Unit III: Synthetic Approaches

Unit III: Synthetic Approaches

Retrosynthesis and Retrosynthetic Analysis:

Elias James Corey brought a more formal approach to synthesis planning (or design) for which he won the Nobel

Prize for Chemistry in 1990. Synthetic planning starts with the product and works backwards towards the starting

materials using standard rules. This process is called retrosynthesis and the art of planning the synthesis of a

target molecule is called retrosynthetic analysis. In this approach, the target molecule is broken into fragments by

breaking bonds through a series of logical disconnection to get the best possible and likely starting materials. It is

exactly the reverse of chemical synthesis. The method of retrosynthetic analysis is very effective, but it requires a

great knowledge of chemical compounds, classes of compounds, chemical reactions, reaction conditions, etc.

Terminologies used in Retrosynthesis:

There are a few terms with which you we should be familiar before we start:

(i) Target Molecule (or TM): The molecule to be synthesized is target molecule.

(ii) Retrosynthetic Arrow: An open-ended arrow (⇒) used to indicate the reverse of a synthetic reaction.

(iii) Disconnection:

Disconnection is an operation involving breaking of bonds between atoms in a molecule to form smaller

fragments. It is opposite to bond formation between atoms. This is also called as transform. A curved line ( ) is

used at the point of disconnection of bond and a double line arrow (⇒) is used for representing disconnection.

(iv) Synthons:

In retrosynthetic analysis, a synthon is an idealized fragments or species (e.g., CH3+ or CH3

-) usually an ion within a

target molecule obtained by disconnection. It may not necessarily correspond to a real molecule. It represents a

potential starting reagent in the retroactive synthesis of that target molecule. The term synthon was coined in

1967 by E. J. Corey. Synthons are classified as donor and acceptor synthons.

(a) Donor Synthons: These are negatively polarized synthons denoted by symbol ‘d’.

Common donor synthons Synthetic equivalents Common donor synthons Synthetic equivalents

R- (Alkyl anions)

CN- (cyanide ion)

RC≡C- (acetylide ion)

RMgX, RLi, R2CuLi

NaC≡N

RC≡CMgX, RC≡CLi

(b) Acceptor Synthons: These are positively polarized synthons denoted by symbol ‘a’.

Common donor synthons Synthetic equivalents Common donor synthons Synthetic equivalents

R+ (Alkyl cations)

Ar+

(Acyl cations)

RCO+ (Acylinium ion)

+CH2-CH2CN

+CH2OH

RCH+OH

RCl, RBr, RI, ROTs

ArN2+X

-

RCOCX (X=Cl, NR2, OR)

CH2=CHCN

HCHO

RCHO

HCO+

HO-C+=O

+CH2-CH2COR

R2C+OH

+CH2CH2OH

+CH2COCH2R

HCOCX (X=Cl, NR2, OR)

CO2

CH2= CHCOR (R=Alkyl, OR)

R2CO

BrCH2COCH2R

According to where the functional group is in relation to the

The d0

Synthon: If the electronegative

acceptor synthons, we call it a d° synthons.

The a1 and d

1Synthons: If the C-1 of the functional group itself is reacting called as d

acceptors) synthon.

The a2

and d2 Synthons: If the C

the reactive one, we have d2 or a

The a3

and d3Synthons: If the C-3 is the reactive one, we call it d

Alkyl Synthons: Alkyl synthons without functional groups are called alkylating synthon.

(v) Synthetic Equivalent:

A synthetic equivalent (SE) is a real molecule or

reagent which generates the synthon.

example: CH3I is the synthetic equivalent of the

synthon CH3+ (i.e., synthon CH3

+ can be obtained

from CH3I). Similarly CH3Li is the synthetic

equivalent of CH3- synthon.

In the planning of the synthesis of phenylacetic

acid, two synthons are identified: a nucleophilic

COOH− group and an electrophilic PhCH

The cyanide anion is the synthetic equivalent for

the COOH− synthon, while benzyl bromide

synthetic equivalent for the benzyl synthon.

