Contents

Preface

General References

1 The Disconnection Approach

A Synthesis of Multistriatin

References

2 Basic Principles: Synthons and Reagents: Synthesis of Aromatic Compounds

A Problem from the Textbook

References

3 Strategy I: The Order of Events

References

4 One-Group C–X Disconnections

References

5 Strategy II: Chemoselectivity

Using Disconnections to Solve Structural and Mechanistic Problems

References

6 Two-Group C–X Disconnections

Counting Relationships between Functional Groups

Synthesis of a Heterocycle

References

7 Strategy III: Reversal of Polarity, Cyclisations, Summary of Strategy

Regioselective Attack on Epoxides

References

8 Amine Synthesis

An Example of a Triamine

Strategic Bond Disconnection

A New Generation Pfizer anti-HIV Drug Maraviroc

References

9 Strategy IV: Protecting Groups

Synthesis without Protection

Protection

An HIV-Protease Inhibitor as an anti-AIDS Drug

Example: Synthesis of Statins (Cholesterol-Lowering Drugs)

References

10 One-Group C–C Disconnections I: Alcohols

An Example of Simple Alkylation

References

11 General Strategy A: Choosing a Disconnection

Summary of Guidelines for Good Disconnections

The Synthesis of an Unusual Amino Acid

Strategy in the Synthesis of Sildenafil (Viagra^®)

