🧫Organic Chemistry II Unit 11 – Organic Synthesis: Retrosynthetic Analysis

Key Concepts and Definitions

• Retrosynthetic analysis involves working backward from a target molecule to identify simpler precursor molecules and reactions that can be used to synthesize the target
• Synthons represent hypothetical building blocks or fragments that can be used to construct the target molecule
• Disconnections are the reverse of synthetic reactions and involve breaking bonds in the target molecule to generate potential precursors
• Forward synthesis is the process of constructing the target molecule from simpler precursors using synthetic reactions
  • Involves working forward from available starting materials to the desired product
• Synthetic equivalent is a real molecule or reagent that can be used to replace a synthon in the actual synthesis
• Functional group interconversions (FGIs) are transformations that convert one functional group into another while preserving the carbon skeleton
• Retrons are structural features or subunits in the target molecule that suggest potential disconnections or synthetic strategies

Retrosynthetic Analysis Fundamentals

• Retrosynthetic analysis begins by identifying the key structural features and functional groups in the target molecule
• Disconnections are made at strategic bonds to generate potential precursors or synthons
  • Bonds are typically broken heterolytically to generate electrophilic and nucleophilic synthons
• The resulting synthons are then analyzed to determine if they are commercially available, easily synthesized, or require further disconnections
• The process is repeated iteratively until simple, readily available starting materials are identified
• Multiple disconnection pathways may be explored to find the most efficient and practical synthetic route
• Retrosynthetic analysis considers factors such as availability of starting materials, number of steps, yield, stereochemistry, and potential side reactions
• The final retrosynthetic plan is then used to guide the forward synthesis of the target molecule

Common Disconnections and Transforms

• Functional group interconversions (FGIs) are commonly used to transform one functional group into another
  • Examples include oxidation of alcohols to aldehydes or ketones, reduction of nitriles to amines, and hydrolysis of esters to carboxylic acids
• Disconnections at carbon-carbon bonds can be achieved through reactions such as aldol condensations, Grignard additions, and Wittig reactions
• Disconnections at carbon-heteroatom bonds include reactions such as nucleophilic substitutions, reductive aminations, and esterifications
• Retro-aldol and retro-Claisen reactions are used to disconnect β-hydroxy ketones and β-keto esters, respectively
• Retro-Diels-Alder reactions are employed to simplify cyclic structures by disconnecting them into diene and dienophile components
• Protecting group strategies are used to temporarily mask reactive functional groups during the synthesis and are later removed
• Stereochemical considerations are important when planning disconnections and synthetic steps to ensure the desired stereoisomers are obtained

Synthetic Strategies and Planning

• Linear synthesis involves a step-wise approach where each reaction builds upon the previous one to gradually construct the target molecule
• Convergent synthesis combines multiple fragments or building blocks simultaneously to form the target molecule more efficiently
• The choice between linear and convergent strategies depends on factors such as the complexity of the target, available starting materials, and potential for side reactions
• Protecting group strategies are employed to selectively mask reactive functional groups and control the reaction site
  • Common protecting groups include silyl ethers for alcohols, benzyl groups for amines, and acetals for aldehydes and ketones
• Retrosynthetic analysis helps identify key intermediates or building blocks that can be synthesized separately and then combined in the forward synthesis
• Synthetic plans should consider the overall yield, number of steps, purification methods, and scalability of the process
• Atom economy and green chemistry principles are important considerations in designing efficient and environmentally friendly synthetic routes

Key Reactions in Organic Synthesis

• Nucleophilic additions to carbonyl compounds include Grignard reactions, organolithium additions, and reductive aminations
• Electrophilic additions to alkenes encompass reactions such as halogenation, hydration, and oxymercuration-demercuration
• Pericyclic reactions, including Diels-Alder cycloadditions and Claisen rearrangements, are useful for forming new carbon-carbon bonds and creating cyclic structures
• Cross-coupling reactions, such as Suzuki, Heck, and Sonogashira couplings, enable the formation of carbon-carbon bonds between sp2-hybridized carbons
• Olefination reactions, like the Wittig reaction and Horner-Wadsworth-Emmons reaction, convert carbonyl compounds into alkenes
• Reduction reactions, such as catalytic hydrogenation, lithium aluminum hydride reduction, and sodium borohydride reduction, are used to reduce functional groups
• Oxidation reactions, including chromic acid oxidation, Swern oxidation, and Dess-Martin periodinane oxidation, are employed to introduce oxygen-containing functional groups

Functional Group Interconversions

• Oxidation reactions convert alcohols to aldehydes, ketones, or carboxylic acids, and alkenes to epoxides or diols
  • Examples include Swern oxidation, Dess-Martin periodinane oxidation, and epoxidation with meta-chloroperoxybenzoic acid (mCPBA)
• Reduction reactions transform carbonyl compounds to alcohols, alkenes to alkanes, and nitro groups to amines
  • Common methods include catalytic hydrogenation, lithium aluminum hydride reduction, and sodium borohydride reduction
• Nucleophilic substitution reactions, such as SN2 and SN1, are used to replace leaving groups with nucleophiles
• Electrophilic addition reactions, like halogenation and hydration, add electrophiles to alkenes or alkynes
• Esterification and amidation reactions convert carboxylic acids into esters and amides, respectively
• Protecting group manipulations involve the selective protection and deprotection of functional groups to control reactivity and site selectivity
• Functional group interconversions are essential tools in retrosynthetic analysis for simplifying target molecules and generating precursors

Practical Applications and Examples

• Retrosynthetic analysis has been successfully applied to the synthesis of complex natural products, such as taxol, strychnine, and vitamin B12
  • These syntheses often involve multiple disconnections, functional group interconversions, and strategic bond formations
• Drug discovery and development rely heavily on retrosynthetic analysis to identify efficient routes to potential drug candidates
  • Optimizing the synthesis of active pharmaceutical ingredients (APIs) is crucial for large-scale production and commercialization
• Retrosynthetic analysis is used in the synthesis of organic materials, such as polymers, dyes, and liquid crystals
  • Designing monomers or building blocks with specific functional groups and properties is essential for achieving desired material characteristics
• Agrochemicals, including pesticides and herbicides, are developed using retrosynthetic approaches to optimize their synthesis and minimize environmental impact
• Fragrance and flavor compounds, such as menthol, vanillin, and citral, are synthesized using retrosynthetic strategies to ensure purity and cost-effectiveness
• Retrosynthetic analysis is a powerful tool for designing and optimizing the synthesis of a wide range of organic compounds with diverse applications

Tips and Common Pitfalls

• Break down complex target molecules into simpler subunits or key structural features to identify potential disconnections
• Consider the availability and cost of starting materials when planning the synthetic route
  • Using commercially available or easily synthesized precursors can greatly simplify the overall synthesis
• Be mindful of functional group compatibility and potential side reactions when proposing disconnections and synthetic steps
• Use retrosynthetic analysis to identify opportunities for convergent synthesis, which can improve efficiency and overall yield
• Consider the stereochemistry of the target molecule and ensure that the proposed synthetic route will lead to the desired stereoisomers
• Protect sensitive functional groups to prevent unwanted reactions and improve selectivity
  • Choose protecting groups that are easily introduced and removed under mild conditions
• Analyze the potential for regioselectivity and chemoselectivity issues in reactions involving multiple functional groups
• Evaluate the scalability and practicality of the proposed synthetic route, considering factors such as yield, purification, and environmental impact
• Consult the literature for known synthetic methods and precedents that can guide the retrosynthetic analysis and inform the synthetic planning
• Iterate and refine the retrosynthetic plan as needed based on experimental results and new insights gained during the forward synthesis