The synthesis of phenylacetic acid determined by

retrosynthetic analysis is thus:

PhCH2Br + NaCN → PhCH2CN + NaBr

PhCH2CN + 2H2O → PhCH2COOH + NH

We can also apply these ideas to the synthesis of the herbicide 2,4

reasonable disconnection of ether is the C

by substitution with an alkoxide anion. We don’t at this stage need to decide

to use, so we just write the synthons.

ccording to where the functional group is in relation to the reactive site, synthons further

If the electronegative heteroatom of the functional group forms covalent bonds with

acceptor synthons, we call it a d° synthons.

1 of the functional group itself is reacting called as d

If the C-2 atom (mainly relative to the carbonyl group) to the functional group is

or a2 synthon.

3 is the reactive one, we call it d3 or a3 synthon.

without functional groups are called alkylating synthon.

is a real molecule or

which generates the synthon. For

I is the synthetic equivalent of the

can be obtained

Li is the synthetic

phenylacetic

are identified: a nucleophilic

PhCH2+ group.

is the synthetic equivalent for

benzyl bromide is the

synthetic equivalent for the benzyl synthon.

The synthesis of phenylacetic acid determined by

CN + NaBr

COOH + NH3

We can also apply these ideas to the synthesis of the herbicide 2,4-D (2,4-dichlorophenoxyacetic acid).

is the C–O bond because we know that ethers can be made from alkyl halides

by substitution with an alkoxide anion. We don’t at this stage need to decide exactly which alkyl halide or alkoxide

can be classified as:

heteroatom of the functional group forms covalent bonds with

1 of the functional group itself is reacting called as d1 (for donors) or a

1 (for

2 atom (mainly relative to the carbonyl group) to the functional group is

without functional groups are called alkylating synthon.

dichlorophenoxyacetic acid). The most

made from alkyl halides

exactly which alkyl halide or alkoxide

Once the retrosynthetic analysis is done, we can go back and use our knowledge of chemistry to think of reagents

corresponding to these synthons.

For example, we should certainly choose the anion of the phenol as the nucleophile and some functionalized

acetic acid molecule with a leaving group in the position.

We can then write out a suggested synthesis in full from start to finish. It isn’t reasonable to try to predict exact

conditions for a reaction: to do that you would need to conduct a thorough search of the chemical literature and

do some experiments. However, all of the syntheses in this chapter are real examples and we shall often give full

details of conditions to help you become familiar with them.

(vi) Functional Group Interconversion:

Functional group interconversion (FGI) is a process of converting one functional group to another either by

substitution or addition or elimination or oxidation or reduction so that the disconnection becomes easier. E.g.,

conversion of an amine into nitro, a crboxylic acid into nitrile, an alcohol to an aldehyde, alkyne to alkene, etc.

A TM containing more than one functional group, one functional group may interfere with desired reaction on

second functional group during a synthesis. This problem can be solved into ways: (a) Use of protecting group (b)

Change in synthetic strategy (using FGI). FGI helps in identifying suitable disconnection. Consider the synthesis of

ketone containing a double bond. Alkenes may be prepared by the dehydration of alcohol.

Although FGI doesn't offer much gain to a synthesis, it facilitates subsequent disconnection of the intermediate.

Problems:

Question1:

Propose a retrosynthetic analysis of the following two compounds. Your answer should include both the synthons,

showing your thinking, and the reagents that would be employed in the actual synthesis.

Compound A:

Answer:

Remember that a conjugated double bond can easily be prepared by dehydration, thus we can perform an FGI to

give the aldol product. The 1,3-diO relationship should make spotting the disconnection very easy. Of course, in

the forward direction the reaction is not quite that simple; we have two carbonyl groups so we must selectively

form the correct enolate but this should be possible by low temperature lithium enolate formation prior to the

addition of cyclohexanone. The aldol condensation is such a common reaction that it is perfectly acceptable to do

the following disconnection:

Compound B:

Answer:

The first disconnection should be relatively simple, break the C–O bond to give the acid and alcohol. The next

stage might be slightly tougher...your best bet is to look at the relationship between the two functional groups; it

is 1,5. This can be formed via a conjugate addition of an enolate. To do this we need two carbonyl groups so next

move is a FGI to form the dicarbonyl. Two possible disconnections are now possible depending on which enolate

we add to which activated alkene. The one I have drawn is simpler, diethyl malonate is commercially available as

is the enone (or it can be prepared by the self-condensation of acetone). Additionally, conjugate addition of

malonates prefers 1,4 to 1,2 addition, which can be an issue with simple carbonyls. Chemoselectivity in the

reduction step is not an issue; NaBH4 does not reduce esters.

