Multicomponent Assembly and Diversification of Novel Heterocyclic Scaffolds Derived from 2-Arylpiperidines

Abstract

A collection of structurally-diverse, polyheterocyclic scaffolds comprising a 2-arylpiperidine subunit was synthesized using a Mannich-type multicomponent assembly process, followed by appropriately sequenced ring-forming reactions. An improved procedure for removal of N-4-pentenoyl groups was developed; one-pot sequences for tandem urea/thiourea formation and N-arylation and tandem enolate arylation/alkylation were discovered. A novel entry to bridged tetrahydroquinoline scaffolds exploiting A^1,3 strain was also invented. Derivatization of several scaffolds was achieved by cross-coupling and N-functionalization.

The continuing demand for small molecules with useful therapeutic properties has necessitated more expedient access to screening libraries containing diverse heterocyclic scaffolds.¹ The identification of so-called privileged substructures, chemotypes that interact with multiple biological targets, and their inclusion in chemical libraries has proven effective toward discovering small molecule probes to study a broad range of known and emerging biological targets.² In order to prepare such compounds, there is a continuing need for developing new approaches that enable the rapid generation of functionalized heterocycles that may be easily elaborated and derivatized.

One successful approach toward this end is the build/couple/pair approach that was pioneered in the context of diversity-oriented synthesis (DOS).³ This strategy involves the construction of molecular frameworks followed by parallel, complexity-generating, ring-forming reactions. We have developed a novel and useful variant of this strategy that features a Mannich-type multicomponent assembly process (MCAP) to generate intermediates that may be quickly elaborated by ring forming reactions that are preprogrammed by the functionality present in the inputs in the MCAP.^4,5 Ideally, one would be able to access different, functionalized scaffolds having privileged structures from a single intermediate that is generated by the multicomponent assembly process. We now report the successful realization of this goal and its application to the diversity-oriented synthesis of compounds having the fused aryl piperidine substructures 1 and 2 in which the secondary amino group serves as a handle for further skeletal diversification (Figure 1). Significantly, the 2-arylpiperidine subunit is present in NK1 receptor antagonists such as 3,⁶ and the cis-fused octahydropyrrolo[3,2-c]pyridine core in 2 is found in CCR 5 antagonists as exemplified by 4.⁷

Figure 1.

MCAP-derived 2-arylpiperidines 1 and 2 and related bioactive compounds 3 and 4

Based upon previous work, we envisioned that dipolar cycloadditions of unsaturated aldehydes would lead to suitable precursors of 1 and 2.⁵ We had also found that acid chlorides are the preferred electrophilic activators of imines toward addition reactions with π-nucleophiles in Mannich-type, multicomponent assembly processes. Because most tertiary amide derivatives of 1 and 2 would be difficult to hydrolyze to give the secondary amines 1 and 2, it was necessary to identify a tertiary amide moiety that could be easily removed. It had been shown that N-4-pentenamides can be cleaved under mild conditions,⁸ so we queried whether the amides 9 and 10 would serve as viable intermediates for preparing 1 and 2. Accordingly, condensation of bromobenzaldehydes 5 and 6 with allylamine gave intermediate imines that were allowed to react in situ with 4-pentenoyl chloride (7) and vinyl ether 8 in the presence of catalytic amounts of TMSOTf to furnish 9 and 10 in very good overall yields (Scheme 1). Heating 9 and 10 with N-methylhydroxylamine and intramolecular dipolar cycloaddition of the intermediate nitrones gave the isoxazolidines 11 and 12 as single diastereomers. The relative configuration of 11 was confirmed by X-ray crystallography. Treatment of 11 and 12 with iodine in a mixture of THF and aqueous HCl led to facile removal of the N-4-pentenoyl group and formation of 13 and 14, respectively. The presence of HCl in this modified protocol for removing N-4-pentenoyl groups was essential to minimize side reactions involving iodohydrin formation that ensued when water alone was used as a co-solvent.

Scheme 1.

Multicomponent assembly of amines 13 and 14

The versatility of the aldehyde 9 as a precursor of other heterocyclic systems was then explored. For example, condensation of 9 with N,O-bis(trimethylsilyl)sarcosine (15) in toluene at room temperature, followed by thermolysis of the putative intermediate oxazolidinone at 135 °C in a sealed tube afforded 17 in 62% yield (Scheme 2).^9,10 The yield of the corresponding reaction of 9 with sarcosine (16) to give 17 was lower yielding, with β-elimination of the amide moiety from the starting aldehyde 9 being a major side reaction. Reaction of 17 with iodine in a mixture of THF and aqueous HCl gave the desired secondary amine 18. Optimal selectivity for cleavage of the amide 17 required a higher concentration of HCl than was used for hydrolysis of amide 11. The amine 18 thus obtained is a versatile substrate for the generation of libraries based on N-functionalizations such as sulfonylation to furnish 19.

Scheme 2.

Access to pyrrolidine-fused scaffolds by an azomethine ylide cycloaddition

We then explored several tactics for two-dimensional diversification of these scaffolds using the (4-bromophenyl)piperidines 12 and 14. For example, Suzuki-Miyaura coupling of 14 was readily effected on treatment with various arylboronic acids using Pd(PPh₃)₄ as the catalyst to give biphenyls as illustrated by the preparation of 20 (Scheme 3).¹¹ The secondary amine moie ty in 20 is amenable to a variety of N-functionalization processes as exemplified by reductive alkylation of 20 to give amine 21.

Scheme 3.

