Solid-phase synthesis of novel 14-membered macroycles for high throughput screening

The preparation of 14-membered macrocycles from a resin-bound orthogonally protected lysine residue is described. Reductive alkylation of the a-nitrogen followed by acylation with an Fmoc-aminoacid provides a protected dipeptide precursor. Removal of the Fmoc-group, acylation with a succinic anhydride, methyltrityl-group removal and macrocyclization provides the desired macrocycles, after TFA cleavage, in excellent yield and purity.

BACKGROUND OF THE INVENTION
 The application of solid-phase combinatorial chemistry and parallel
 synthesis techniques for the preparation of non-peptide drug-like
 molecules has greatly expanded the diversity of agents available for
 biological screening in the pharmaceutical industry. There has been
 considerable effort to increase the size and diversity of corporate sample
 collections to feed HTS (high throughput screening) programs. HTS can
 potentially deliver a plethora of lead structures, active in novel
 therapeutic targets, for the medicinal chemist to explore. While there
 have been many reports in the literature describing methods to prepare
 acyclic and cyclic (5-, 6- and 7-membered cycles and fused-cycles)
 compound libraries for HTS; as well as for lead optimization programs,
 there has been few reports describing the solid-phase synthesis of
 macrocylic compound libraries..sup.1 To achieve a truly diverse sample
 collection for screening, it is desirable that the sample collection
 contain molecules of varying degrees of conformational flexibility as
 listed in table I.
 TABLE I
 Conformational
 Molecules Flexibility Examples
 Acyclic High oligonucleotides.sup.1a, peptides.sup.1b,
 peptoids.sup.1c,
 b-peptoids.sup.1d, oligocarbamates.sup.1e
 Macrocyclic Medium cyclic peptides.sup.1f, macrocycles.sup.1g
 Cyclic Low benzodiazepines.sup.1h, hydantoins.sup.1i,
 diketopiperazines.sup.1j, 2-
 alkylthiobenzimidazoles.sup.1k
 Acyclic molecules, because they can adopt multiple low energy
 conformations, would be expected to be fairly promiscuous and provide a
 higher hit rate relative to the more restricted molecules. Acyclics;
 however, provide little information about the required spatial arrangement
 of pharmacophoric groups. On the other hand, cyclic conformationally
 restricted molecules would be expected to provide valuable structural
 information concerning the binding requirements. Macrocylic compounds,
 neither completely rigid nor flexible, would be unique in their coverage
 of 3-d space and would be a valuable addition to our sample collection.
 The fact that cyclic peptides have long been of interest due to their
 attractive biological profile provides further incentive for synthetic
 investigation. It has been well documented that cyclic peptides often
 display increased selectivity, better bioavailability, and less
 susceptibility to proteolytic degradation than corresponding polypeptides.
 The preparation of cyclic peptides on a solid support has been established
 as an efficient method of synthesis that avoids undesired cross coupling
 reactions common to macrocyclization by providing a "pseudo-diluted"
 environment..sup.2
 SUMMARY OF THE INVENTION
 The invention relates to an efficient solid-phase synthesis of novel
 14-membered ring dipeptide derived macrocycles from readily available
 building blocks..sup.3 Thus, the invention relates to a process for
 preparing a macrocyclic compound selected from those of the formula:
 ##STR1##
 wherein:
 R.sub.1 is selected from naphthyl, diphenyl, and phenoxyphenyl;
 R.sub.2 is selected from C.sub.1 -C.sub.6 alkyl, benzyl, C.sub.1 -C.sub.6
 alkylamino, CH.sub.2 SCH.sub.2 Ph, and CH.sub.2 (4-MeO)Ph;
 R.sub.3 is selected from hydrogen and C.sub.1 -C.sub.6 alkyl; and
 R.sub.4 is selected from hydrogen, phenyl, C.sub.1 -C.sub.6 alkyl;
 or R.sub.3 and R.sub.4 taken together form a 5-6 membered carbocyclic ring;
 which comprises:
 preparing a resin-bound protected dipeptide precursor of the formula:
 ##STR2##
 from a resin-bound orthogonally protected lysine residue by reductive
 alkylation of the .alpha.-nitrogen followed by acylation with an
 Fmoc-aminoacid to provide the protected dipeptide precursor; and
 removing the Fmoc-group, acylating with a succinic anhydride, removing the
 methyltrityl-group followed by macrocyclization to provide the desired
 macrocycle of formula I.
 This approach uses the side-chain functionality of lysine in the final ring
 closure..sup.4 In addition, this route proved well suited for sort and mix
 combinatorial chemistry..sup.5 With the IRORI microkans and our visual
 tagging method, a diverse three dimensional library of 500 single pure
 compounds was generated..sup.6,7