Question 2:

Give the retrosynthetic analysis for the following three compounds. Pay special attention to the relationship

between the functional groups.

Answer: In first case the unsaturated compound so we are looking at either aldol condensation or a simple

Wittig reaction. Sometimes you will see double bond disconnections drawn with a double charge synthon.

In the second case there is no simple enolate disconnections so we have to look slightly further a field. Whilst we

can go via an alkyne, the best route probably involves FGI to a nitrile and then simple C–C bond formation by a

substitution with a cyanide anion. Alkylation of an enolate offers the most rapid approach to the third structure.

Not much needs to be said about this one.

Acyiation of an enolate offers the most rapid approach to the third structure.

Question 3:

How would you synthesize: from .

Answer:

Question 4:

Paracetamol, for example, is an amide that can be disconnected either to amine + acyl chloride or to amine +

anhydride.

Paracetamol

Which reagent is best can often only be determined by experimentation?

Answer: Commercially, paracetamol is made from para-

aminophenol and acetic anhydride largely because the by-product,

acetic acid, is easier to handle than HCl. We can depict both

anhydride and acyl chloride in this scheme as an ‘idealized

reagent’, an electrophilic acetyl group MeCO+

(synthon).

Synthetic Planning:

Synthetic planning is a construction process that involves converting simple and commercially available molecules

into complex molecules using specific reagents associated with known reactions in the retrosynthetic scheme.

The overall yield in a multistep synthesis is the product of the yields for each separate step.

Syntheses can be grouped into two broad categories: (i) Linear syntheses (ii) Convergent syntheses

1. Linear Synthesis:

In linear synthesis, the target molecule (TM) is synthesized through a series of linear transformations. The TM is

assembled in a stepwise manner. E.g.,

For the above seven step synthesis, there are total eight components (A to H). If the yield of the intermediate at

each step is 80% then,

Overall yield of H = ��

�� x

��

�� x

��

�� x

��

�� x

��

�� x

��

�� x

��

�� =

��

�� = 0.21

Therefore, overall yield % of H = 21%.

In another example of linear synthesis:

For the above three step synthesis, there are total four components (A to D). If the yield of the intermediate at

each step is 40% then,

Overall yield of D = ��

�� x

��

�� x

��

�� x

��

�� =

�.

�� = 0.0256

Therefore, overall yield of D = 2.56%.

In linear synthesis the overall yield quickly drops with each reaction step.

2. Convergent Synthesis:

A convergent synthesis is a strategy that aims to improve the efficiency of multistep organic synthesis. In this

case, the key fragments of the target molecule are synthesized separately or independently and then brought

together at a later stage in the synthesis to make the target molecule. E.g.,

In above two sequences convergent synthesis, there are total nine components (A to I), one with three steps and

other with four steps. As the four steps sequence is longer, therefore:

Overall yield of G-H-I = ��

�� x

��

�� x

��

�� x

��

�� =

�� = 0.65

Therefore, overall yield % of G-H-I = 65%.

In another two sequences convergent synthesis:

There are total five components (A to E), both sequences with two steps. Therefore:

Overall yield of E = �

�� x

�

�� x = 0.25

Therefore, overall yield of E = 25%. The overall yield of E (25%) looks much better.

Convergent synthesis is applied in the synthesis of complex molecules and involves fragment coupling and

independent synthesis. This technique is more useful if the compound is large and symmetric, where at least two

aspects of the molecule can be formed separately and still come together. It is shorter and more efficient than

a linear synthesis leading to a higher overall yield. It is flexible and easier to execute due to the independent

synthesis of the fragments of the target molecule.

