 Good morning everyone and welcome back to this NPTEL lecture series on Classics in Total Synthesis Part 1. So we have been discussing about three-membered ring containing natural products and total synthesis and recently we started discussing about four-membered ring and non-natural products. We just completed the discussion on the synthesis of Cubane. So now we will move to some interesting naturally occurring compounds and how they have been synthesized. So when we talk about four-membered ring immediately one natural product which would come to all your mind is penicillin. So penicillin as you know it has a great history and it was discovered by Sir Alexander Fleming in 1920 and that was considered as one of the greatest discoveries because during the world war many people died and with the isolation of penicillin later you know they could change the treatment for people who were infected seriously during the war. And this penicillin if you look at the history of penicillin after seeing the potential of penicillin USA and UK they come up with a real you know high level of target and what they wanted to do was first of all so they know when they isolated penicillin they know it is an excellent antibiotic but they did not know the structure. So the first objective was to elicitate the structure what is the correct structure of penicillin. Once you isolate once you elicitate the structure then the next step is whether we can we should be able to synthesize this chemically chemical synthesis in the laboratory is a second objective. Once it is done at the laboratory level then it should be possible to scale up and at the same time they were also looking at the third option that is whether penicillin can be synthesized or can be made on a large scale through fermentation. So these were the three major objectives during the second world war how to produce more penicillins because this was very much required as you know many people died in the second world war not due to the opposition but due to the infection. So that was a serious problem and they wanted to address this as early as possible. But however as you know this is not an easy task it took more time and the synthesis of penicillin somehow could not be achieved during the second world war. It took almost 10 years later to achieve the first total synthesis and from the stretcher point of view the correct stretcher was elicited with the help of X-ray and that was done by Nobel laureate Professor Dorothy and if you have a close look at this penicillins you can see one core stretcher is a beta-lactam the four-membered beta-lactam and that is fused with say a five-membered ring that is fused with a five-membered ring. So this is the core stretcher for all penicillins and what you also see is this amino group adjacent to the carbonyl group, amino group adjacent to the carbonyl group and which is acylated okay which is acylated with various acyl group okay and when you look at this four-membered lactam okay this four-membered lactam it is quite unstable compared to normal amides or normal lactams as you know cyclic amides are called lactams and compared to five-membered six-membered lactams the four-membered lactam is quite unstable and of course when it is unstable you can also react faster. So what Woodward told was it is not like a normal amide okay so if you look at normal amide you can see this can exist like this whereas in the case of four-membered lactam okay it cannot exist like this so that is where the reactivity of beta-lactam comes into play and if you look at literature how this beta-lactam were made okay. So the beta-lactam the parent stretcher that is the four-membered compound with NH that is this compound is called acetidines okay. So beta-lactam means acetidine with a carbonyl group at two position okay a common name we always call it as beta-lactam but you know IUPAC names are you know different four-membered the beta-lactam are called acetidinones okay from the IUPAC and as we have seen penicillins and there are many other substituted penicillins which are well known as antibacterial agents from the spectroscopic point of view when you take IR of all this beta-lactam you will see a significant absorption at between 1735 to 1765 normally when you look at simple amides or six-numbered lactam you will see a strong absorption at 1660. So you will see clear difference close to 100 okay so that tells the presence of beta-lactam when you make beta-lactam the best way to see is just to take IR and then see oh you have got beta-lactam okay because you will see a clear absorption around 1750 that will confirm that you have made beta-lactam. So how these beta-lactam were made before I talk about the total synthesis of penicillin okay how these beta-lactam are generally prepared. So there are five common reactions which you know have been successfully used to make this beta-lactam one one can use cyclization okay intramolecular cyclization to form this four number ring two one can also think about using cycloaddition reaction okay so you have four number ring then immediately you can also think about using 2 plus 2 cycloaddition reaction three ring expansion that means you know you have a three-numbered ring from three-numbered ring you can expand to four-numbered ring and four this also quite frequently used this beta-lactam is insertion reaction okay so either you can do carbon insertion or nitrogen insertion the last one is from acetidates okay so these