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Transition Metal Mediated Synthesis of Conformationally Constrained Phenylalanine Analogues and Their Applications

Susan E. Gibson (née Thomas),a Nathalie Guillo,a Richard J. Middletona and Matthew J. Tozerb

Department of Chemistry, Imperial College of Science, Technology and Medicine, South Kensington, London SW7 2AY, UK

b James Black Foundation, 68 Half Moon Lane, Dulwich, London SE24 9JE

This contribution describes our recent work on the synthesis of a new series of conformationally constrained analogues of phenylalanine together with the first example of an application of the series in a bioactive system. After an introductory outline of some of the biological aspects of conformationally constrained amino acids, the synthesis of the conformationally constrained phenylalanines using routes based on i) palladium catalysed Heck cyclisations, and ii) radical cyclisations will be described. This will be followed by a description of an application of this series of compounds which will detail i) the chemical synthesis of several potential cholecystokinin-B/gastrin receptor antagonists, and ii) the results of the biological tests carried out on the compounds produced.


Restricting the conformational freedom of peptides may lead to i) increased binding affinity at receptors, increased selectivity at receptors, iii) increased stability with respect to enzymatic degradation, and iv) increased insight into the bioactive conformation of the peptide. One method of constraining the conformational freedom of peptides (or peptide derivatives) is to introduce conformationally constrained amino acids, four examples of which are illustrated in Scheme 1.

Scheme 1

Focussing on phenylalanine, several examples of conformationally constrained analogues of this amino acid are depicted in Scheme 2. Of particular note, in the context of this contribution, is the compound on the far right i.e. 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid, commonly referred to as Tic.

Scheme 2

Incorporation of Tic into a peptide not only restricts the conformational freedom of the aromatic ring but also places constraints on the peptide backbone. These effects provide valuable insight into the bioactive conformation of the peptide ligand and often lead to potent and selective compounds. For example, replacement of a Phe residue with a Tic residue in the octapeptide depicted in Scheme 3 led to a substantial increase in affinity and selectivity for the u opioid receptor, and the dipeptide represented in Scheme 4 is currently showing significant promise in in vivo tests.

Scheme 3

Scheme 4

In view of the extensive use of Tic in studies of this type, we envisaged that 2,3,4,5-tetrahydro-1H-3-benzazepine-2-carboxylic acid (Sic), 1,2,3,4,5,6-hexahydro-3-benzazocine-2-carboxylic acid (Hic) and 2,3,4,5,6,7-hexahydro-1H-3-benzazonine-2-carboxylic acid (Nic), together with Tic, would provide a useful series of compounds for probing bioactive conformations of peptide or peptide-derived ligands (Scheme 5). We thus embarked on the synthesis of Sic, Hic and Nic in order to test our hypothesis.

Scheme 5



Our retrosynthetic analysis of the cyclic amino acids Sic, Hic and Nic is shown in Scheme 6.

Scheme 6

Our initial targets were thus a series of iodoaldehydes, the synthesis of which was carried out using standard procedures as depicted in Scheme 7.

Scheme 7

Reductive amination of the iodoaldehydes with (+/-)-serine methyl ester was then followed by either Ac or BOC protection and the introduction of a double bond to give two sets of substrates for the proposed intramolecular Heck reaction (Scheme 8).

Scheme 8

At this point, it seemed appropriate to consider the cyclisation mode that the proposed Heck reactions would follow i.e. would an endo pathway dominate to give intermediates with b-hydrogens that would allow the Heck cyclisation to proceed to give the required products, or would an exo pathway be favoured leading to intermediates without appropriate b-hydrogens which would possibly not only give undesired products but also consume precious palladium catalyst (Scheme 9)?

Scheme 9

A literature survey at this point in the project revealed that although there were several hundreds of reported examples of intramolecular Heck reactions giving five- and six-membered rings (almost always via an exo mode of cyclisation), and some examples of the formation of macrocycles, examples of the formation of medium-sized rings were extremely rare (Scheme 10). Furthermore, from the small number of examples known it was unclear which mode of cyclisation was favoured.

Scheme 10

The outcome of one of our many early Heck reactions is depicted in Scheme 11. Relatively harsh conditions and use of Et3N as a base resulted in a mixture of products derived from both endo and exo modes of cyclisation.

