Enantioselective Synthesis of 3,5,6-Substituted Dihydropyranones and Dihydropyridinones using Isothiourea-Mediated Catalysis.

The scope of dihydropyranone and dihydropyridinone products accessible by isothiourea-catalyzed processes has been expanded and explored through the use of 2-N-tosyliminoacrylates and 2-aroylacrylates in a Michael addition-lactonization/lactamization cascade reaction. Notably, to ensure reproducibility it is essential to use homoanhydrides as ammonium enolate precursors with 2-aroyl acrylates, while carboxylic acids can be used with 2-N-tosyliminoacrylates, delivering a range of 3,5,6-substituted dihydropyranones and dihydropyridinones with high enantioselectivity (typically >90 % ee). The derivatization of the heterocyclic core of a 3,5,6-substituted dihydropyranone through hydrogenation is also reported.

Introduction

The synthesis of small, functionalized chiral heterocycles through asymmetric catalysis remains a prominent area of research in synthetic methodology. The recognition of endocyclic enol dihydropyranones and dihydropyridinones as key constituents within natural products and bioactive compounds aids the appeal of catalytic routes to produce these molecules.1 Classical routes involve uncatalyzed Diels–Alder reactions and more recently metal‐catalyzed π‐olefin and π‐alkyne cyclizations.2 However, the current state‐of‐the‐art methods for the production of chiral dihydropyranones and dihydropyridinones with high diastereo‐ and enantiocontrol remains the use of organocatalytically generated enolate equivalents. Typical processes within this area have utilized N‐heterocyclic carbene (NHC)‐generated azolium enolates,3 enamine catalysis,4 or cinchona alkaloids5 and isothiourea‐generated ammonium enolates in formal [4+2]‐cycloaddition/Michael addition‐cyclization reactions with electron‐deficient olefins using a wide range of enolate precursors and strategies.6 To date, intermolecular reactions within these systems have typically utilized β‐substituted electron‐deficient enones or α,β‐unsaturated ketimines to form 3,4,6‐substituted dihydropyranones and dihydropyridinones with excellent diastereo‐ and enantiocontrol (typically >90:10 d.r., >95 % ee; Figure 1).

Figure 1

Enantioselective synthesis of dihydropyranones and dihydropyridinones – overview of the field.

Within this area, following pioneering work by Romo and co‐workers,7 our laboratory has developed an isothiourea‐catalyzed8 Michael‐addition cyclization protocol using bench stable carboxylic acids as starting materials that has been applied to the synthesis of a variety of heterocycles.9 Following our previous report on the synthesis of di‐, tri‐, and tetra‐substituted pyridines using 2‐N‐tosyliminoacrylates, we considered using β‐unsubstituted Michael acceptors (2‐N‐tosyliminoacrylates and 2‐aroylacrylates) in enantioselective isothiourea‐catalyzed Michael addition‐cyclization cascades to generate 3,5,6‐trisubstituted dihydropyranones and dihydropyridinones containing a single stereocenter (Figure 2).10 To the best of our knowledge, only limited precedent with such β‐unsubstituted acrylate acceptors in organocatalytic [4+2]‐cycloaddition type processes have been reported.11

Figure 2

Enantioselective synthesis of dihydropyranones and dihydropyridinones – this work.

At the onset of these investigations the highly reactive nature of β‐unsubstituted 2‐N‐tosylimino‐ and 2‐aroylacrylates, and the assumed relative rate of a competitive racemic base‐promoted background reaction, were envisioned as problems to overcome to obtain high enantioselectivity in these reactions. In this study, the scope and limitations of this approach are investigated and explored, with the key finding being the necessity to use homoanhydrides as ammonium enolate precursors with 2‐aroylacrylates, while carboxylic acids can be used with 2‐N‐tosyliminoacrylates.

Results and Discussion

Michael Addition‐Lactonization using 2‐Aroylacrylates; Optimization and Generality

Preliminary studies began with optimization of the isothiourea‐catalyzed Michael addition‐lactonization using 2‐aroylacrylate 1 and phenylacetic acid 2 as a model system.12 Employing in situ mixed anhydride formation using pivaloyl chloride and phenylacetic acid 2 with 2‐aroylacrylate 1 catalyzed by (+)‐BTM (benzotetramisole) 4 (5 mol %) at −78 °C after 1 h afforded dihydropyranone 3 in an excellent yield of 79 % and 93 % ee (Scheme 1). However, disappointingly this method did not prove general when applied to subsequent substrates. In all cases the desired dihydropyranone products were formed in typically excellent yield but with no enantiocontrol. For example, treatment of 4‐bromophenyl acetic acid 5 and 2‐aroylacrylate 1 with iPr2NEt, pivaloyl chloride, and (+)‐BTM 4 at −78 °C gave the desired dihydropyranone 6 in a good yield of 80 % but with 0 % ee. Attempted optimization through syringe pump addition of Michael acceptor 1 (0.25 m in CH2Cl2) gave marginal improvement with typical enantioselectivities of approximately 20 % ee observed with isothiourea catalyst 4 (Scheme 1).

