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Retrosynthesis
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- 2. Content Retro synthesis of: •Acyclicsaturated systems • Acyclicunsaturated systems • Monocyclic compound • Bicycliccompound • Aromatic compounds
- 3. Retrosynthetic analysis isa problem-solving technique for transforming the structure of a synthetic target (TGT) molecule to a sequence of progressively simpler structures along a pathway which ultimately leads to simple or commercially available starting materials for a chemicalsynthesis.
- 4. Criteria for agood disconnection • A good mechanism • Greatest simplification • Recognisable starting material
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- 6. Analysis Synthesis Cyanide is agoodanionand cation is stabilised by the lone pair of electron on oxygen
- 7. The acetelyne anionis the most stable anion Analysis Synthesis Ph OH Me CH
- 8. The reactions mentionedin previous slides doesn’t gives us stable anion Et-Synthon synthetic equivalent of Et– the grignard reagent (Et-MgBr) or the alkyllithium(Et- Li). Analysis Synthesis
- 9. Route a) givesanother target to be made Route b) gives to equal simplified pieces – acetone and cyclohexyl bromide
- 10. Disconnection of tertiaryalcohol with two group same Analysis Removing both the group at single disconnection getting anester and two grignard’s reagents Synthesis
- 11. Double bond insix membered ring with an electron withdrawing group, we use diels-alder disconnection Analysis
- 12. Disconnection of analcohol with one group as a hydrogen Synthon - H- Synthetic equivalents -sodium borohydride and lithium aluminiumhydride The disconnection involving H- does not alter thecarbon skeleton;they are not disconnections but functionalgroup interconversion (FGI). Analysis
- 13. Sodium boromohydride onlyreducesthe aldehyde and lithium aluminium hydride reduces almost allcarbonyl compounds neither reagents reduces the double bond Analysis One or both the hydrogen can be removed and either starting materialscan be made by diels-alder reactions. Synthesis
- 14. Synthesis of mostcompounds is often best by first going back to alcohols by a straightforward FGI and disconnections List of compounds you can make alcohols from FGI.
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- 18. Reagentfor Synthon Ais an epoxide. Works wellwithmono-substituted epoxides Analysis Synthesis It is a poor tactic to chop off carbon at once, also this route gives us only 40% yeild asthe benzyl grignard reagents easily gives radicalsand polymerises.
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- 20. Designing an alkenesynthesisis to add water across the double bond thus on dehydration A willgive cyclohexanone but B willgive another Alkene Analysis
- 21. Analysis Synthesis Dehydration of Agives C (another alkene)but dehydration of B gives only thetarget molecule and notless substitutedD
- 22. Alternate route fordisconnection of olefins is from the double bond. Corresponds to wittig reactions Analysis
- 23. The halide inroute B is easilyrecognisable and the aldehyde can be synthesised usingthe diels-alder reaction The route A can also be used butthe startingmaterials are not so readily available. Analysis Synthesis
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- 25. Disconnectionof bond joiningaromatic ring and aliphaticrind is carried out by friedel-crafts reaction usingacetyl chloride and aluminium chloride Analysis
- 26. Routea is betteras it will give us an acyl rather thanalkyl halideand an activated benzenering Analysis
- 27. Routeb disconnection isnotpossible as nitro group is metadirecting and also a nitrobenzene will not react under friedel-crafts Routea is fine as MeO is ortho-directing. Analysis
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- 30. An acid isan hydroxyl compound and hence canbe disconnected as alcohols Analysis Acid derivatives are made directly from acids or conversion of other acid derivatives depending on their stability.
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- 33. Compounds with nofunctional groups are made by hydrogenation of a double bond
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- 37. The grignard reagentmight attack ketone giving wrong product To avoidthis, protect the ketone by reversible FGI. A common method is to make a cyclicketal Any functional group can be a protecting group providing •It can be added easily •It can be easily removed •It should not react with the reagent
- 38. This reactions givesa poor yield The product is as reactive asthe starting material and further reaction occurs
- 39. So we activateoneposition of carbonyl group by adding CO2Et group using ester Now removing the activated group just asthe protecting group. Here, by hydrolysis anddecarboxylation.
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- 42. Application of disconnections In1936, robinson carried out thisreaction to get thealcohol A He got an alcohol B Even with modern spectroscopic method the quickest way to identify a compound is to synthesise and compare the N.M.R, Fingerprint and I.R. region. Analysis Synthesis
- 43. References • Stuart Warren.Organicsynthesis:thedisconnectionapproach.Wiley India Pvt. Ltd, Publishers. • Stuart Warren. Designing organicsyntheses:Aprogrammed introductiontothesynthonapproach. Wiley India Pvt. Ltd, Publishers. • E. J. Corey. Thelogicofchemicalsynthesis.Jon Wiley and Sons, Publishers
- 44. "Long multistep syntheses of20 or more steps could be undertaken with confidence despite the Damocles sword of synthesis - only one step need fail for the entire project to meet sudden death.“ - E. J. Corey. The logic of chemical synthesis. Jon Wiley and Sons, Publishers