Umpolung of Reactivity:

The heteroatoms in organic molecules polarize carbon skeletons by virtue of their electronegativity. Therefore, in

standard organic reactions, the majority of new bonds are formed between atoms of opposite polarity. This can

be considered to be the normal mode of reactivity. One consequence of this natural polarization of molecules is

that 1,3- and 1,5- heteroatom substituted carbon skeletons are extremely easy to synthesize (Aldol reaction,

Claisen condensation, Michael reaction, Claisen rearrangement, Diels-Alder reaction), whereas 1,2-, 1,4-, and 1,6-

heteroatom substitution patterns are more difficult to access via normal reactivity. It is therefore important to

understand and develop methods to induce umpolung in organic reactions. The concept was introduced by D.

Seebach and E.J. Corey. The umpolung has been extended to the reversal of any commonly accepted reactivity

pattern. For example, reaction of R-C≡CX (X = halide) as a synthon for R-C≡C+ (i.e. electrophilic acetylene) is an

umpolung of the normal more common acetylide, R-C≡C-(i.e. nucleophilic) reactivity.

Umpolung (means polarity inversion) in organic chemistry is the chemical modification of a functional group with

the aim of the reversal of polarity of that group. This modification allows secondary reactions of this functional

group that would otherwise not be possible. Polarity analysis during retrosynthetic analysis tells a chemist when

umpolung tactics are required to synthesize a target molecule. The umpolung chemistry reverses the normal

traditional reactivity patterns imposed by hetero-atoms in alkyl chains is not only intellectually interesting but

synthetically useful as well.

1,3-Dithiane (A thioacetal) Normal reactivity Masked acyl anion (Umpolung)

Types of Umpolung:

1. Carbonyl Umpolung:

A classic example of polarity inversion is observed in dithiane chemistry. Ordinarily the oxygen atom in the

carbonyl group is more electronegative than the carbon atom and therefore the carbonyl group reacts as

an electrophile at carbon. This polarity can be reversed when the carbonyl group is converted into a dithiane or

a thioacetal. In synthon terminology the ordinary carbonyl group is an acyl cation and the dithiane is a

masked acyl anion. When the dithiane is derived from an aldehyde such as acetaldehyde the acyl proton can be

abstracted by n-butyllithium in THF at low temperatures (-300C to -40

0C) 2-lithio-1,3-dithiane (a masked acyl

anion) is generated.

2-Lithio-1,3-dithiane (Corey-Seebach Reagent)

The 2-lithio-1,3-dithiane reacts as a nucleophile in nucleophilic displacement followed by hydrolysis of the

dithiane group the final reaction products are α-alkyl-ketones or α-hydroxy-ketones, as:

The overall reaction can be represented as:

The nucleophilic displacement of 2-lithio-1,3-dithiane with alkyl halides (such as benzyl bromide), with other

carbonyl compounds (such as cyclohexanone), oxiranes such as phenyl-epoxyethane, shown below. After

hydrolysis of the dithiane group the final reaction products are α-alkyl-ketones or α-hydroxy-ketones.

(i) Reaction with Alkyl Halides:

(ii) Reaction with Carbonyls:

(iii) Reaction with Epoxides:

(iv) Reaction with CO2:

(v) Reaction with Nitriles:

+

+

+

2. Amine Umpolung:

The nitrogen atom in the amine group is reacting as a

Primary (1°) amine Secondary (2°) amine

This polarity can be reversed when a primary or secondary amine is substituted with a good

a halogen atom or an alkoxy group). The resulting N

nitrogen atom and react with a nucleophile

Grignard reagent).

Revesible umpolung of primary amine

3. Cyanide Umpolung:

The cyanide ion, a canonical umpolung reagent, is unusual in that a carbon triply bonded to nitrogen

expected to have a (+) polarity due to the higher electronegativity of the nitrogen atom. Yet, the negative charge

of the cyanide ion is localized on the carbon, giving it a (

umpolung in many reactions where cyanide is involved. For example, cyanide is a key catalyst in the

condensation, a classical example of polarity inversion.