Scheme 1

Initial results of the Michael addition‐lactonization. [a] Syringe pump addition of 1 (0.25 m in CH2Cl2) over 2 h.

To investigate the lack of enantiocontrol using 4‐bromophenyl acetic acid 5 in this process the feasibility of a competitive base‐mediated racemic reaction process was probed. Direct treatment of 4‐bromophenyl acetic acid 5 with pivaloyl chloride and iPr2NEt, followed by addition of 2‐aroylacrylate 1 without the inclusion of the isothiourea Lewis base yielded dihydropyranone 6 in an isolated yield of 52 %. This is consistent with a significant base‐mediated reaction under these conditions presumably enhanced by the high reactivity of the 2‐aroylacrylate (Scheme 2).13

Scheme 2

Control experiment.

Alternative reaction conditions were explored to prepare the target products with reproducibly high enantioselectivity. We have previously demonstrated the use of homoanhydrides as alternative ammonium enolate precursors to carboxylic acids, with one advantage of this approach being the reduced levels of added organic base necessary for catalysis in comparison to the in situ mixed anhydride approach.9g The use of homoanhydride 7 as the enolate precursor offered a major breakthrough in this system, delivering products with high and reproducible levels of enantioselectivity under optimized reaction conditions (Table 1).

Table 1

Optimization studies of the Michael addition‐lactonization.

Entry	Lewis base [mol %]	T [°C]	t [h]	Base	Yield [%][a]	ee [%][b]
1	8 (5)	rt	0.2	iPr2NEt	81	46
2	9 (5)	rt	0.2	iPr2NEt	80	78[c]
3	4 (5)	rt	0.2	iPr2NEt	88	88
4	4 (5)	−78	1	iPr2NEt	88	91
5	4 (5)	−78	1	Et3N	85	0
6	4 (5)	−78	1	Cs2CO3	58	57
7	4 (5)	−78	1	Na2CO3	81	83
8[d]	4 (5)	−78	2	iPr2NEt	88	70

[a] Yield of isolated product. [b] Determined by chiral HPLC analysis. [c] (S)‐enantiomer obtained. [d] Syringe pump addition of 1 (0.25 m in CH2Cl2) over 2 h.

Homoanhydride 7 and 2‐aroylacrylate 1 were used as a model system for reaction optimization. Lewis base screening showed 5 mol % (+)‐BTM 4 to be optimal, forming product 6 in 88 % yield and excellent 91 % ee at −78 °C (Table 1, entry 4). The nature of the base used in this process also proved key to high enantioselectivity, as Et3N gave the product in 85 % yield, but in racemic form, consistent with a competitive base‐catalyzed background reaction (Table 1, entry 5). The inorganic base Cs2CO3 gave 6 in poor yield and ee, whereas Na2CO3 proved moderately successful, and yielded 6 in 81 % yield and 83 % ee (Table 1, entries 6 and 7). Syringe pump‐addition of Michael acceptor 1 in CH2Cl2 over 2 h gave no improvement, and formed 6 in 70 % ee (Table 1, entry 8).

With optimized reaction conditions in hand, the scope of this reaction process was evaluated (Table 2).14 The use of a homoanhydride containing an electron‐donating 4‐MeOC6H4 substituent was tolerated and gave 10 in 83 % yield and 91 % ee, while 4‐substituted halogenated aromatics could also be installed in high yield and ee (products 6 and 11). Pleasingly, sterically demanding enolate precursors such as the 1‐naphthyl and o‐tolyl homoanhydrides could be applied, producing 12 in an excellent yield of 81 % and 97 % ee and 13 in 81 % yield with moderate 50 % ee. Assessing the scope of aroyl acrylate Michael acceptors, the electron‐rich 4‐MeOC6H4 aryl unit was incorporated in 14 and isolated in 79 % yield and 91 % ee. Dihydropyranone 15 was synthesized in 65 % yield and 81 % ee, however, in this case Na2CO3 was required as the base to obtain good enantioselectivity.15 Furthermore, 4‐methoxyphenyl acetic anhydride was explored with heteroaryl groups such as 2‐furyl, and gave 16 in 61 % yield and 99 % ee. Halogenated aromatics can be installed at the 6‐position to afford 17 in 61 % yield but with moderate 68 % ee. Finally, the naphthyl substituent was employed and afforded 18 in 71 % yield and 86 % ee. The absolute configuration of 12 was confirmed by X‐ray diffraction with all other examples assigned by analogy (Figure 3).16

Table 2

Substrate scope of the Michael addition‐lactonization.

