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Convergent and Fast Route to Piperazines via IMCR

Piperazines and their keto analogs are amongst the most important backbones in today’s drug discovery industries (Scheme 1). Owing to the high number of positive hits encountered in biological screens with this heterocycle and its congeners, the piperazine template certainly deserves the title of "privileged scaffold" in medicinal chemistry. (Privileged scaffolds are molecular backbones with versatile binding properties representing a frequently-occurring binding motif, and providing potent and selective ligands for a range of different biological targets). Moreover, the piperazine scaffold occurs regularly in complex natural products. Thus, it is no wonder that there is a plethora of different synthetic methods that allow for the fast and efficient assembly of these heterocyclic systems (for an excellent review see: C. J. Dinsmore et al. Tetrahedron 2002, 58, 3297. ).

Scheme 1. Piperazines and the different possible isomers of ketopiperazines, along with some biologically active synthetic piperazines: ABT-724 (1) a selective dopaminergic D4 agonist (erectile dysfunction treatment, Abbott), Indinavir (2) a HIV protease inhibitor (Roche), a thermolysin inhibitor 3 (Affymetrix), an oxytocin antagonist 4 (GSK), a CCR5 receptor antagonist 5 (Ono), and CCK-B (cholecystokine) receptor antagonist 6.

Many different syntheses of ketopiperazines have been described in the past. However, multicomponent reactions (MCRs) seem to be particularly well suited to assemble piperazines, since >10 different methods have been published so far.

Piperazines, piperazines, piperazines…

Scheme 2 summarizes several useful ways to synthesize arrays that contain a variety of piperazine scaffolds. As part of the so called UDC strategy (Ugi/Deboc/Cyclize), Hulme et al. described several new scaffolds which can be easily built up by using a Boc-protected starting material. For example, Boc-protected α-aminoacids and cyclohexenyl isocyanide yield highly-substituted diketopiperazines (DKPs) in one step using strategy A (C. Hulme et al. Tetrahedron Lett. 1998, 39, 1113. ).

Scheme 2. Piperazine synthesis by IMCRs. Throughout this article, the following color coding is used in the schemes: blue for the amine component, red for the acid component, green for the oxo component and magenta for the isocyanide component. A: 1) cyclohexenyl isocyanide, R2CHO, HOOCCHR1NR4Boc, R3NH2; 2) TFA, DCM; 3) NEt3; B: 1) H2NCHR4COOR, R1NC, R2CHO, BocHNCHR3COOH, 2) TFA, DCM, 3) toluene, EtOH, reflux; C: HCOCOOEt, R3HNCH2CH2NH2, R1COOH, R2NC; D: 1) chloroacetaldehyde, R1NC, R2HNCH2CH2NH2, R3COOH; 2) base; E: 1) R4NC, HCOCOOEt, R1NH2, BocR2NCHR3COOH; 2) TFA, DCM; 3) toluene, EtOH, reflux; F: 1) HOOCCHR1NH2, R2CHO, R3NC, MeOH; 2) tBuOK, THF, Δ.

Another one-pot DKP synthesis using an IMCR was introduced by Szardenings et al. who reacted an α-aminoacid ester, an N-protected α-aminoacid, an aldehyde or a ketone, and an isocyanide to yield the desired DKP after the primary U-4CR and secondary deprotection and cyclization, according to strategy B (A. K. Szardenings, Tetrahedron 1997, 53, 6573. ). In C, the use of glyoxylic acid esters, in conjunction with isocyanides, ethylenediamine, and acids according to Ugi et al. (I. Ugi et al. Heterocycles 1998, 49, 29.), or with mono Boc-protected ethylendiamines, carboxylic acids, and isocyanides respectively according to Hulme et al. also yields ketopiperazines (C. Hulme et al. Tetrahedron Lett. 1999, 40, 5295. ). Rossen et al. from Merck reacted mono Boc-protected ethylenediamines, carboxylic acids, isocyanides and chloroacetaldehyde smoothly to yield piperazines D (K. Rossen et al. Tetrahedron Lett. 1997, 38, 3183. ). Once again, glyoxylic acid esters provide a rapid route to DKPs in conjunction with N-Boc-protected α-aminoacids, isocyanides and primary amines via Boc-deprotection and cyclization E (C. Hulme et al. Tetrahedron Lett. 1999, 40, 5295. ). Ugi et al. used unprotected α-aminoacids, isocyanides, aldehydes in methanol with a subsequent base-induced cyclization to yield 2,6-DKP F (I. Ugi et al. Heterocycles 1998, 47, 965. DOI). In the case of a ketone as the oxo-component, they found spontaneous cyclization to the 2,6-DKP.