The net result of the benzoin reaction is that a bond has been formed between two carbons that are normally

electrophiles.

group is reacting as a nucleophile by way of its lone pair.

Secondary (2°) amine tertiary(3°) amine

This polarity can be reversed when a primary or secondary amine is substituted with a good

). The resulting N-substituted compound can behave as an

nucleophile as for example in the electrophilic amination of

Revesible umpolung of primary amine

The cyanide ion, a canonical umpolung reagent, is unusual in that a carbon triply bonded to nitrogen


of the cyanide ion is localized on the carbon, giving it a (-) formal charge. This chemical ambivalence results in

reactions where cyanide is involved. For example, cyanide is a key catalyst in the

, a classical example of polarity inversion.

of the benzoin reaction is that a bond has been formed between two carbons that are normally

This polarity can be reversed when a primary or secondary amine is substituted with a good leaving group (such as

substituted compound can behave as an electrophile at the

as for example in the electrophilic amination of carbanions (e.g.,

The cyanide ion, a canonical umpolung reagent, is unusual in that a carbon triply bonded to nitrogen would be


) formal charge. This chemical ambivalence results in

reactions where cyanide is involved. For example, cyanide is a key catalyst in the benzoin

of the benzoin reaction is that a bond has been formed between two carbons that are normally

Protecting Groups:

In many preparations of delicate organic compounds, some specific parts of their molecules cannot survive the

required reagents or chemical environments. Then, these parts, or groups, must be protected. A protecting group

(PG) is a molecular framework that is introduced onto a specific functional group (FG) in a poly-functional

molecule to block its reactivity under reaction conditions needed to make modifications elsewhere in the

molecule. A good protecting group should be such that:

(i) It should be readily, but selectively introduced to the desired functional group in a poly-functional molecule.

(ii) It should be stable / resistant to the reagents employed in subsequent reaction steps in which the group

being masked (protected) is desired to remain deactivated (protected).

(iii) It should be capable of being selectively removed under mild conditions when its protection is no longer

required.

The most reactive functional groups commonly requiring protection:

(i) Protection of Alcohol:

• Acetyl (Ac): Removed by acid or base (see Acetoxy group).

• Benzoyl (Bz): Removed by acid or base, more stable than Ac group.

• Benzyl (Bn): Removed by hydrogenolysis. Bn group is widely used in sugar and nucleoside chemistry.

• β-Methoxyethoxymethyl ether (MEM): Removed by acid.

• Dimethoxytrityl, [bis-(4-methoxyphenyl)phenylmethyl] (DMT): Removed by weak acid. DMT group is

widely used for protection of 5'-hydroxy group in nucleosides, particularly in oligonucleotide synthesis.

• Methoxymethyl ether (MOM): Removed by acid.

• Methoxytrityl [(4-methoxyphenyl)diphenylmethyl] (MMT): Removed by acid and hydrogenolysis.

• p-Methoxybenzyl ether (PMB): Removed by acid, hydrogenolysis, or oxidation.

• Methylthiomethyl ether: Removed by acid.

• Pivaloyl (Piv): Removed by acid, base or reductant agents. It is substantially more stable than other acyl

protecting groups.

• Tetrahydropyranyl (THP): Removed by acid.

• Tetrahydrofuran (THF): Removed by acid.

• Trityl (triphenylmethyl, Tr): Removed by acid and hydrogenolysis.

• Silyl ether (most popular ones include trimethylsilyl (TMS), tert-butyldimethylsilyl (TBDMS), tri-iso-

propylsilyloxymethyl (TOM), and triisopropylsilyl (TIPS) ethers): Removed by acid or fluoride ion. (such as

NaF, TBAF (tetra-n-butylammonium fluoride, HF-Py, or HF-NEt3)). TBDMS and TOM groups are used for

protection of 2'-hydroxy function in nucleosides, particularly in oligonucleotide synthesis.

