Facile and Unified Approach to Skeletally-Diverse, Privileged Scaffolds

Abstract

A novel strategy has been developed to generate a diverse array of privileged scaffolds from readily available tetrahydropyridine precursors that may be prepared by a multicomponent assembly process (MCAP) followed by a ring-closing metathesis (RCM). The functionality embedded in these key intermediates enables their facile elaboration into more complex structures of biological relevance by a variety of ring-forming processes and refunctionalizations.

The demand to identify new, selective agents to treat human diseases and to serve as tools to interrogate biological function has led to various approaches for generating molecular libraries for biological screening. Although traditional combinatorial synthesis has been employed to produce large numbers of novel compounds, these libraries have historically suffered from low hit rates and poor specificity, a consequence that has been attributed to poor physiochemical properties and insufficient structural diversity, coupled with a lack of stereocenters and molecular rigidity.¹ Accordingly, recent efforts in library design have focused upon developing more effective strategies. For example, libraries based upon so-called privileged substructures typically exhibit higher hit rates in a variety of biological assays.^1,2 A single privileged scaffold can be easily modified via manipulation of either functional groups or ring substitution patterns. Because such alterations often induce marked changes in potency and target affinity, libraries based upon privileged scaffolds are well-suited for identifying new lead compounds for drug discovery.

We recently became interested in discovering new strategies for diversity-oriented synthesis (DOS)³ to prepare collections of biologically active small molecules having privileged ring systems as well as substructures found in natural products. One such strategy combines a multicomponent assembly process (MCAP) involving three or more reactants to prepare pivotal intermediates that are transformed into heterocyclic scaffolds by various ring-forming reactions that are directed by selective pairing of functional groups.^4,5,6 We have now developed a useful extension of this strategy, wherein tetrahydropyridines, that are accessed via a MCAP and a subsequent ring-closing metathesis (RCM), are transformed into a number of privileged scaffolds. We now present some of the details of these investigations.

In order to develop this new approach to scaffold generation, a suitably-functionalized tetrahydropyridine, such as 3 was needed. Accordingly, reaction of 2-bromo-6-chlorobenzaldehyde (1),⁷ allylamine, Cbz–Cl, and allylzinc bromide in a Mannich-like MCAP gave diene 2 in 89% yield (Scheme 1). Cyclization of 2 via a RCM reaction delivered the pivotal intermediate 3. Tactics for its elaboration into various heterocyclic scaffolds, especially privileged substructures, were then explored.

Scheme 1.

Synthesis of tetrahydropyridine 3.

The isoindolinone ring system, which is found in the potent anti-viral natural product stachyflin,⁸ is one important member of the family of privileged scaffolds.⁹ Intrigued by the possibility of preparing isoindolinones from 3 via a Parham cyclization,¹⁰ we found that treatment of 3 with n-BuLi at –100 °C gave isoindolinone 4 in 60% yield (Scheme 2). In order to illustrate possible tactics for diversifying 4, it was subjected to Suzuki cross-coupling reactions with electron-rich and electron-deficient arylboronic acids to give biaryls 5 and 6; hydrogenation of 5 provided saturated amide 7. Relative to possibilities for biological activity, it is notable that cyclohexyl-fused isoindolinones similar to 4 possess potent urotensin-II receptor antagonist activity,¹¹ and biarylisoindolinones similar to 5–7 exhibit KDR inhibitory activity.¹² Indeed, biaryls are privileged scaffolds that are present in 4.3% of all known drugs.^1,13

Scheme 2.

Synthesis of isoindolinone scaffold 4 and subsequent Suzuki cross-coupling.

Tetrahydropyridine 3 underwent facile Heck cyclization under Jeffrey's conditions¹⁴ and microwave irradiation to provide the enecarbamate 10, a versatile intermediate that is nicely functionalized for a number of diversification reactions (Scheme 3). For example, electron-rich enecarbamates are excellent inputs in imino Diels-Alder reactions, such as the Povarov reaction.^15,16 Although Povarov reactions involving substrates having the structural complexity of 10 are not known, we discovered that the reaction of 10 with p-toluidine and ethyl glyoxylate in the presence of Sc(OTf)₃ gave a readily separable mixture (1.2:1.0) of diastereomeric tetrahydroquinolines 11 and 12 in 84% yield. Formation of a mixture of stereoisomers was not unexpected because Povarov reactions often proceed with low stereoselectivity. The relative stereochemistry of 11 was verified by single X-ray crystallographic analysis, whereas the structure of 12 was tentatively assigned based upon a value of J_H2-H3 = 3.4 Hz, which is consistent with the proposed stereochemistry and not with the trans-diaxial relationship expected for the C(3)-epimer.¹⁷ The indanyl quinoline 14, the structure of which was secured by X-ray crystallography, was also isolated in 5–10% yield; 14 is presumably formed from 11 and/or 12 via an elimination-oxidation sequence.¹⁸ Oxidation of the mixture of 11 and 12 with DDQ furnished 13 in 70% yield. It is noteworthy that 11–13 embody fused privileged substructures that can be diversified through elaboration of multiple functional handles.

Scheme 3.

Imino Diels-Alder reaction of enecarbamate 10.