are the five common types of reactions which are routinely used and regularly used to make various beta-lactam first let us start with cyclization reaction. Cyclization reaction normally if you have a beta amino acid okay if you have a beta amino acid then if you treat this beta amino acid with acyl chlorides PCL3 SOCl2 then it can form beta-lactam but isolation is very important because these are quite unstable should quickly isolate to get the corresponding beta-lactam however when you have beta amino propionic acids okay then if you heat it for a long time if you heat it for a long time there is a possibility of beta elimination so when beta elimination takes place you will get the amine and the carboxylic acid I will just show that example so here you know you see this is alpha beta okay so beta amino acid this untreatment with acyl chloride okay so basically so acylation takes place at the carboxylic acid followed by the nucleophilic attack of the nitrogen you get the corresponding beta-lactam and if you heat it if you heat it it undergoes a retro-mickel okay it undergoes a retro-mickel to give the corresponding amine and alpha beta unsaturated carboxylic acid so this is the major drawback when you do such cyclisation reaction you should never heat it okay when you have beta amino acid then same beta amino acid as I said acyl chloride PCL3 SOCl2 could be used for cyclisation so PCL3 will give you the same beta-lactam and if you protect the nitrogen if you protect the nitrogen and then treat with thionyl chloride so what will happen this carboxylic acid intramolecularly can attack the amine okay so then once it is here then as you can see here this nitrogen lone pair can attack intramolecularly to the carbonyl and it can rearrange to give 4-membered lactone and the carboxylic acid is isobutric acid and this isobutric acid okay still SOCl2 is there is not it so what will happen it will go to corresponding acid chloride isobutric acid chloride okay so you can use PCL3 you can use SOCl2 you can also use CH3 COCl to form this type of beta-lactam starting from beta amino acid only thing is you should not heat it when you heat it you will get the amine and the alpha-beta and such a carboxylic acid now if you look at this example okay again cyclisation takes place but for NH2 to cyclise NH2 to cyclise you need a base and a hindered base here a grignard is used to remove this proton and that can cyclise to give a 4-membered lactone okay so the hindered grignard reagent is used as a nucleophile base to form this 4-membered this form and in this example as you can see here this is the most acidic proton is not it so if you treat with base okay even pyethylamine or sodium ethoxide so it can pick up the proton here and intramolecular SN2 type cyclisation should give the corresponding beta-lactam so this particular example is slightly different than the 3 examples which I discussed so the earlier example amine the primary amine primary amine intramolecularly attacks the carbonyl group okay carbonyl group having a leaving group here already you can see the amide bond the amide bond is already formed the earlier cases amide bond that is a lactam bond is formed during the key reaction here in this case the SN2 substitution takes place okay anion is generated and you have a good leaving group that is chloride or bromide or iodide the intramolecular SN2 like reaction takes place to give corresponding beta-lactam then one can also use beta-gamma unsaturated hydroxamates beta-gamma unsaturated hydroxamates what is that you can see so this is called hydroxamates okay then you have alpha beta gamma beta gamma unsaturated hydroxamate now on the double bond if you add iodine if you add bromine so then it can form the corresponding iodonium or bromonium ion once this is formed then intramolecularly nitrogen can attack and open the corresponding iodonium or bromonium ion so that will give you the corresponding 4 umbered lactam that is beta lactam okay so basically you are doing iodolactamization or have bromolactamization okay so that is how you make this beta-lactam and this also can be easily cleaved NO bond can be cleaved under hydrogen analysis condition to get the free NH then one can also think about using cycloaddition reaction so 2 plus 2 cycloaddition reaction so when you want to use 2 plus 2 cycloaddition to form beta-lactam so one portion should be alkene the other portion should be isocyanate okay so isocyanates are easy to make okay so you have isocyanate and then double bond then this can undergo the spontaneous 2 plus 2 cycloaddition to give the corresponding your beta-lactam if you use chlorosulfonyl isocyanate okay chlorosulfonyl isocyanate you will get this beta-lactam and then the chlorosulfonate group can be easily removed if you take this and then treat with water just water that is sufficient that will cleave the SO2CM ring expansion so ring expansion is a very interesting and important reaction because as such 4-membered ring itself is strained and you are getting this 4-membered ring from another strained compound that is 3-membered ring so from that angle one strain to another strain it takes place okay say for example if you have the cyclopropane okay cyclopropane having a hydroxyl group and amine attached to the same carbon hydroxyl and amine attached to the same carbon okay now if the NH is converted to NCL NCL then treatment with silver salts as you know when you take any N halide treat with silver salt first