• Methyl ethers: Cleavage is by TMSI in dichloromethane or acetonitrile or chloroform. An alternative

method to cleave methyl ethers is BBr3 in DCM

• Ethoxyethyl ethers (EE): Cleavage more trivial than simple ethers e.g. 1N hydrochloric acid

(ii) Protection of Carbonyl Groups:

• Acetals and Ketals: Removed by acid. Normally, the cleavage of acyclic acetals is easier than of cyclic

acetals.

• Acylals: Removed by Lewis acids.

• Dithianes: Removed by metal salts or oxidizing agents.

(iii) Protection of Carboxylic Acids:

• Methyl esters: Removed by acid or base.

• Benzyl esters: Removed by hydrogenolysis.

• tert-Butyl esters: Removed by acid, base and some reductants.

• Esters of 2,6-disubstituted phenols (e.g. 2,6-dimethylphenol, 2,6-diisopropylphenol, 2,6-di-tert-

butylphenol): Removed at room temperature by DBU-catalyzedmethanolysis under high-pressure

conditions.

• Silyl esters: Removed by acid, base and organometallic reagents.

• Orthoesters: Removed by mild aqueous acid to form ester, which is removed according to ester

properties.

• Oxazoline: Removed by strong hot acid (pH < 1, T > 100 °C) or alkali (pH > 12, T > 100 °C), but not

e.g. LiAlH4, organolithium reagents or Grignard (organomagnesium) reagents.

(iv) Protection of Amines:

• Carbobenzyloxy (Cbz) group: Removed by hydrogenolysis

• p-Methoxybenzyl carbonyl (Moz or MeOZ) group: Removed by hydrogenolysis, more labile than Cbz

• tert-Butyloxycarbonyl (BOC) group (common in solid phase peptide synthesis): Removed by concentrated

strong acid (such as HCl or CF3COOH), or by heating to >80 °C.

• 9-Fluorenylmethyloxycarbonyl (Fmoc) group (Common in solid phase peptide synthesis): Removed by

base, such as piperidine.

• Acetyl (Ac) group is common in oligonucleotide synthesis for protection of N4 in cytosine and N6

in adenine nucleic bases and is removed by treatment with a base, most often, with aqueous or

gaseous ammonia or methylamine. Ac is too stable to be readily removed from aliphatic amides.

• Benzoyl (Bz) group is common in oligonucleotide synthesis for protection of N4 in cytosine and N6

in adenine nucleic bases and is removed by treatment with a base, most often with aqueous or gaseous

ammonia or methylamine. Bz is too stable to be readily removed from aliphatic amides.

• Benzyl (Bn) group – Removed by hydrogenolysis

• Carbamate group – Removed by acid and mild heating.

• p-Methoxybenzyl (PMB) – Removed by hydrogenolysis, more labile than benzyl

• 3,4-Dimethoxybenzyl (DMPM) – Removed by hydrogenolysis, more labile than p-methoxybenzyl

• p-methoxyphenyl (PMP) group – Removed by ammonium cerium(IV) nitrate (CAN)

• Tosyl (Ts) group – Removed by concentrated acid (HBr, H2SO4) & strong reducing agents (sodium in

liquid ammonia or sodium naphthalenide)

• Troc (trichloroethylchloroformate ) group – Removed by Zn insertion in the presence of acetic acid

• Other Sulfonamides (Nosyl and Nps) groups:

(v) Phosphate Protecting Groups

• 2-cyanoethyl: Removed by mild base. The group is widely used in

• Methyl (Me): Removed by strong nucleophiles

(vi) Terminal Alkyne Protecting G

• Propargyl alcohols in the Favorskii reaction

• Silyl groups, especially in protection of the

The commonly encountered functional groups in organic synthesis that are reactive to nucleophilic or

electrophilic reagents whose selective transformation may present challenges do regularly require deactivation by

masking with a protecting group. The common protecting groups for alcohols are ether

are among the least reactive of the organic functional groups. The ethe

grouped in the following categories:

(i) Acetal protecting groups These protections replace the acidic proton on an alcohol with an un

ether moiety.