Figure 3

Molecular representation of X‐ray structure 12.

Following the development of this procedure for the synthesis of 3,5,6‐substituted dihydropyranones, the utility and further elaboration of these products was explored. The isothiourea‐catalyzed Michael addition‐lactonization could be readily carried out on a reasonable laboratory scale (4.27 mmol), thereby providing 1.27 g of 10 in 87 % ee. Dihydropyranone 10 could be transformed into pyranone 19 through a substrate controlled Pd/C‐catalyzed hydrogenation to afford product 19 in 60 % yield with >95:5 d.r. (Scheme 3). Pyranone 19 was further derivatized by a reductive ring‐opening, providing triol 20 in 88 % yield and high diastereoselectivity. Nuclear Overhauser effect (NOE) experiments confirmed the relative configuration of 19, however, the ee determination of 19 or triol 20 by chiral HPLC or GC was not possible in our hands.17

Scheme 3

Derivatization of dihydropyranone 10.

Michael Addition‐Lactamization using 2‐N‐tosyliminoacrylates; Optimization and Generality

Following the successful synthesis of dihydropyranones through an isothiourea‐catalyzed Michael addition‐lactonization process, the method was extended to explore the structurally related dihydropyridinone motif. Initial investigations began with optimization using 2‐N‐tosyliminoacrylate 21 and carboxylic acid 2. Treatment of 2 with pivaloyl chloride and iPr2NEt followed by subsequent addition of Michael acceptor 21 and HyperBTM 8 (10 mol %) in CH2Cl2 at room temperature gave dihydropyridinone 22 in 60 % yield and 80 % ee after 30 min (Table 3, entry 1). A screen of isothiourea catalysts revealed (−)‐tetramisole 9 to be optimum, providing dihydropyridinone 22 in 62 % yield and 84 % ee (Table 3, entry 2). Lowering the reaction temperature to −78 °C gave 22 in a 74 % yield and an improved 91 % ee (Table 3, entry 5). The reaction was sensitive to solvent choice in terms of both conversion and enantioselectivity. For example, THF gives a poor 40 % yield of 22 in 86 % ee (Table 3, entry 7), while MeCN at −30 °C provides 22 in 70 % yield but in racemic form (Table 3, entry 8). Finally, the catalyst loading for 9 could be lowered to 5 mol % without compromising the yield or ee (Table 3, entry 9).

Table 3

Optimization studies of the Michael addition‐lactamization.

Entry	Lewis base [mol %]	T [°C]	t [h]	Solvent	Yield [%][a]	ee [%][b]
1	8 (10)	rt	0.5	CH2Cl2	60	80[c]
2	9 (10)	rt	0.5	CH2Cl2	62	84
3	4 (10)	rt	0.5	CH2Cl2	60	80[c]
4	8 (10)	−78	16	CH2Cl2	71	86[c]
5	9 (10)	−78	16	CH2Cl2	74	91
6	4 (10)	−78	16	CH2Cl2	70	85[c]
7	9 (10)	−78	16	THF	40	86
8	9 (10)	−30	16	MeCN	70	0
9	9 (5)	−78	16	CH2Cl2	74	91

[a] Yield of isolated product. [b] Determined by chiral HPLC analysis. [c] (R)‐enantiomer obtained.