The enecarbamate 10 can be easily converted into a number of nor-benzomorphans, a privileged skeleton whose members exhibit a range of neurological activities such as AChE inhibitory¹⁹ and codeine-like analgesic activity (Scheme 4).²⁰ In the event, the olefinic moiety in 10 was first reduced selectively under ionic conditions to furnish 15 in 94% yield.²¹ Exemplary cross-coupling reactions of 15 with arylboronic acids and amines gave the biaryls 16 and 17 and the aniline 18 in good yield. In an application of a known, but rarely used reaction,²² we found that treating 18 with TMSI, followed by workup with aqueous sodium bicarbonate, yielded benzylamine 19.

Scheme 4.

Synthesis of nor-benzomorphan analogs 16-19.

Tetrahydrobenzo[1,5]oxazocines are known to exhibit a diverse array of important biological properties, including CNS and analgesic activity,²³ as well as hepatitis C inhibitory activity.²⁴ Accordingly, we developed a novel entry to such compounds as exemplified by a facile synthesis of benzoxazocine 23 that featured a ring-closing etherification (Scheme 5). We first prepared the piperidine 21 as a single diastereomer by highly selective vicinal dihydroxylation of 20 from the more hindered olefin face employing the conditions of Woodward.²⁵ When 21 was subjected to a copper-catalyzed intramolecular etherification,²⁶ benzoxazocine 23 was obtained in 87% yield. With both a free hydroxl group and protected nitrogen, benzoxazocine 23 is ideally-suited for analog synthesis, as exempified by allyl and benzyl ethers 24 and 25, respectively.

Scheme 5.

Tetrahydrobenzo[1,5]oxazocine 23 via an intramolecular Ullmann etherification.

Compounds containing the 1,2,3,4-tetrahydrobenzo [h][1,6]naphthyridine motif exhibit a wide range of biological properities, including selective 5-HT₄ antagonist activity,²⁷ gastric (H⁺/K⁺)-ATP-ase inhibition,²⁸ and broad-spectrum antibacterial activity.²⁹ We thus queried whether we might be able to access this ring system from 3 via double-bond isomerization, followed by a Povarov reaction. Thermal, palladium-catalyzed isomerizations of tetrahydropyridines to enecarbamates are known,³⁰ but we found that heating 3 in the presence of 10% Pd/C using conventional heating methods gave irreproducible yields of 26; reaction times were also lengthy. On the other hand, when this reaction was promoted with microwave heating (300 W, 120 °C), 26 was obtained in 77% yield after only 50 min; it is notable that there was no observable loss of halogen under these conditions (Scheme 6). When 26 was allowed to react with ethyl glyoxylate and either o-bromoaniline or pchloroaniline in the presence of Sc(OTf)₃, the corresponding tetrahydroquinolines 27 and 28 were formed as mixtures of diastereomers in 80% and 73% yield, respectively. Oxidation of 27 and 28 with DDQ gave the corresponding tetrahydrobenzonaphthyridines 29 and 30, each of which has multiple functional handles for further diversification. Moreover, varying the aniline and aldehyde inputs in the Povarov MCR would further expand the range of possible analogs.

Scheme 6.

Preparation of hydrobenzonaphthyridines 29 and 30.

Pyridazines display a wide range of biological activities and have shown promise as 11β-HSD1 inhibitors for treating type II diabetes³¹ and as effective antitumor agents.³² During the course of our efforts to synthesize novel heterocyclic scaffolds for DOS, we sought to extend our MCAP/RCM approach to access novel fused pyridazine ring systems such as 35. The synthesis of this scaffold began by preparing the enyne 32 using propargylamine in an MCAP reaction (Scheme 7). A subsequent enyne RCM proceeded in excellent yield to furnish the diene 33, which underwent a Diels-Alder reaction with di-tert-butyl azodicarboxylate to afford cycloadducts 34 as an inseparable mixture of diastereomers. We initially tried to prepare the pyridazine 35 from 34 by the acid-promoted removal of the N-Boc protecting groups followed by oxidation using a variety of oxidants. However, all of our efforts were unsuccessful, and the diene 33 was invariably obtained in near quantitative yield, presumably through a retro Diels-Alder pathway involving extrusion of molecular nitrogen. While exploring various alternative routes to generate an aromatic pyridazine ring, we fortuitously discovered that treatment of 34 with bromine provided 35 via an unprecedented tandem sequence of bromination, N-Boc deprotection, and aromatization. Pyridazine 35 bears several functional handles for further elaboration and derivatization.

Scheme 7.

Pyridazine 35 via a tandem bromination, deprotection, oxidation sequence.

In summary, we have developed a novel strategy for the diversity oriented synthesis of a variety of heterocyclic scaffolds, many of which embody privileged substructures that are suitably functionalized for further diversification. The key feature of the approach is a multicomponent assembly process followed by a ring closing metathesis to give substituted tetrahydropyridines. These tetrahydropyridines then serve as pivotal intermediates for the facile generation of numerous functionalized scaffolds of biological relevance. Further applications of this and related approaches to the syntheses of novel compound libraries are in progress, and the results of these investigations will be reported in due course.