohols (LiAlH4) or aldehydes (DIBAL-H at -78 degrees C), and aldehydes or ketones to alcohols (NaBH4 or LiAlH4). Amides are reduced to amines by LiAlH4. Alkynes can be selectively reduced to cis-alkenes (H2/Lindlar catalyst), trans-alkenes (Na/NH3), or fully to alkanes (excess H2/Pd-C).
Leaving group installation converts alcohols to alkyl halides (HBr, PBr3, SOCl2) or tosylates (TsCl/pyridine), providing versatile substrates for SN2 reactions. Amines are best synthesized by reductive amination or the Gabriel synthesis.
<image>A retrosynthesis example showing the target molecule 4-phenyl-2-butanone (C6H5CH2CH2COCH3). Step 1: Retrosynthetic disconnection at the bond between the alpha-carbon and the phenyl-bearing carbon, revealing an acetone enolate synthon (nucleophilic) and a phenylacetaldehyde synthon (electrophilic). This maps to an aldol condensation followed by reduction. Step 2: An alternative disconnection at the C-C bond alpha to the carbonyl on the other side, showing an acetoacetic ester synthesis approach -- alkylation of ethyl acetoacetate with a benzyl halide, followed by hydrolysis and decarboxylation. Both routes are drawn with retrosynthetic arrows, and the forward synthetic steps are written below each. Reagents are specified at each step. A caption reads: "Retrosynthetic analysis often reveals multiple routes to the same target; the best route is chosen based on selectivity, yield, and availability of starting materials."</image>
IV. Protecting Groups
When a molecule contains multiple functional groups, a reagent intended for one group may inadvertently react with another. The solution is to temporarily convert the sensitive functional group to an unreactive form (protect), carry out the desired reaction, and then restore the original group (deprotect).
Alcohols are commonly protected as silyl ethers (TBS or TMS, installed with R3SiCl and base, removed with fluoride or dilute acid) or as acetals (installed by reaction with an aldehyde or ketone under acid catalysis, removed by aqueous acid). Carbonyls are protected as cyclic acetals (1,3-dioxolanes, formed with ethylene glycol and acid, removed with aqueous acid), which render the carbonyl inert to nucleophilic addition, reduction, and Grignard reagents. Amines are protected with Boc (installed by Boc2O, removed by TFA), Cbz (installed by CbzCl, removed by H2/Pd), or Fmoc (removed by piperidine). Carboxylic acids are protected as esters, removed by hydrolysis or hydrogenolysis.
The essential principle is that the protecting group must be stable under the conditions of all subsequent reactions and must be selectively removable at the end without disturbing the rest of the molecule.
V. Stereochemical Control in Synthesis
Many target molecules contain stereocenters, requiring the synthesis to control both absolute and relative stereochemistry. This is achieved by selecting reactions with known stereochemical outcomes. SN2 proceeds with inversion of configuration. Epoxidation followed by ring opening gives predictable anti addition. Hydroboration-oxidation gives syn addition with anti-Markovnikov regiochemistry. The Diels-Alder reaction preserves diene and dienophile geometry and offers endo/exo selectivity. The Sharpless asymmetric epoxidation provides enantioselective epoxidation of allylic alcohols using a chiral titanium tartrate catalyst.
Additional strategies include starting from chiral pool molecules (amino acids, sugars, terpenes) that already possess the desired stereochemistry, employing chiral auxiliaries or chiral catalysts for enantioselective transformations, and using enzymatic resolution to separate racemic mixtures.
VI. Evaluating Synthetic Routes
A well-designed synthesis is short (fewer steps mean higher overall yield and less waste), convergent (large fragments are assembled late in the synthesis rather than building linearly one step at a time), selective (unwanted side reactions and protecting group manipulations are minimized), and practical (using readily available, inexpensive starting materials).
Atom economy, a key green chemistry metric, measures what fraction of the atoms in the reactants end up in the desired product. Overall yield is the product of the yields of all individual steps; even with 90% yield per step, a 10-step linear synthesis gives only 35% overall yield. Comparing two synthetic routes involves evaluating the number of steps, estimated overall yield, selectivity issues (regiochemistry, stereochemistry), and the need for protecting groups.
<image>A side-by-side comparison of a linear synthesis versus a convergent synthesis for a hypothetical 8-carbon target molecule. Left side: "Linear Synthesis" -- a chain of 8 sequential steps starting from a 2-carbon starting material, adding fragments one at a time. Each step is numbered; the overall yield calculation shows 0.9^8 = 43% if each step is 90%. Right side: "Convergent Synthesis" -- two parallel 3-step sequences each build a 4-carbon fragment from 2-carbon starting materials, then the two fragments are joined in a single coupling step, followed by one final transformation (total: 4 steps in the longest linear sequence). The overall yield calculation shows 0.9^4 = 66% for the longest branch. The convergent route is highlighted as more efficient. A caption reads: "Convergent synthesis strategies maximize overall yield by keeping the longest linear sequence short and assembling large fragments in a late-stage coupling step."</image>
VII. Practice Framework for Synthesis Problems
When facing a synthesis problem, a systematic approach prevents errors. Begin by comparing the target to the starting material: identify which bonds have been made or broken, what new functional groups are present, and whether any new stereocenters have appeared. Work retrosynthetically, identifying the last reaction in the sequence first. Determine which functional group interconversions are needed, and check whether any functional groups would interfere with planned reagents (necessitating protecting groups). Write the forward synthesis with specific reagents and conditions for each step, then verify the stereochemistry and regiochemistry of every step.
Common pitfalls include forgetting that Grignard reagents react with acidic protons (requiring prior protection of OH, NH, and COOH groups), attempting SN2 reactions on tertiary substrates (which give elimination instead), ignoring stereochemistry when the product has stereocenters, and failing to account for carbocation rearrangements in SN1 and E1 pathways.