thing is silver chloride you know it will come out and then you will have a positive charge on the nitrogen then the cyclopropane will open up and migrate to the positive charge on the nitrogen and now the positive charge will be on the carbon having hydroxyl group so the loss of proton will give you the 4-membered ring okay so this is another interesting way to make the 4-membered beta-lactam okay then as I said another interesting reaction to make beta-lactam is carbene insertion okay if you can generate carbene carbene are generally made from diacyl compound okay so if you have diacyl compound and then treat with rhodium di rhodium tetracetate okay so then it can generate in seed to carbene or carbenides since you are using rhodium metal it is a rhodium carbenide that can you know immediately undergo a carbene insertion at this carbon to give the corresponding beta-lactam okay so now we discussed quite a few methods for making beta-lactam okay now we will move to how penicillin was discovered okay the penicillin the antibiotic a well-known antibiotic a great history behind this isolation of penicillin and the use of penicillin but how the first total synthesis of penicillin was accomplished because the shegan from MIT and his group they spent several years you know you can see when it was isolate 1920 okay so they spent many years on the total synthesis of penicillin and finally they sustained efforts led to the first total synthesis in 1957 okay let us see how he has synthesized and what is what was his retosynthetic plan the first bond to be disconnected obviously was the lactam bond okay you cleave the lactam bond and you get a carboxylic acid and amine okay so you have a carboxylic acid and amine so basically what you are going to do is you are going to make Cn bond okay so when you have amine and carboxylic acid you can use many coupling region okay then if you look at the right hand side so that is a protected aldehyde isn't it that is a protected aldehyde okay so you need aldehyde and then the aldehyde if you treat with this amino thiol okay then it can protect and before that if you look at this amine amine is already protected and to start with you need a protected amine so normally they use thalymide because thalymide if you treat with hydrazine so the thalyl group can be cleaved and then you will get back NH2 so for protection of NH2 olden days they used to use thalic and hydrogen method okay and this can be easily cleaved as I said this aldehyde and this amino thiol okay so they can this aldehyde can be protected to give this right hand side portion and this upon hydrolysis with NH hydrogen will give you NH2 so the first target is to make this D penicillamine hydrochloride in optically active form okay that was the first task for Sheehan to accomplish. So he started with resemic valine and then treated with chloroacetylchloride so you have NH2 the NH2 is acetylated with chloroacetylchloride so next step is a very interesting reaction this upon treatment with acetic anhydride okay this treatment with acetic anhydride he got a very interesting combo okay how did this happen let us see so first the chloroacetylchloride treatment with acetic anhydride this carboxylic acid is acetylated the first step is the acetylation of carboxylic acid that means mixed anhydride of this carboxylic acid and acetic acid so that gives this intermediate now the lone pair on the nitrogen moves to this carbonyl and this carbonyl oxygen intramolecally attacks the anhydride and gives this 5 ohm battery okay you can see the 5 ohm battery I will leave it for a second for better understanding okay. So now the OAC minus OAC minus which came out can pick up this hydrogen because this hydrogen is acetic is not it this hydrogen is acetic so OAC minus picks up this hydrogen and forms this dienolate and you can see if this molecule there is a push pull factor you have a negative charge on the oxygen and you also have a leaving group here so this is a classical example for such elimination so you get this unstable compound okay this unstable compound immediately will isomerize okay that will give you the product which I showed in the previous slide okay. So valine upon treatment with chloroacetylchloride followed by acetylation in 2 steps you got this interesting product good so how this can be it can be converted into penicillin so treat with hydrogen sulphide so hydrogen sulphide sulphur you know they tend to add 1 4 so hydrogen sulphide in the presence of sodium methoxide that means you are adding SH minus so SH minus can add in a 1 4 fashion at this carbon so that will lead to your 5-amper ring and this exocyclic double bond is missing but you introduce the SH as you know in penicillin you need that thiol and then hydroxyl group that is a deep penicillin. So now if you treat with sodium methoxide if you treat with sodium methoxide this 5-amper ring will open how it will open this is a mechanism first methoxy adds to the carbonyl and then O minus when it comes back you know it opens a 5-amper ring and it gives this open chain combo okay is it chiral is it chiral no it is resemic because we started with resemic combo okay so you have this then how you can resolve it the penicillin is an optical active combo how you can resolve so first you protect this NH and this SH okay and for that you have to remove the acetyl group so the acetyl group you remove with HCl so you get NH2 that NH2 on treatment with acetone you protect the combo