The generality of this process was next examined using (−)‐tetramisole 9 (5 mol %) as the Lewis base. Initially 2‐N‐tosyliminoacrylate 21 was treated with a range of commercially available carboxylic acids under the previously optimized reaction conditions (Table 4). However, some examples gave poor conversion of the desired dihydropyridinone products even after extended reaction times at −78 °C. Therefore, a more general procedure was developed by allowing the reaction to warm over 16 h from −78 °C to room temperature, giving full conversion of the Michael acceptor with a range of acetic acids.18 Electron‐rich aromatic substituents such as 4‐MeOC6H4 and 4‐Me2NC6H4 are tolerated and desired products 23 and 24 are afforded in good yields of 76 % and 71 % with excellent 95 % and 94 % ee, respectively. Electron‐deficient aryl units were also tolerated, and the CF3‐bearing dihydropyridinone 25 was isolated in 73 % yield and 93 % ee. 3‐Tolylacetic acid produced dihydropyridinone 26 in 69 % yield and high 95 % ee. Halogen‐substituted aryl substituents could also be incorporated, giving product 27 in 61 % yield, albeit a reduced 71 % ee was obtained. Pleasingly, heteroaryl groups were tolerated and delivered dihydropyridinone 28 in 65 % yield and excellent 90 % ee. Next, the scope of 2‐N‐tosyliminoacrylate Michael acceptors was explored. Unfortunately, only electron‐rich aryl units could be included at the 6‐position of the dihydropyridinone products owing to a limitation in the synthesis of the 2‐N‐tosyliminoacrylates.19 For example, the 4‐MeOC6H4 aryl group could be included to give product 29 in 63 % yield and 98 % ee. Also, 4‐tolyl and 3,5‐xylyl groups were well tolerated and afforded the corresponding products 30 and 31 in 60 % yield, 97 % ee and 59 % yield and 90 % ee, respectively. Finally, 2‐naphthyl substitution was possible, 32 was formed in 69 % yield and 91 % ee. This Michael addition‐lactamization method was also performed on reasonable laboratory scale (4.17 mmol), thus providing 1.30 g of 22 with excellent 99 % ee. The absolute configuration of 25 was determined by X‐ray diffraction, with all other products assigned by analogy (Figure 4).20

Table 4

Substrate scope of the Michael addition‐lactamization.

[a] 4.17 mmol scale.

Figure 4

Molecular representation of X‐ray structure 25.

Following our previous studies a proposed mechanism for the processes described above begins with N‐acylation of isothiourea catalyst with either the homoanhydride (with aroyl acrylates) or in situ formed mixed anhydride (with imino acrylates) to form an acyl ammonium species 33 (Figure 5). Subsequent deprotonation gives (Z)‐ammonium enolate 34, which is stabilized by a proposed n O to σ*C−S interaction or favorable electrostatic stabilization between the enolate oxygen and sulfur atom on the catalyst framework.21 Enantioselective Michael addition to an aroyl acrylate or imino acrylate, followed by lactamization/lactonization, provides the corresponding heterocyclic products 35 and releases the catalyst.

Figure 5

Proposed catalytic cycle.

Conclusions

In conclusion, the isothiourea‐catalyzed Michael‐addition lactamization/lactonization of 2‐[aryl(tosylimino)methyl]acrylate or 2‐aroylacrylates from arylacetic acids or homoanhydrides, respectively, produces stereodefined 3,5,6‐substituted dihydropyridinones or dihydropyranones in high yield and enantioselectivity. Using these products to provide further complex chiral building blocks has been demonstrated through the use of hydrogenation or ring‐opening processes. Further studies within our laboratory are focused towards the continued development of isothioureas and other Lewis bases in catalysis.

Experimental Section

General procedure: Isothiourea‐catalyzed Michael addition‐lactonization

To a solution of requisite homoanhydride (1.25 equiv) in CH2Cl2 (0.31 m in homoanhydride) at −78 °C was added Lewis base catalyst (5 mol %) and the reaction stirred for 20 min. A solution of Michael acceptor (1.0 equiv) in CH2Cl2 (0.25 m), pre‐cooled to −78 °C, is added followed by a solution of iPr2NEt (1.25 equiv) in CH2Cl2 (0.31 m), also pre‐cooled to −78 °C, and reaction stirred until complete by TLC analysis. The reaction was quenched with HCl (1 m in H2O), extracted with CH2Cl2 (×3), dried over MgSO4, and concentrated under reduced pressure to give the crude residue. Products were purified by Biotage Isolera 4 and kieselgel 60 (0.040–0.063 mm) silica grade in the solvent system reported.

General procedure: Isothiourea‐catalyzed Michael addition‐lactamization

To a solution of requisite carboxylic acid (2.0 equiv) in CH2Cl2 (0.1 m in carboxylic acid) at 0 °C was added iPr2NEt (3.0 equiv) and pivaloyl chloride (3.0 equiv). The reaction was left to stir for 10 min before being cooled to −78 °C at which point Lewis base catalyst (5 mol %), Michael acceptor (1.0 equiv), and iPr2NEt (1.0 equiv) were added and the reaction was warmed to room temperature over 16 h. The reaction was quenched with HCl (1 m in H2O), extracted with CH2Cl2 (×3), dried over MgSO4, and concentrated under reduced pressure to give crude residue. Products were purified by column chromatography in the solvent system reported.

The data underpinning the work in this manuscript can be found at http://dx.doi.org/10.17630/6e0ad60a‐ddf5‐459c‐bc45‐cdd80fd518b8

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Supplementary

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