Szardenings et al. from Affymax made use of strategy B in order to discover thiol-containing DKPs as novel heterocyclic inhibitors of matrix metalloproteinases (MMPs), collagenase-1 and gelatinase-B (Szardenings et al. J. Med. Chem. 1999, 42, 1348. ). Screening arrays of thousands of Ugi DKPs yielded low-nM, selective collagenase inhibitors active in the assay (Scheme 3).

Similarily, chemists from Array Biopharmaceuticals used strategy A to synthesize 4000 DKPs (A. Kennedy et al. Org. Lett. 2002, 4, 1167. ). In this case, a convertible solid phase-bound carbonate isocyanide was used.

Scheme 3. Solid phase synthesis of DKP arrays (TentaGel S OH). The gray scale in the box corresponds to the IC50 against collagenase-1 (see the "thermometer" for gray scale).

Strategy D allows for the practical and rapid synthesis of the HIV protease inhibitor CrixivanŽ (2) (K. Rossen et al. Tetrahedron Lett. 1998, 39, 6823. ). Generally speaking, commercial HIV protease inhibitors are complicated molecules that are lengthy to synthesize and are filled with stereocenters. None of them are accessible by a less than a 20-step linear synthesis. Therefore, the cost of goods (COG) is quite high, and methods that allow a shorter synthesis would be welcome. Merck process research chemists developed a short alternative synthesis of the piperazine intermediate for the HIV protease inhibitor 2. It consists of a novel, two-step preparation of a tetrahydropyrazine, chiral hydrogenation and subsequent formamide deprotection without concomitant racemization (Scheme 4).

Scheme 4. Enantioselective preparation of the key chiral building block piperazine 17 in the synthesis of the HIV protease inhibitor CrixivanŽ. It involves a U-4CR leading to 16 and subsequent HCl elimination giving an α,β-unsaturated α-aminoacid, with subsequent piperazine ring closure, chiral hydrogenation and mild deprotection.

Abbott chemists used a U-4CR combining a pair of bifunctional Ugi-compatible starting materials, α-azidocarboxylic acid (20) and propargyl amine (19) to 22 in order to perform a subsequent [3+2] cycloaddition (Huisgen reaction) to yield bicyclic dihydrotriazolo[1,5-a]pyrazinone 23 (I. Akritopoulou-Zanze et al. Tetrahedron Lett. 2004, 45, 8439. ).

Scheme 5. Triazole-fused piperazines can be synthesized by the sequence U-4CR followed by [3+2] cycloaddition.

Priaton workers synthesized a novel bifunctional isocyanide 24 with a leaving group, and used this compound to prepare libraries of fused tetrazolopiperazines 28 by the sequence U-4CR plus intramolecular SN2 reaction (M. Umkehrer et al. Tetrahedron Lett. 2004, 45, 6421. ).

Scheme 6. An isocyanide with an internal leaving group allows for the one-pot assembly of tetrazolopiperazine 28.

Recently, Marcaccini et al. described another complementary approach towards DKPs (S. Marcaccini et al. Tetrahedron Lett. 2001, 42, 2727. ). A U-4CR between amines, aldehydes, chloroacetic acid and isocyanides affords the Ugi intermediate, which is cyclized to the title compounds upon treatment with ethanolic KOH under ultrasonication. A limitation of this synthesis is that the U-4CR products arising from aliphatic aldehydes only gave complex reaction mixtures during the cyclization. Thus, 29 can be synthesized in 48% yield over the two steps in a straightforward manner.

Scheme 7. U-4CR with chloroacetic acid followed by ring closure under basic conditions with ultrasound allow for large arrays of DKP.

In addition to these, another new approach to DKPs was published using dipeptides, aldehydes and isocyanides (S. Cho et al. Mol. Div. 2003, 6, 283. ). For example, L-Ala-L-Phe (30) reacts with isobutyraldehyde (31) and TOSMIC (32) to form 34 in 45% yield with a dr of 3:1. The intramolecular reaction is assumed to proceed via a 9-membered intermediate α-adduct 33, which collapses to the 6-membered DKP, by an intramolecular transacylation.

Scheme 8. Dipeptides serve as starting materials for a novel DKP synthesis involving U-4CR.

Finally Sollis from Glaxo-Smith-Kline described a short and novel stereospecific synthesis of trisubstituted 2,5-DKPs (S. L. Sollis J. Org. Chem. 2005, 70, 4735. ) to gain access to orally bioavailable oxytocin antagonists similar to 4. Different MCR strategies allowed for the stereoselective synthesis of all possible diastereomers!