References

12 Strategy V: Stereoselectivity A

References

13 One-Group C–C Disconnections II: Carbonyl Compounds

References

14 Strategy VI: Regioselectivity

Other Examples of Regioselectivity

A Heterocyclic Example

References

15 Alkene Synthesis

A Pharmaceutical Example

The Importance of Experimental Work

References

16 Strategy VII: Use of Acetylenes (Alkynes)

Examples from the Textbook Chapter

Synthesis of a Cyclic Ketone by Hydration of an Acetylene

An Interesting Mechanism and a Useful Separation

Electrophilic Acetylenes

Alkynes in Synthesis

References

17 Two-Group C–C Disconnections I: Diels-Alder Reactions

References

18 Strategy VIII: Introduction to Carbonyl Condensations

References

19 Two-Group C–C Disconnections II: 1,3-Difunctionalised Compounds

A Synthesis of the Enzyme Inhibitor Elasnin

References

20 Strategy IX: Control in Carbonyl Condensations

Three Examples

References

21 Two-Group C–C Disconnections III: 1,5-Difunctionalised Compounds Conjugate (Michael) Addition and Robinson Annelation

References

22 Strategy X: Aliphatic Nitro Compounds in Synthesis

The Synthesis of an ACE Inhibitor

References

23 Two-Group Disconnections IV: 1,2-Difunctionalised Compounds

Acyl Anion Equivalents

Some Problems

α-Functionalisation of Carbonyl Compounds

References

24 Strategy XI: Radical Reactions in Synthesis

The Mechanism of Allylic Bromination with NBS

Application of NBS in Synthesis

Carbon–Carbon Bond-Forming Reactions

A Pharmaceutical Example

References

25 Two-Group Disconnections V: 1,4-Difunctionalised Compounds

Buying the 1,4-diCO Relationship

Troubles and Triumphs with Homoenolates

A General Synthesis of Partly Protected Succinic Acids

A Remarkable Reaction from the Textbook

References

26 Strategy XII: Reconnection

Synthesis of 1,2-and 1,4-diCO Compounds by Oxidative C=C Cleavage

Oxidative Cleavage of Aldol Products

Cleavage of Aldol Products by Retro-Aldol Reaction

References

27 Two-Group C–C Disconnections VI: 1,6-diCarbonyl Compounds

Problems from the Textbook

The Synthesis of Acorenone B

The Synthesis of a Symmetrical Keto-di-Acid

Oxidative Cleavage by the Baeyer-Villiger Rearrangement

References

28 General Strategy B: Strategy of Carbonyl Disconnections

The Synthesis of Long Chain Fatty Acids

The Synthesis of a Furan

The Synthesis of a Modern Drug Candidate

References

29 Strategy XIII: Introduction to Ring Synthesis: Saturated Heterocycles

Cyclisation Reactions

A Bicyclic Amine

References

30 Three-Membered Rings

Cyclisation and Carbene Strategies Compared

Cyclopropanes from Electrophilic Alkenes

The Synthesis of Halicholactone

References

31 Strategy XIV: Rearrangements in Synthesis

Diazoalkanes

The Pinacol Rearrangement

The Favorskii Rearrangement

References

32 Four-Membered Rings: Photochemistry in Synthesis

An Example from Chapter 31

Development of Material from the Textbook

Photochemical Cycloadditions

Four-Membered Rings by Ionic Reactions

References

33 Strategy XV: The Use of Ketenes in Synthesis

Do Ketenes Exist?

The Synthesis of α-Lactones

Ketenes as Intermediates

[2 + 2] Thermal Cycloadditions of Ketenes

References

34 Five-Membered Rings

An Intermediate in the Synthesis of Coriolin

Asymmetric Synthesis from Terpenes

Cyclisation of Alkyl Lithiums onto Alkenes

References

35 Strategy XVI: Pericyclic Reactions in Synthesis: Special Methods for Five-Membered Rings

Electrocyclic Reactions

Sigmatropic Rearrangements

References

36 Six-Membered Rings

A Synthesis from the Textbook Chapter

The Diels-Alder Route

The Birch Reduction Route

References

37 General Strategy C: Strategy of Ring Synthesis

Development of Some Chemistry from the Textbook

References

38 Strategy XVII: Stereoselectivity B

The Prelog-Djerassi Lactone

A Pharmaceutical Example

The Synthesis of a Cage Molecule

Conformational Control

References

39 Aromatic Heterocycles

The Mechanism of the Stetter Synthesis of 1,4-diCarbonyl Compounds

The Synthesis of Five-Membered Heterocycles

Mechanisms in Heterocyclic Chemistry

Pyrazole, Imidazole and Quinoline

References

40 General Strategy D: Advanced Strategy

The Synthesis of Methoxatin

The Key Reaction Strategy: Diels-Alder Reactions

References

Index

This edition first published 2009
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Library of Congress Cataloging-in-Publication Data

Warren, Stuart.
Workbook for organic synthesis: the disconnection approach/Stuart Warren and Paul Wyatt. – 2nd ed.
p. cm.
Includes bibliographical references and index.
ISBN 978-0-470-71227-6 – ISBN 978-0-470-71226-9
1. Organic compounds – Synthesis – Textbooks. I. Wyatt, Paul. II. Title.
QD262.W93 2009
547'.2 – dc22
2009030810

A catalogue record for this book is available from the British Library.

ISBN 978-0-470-7-12276 (h/b) 978-0-470-7-12269 (p/b)

Preface

In the 26 years since Wiley published Organic Synthesis: The Disconnection Approach and the accompanying Workbook, this approach to the learning of synthesis has become widespread while the books themselves are now dated in content and appearance. In 2008, Wiley published the second edition of Organic Synthesis: The Disconnection Approach by Stuart Warren and Paul Wyatt for which this is the accompanying Workbook.

This workbook contains further examples, problems (and answers) to help you understand the material in each chapter of the textbook. The structure of this second edition of the workbook is the same as that of the textbook. The 40 chapters have the same titles as before but all chapters have undergone a thorough revision with some new material. The emphasis is on helpful examples and problems rather than novelty. Many of the problems are drawn from the courses we have given in industry on ‘The Disconnection Approach’ where they have stimulated discussion leading to deeper understanding. It makes sense for you to have the relevant chapter of the textbook available while you are working on the problems. We have usually devised new problems but some of the problems in the first edition seemed to do such a good job that we have kept them. Usually, the answers are presented in a different and, we hope, more helpful style.

It is not possible to learn how to design organic syntheses just from lectures or from reading a textbook. Only by tackling problems and checking your answers against published material can you develop this skill. We should warn you that there is no single ‘right answer’ to a synthesis problem. Successful published syntheses give some answers that work, but you may well be able to design others that have a good chance of success. The style of this second edition is to give more discussion of alternative routes.