Biaryl synthesis via Suzuki coupling

Although the secondary amine 14 can be used as a starting material in carbon-carbon bond forming cross-couplings, it was necessary to employ the corresponding amide 12 in order to engage the aryl bromide moiety in carbon-heteroatom bond forming reactions. For example, the amide 12 underwent Ullmann-type coupling with imidazole in the presence of CuBr and the β-ketoester ligand 22 to afford the N-arylimidazole 23 (Scheme 4).¹² N-Arylimidazoles are structures of biological interest and are found in several compounds with anticonvulsant activity.¹³ The analogous Buchwald-Hartwig coupling of 12 with piperidine using Pd(OAc)₂ and the phosphine ligand 24 provided the N-arylpiperidine 25.¹⁴ Subsequent removal of the pentenamide group from 25 delivered the amine 26,. That 26 is a useful substrate for diversification by N-functionalization, is illustrated by formation of the urea 27 on treatment of 26 with ethyl isocyanate.

Scheme 4.

Cross-coupling of 12 with nitrogen nucleophiles and N-refunctionalization

In addition to the obvious possibilities for generating libraries from the parent scaffolds 11, 12, and 17 and their derived amines, we were intrigued by the opportunity of using these cycloadducts as precursors of more complex heterocycles. For example, a one-pot sequence was developed in which a mixture of 13, phenyl isocyanate, Pd(OAc)₂/BINAP and Cs₂CO₃ was stirred at room temperature; once urea formation was complete, the mixture was simply heated to provide 29 via a palladium-catalyzed intramolecular N-arylation (Scheme 5).¹⁵ It is important to ensure complete formation of the intermediate urea 28 before heating; mixtures of cyclized and uncyclized ureas are otherwise obtained. The dihydroquinazolin-2-one substructure present in 29 is found in Na⁺/Ca²⁺ exchange inhibitors and antipsychotic agents.^16,17 In a related transformation, the amine 13 was subjected to a novel one-pot sequence of thiourea formation and palladium-catalyzed cyclization to give the 2-imino-1,3-benzothiazinane 31 via the intermediate thiourea 30.¹⁸ It is necessary to use Pd[(t-Bu₃P)]₂ in order to obtain good yields in this latter process.

Scheme 5.

Synthesis of the dihydroquinazolin-2-one 26 and 2-imino-1,3-benzothiazinane 28

It occurred to us that we might also form carbon-carbon bonds to form new rings via enolate arylations. In this context, we discovered that reaction of 32, which was obtained by N-acylation of amine 13 with phenylacetyl chloride, with excess LDA in the presence of DMPU effected the desired cyclization, presumably via a benzyne intermediate (Scheme 6).¹⁹ In situ methylation of the intermediate enolate 33 resulting from this process proceeded from the sterically more accessible face to give 34 with >95:5 dr (LC-MS).^20,21 Although quenching 33 with 3,5-di-tert-butyl-4-hydroxytoluene (BHT) occurred with high diastereoselectivity, the resulting lactam 35 underwent facile epimerization to give mixtures (ca. 2:1) of diastereomers. The biological relevance of compounds such as 34 arises from the observation that 4,4-disubstituted dihydroisoquinolin-3-ones have vasorelaxant properties,²² and several 4-aryltetrahydroisoquinolines exhibit central nervous system activity.²³ Reductive cleavage of the isoxazolidine N,O-bond of 34 with nickel boride provided the amino alcohol 36, which is primed for further diversification by a variety of N-functionalizations.

Scheme 6.

Dihydroisoquinolin-2-one synthesis by a diastereoselective tandem enolate arylation/alkylation

Several strategies for further transforming amino alcohols derived from isoxazolidines may be illustrated beginning with the cycloadduct 37, which was prepared as previously described.⁵ Although reductive cleavage of the N, O-bond in 37 with nickel boride was accompanied with extensive dehalogenation, use of zinc in aqueous HCl at 0 °C gave 38 in 83% yield (Scheme 7).²⁴ N-Acylation of 38 proceeded with concomitant O-acylation; however, the simple expedient of protecting the primary hydroxyl group in 38 in situ as a TMS ether enabled highly efficient amide formation as exemplified by a one-pot synthesis of 39.

Scheme 7.

Synthesis and diversification of amino alcohol 38 exploiting A^1,3 strain of N-acyl-2-arylpiperidines

The cis-relationship between the methylamino group and the 2-bromphenyl substituent in 38 can be exploited by an intramolecular Buchwald-Hartwig coupling, which was achieved by heating 38 in the presence of Pd(OAc)₂/BINAP and Cs₂CO₃, to give the bridged tetrahydroquinoline 40 (Scheme 7). The preferred axial orientation of the aryl group in 38 that is enforced by A^1,3 strain involving the N-acetyl group presumably facilitates cyclization.²⁵ The conformationally-constrained bicyclic tetrahydroquinoline framework in 40 is a promising scaffold for DOS as exemplified by using the primary alcohol in 40 as a functional handle in a Mitsunobu reaction to give the succinimide 41.

In summary, we have significantly expanded the scope of our Mannich-type, multicomponent assembly process followed by dipolar cycloadditions to generate diverse collections of 2-arylpiperidines. New aspects of the process include use of the 4-pentenoyl group as an activating electrophile in the MCAP and as an amine-protecting group that is easily removed using iodine in the presence of aqueous acid. This new tactic for deprotecting N-4-pentenoyl groups will render it more attractive for future applications. The versatility of the initially-formed cycloadducts as precursors of diverse heterocyclic cores that are suitably functionalized for further transformations was established. Indeed, the methods developed for tandem urea or thiourea formation and cyclization as well as enolate arylation and alkylation will be broadly applicable in DOS. The novel entry to bridged tetrahydroquinoline scaffolds exploiting A^1,3 strain is also expected to be generally useful. The application of these techniques to the synthesis of pilot-scale compound libraries is ongoing, and further results will be reported in due course.