(ii) Silyl ether protecting groups.

For example, lithium aluminium hydride

alcohols. It will always react with carbonyl

reduction of an ester is required in the presence of a carbonyl, the attack of the hy

prevented. The carbonyl is converted into an

a protecting group for the carbonyl. After the

reacting it with an aqueous acid), giving back the original carbonyl. This step is called

Consider the reduction of ester group to primary alcohol

LiAlH4 will also reduce the ketone as well which we don't want, as:

Unprotected ketone during reduction of

We can avoid this problem if we change the ketone to a different functional group first using protecting group

(like a cover over the ketone). In reality, the molecular cover is a prote

of keto-ester to primary alcohol, we protect the ketone as an acetal (which is ether and doesn't react with LiAlH

Acetal protection of a ketone during reduction of an

(Nosyl and Nps) groups: Removed by samarium iodide, tributyltin hydride

roups:

emoved by mild base. The group is widely used in oligonucleotide synthesis

emoved by strong nucleophiles e.c. thiophenole/TEA.

Terminal Alkyne Protecting Groups:

Favorskii reaction,

ially in protection of the acetylene itself.


ransformation may present challenges do regularly require deactivation by

masking with a protecting group. The common protecting groups for alcohols are ether-protecting groups. Ethers

are among the least reactive of the organic functional groups. The ether protecting groups of alcohols can be

Acetal protecting groups These protections replace the acidic proton on an alcohol with an un

lithium aluminium hydride is a highly reactive but useful reagent capable of reducing

carbonyl groups and this cannot be discouraged by any means. When a

reduction of an ester is required in the presence of a carbonyl, the attack of the hydride on the carbonyl has to be

prevented. The carbonyl is converted into an acetal, which does not react with hydrides. The acetal is then called

for the carbonyl. After the step involving the hydride is complete, the acetal is removed (by

reacting it with an aqueous acid), giving back the original carbonyl. This step is called de-protection

Consider the reduction of ester group to primary alcohol in the presence of a carbonyl group

reduce the ketone as well which we don't want, as:

during reduction of an ester


(like a cover over the ketone). In reality, the molecular cover is a protecting group. In the reduction of ester group

ester to primary alcohol, we protect the ketone as an acetal (which is ether and doesn't react with LiAlH

during reduction of an ester

tributyltin hydride.

oligonucleotide synthesis.


ransformation may present challenges do regularly require deactivation by

protecting groups. Ethers

r protecting groups of alcohols can be

Acetal protecting groups These protections replace the acidic proton on an alcohol with an un-reactive

is a highly reactive but useful reagent capable of reducing esters to

groups and this cannot be discouraged by any means. When a

dride on the carbonyl has to be

, which does not react with hydrides. The acetal is then called

step involving the hydride is complete, the acetal is removed (by

protection.

yl group. In this reduction,


cting group. In the reduction of ester group

ester to primary alcohol, we protect the ketone as an acetal (which is ether and doesn't react with LiAlH4).

Consider another example:

In reality, the molecular cover is a protecting group. In the reduction of ester group of keto-ester to primary

alcohol, we protect the ketone as an acetal (which is ether and doesn't react with LiAlH4).

Then we can reduce the ester to the primary alcohol.

Finally we can remove the protecting group:

Overall, this gives us the complete scheme:

Asymmetric Synthesis (Enantioselective or Chiral or Stereoselective Synthesis):

Asymmetric synthesis, also called chiral synthesis, enantioselective synthesis or stereoselective synthesis, is the

selective organic synthesis which introduces one or more new and desired elements of chirality. In this case an

achiral substrate molecule is converted into a chiral unit in such a manner that unequal amounts of stereoisomers

are produced. As the different enantiomers or diastereomers of a molecule often have different biological activity

therefore it is important in the field of pharmaceuticals. The preparation of 2-hydroxypropanenitrile from CH3CHO

and HCN in the absence of any chiral reagent produces an excess of one enantiomer over the other. This would

constitute an absolute asymmetric synthesis that is, creation of preferential chirality (optical activity) in a

symmetrical environment from symmetrical reagents:

The cyanide ion has an exactly equal chance of attacking above or below the plane of the ethanal molecule,

producing equal numbers of molecules of the enantiomers.