Stuart Warren and Paul Wyatt
2009

General References

Full details of important books referred to by abbreviated titles in the chapters to avoid repetition.

Clayden Organic Chemistry: J. Clayden, N. Greeves, S. Warren and P. Wothers, Organic Chemistry, Oxford University Press, Oxford, 2000.

Disconnection Textbook: S. Warren and P. Wyatt, Organic Synthesis: The Disconnection Approach, Second Edition, Wiley, Chichester, 2008.

Drug Synthesis: D. Lednicer and L. A. Mitscher, The Organic Chemistry of Drug Synthesis, Wiley, New York, seven volumes, from 1977.

Fieser, Reagents: L. Fieser and M. Fieser, Reagents for Organic Synthesis, Wiley, New York, 20 volumes, 1967–2000, later volumes by T.-L. Ho.

Fleming, Orbitals: Ian Fleming, Frontier Orbitals and Organic Chemical Reactions, Wiley, London, 1976.

Vogel: B. S. Furniss, A. J. Hannaford, P. W. G. Smith, and A. R. Tatchell, Vogel’s Textbook of Practical Organic Chemistry, Fifth Edition, Longman, Harlow, 1989.

1

The Disconnection Approach

We start with a few simple problems to set you at ease with disconnections. Problem 1.1: Here is a two-step synthesis of the benzofuran 3. Draw out the retrosynthetic analysis for the synthesis of 2 from 1 showing the disconnections and the synthons.

Answer 1.1: As this is a simple S_N2 reaction, the disconnection is of the C–O bond 2a and the synthons are nucleophilic phenolate anion 4, which happens to be an intermediate in the reaction, and the cation 5, which happens not be an intermediate in the reaction but is represented by the α-bromoketone 6.

Problem 1.2: Draw the mechanism of the cyclisation of 2 to 3. This is an unusual reaction and it helps to know what is going on before we analyse the synthesis. Answer 1.2: The first step is an acid-catalysed cyclisation of the aromatic ring onto the protonated ketone 7. Loss of a proton 8 completes the electrophilic aromatic substitution giving the alcohol 9.

Now protonation of the alcohol leads to loss of water 10 to give a stabilised cation that loses aproton 11 to give the new aromatic system 3. Problem 1.3: Now you should be in a position to draw the disconnections for this step.

Answer 1.3: We hope you might have drawn the intermediate alcohol 9. Changing 3 into 9 is not a disconnection but a Functional Group Interconversion (FGI) – changing one functional group into another. Now we can draw the disconnection revealing the synthons 12 represented in real life by 2.

A Synthesis of Multistriatin

In the textbook we gave one synthesis of multistriatin 17 and here is a shorter but inferior synthesis as the yields are lower and there is little control over stereochemistry.¹ Problem 1.4: Which atoms in the final product 17 come from which starting material and which bonds are made in the synthesis? Hint: Arbitrarily number the atoms in multistriatin and try to trace each atom back through the intermediates. Do not be concerned over mechanistic details, especially of the step at 290°C.

Answer 1.4: However you numbered multistriatin, the ethyl group (7 and 8 in 17a) finds the same atoms in the last intermediate 16a and the rest falls into place. It then follows which atoms come from 14 and which from 15. Finally, you might have said that C-4 in our diagrams comes from formaldehyde.

So the disconnections also fall into place. Just one C–O bond was disconnected at first 17b then one C–O and one C–C 16b and finally the alkene was disconnected 14b in what you may recognise as an aldol reaction with formaldehyde. If you practise analysing published syntheses like this, you will increase your understanding of good bonds to disconnect.

References

1. W. E. Gore, G. T. Pearce and R. M. Silverstein, J. Org. Chem., 1975, 40, 1705.

2

Basic Principles: Synthons and Reagents: Synthesis of Aromatic Compounds

This chapter concerns the synthesis of aromatic compounds by electrophilic and nucleophilic aromatic substitution. All the disconnections will therefore be of bonds joining the aromatic rings to the sidechains. We hope you will be thinking mechanistically, particularly when choosing which compounds can undergo nucleophilic aromatic substitution and the orientation of electrophilic aromatic substitution. Any textbook of organic chemistry¹ will give you the help you need.