okay and as you can see here this is resemic combo DL and they try to resolve at this stage but the resolution was difficult so what they did they treated with formic acid and acetic invader so formic acid and acetic invader they could introduce this aldehyde the N was protected okay now this resemic mixture was resolved with a with an alkaloid called bruisine okay the resemic mixture was resolved with an alkaloid called bruisine okay after result resolution so then we just add acid to get back this chiral okay so this is optically active okay this upon hydrolysis with HCl so what will happen this NCHO the CHO will go and then you will get NH2 since you are using HCl it is formed as the corresponding hydrochloride salt deep penicillamine hydrochloride salt okay the other one so now you have the deep penicillamine hydrochloride and you need the aldehyde from the other side so first you start with terributyl thalimido acetyl okay so this is very easy to prepare you take thalimide and then treat with corresponding bromocombo okay so then simple nucleophilic substitution you will get this combo this on treatment with sodium terributoxide and to introduce this CHO okay introduce this CHO you treat with terributyl formate terributyl formate so you get the corresponding terributyl thalimido melon aldehyde okay now you mix these two okay to protect the aldehyde so when you do that you get a mixture why you get a mixture if you look at this carbon that is resemic isn't it so you get a mixture at this carbon so you can call this as D alpha and you can call this as D gamma but both could be separated so take that D alpha and what you need you need NH2 and this thalimide protecting group can be easily cleaved by treatment with by treating with hydrogen okay so you treat with hydrogen the thalimide protecting group goes you recover your NH2 now if you treat with HCl you isolate the corresponding hydrochloride salt okay then you treat with base that is triethylamine and then do the acetylation so this acetylation you do with phenoxy acetylchloride okay phenoxy acetylchloride so now what you need to do you have to hydrolyze this terributyl ester then do the coupling you have NH carboxylic acid afterwards then you have to couple so HCl will remove the terributyl ester and then you treat with pyridine that is just to you know when you treat with this NH also will be in the form of NH HCl so you have to treat with pyridine get back the free amine once you have the free amine treat with DCC okay so the DCC is a good coupling reagent for making lactum but before that you have to treat with one equivalent of potassium hydroxide to deprotonate this carboxylic acid okay so that becomes CO2- then you add this DCC then the DCC undergoes the intramolecular lactum formation okay so that is how you get the penicillin 5 potassium salt there is one side reaction which takes place in this synthesis so when you have this when you treat with DCC what you get is this 5 umbered acetyl lactum okay instead of this nitrogen attacking what happens the nitrogen load pair on the acyl group okay that comes and then the carbonyl group attacks and it forms the 5 umbered acetyl lactum so this also you get a decent amount as a side product so this 6 amino penicillinic acid that is without the acyl group so how one can make because if you look at the earlier synthesis so we went with acyl group finally only we did the coupling is not it so here what you do the NH2 first you protect it as the tritile NH tritile group then you do the coupling okay you can couple with either DCC or diisopropyl carbodiamide to get the beta lactum now you need to remove the benzyl group and then pritile group so both can be done in one step that is upon hydrogen analysis followed by HCl treatment one gets the 6 amino penicillinic acid so to summarize what Sighan has done there was the first and very elegant total synthesis of penicillin and you can see way back in 1957 such a unstable reactive final compound penicillin 5 was made by Sighan and his group in 1957 and the synthesis started with commercially available valine okay and also from thalymide just you take thalymide and then you know treat with bromoethyl acetate to get the other starting material so it was very simple and straightforward starting materials which are commercially available and we had used this successfully to make the total synthesis of penicillin and the key reactions when you talk about the key reaction the Michael addition of SH minus was one of the key reactions and the coupling reagent was used to couple the amine and carboxylic acid intramolecularly to form the 4-hombard ring overall the total synthesis was accomplished in 8 steps with a overall yield of close to 0.7 percent so considering that this is the first total synthesis this was a significant achievement of course there were many total synthesis of penicillin later but the long run the huge quantities of penicillin and its derivatives were made only through fermentation method after fermentation once you get the penicillin then one can do lot of synthetic modification to get other penicillin like natural products but large quantities of penicillins are made only by fermentation. So with this I will complete the total synthesis of penicillin and we will discuss about other natural products belonging to this in the next couple of classes for example thynamycin and lactocysteine with these two we will discuss in the next couple of classes. Thank you.