123.312 Advanced Organic Chemistry: Retrosynthesis Tutorial
Question 1. 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
O
Answer:
FGI OH C–C OH
O dehydration O aldol O ! !
O
O
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:
O O ! !
O
O
Compound B
O
O
Answer
O O O O ! FGI O O HO 1 2 OH HO O 3 5 reduction 4
C–C
O O
HO ! !
O O O EtO OEt
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.
+ O O H , O O O O base NaBH4 EtO C H2O EtO2C 2 O O EtO OEt CO Et 2
Question 2. Give the retrosynthetic analysis for the following three compounds. Pay special attention to the relationship between the functional groups.
CO2H CO2H CO2H
Answers: The first is the easiest; it is an !,"-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…I’m not convinced it helps but if it allows you to rationalise what is going on more readily then use it!
FGI C=C CO2H CO2Et CO2Et hydrolysis ! !
PPh3 CHO CO2Et
The second is probably the hardest; 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.
FGI C–C CO H N 2 N hydrolysis ! !
Br NaCN
Alkylation of an enolate offers the most rapid approach to the third structure. Not much needs to be said about this one.
FGI C–C CO H CO Et 2 2 CO2Et hydrolysis ! !
CO2Et Br CO2Et
Question 3. How would you make these compounds?
H OH CO2H NH2 N
Answers
The first is simply a case of reduction amination. We cannot form an amide so it has to proceed by the imine.
H FGI N N C=N O H2N
reduction
The next isn’t much harder…we have an alcohol, this should yell Grignard addition to a carbonyl and hence the disconnections are:
OH O C–C BrMg Br
This one is potentially a little harder…but not much. The best route to the acid is via alkylation of diethyl malonate. The latter is easily enolised, will only undergo two additions, is fairly robust yet will readily undergo decarboxylation.
CO2H FGI EtO2C CO2Et C–C Br Br EtO2C CO2Et decarboxylation
The final compound is a primary amine. This could either be prepared by reductive amination of the appropriate ketone (made from oxidation of the secondary alcohol made earlier) or by substitution of an appropriately derivatised secondary alcohol (tosylation of the secondary amine) with azide followed by reduction.
NH2 FGI NH O C=N
reduction
NH2 FGI N3 OTs OH C–N C–S
reduction
Question 4. Perform the retrosynthetic analysis of the following compound. Remember, your planned synthesis must be synthetically possible and shouldn’t suffer from regio- or chemoselectivity issues.
O O NEt2
O NH2
Answer
O O O OH O OH NEt2 FGI C–O
O O reduction O
NH2 remove the ester NH2 NO2 (with the reactive functionality) the amine can be problematic we can now remove the ether. so we convert it to the less Attepts to do this earlier reactive nitro group. This also C–O would have met with failure prepares the way for its due to alkylation of the amine eventual disconnection O OH O OH O OH FGI C–N
diazonium + – OH N2 BF4 OH NO2 I would stop here as I don't know how much the phenol group is ortho, para aromatic chemistry you have done. But if you directing but should favour the FGI have done enough then we can take the leaser hindered position (and diazonium synthesis all the way back to the acid we might be able to argue about H-bonding)
O OH O OH O OH FGI C–N
reduction NH2 NO2
remember, the acid is electron withdrawing so is meta directing
Question 5. (a) How would you synthesise
From
Answer:
i. BH3 ii. H2O2 / NaOH PBr3 OH Br
Remember, we need to get anti-Markovnikof addition of the hydroxyl group so we use hydroboration / oxidation.
(b) How would you synthesise
OH
From
Answer:
i. BH3 Br2, ii. H2O2 / t-BuOK hv Br NaOH OH The key to this one is functionalisation of the hydrocarbon. This is achieved by radical bromination, then its plain sailing.
(c) How would you synthesise
OH
From
Br
Answer:
Mg OH Br MgBr Et2O CH3CHO
Lets be honest, if you can’t do this one then you’re in trouble!
(d) How would you synthesise
HO
From
HO
Answer:
PCC EtMgBr HO O HO Likewise, this one is not that taxing but hopefully gets you thinking about functional group interconversions.
(e) How would you synthesise
OH
From
Br
Answer:
t-BuOK mCPBA EtMgBr OH Br O
This one is quite hard. But again, it is all about FGI and recognising where the original carbons are. I recommend number you carbons and then trying to identify relationships between functional groups and this numbering.
(f) How would you synthesise
Br
From
OH
Answer:
HBr OH acid ROOR dehydration Br Quick, but not necessarily straightforward; the more reactions you know the easier this becomes. Here we require anti-Markovnikov addition of the HBr. Therefore, we add peroxide to allow a radical reaction.
(g) How would you synthesise
NH2 O
From
O
Answer:
HBr PPh3 Br N3 NH2 O ROOR NaN3 H2O O O O
Again, need anti-Markovnikov so use radical bromination. The add nitrogen via the azide. Of course, there are other answers (hydroboration , oxidation and reductive amination??)
(h) How would you synthesise
Ph N H
From
O
OEt
Answer:
O O O O + LiAlH Ph H3O SOCl2 PhNH2 Ph 4 N OEt OH Cl N H H
Hopefully, this one doesn’t cause too many problems.
(i) How would you synthesise
OH Et Et
From
EtO2C CO2Et
Answer:
+ i. base i. base i. H3O OH ii. Et–I EtO2C CO2Et ii. Et–I EtO2C CO2Et ii. heat Et CO2H LiAlH4 EtO2C CO2Et Et Et Et Et Et Et This one is all about recognising which functional groups are required and how they can be interconverted.
(j) How would you synthesise
OH OH
From
O
Answer:
i. base O OH OH OH O ii. EtCHO NaBH4
Don’t get fooled by how similar molecules may appear. Count the atoms, look for real relationships. In this case we have a 1,3-diol derived from a ketone; start thinking about an aldol reaction right away.
(k) How would you synthesise
Br
From
OH
Answer:
HBr OH PCC O MeMgBr acid ROOR OH Br