Multi-step Organic Synthesis Conversion of existing molecules into other useful molecules.
Pharmaceuticals Natural sources: petroleum natural gas Polymers natural products ®®® Gasoline Commercial sources Textiles Etc. Etc.
• Little molecules ----> big molecules • Synthesis of unnatural products • Modification of existing structures • Synthesis of rare and useful natural products
Example: Paclitaxel (taxol): anticancer drug and rare natural product
O
O O O O H H OH H N O H H OH HO H Ph O O O O O Example #2: Synthesis of indynaprost from cyclopentadiene
O HO C into 2 H H
HO HO Cyclopentadiene Indynaprost (Experimental platelet aggregation inhibitor)
8 Lecture Supplement: Multi-step Organic Synthesis Fundamental Synthesis Operations
Þ C-C bond formation Þ functional group interconversions (FGI): · oxidation · substitution · addition · reduction · elimination · others Þ skeletal rearrangements Þ protection/deprotection
Keys to Doing Synthesis Problems • Use reactions you know (flash cards)
• Keep an eye on the requirements of the problem: Can I solve the problem in one step from where I am?
• Think backwards: retrosynthesis (E.J. Corey, Harvard, Nobel Prize in Chemistry, 1990) Retrosynthesis (Corey’s definition): “a problem solving technique for transforming the structure of a synthetic target molecule to a sequence of progressively materials along a pathway which ultimately leads to a simple or commercially available starting material for chemical synthesis”
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Sample Problem #1
H O
from
Target molecule Starting material
Hint: Unless otherwise specified, any other reagents can also be used.
Lecture Supplement: Multi-step Organic Synthesis 9 H O Can the target molecule be made from the starting material one step ??? in one reaction? Ph Ph
Analysis: No change in carbon count FGI: alkene ® aldehyde
How to make an aldehyde? • Alkene ozonolysis (alters carbon count) • Oxidation of a primary alcohol (carbon count unchanged)
Hint: Flash cards may suggest reactions to carry out the desired FGI.
H O OH oxidation
Ph Ph
means: "thinking backwards" "the molecule on the left could be made from the molecule on the right by the reaction above the arrow"
Can the new target molecule (the alcohol) be made in one step from the starting alkene?
OH hydroboration
Ph Ph
Complete Retrosynthesis:
H O OH oxidation hydroboration
Ph Ph Ph
Forward Direction: · Fill in the reaction details · Carefully check each step: stereochemistry, carbocation rearrangements, etc.
10 Lecture Supplement: Multi-step Organic Synthesis H O oxidation OH hydroboration
Ph Ph PCC Ph 1. BH3 2. H2O2, NaOH
Or: 1. BH3 OH H O 2. H2O2, NaOH PCC Ph Ph Ph
Sample Problem #2
CH3S HO from Ph OCH2CH3 Ph
Can the target molecule be made from the starting material in one reaction?
Analysis: Target has three more carbons than starting material, but no new C-C bonds. FGI: Alcohol ® thioether Alkene ® ether
How to make an ether? - · SN2 with CH3CH2O - · SN2 with RO and CH3CH2I · Alkoxymercuration
CH3S Williamson CH3S
Ph OCH2CH3 Ph OH
Can the target molecule be made from the new starting material in one reaction?
How to make an alcohol? - · SN2 with HO · Alkene hydration · Nucleophilic addition to epoxide
CH3S oxymercuration CH3S
Ph OH Ph
Lecture Supplement: Multi-step Organic Synthesis 11 Can the target molecule be made from the new starting material in one reaction?
How to make a thioether? - · SN2 with RS (Must first convert OH into LG)
- TsO HO CH3S CH3S SN2 TsCl
Ph Ph Ph
Complete Retrosynthesis
CH S CH S 3 Williamson 3 oxymercuration CH3S
Ph OCH2CH3 Ph OH Ph
- CH3S SN2 TsO TsCl HO
Ph Ph
Forward Direction
HO TsO CH S TsCl Na SCH3 3 1. Hg(OAc)2, H2O pyridine 2. NaBH Ph Ph Ph 4
CH S CH S 3 1. NaH 3
2. CH3CH2I Ph OH Ph OCH2CH3
Shorter Forward Direction: Alkoxymercuration achieves the alkene ® ether conversion in just one step
1. Hg(CF3CO2)2, HO TsO CH S TsCl Na SCH3 3 CH3CH2OH pyridine 2. NaBH Ph Ph Ph 4
CH3S
Ph OCH2CH3
Hint: Thorough reaction knowledge increases flexibility and simplifies synthesis problems. Some problems might not be doable without good mastery of reactions!
12 Lecture Supplement: Multi-step Organic Synthesis Sample Problem #3
C N into
O Can the target be made in one step from the given starting material?
Analysis: New C-C bond between ring and nitriles FGI: Alkene ® ether + nitrile + another alkene, with trans stereochemistry
How to make a nitrile? - · SN2 with CN and alkyl halide, alkyl sulfonate, or epoxide
How to make an ether? - · SN2 with RO · oxymercuration with ROH · Williamson ether synthesis
Several routes to consider. Pick one and explore it! The trans relationship of the ether and nitrile groups suggests an epoxide opening.
C N C N - Williamson CN (SN2) O
O OH
Can the new target molecule (the epoxide) be made from the original starting material (alkene) in one step?
RCO3H O
Complete Retrosynthesis
C N C N - Williamson CN (SN2) RCO H O 3
O OH Forward Direction
C N C N CH3CO3H KCN 1. NaH O CH3OH 2. OH Br O
Lecture Supplement: Multi-step Organic Synthesis 13