Retrosynthesis Introduction

RETROSYNTHESIS
INTRODUCTION
PRESENTED BY:
A.S MADHU PRASANTH
Ist M.PHARM
DEPARTMENT OF PHARMACEUTICAL CHEMISTRY
JSS COLLEGE OF PHARMACY, MYSORE

INTRODUCTION
1
TERMINOLOGIES
2
FUNCTIONAL GROUP INTERCONVERSION
4
DISCONNECTION
3
5
CONTENTS
APPLICATIONS

INTRODUCTION
Retrosynthesis is the process of working backward from the target molecule in order to device suitable synthetic route.
OR
The process of mentally breaking down a molecule into a starting material.
OR
It is the problem solving technique for transforming the structure of a synthetic target molecule to a sequence of progressively simple
structure along a pathway which ultimately leads to simple or commercially available starting materials for a chemical synthesis.
 Retrosynthesis is denoted by meaning could be made from.
 It is different from the synthesis in which the normal synthesis is a forward process, starting from precursors and ending to form the target organic
compound, whereas in retrosynthesis is a reverse process, starting from the target organic compound and going back to its relevant precursors.
 Retrosynthesis is used to a) To plan the synthesis of that organic compound.
b) To know is there any synthesis path to the target.
c) To increase the yield of the compound by choosing another synthesis strategy.
C A + B
 Here the C is the compound where it can be synthesized from the precursors A and B.

TERMINOLOGIES
Target molecule (TM) : The molecule whose synthesis is being planned. Usually written TM and identified by the frame number.
Disconnection : An imaginary bon cleavage corresponding to a reverse of a real reaction.
Functional group inversion (FGI) : The process of converting one functional into another by substitution, addition, elimination, reduction or
oxidation.
Synthon : Idealized fragment with an associated polarity (represented by a + or - ) resulting from a disconnection, which stand for reagents we are
going to use in the forward synthesis. Synthon cannot itself be used, often because it is too unstable.
Reagent : A real chemical compound used as equivalent of a synthon. It is also called as synthetic equivalents.
Synthesis tree : Set of all the possible disconnections and synthons leading from the target to the starting material of a synthesis.

Some Synthons and Synthetic Equivalents :
Synthons are classified into donor or acceptor synthons.
The negatively polarized synthon is called as donor synthon and it denoted by “d”.
The positively polarized synthon is called as acceptor synthon and it is denoted by “a”.
We can classify the synthons further according to where the functional group is in relation to the reactive site.
Some examples of synthons are :
• This synthon is corresponds to an aldehyde, we can an a1 synthon because it is an acceptor that carries a functional group on the same carbon as its
reactive center.
• It is an d2 synthon because it is a donor whose reacting site is in the 2-position relative to the carbonyl group.

• Some other synthons are a2 and a3 synthons in which the a2 synthon is equivalent to the epoxide.
• Some synthons and their synthetic equivalents used are
Synthons Synthetic Equivalents

PRINCIPLES
The basic principles involved in the retrosynthesis are disconnection and functional group interconversion.
DISCONNECTION:
 It is an imaginary bond cleavage, corresponding to the reverse of a synthetic reaction.
 As a result of disconnection usually negative ion and positive ion are formed which are called as synthons.
 Disconnection is shown by a wavy line like .
 The hardest task in designing a retrosynthetic analysis is spotting where to make the disconnections, so there are some guidelines.
• GUIDELINE – 1
Disconnections must correspond to known, reliable reactions and it’s the most important thing to bear in mind when working out a
retrosynthesis.
When we disconnect the ether we chose next to the oxygen atom because we know about the synthesis of ether. We chose not to disconnect on the aryl side
of the oxygen atom because we know no reliable reaction corresponding to nucleophilic attack of an alcohol on an unactivated aromatic ring.
• GUIDELINE – 2
For compound consisting of two parts joined by a heteroatom, disconnect next to the heteroatom.
In all the retrosynthetic analyses if there is a heteroatom (N, O, S) joining the rest of the molecule together, so in each case have to make the disconnection
next to the N, O, S. This guideline works for esters, amides, ethers, amines, acetals, sulfides, and so on because these compounds are often mad by a
substitution reaction.