Problem 2.1: Compound 1 was needed² for an exploration of the industrial uses of HF. Suggest how it might be made. Hint: consider which of the three substituents you would rather not add to the ring.

Answer 2.1: We can add the nitro group by nitration and the isopropyl group by Friedel-Crafts alkylation (as it is a secondary alkyl group) but we would rather not add the OMe group as there is no good reagent for MeO⁺. So we disconnect first the most deactivating group (nitro) 1a and then the isopropyl group 2.

Before writing out the synthesis, we should check that the orientation of the substitution will be what we want. The OMe group is ortho, para-directing so alkylation will go mainly para because of steric hindrance. Now we have a competition as isopropyl is also ortho, para-directing but, since OMe has a lone pair of electrons conjugated with the benzene ring, it will dominate so everything is fine. We therefore suggest:

Did you consider the alternative strategy? That is, disconnect the isopropyl group first 1b to give a new intermediate 4 and disconnect the nitro group second. The starting material, anisole 3, is the same in both routes.

Again we should check the orientation. Nitration of anisole will give a mixture of ortho 4 and para 5 products so much depends on the ratio and whether they can easily be separated. The Friedel-Crafts reaction will go ortho or para to the OMe group and meta to the nitro group so that is all right. However the deactivating nitro group might make the reaction difficult.

So what did the chemists prefer? One published synthesis² used HF as a catalyst to alkylate ortho-nitro-anisole 4 with isopropanol. The yield was a respectable 84%. This made sense as they had a supply of 4. If anisole is nitrated with the usual HNO₃ /H₂ SO₄ , a 31:67 ratio of ortho:para products is obtained. If the nitrating agent is an alkyl nitrite in MeCN, the ratio improves to 75:25. The best route nowadays is probably the nitration of available para-isopropyl phenol 6, probably quantitative, and methylation of the product 7 with, say, dimethyl sulfate.

Problem 2.2: These compounds 8 and 9 each have two benzene rings linked by a heteroatom and both are used to make anti-inflammatory drugs. An obvious strategy is to disconnect one C–X bond in each case and combine the two compounds by nucleophilic aromatic substitution. Suggest a synthesis for each compound.

Answer 2.2: The two disconnections 8a and 8b illustrate the types of molecules needed for the first problem. In each case X is a leaving group such as a halogen and the phenols 11 and 12 would be used as their anions.

To be successful, nucleophilic aromatic substitution needs an electron-withdrawing group ortho or para to the leaving group. A chloride, as in 13 is not adequate but the ketone in 10 is perfectly placed. The reported synthesis³ uses 10;X = Cl with 11 and Cu/NaOH as catalyst. We might nowadays prefer available 10;X = F with the anion of the phenol.

The other compound 9 is easier in one way as both disconnections 9a and 9b are feasible. Each ring 14 and 15 has an electron-withdrawing CO₂ H group in the right position (ortho to the leaving group X). Compound 17 has another leaving group (Cl) that is para to the CO₂ H group so it could react. On the other hand, compound 15 could react with itself and polymerise as it has the nucleophilic amine and the activated chloride in the same molecule.

The reported synthesis⁴ uses 16 and 17; X = Cl relying on the CO₂ H group to provide regioselectivity at the more electrophilic ortho position. It is possible⁵ that the fluoro-compound 17; X = F would be a better way.

Problem 2.3: Chagas disease causes some 50,000 deaths annually in South America. Drugs based on the structure 18 are urgently needed. You are not expected to understand the chemistry used to make the strange heterocyclic ring but you might appreciate that it could come from an ortho-nitro aniline such as 19 or an activated halide such as 20. Suggest syntheses for these starting materials.

Answer 2.3: In both cases, the initial disconnection of the nitro group 19a and 20a is very appealing. The starting materials 21 and 22 should be easily made and nitration will go ortho to NH₂ rather than Me in 21 and ortho to Cl and meta to the deactivating aldehyde in 22.