Scheme 11

In contrast, the somewhat milder conditions defined in Scheme 12 using NaHCO3 as the base led to only the isolation of the desired endo product.

Scheme 12

An explanation for these observations is depicted in Scheme 13. It is envisaged that the Et3N acts as a souce of hydride (an established process) and so opens up the pathways leading to the exo products. In the absence of Et3N these pathways cannot be accessed and only the endo product is observed. However, the exo cyclisation still occurs leading to a build-up of the palladium intermediate lacking b-hydrogens (or a derivative thereof) and the consumption of palladium catalyst.

Scheme 13

This interpretation suggests that as the ring size increases and the endo mode of cyclisation becomes more favourable than the exo mode of cyclisation (for stereoelectronic reasons), then the reaction should require less catalyst and give higher yields. This prediction is consistent with the results depicted in Scheme 14 which illustrate that decreasing amounts of palladium catalyst are required to generate essentially the same yield of product as the ring size increases.

Scheme 14

At this stage in the project, although we were pleased that the intramolecular Heck reaction was providing the seven-, eight- and nine-membered rings we desired, we were still concerned about the quantity of palladium catalyst that was being consumed. We thus carried out an extensive set of experiments designed to optimise the conditions with a view to reducing the catalyst loadings. The results of our experiments with the seven- eight- and nine-membered rings are depicted in Schemes 15-17 respectively and summarised in Scheme 18.

Scheme 15

Scheme 16

Scheme 17

Scheme 18

The figures given in Scheme 18 demonstrate quite clearly that as the size of the ring being generated increases, the reaction requires less catalyst and delivers a higher yield of product. Thus the eight- and nine-membered Heck products are generated in synthetically acceptable yield using acceptable catalyst loadings.

In view of the moderate yield obtained for the formation of the seven-membered ring, however, we decided to examine a second approach to the synthesis of the desired medium-sized rings. Given the nature of the substrates we had in hand, we chose to determine whether or not radical cyclisations would provide the required rings in good yield (Scheme 19).

Scheme 19

Once again, a survey of the literature revealed that although radical cyclisations had been used to generate several hundred five- and six-membered rings, examples of their use to generate seven-, eight- and nine-membered rings were relatively rare (Scheme 20).

Scheme 20

We were thus gratified to find that treatment of our unsaturated iodoarenes with Bu3SnH in the presence of AIBN gave the desired cyclised products in acceptable yield (Scheme 21).

Scheme 21

Comparison of the yields obtained for the intramolecular Heck reaction and the corresponding radical cyclisations reveal that the two methods are complementary, the Heck reaction being more favourable for the formation of the larger rings and the radical reaction providing better yields of the smaller rings (Scheme 22).

Scheme 22

Once good methods for obtaining the medium-sized rings of Sic, Hic and Nic were in place, their synthesis was completed in a straightforward manner as shown in Scheme 23.

Scheme 23


The diacid depicted in Scheme 24 is a cholecystokinin-B/gastrin receptor inhibitor developed by the James Black Foundation.

Scheme 24

In order to determine the effect on biological activity of replacing the phenylalanine residue in the antagonist with Tic, Sic, Hic and Nic and ultimately perhaps to learn more about the bioactive conformation of the antagonist, the corresponding Tic, Sic, Hic and Nic analogues were made according to the route depicted in Scheme 25.

Scheme 25

The results of two biological assays of the parent antagonist, the four analogues and a commonly used literature reference compound are given in Scheme 26. These demonstrate that although the Tic, Sic and Hic analogues of the antagonist 3a-c show much lower binding affinities than the parent compound 2, compounds 4d containing Nic displayed almost identical binding characteristics to those of 2. Molecular modelling studies designed to incorporate this information into the current model of the bioactive conformation of 2 are now underway.

Scheme 26


We have used transition metal catalysed reactions to synthesise several new conformationally constrained phenylalanine analogues which together with the well-established Tic form a series in which the members of steadily changing conformational freedom. Our first application in a biologically active system has generated promising results which suggest that the series of compounds will prove to be considerably more powerful than any one individual member. We are currently examining further biological applications of Sic, Hic and Nic.