Scheme 9. All enantiomers and diastereomers of the oxytocin antagonist 35 can be synthesized by a U-MCR with subsequent transformation

Note that the several different described ways to assemble the (mono/di-keto)piperazine moiety utilize an MCR and subsequent cyclization strategy, but their outcome is quite different in terms of molecular connectivity and starting materials! This is of great use for library applications as well as for medicinal chemistry problems.

Applications of IMCRs in the synthesis of piperazine natural products

(Keto)piperazines are core fragments in a variety of natural products (Scheme 10).

Scheme 10. Examples of piperazine-containing bioactive natural products are the antibiotic bicyclomycin (36), cytotoxic chetracin A (37), ergotamine (38), antitumor airotin (39), and blue cheese constituent marcfortine C (40).

A three step diketopiperazine synthesis using an Ugi reaction and a convertible isocyanide, diphenylmethyl isocyanide, was reported by Boehm and Kingsbury in their polyoxin antibiotic synthesis (J. C. Boehm, W. D. Kingsbury J. Org. Chem. 1986, 51, 2307.).

Scheme 11. A central step in the total synthesis of the antifungal Polyoxin 46 constitutes a U-4CR to yield diketopiperazine 45. This sequence involves U-4CR, conversion of the amide via an N-nitrosamide to the benzhydryl ester followed by hydrogenation with concomitant DKP formation.

Three masterpieces from Fukuyama et al. clearly show the usefulness of IMCRin natural product synthesis.

The antibiotic bicyclomycin (36), was isolated from Streptomyces sapporonensis and is very active against gram-negative bacteria, and many derivatives were synthesized (Fukujama et al. Tetrahedron Lett. 1981, 22, 4155. ). A key position in the synthesis of numerous analogs is occupied by the U-4CR of the α,β-unsaturated carboxylic acid 47, the amine 50, the α-hydroxyaldehyde 48, and the isocyanide 49 to form 51.

Scheme 12. IMCR reaction in Fukuyama’s bicyclomycin synthesis.

Ecteinascidin 743 (52) is a highly complex natural product isolated from the marine tunicate Ecteinascidin turbinata. Recently, this compound entered Phase II clinical studies under orphan drug status, for the treatment of soft tissue sarcoma and ovarian, breast, endometrial, prostate and non-small cell lung cancers. Initial data suggest it to be a highly efficient anticancer drug, especially for use in children. It has a novel mode of action that combines selectively inhibiting gene expression, including oncogenes, involved in cell growth and drug resistance, inhibiting genetic repair pathways and inhibiting cell cycle progression, finally inducing apoptosis. ET 743 is a complex natural product with ten condensed rings, 3 highly oxidized benzenes, 4 five-membered heteroaliphatic rings, and a 10-membered ring containing thioether and ester groups. Being such a potent natural product, ET 743 is the target of several synthetic groups.
A particularly impressive demonstration of the convergent character of IMCRs is the so fare only published total synthesis of the antitumor marine natural product ecteinascidin 743 (52) (T. Fukuyama et al. J. Am. Chem. Soc. 2002, 124, 6552. ).

Scheme 13. A key step in Fukuyama’s ecteinascidin 743 total synthesis involves an IMCR and DKP formation: a: MeOH, reflux, 90%; b: TBAF, THF, room temperature, 89%; c: Ac2O, pyridine, DMAP, room temperature, 93%; d: TFA, anisole, DCM, room temperature; e: EtOAc, reflux, 87% (2 steps).

Recently Fukuyama described the total synthesis of the pentacyclic quinine natural product lemonomycin (65) using an IMCR strategy for the stereoselective construction of the 3,8-diazabicyclo[3.2.1] skeleton involving a U-4CR and DKP synthesis (Scheme 14) (T. Fukuyama et al. Chem. Commun. 2005, 394. ).

Scheme 14. IMCR in the total synthesis of cytotoxic quinone natural product lemonomycin.

Outlook

A general feature of IMCR is that a particular backbone can be assembled in different and complementary ways. This greatly helps medicinal, combinatorial and natural product chemists to cover a certain activity space in an optimum fashion. The present highlight summarizes 12 convergent IMCR routes towards piperazines. Moreover it can be anticipated that even more MCR routes toward piperazines as privileged scaffolds will be discovered in the future!

A. Doemling, Org. Chem. Highlights 2005, July 5.
URL: http://www.organic-chemistry.org/Highlights/2005/05July.shtm