The synthesis of 19 is straightforward⁶ as the amine 21 is available from the nitration and reduction of toluene. Amide 23 formation reduces the reactivity of the amine so that mono-nitration and hydrolysis give 19. Nitration of 23 gives 19.

The aldehyde 22 is more difficult as we should need to chlorinate benzaldehyde in the para position to get 22. One solution is to oxidise para chloro-toluene 24, available⁷ from 21 via the diazonium salt with, for example, chlorine to give 25 that can be hydrolysed⁸ to the aldehyde 22.

A Problem from the Textbook

When discussing the synthesis of saccharine in chapter 2 of the textbook, we said; ‘In practice chloro-sulfonic acid is used as this gives the sulfonyl chloride directly. You may be surprised at this, thinking that Cl might be the best leaving group. But there is no Lewis acid here. Instead the very strong chloro-sulfonic acid protonates itself to provide a molecule of water as leaving group.’ The reaction gives a mixture of the ortho-27 and para-28 products. Problem 2.4: With those hints, draw a mechanism of the chlorosufonation.

Answer 2.4: ‘Strong’ means a strong acid here so chloro-sulfonic acid 29 protonates itself to give a cation that loses water 30 to give the reactive cation 31. This is attacked by toluene in the ortho-and para-positions to give e.g. 32 that loses a proton to give 28.

References

1. Clayden, Organic Chemistry, chapters 22 and 23.

2. W. S. Calcott, J. M. Tinker and V. Weinmayr, J. Am. Chem. Soc., 1939, 61, 1010.

3. Drug Synthesis, vol 4, p. 42.

4. Drug Synthesis, vol 3, p. 38.

5. S. M. Kelly and H. Schad, Helv. Chim. Acta, 1985, 68, 1444.

6. W. Porcal, A. Merlino, M. Boiani, A. Gerpe, M. Gonz´alez and H. Cercetto, Org. Process. Res. Dev., 2008, 12, 156.

7. Vogel, p. 931.

8. W. L. McEwen, Org. Synth. Coll., 1943, 2, 133.

3

Strategy I: The Order of Events

You should refer to the Guidelines from the textbook when you solve the problems in this chapter.

Guideline 1: Consider the effects of each functional group on the others. Add first (that is disconnect last) the one that will increase reactivity in a helpful way.

Guideline 2: Changing one functional group into another may alter reactivity dramatically.

Guideline 3: Some substituents are difficult to add so it is best to start with them already present.

Guideline 4: Some disubstituted compounds are also readily available and they may contain a relationship (especially ortho) that is difficult to achieve by electrophilic substitution.

Guideline 5: Some groups can be added to the ring by nucleophilic substitution.

Guideline 6: If a series of reactions must be carried out, start with one that gives a single product unambiguously and not one that would give a mixture.

Remember that these guidelines may conflict or even contradict each other. THINK! Problem 3.1: Suggest syntheses of 1 and 2 needed as intermediates: 1 in the synthesis of some brominated acids¹ and 2 to study the mechanism of enzymatic ester hydrolysis.²

Answer 3.1: With two electron-withdrawing groups in 1, some FGI is needed to control the orientation and gain some reactivity. There are good ways to introduce Br and NO₂ but no easy way to introduce CO₂H. FGI of CO₂H to Me with oxidation in mind would give an ortho, para-directing group where we need it 3. Now we might disconnect NO₂ 3a or Br 3b as there are good reagents for adding both. There might be some doubt as to where 4 would be nitrated as both Me and Br are ortho, para-directing, but there is no doubt where 5 will be brominated as Me is ortho, para-directing while NO₂ is meta-directing.

So the synthesis was nitration of toluene (actually 5 is available), separation of 5 from the ortho isomer, bromination of 5, and oxidation of 3 to give the target molecule.¹

No doubt the CHO group could also be formed by oxidation of a CH₃ group but it can be inserted next to a phenolic OH by the Reimer-Tiemann reaction.³ Now we can disconnect the t-Bu group with Friedel-Crafts alkylation in mind.