• GUIDELINE - 3
Consider alternative disconnections and choose routes that avoid chemoselectivity problems – often this means disconnecting reactive groups first.
Disconnection (e) requires alkylation of a compound that is itself an alkylating agent.
Disconnection (f) is much more satisfactory, and leads to a compound that is easily disconnected to 4-hydroxyphenol (para-cresol) and 1,2-dibromoethane.
Using this guideline, we can say that it’s best to disconnect the bromoethyl group (f) before the benzyl group because the bromoethyl group is more reactive
and more likely to cause problems of chemoselectivity.

FUNCTIONAL GROUP INTERCONVERSIONS (FGI):
It is the process of converting one functional group into another by substitution, addition, elimination, reduction or oxidation.
The FGI is needed because when a target molecule containing more than one functional group, one functional group may interfere with desired reaction
on second functional group during a synthesis.
For convenience these functional groups are initially divided into three major classes depending upon their oxidation level.
Carboxylic
acid and their
derivatives
• Compounds in this class are the highest oxidation level of organic compounds.
• Carboxylic acid(RCOOH), Esters/Lactone(RCOOR), Amide/lactam(RCONHR).
Aldehydes,
ketones and
their
derivatives
• Functional groups in this class are at lower oxidation level then the above class.
• Aldehydes(RCHO), Ketones(RCOR), Imines(R-C=N-RI), Oximes(RC=N-OH).
Alcohols and
their
derivatives
• Apart from Alcohols(R-OH) themselves, this includes.
• Amines(RNH2), Thiols(RSH), Ethers(ROR), Alkyl halides(RX).

Transformations of carboxylic acid derivatives
Esters Acyl chloride Amides
Transformations of aldehyde derivatives

ALKANES
ALKYL
HALIDES
AMINES
CARBOXYLIC
ACIDS
ETHERS
ALCOHOLS
ALKENES
ALKYNES NITRILES
CARBONYL
COMPOUNDS
ACYL
DERIVATIVES
Halogenation
Oxidation Replacement of
OH with X, amino
Hydration
Dehydration
Oxidation
WES Ammonia
WES-Williamson Ether Synthesis
FUNCTIONAL GROUP INTERCONVERSION

The antihypertensive drug ofornine contains and amide and amine functional group and we need to decide which to disconnect first.
If we disconnect the secondary amine first (b), we will have chemo selectivity problems constructing the amide in the presence of resulting NH2 group.
If we follow the disconnection (a), we will have the chemo selectivity problems because we have to construct an amine in the presence of acyl chloride.
However, we shall want to make acyl chloride from carboxylic acid, which can then easily be disconnected to 2-aminobenzoic acid and 4-
chloropyridine.

Retrosynthesis of Paracetamol
=
=
The p-aminophenol reacts with the acetic anhydride through the nucleophilic addition-elimination reaction.
The NH2 is a poor electrophile it usually acts as an nucleophile. Therefore an equivalent electrophile for NH2 would be NO2 which can then be
reduced to NH2. The NO2 reacts with phenol through the electrophilic substitution reaction.

REFERENCES
1. Jonathan Clayden, Nick Greeves, Stuart Warren. Organic Chemistry. Second Edition. Oxford University Press.
2. Stuart Warren, Paul Wyatt. Organic Synthesis: The Disconnection Approach. Second Edition. A. John Wiley and Sons, Ltd.,
Publication.
3. Retrosynthesis, In Wikipedia. https://en.m.wikipedia.org/wiki/Retrosynthetic_analysis.
4. Paracetamol, In https://chembam.com/resources-for-students/the-chemistry-of/retrosynthesis-paracetmol.

Retrosynthesis Introduction

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Retrosynthesis Introduction