Objectives behind studying asymmetric synthesis are:

To gain an appreciation of the types of asymmetric reactions, that may be employed in organic synthesis.

To an understand the origins of the enantioselectivities and the mechanisms of the reactions.

To be able to propose asymmetric syntheses of organic molecules of medium complexity.

Strategies of Asymmetric Synthesis:

There are two main strategies of asymmetric synthesis: Chiral pool synthesis or chiron approach and Chiral

auxiliary approach.

(a) Chiral Pool Synthesis or Chiron Approach:

Chiral pool refers to a collection of enantiomerically pure molecules available from nature. Chiral pool

synthesis is one of the simplest and oldest approaches for enantioselective synthesis. This can meet the

criteria for enantioselective synthesis when a new chiral species is created, such as in an SN2 reaction.

Common chiral starting materials derived from nature include amino acids, chiral carboxylic acids and

monosaccharides. Chiral pool substrates that are commonly used in organic synthesis contain functional

groups that are poor leaving groups.

(b) Chiral Auxiliary Approach:

A chiral auxiliary is a chiral molecular unit that can be temporarily incorporated in an achiral substrate to guide

selective formation of one of a possible pair of enantiomers. Chiral auxiliaries are optically active compounds and

introduce chirality in otherwise achiral starting materials. Structures of some chiral auxiliaries are given below:

Examples of chiral auxiliaries used in the alkylation of enolates.

A chiral auxiliary physically blocks one of two possible trajectories of attack on an achiral substrate, leaving only

the desired trajectory open for reaction. Since the chiral auxiliary is enantiopure, the two trajectories are not

equivalent but diastereomeric. The temporary stereocenter introduced by the chiral auxilliary directs the

formation of a second stereocenter. The stereochemistry of the new chiral centre can be rationalized based on

steric considerations.

Schematic presentation of chiral auxiliary

General scheme for employing a chiral auxiliary in asymmetric synthesis

Qualities of a Good Chiral Auxiliary:

(a) It needs to be available in both enantiomeric forms.

(b) It needs to be easy and quick to synthesize.

(c) It must be readily incorporated onto an achiral substrate.

(d) It should provide good levels of asymmetric induction leading to high enantiomeric excess (ee). Steric bias

plays a major role in facial differentiation.

Schematic presentation of chiral auxiliary

General scheme for employing a chiral auxiliary in asymmetric synthesis

Qualities of a Good Chiral Auxiliary:

eeds to be available in both enantiomeric forms.

and quick to synthesize.

ust be readily incorporated onto an achiral substrate.

It should provide good levels of asymmetric induction leading to high enantiomeric excess (ee). Steric bias

plays a major role in facial differentiation.

It should provide good levels of asymmetric induction leading to high enantiomeric excess (ee). Steric bias

(e) It needs to be selectively cleaved from the substrate under mild conditions.

(f) It must be recoverable and re-useable.

Advantages of Using Chiral Auxiliaries:

The levels of diastereofacial control in the reactions are usually high leading to high ee.

The diastereomers generated from the use of chiral auxiliaries can be separated by the use of conventional

methods (such as chromatography and crystallization).

Chiral auxiliaries can be recycled (re-used) thus reducing the expenses of buying the chiral reagent routinely.

The sense of configuration at the newly formed chiral centre can be determined by X-ray crystallography.

Disadvantages of Using Chiral Auxiliaries:

Both enantiomers of a chiral auxiliary are usually not readily available. More often, one enantiomer may be

far more expensive than the other.

Chiral auxiliaries need to be synthesized.

As with protecting groups, there are extra steps associated with the use of chiral auxiliaries. The chiral

auxiliary has to be introduced and then removed once it purpose has been accomplished.

A stoichiometric amount of the chiral template (chiral auxiliary) is usually required.

Documents

Unit III: Synthetic Approaches