The large t-Bu group much prefers the para position and the Reimer-Tiemann reaction using chloroform as a source of dichlorocarbene (Textbook chapter 30) goes ortho to the conjugating OH group.^2,4

Example and Problem 3.2: Bumetamide 7 is a diuretic from Leo Pharmaceutical Products in Denmark. The synthesis⁵ was planned by a number of FGIs to give 8 and then a C–O disconnection to give 9 as a suitable starting material. Problem 3.2: Suggest why these FGIs were chosen as a preliminary to disconnection.

Answer 3.2: The PhO group must be added by nucleophilic aromatic substitution so electron-withdrawing groups are essential. We have two (SO₂X and CO₂H) in the right positions, ortho and para to Cl in 9, and could have a third if NH₂ is replaced by NO₂. Problem 3.3: Suggest a synthesis of 9.

Answer 3.3: Two of the substituents in 9 (SO₂OH and Cl) can be added by electrophilic substitution and we have seen some ways to add the CO₂H group. The most obvious thing to do is to replace NH₂ by NO₂ 10 and disconnect both NO₂ and SO₂OH giving p-chlorobenzoic acid 11 as starting material. This compound is available but could be made by chlorination of toluene and oxidation of the methyl group.

Now we need to decide in which order to add the two substituents. The orientation will be decided by the Cl group as it is ortho, para-directing. In the published synthesis⁵ chlorosulfonation is used followed by nitration and the sulfonamide 13 is formed before the nitro group is reduced to the amine.

With three groups to help nucleophilic substitution, phenoxide was added and catalytic hydrogenation of 14 to the amine 15 was followed by reductive amination (chapter 8) with PrCHO to give bumetamide 7.

References

1. K. Friedrich and H. Oster, Chem. Ber., 1961, 94, 834.

2. R. Breslow, M. F. Czarniecki, J. Emert and H. Hamaguchi, J. Am. Chem. Soc., 1980, 102, 762.

3. Vogel, pp. 992 and 997.

4. J. H. Simons, S. Archer and H. J. Passino, J. Am. Chem. Soc., 1938, 60, 2956.

5. P. W. Feit, H. Bruun and C. K. Nielsen, J. Med. Chem., 1970, 13, 1071; P. W. Feit, Ibid., 1971, 14,

4

One-Group C–X Disconnections

If you have also read chapter 6, you will realise that acid derivatives such as esters 1 or amides 3 are usually made by acylation so that the C–O or C–N bond that is disconnected is the one between the heteroatom and the carbonyl group. In this way we are really using two-group disconnections for these compounds. The synthesis might combine an alcohol or an amine with an acid chloride 2.

Problem 4.1: Suggest which C–X bond would be your first choice for disconnection in these two compounds, explaining your reasons. Draw your proposed starting materials.

Answer 4.1: Though there are many C–X bonds in both molecules, the first disconnection should be of the ester 4a and of the amide 5a both because we know of good ways to make these functional groups and because the disconnections are in the middle of the molecules. You might have drawn 6 and 8 as acid chlorides or as acids, as we have done, deciding to work out the reagents later. Problem 4.2: What difficulties do you foresee in carrying out the reaction?

Answer 4.2: Both 6 and 7 have acid groups, so we shall have to activate the CO₂H group in 6 and perhaps protect the CO₂H group in 7. The situation for 8 + 9 is worse: not only does each compound have a CO₂H group, but 8 also has two nucleophilic groups (OH and NH₂). Again protection and activation will be needed. This second case is not as bad as it seems as 5 is a dipeptide and standard peptide coupling procedures can be used.¹ Stereochemistry is not a problem as the bond-forming steps do not affect any chiral centre.

We shall concentrate mainly on ethers and sulfides where true one-group C–X disconnections will be needed though mechanistic arguments will still be necessary. Problem 4.3: Suggest a synthesis for the ethers 10 and 11.

Answer 4.3: The first 10 is easy: we much prefer the disconnection on the alkyl side as the aromatic ring is not activated for nucleophilic substitution while the halide 12 is allylic and therefore electrophilic.

The second 11 requires more thought: The same disconnection 11a gives a primary halide 14 but it has a quaternary centre joined to it and there will be considerable steric hindrance to an S_N2 reaction. In addition, the amine in 15 is more nucleophilic than the phenolic OH group. Is there an alternative?

The amine 11 could be made by reduction of a nitro group and now the alternative disconnection 16 corresponding to nucleophilic aromatic substitution becomes possible.² There is no longer any ambiguity as there is only one nucleophilic group. In addition, the halide 14 would have to be made from the alcohol 17. Compounds derived from 11 are used in the treatment of diabetes.

Problem 4.4: Suggest a synthesis of fluoxetine 19, better known as the antidepressant Prozac®.

Answer 4.4: We should rather disconnect the ether in the middle of the molecule than the amine at the end and here again either C–O disconnection 19a or 19b would serve. The electrophile 21 (X is a leaving group) is benzylic and reactive while the CF3 group activates the ring for SNAr by stabilising the intermediate anion in the same way as the nitro group.

No doubt either synthesis will work but we could consider that the reaction at the chiral centre 19a might lead to some racemisation while reaction of 23 does not involve the chiral centre. The synthesis has been carried out with a single enantiomer of 23 using NaH as base in an amide solvent.³ The base gives the anion 24 so that oxygen becomes more nucleophilic than nitrogen.

The question remains: how do we make the aminoalcohol 23? Using a one-group disconnection of the C–N bond, we can displace a leaving group X from 25 and a search of available starting materials reveals the chloroketone 26.

Reduction of the ketone and reaction of the chloride 27 with NaI in acetone (NaCl precipitates from acetone and drives the equilibrium to the right) gave the corresponding iodide 28. Reaction of 28 with an excess of MeNH₂ as its available aqueous solution gave 23 in quantitative yield.³

An alternative is to add the second aromatic ring by a Mitsunobu reaction and displace chloride afterwards with aqueous MeNH₂. If a single enantiomer, e.g. (R)-(+)-27, is used, the inverted product (S )-(–)-29 is formed stereospecifically by the Mitsunobu reaction.⁴

A related route starts with the epoxidation of cinnamyl alcohol 30 and regioselective reduction of the epoxide 31 by Red-Al, NaH₂Al(OCH₂CH₂OMe)2 to give 32 because the aluminium complexes to the primary alcohol and delivers hydride to the nearer end of the epoxide. Mesylation and displacement with aqueous MeNH₂ complete the synthesis.⁵

Problem 4.5: Suggest a synthesis of febantel 34 used as an anthelmintic to combat tapeworms and the like.

Answer 4.5: If we do the obvious amide disconnection first 34a, we have a serious problem of chemoselectivity as we shall have to acylate one of two very similar amines 35.But if we change the other amine into a nitro group 36, the problem disappears and also suggests how we might make the sulfide.

Amide disconnection 36a reveals a simple nitro-amine with the PhS group in just the right position for C–S disconnection 37 with the nitro group activating a nucleophilic aromatic substitution of a suitable leaving group X.

As it happens, the chloro-compound 38;X = Cl is available, though it could easily be made by nitration of meta-chloro-aniline 39. Displacement of chloride with the anion of PhSH gives 37. Acylation with methoxyacetyl chloride and reduction of the nitro group gives febantel.⁶

References

1. Clayden, Organic Chemistry, chapter 52, Polymerization.

2. T. Sohda, K. Mizuno, E. Imayima, Y. Sugiyama, T. Fujita and Y. Kawamatsu, Chem. Pharm. Bull., 1982, 30, 3580.

3. D. W. Robertson, J. H. Krushinski, R. W. Fuller and J. D. Leander, J. Med. Chem., 1988, 31, 1412.

4. M. Srebnik, P. V. Ramachandran and H. C. Brown, J. Org. Chem., 1988, 53, 2916.

5. Y. Gao and K. B. Sharpless, J. Org. Chem., 1988, 53, 4081.

6. Drug Synthesis, 4, 35.