Asymmetric Synthesis of Tetrahydropyridines via an Organocatalytic One-Pot Multicomponent Michael/Aza-Henry/Cyclization Triple Domino Reaction

Abstract

A low loading of a quinine-derived squaramide efficiently catalyzes the triple-domino Michael/aza-Henry/cyclization reaction between 1,3-dicarbonyl compounds, β-nitroolefins, and aldimines to provide tetrahydropyridines bearing three contiguous stereogenic centers in good yields, excellent enantiomeric excesses, and up to high diastereomeric ratios.

The tetrahydropyridine ring belongs to a widespread substructure in naturally occurring compounds and some synthetic bioactive molecules.¹ The tetrahydropyridine I is an aroma compound generated from the Maillard reaction (Figure 1).² Arecoline II is a nicotinic acid based alkaloid possessing a stimulating effect due its agonistic influence on the muscarinic acetylcholine receptors.³ Betanin III, a plant pigment, is another natural product containing a tetrahydropyridine unit used as a food additive.⁴ The synthetic tetrahydropyridine derivative IV has shown proinflammatory protein inhibition activity.⁵ In addition, these heterocycles also serve as precursor for the synthesis of valuable piperidine derivatives.⁶

Figure 1.

Selected bioactive tetrahydropyridine derivatives.

In the last 10 years, many interesting organocatalytic cascade sequences have emerged as powerful tools for the synthesis of valuable carbo- and heterocycles bearing multiple stereogenic centers in an highly stereoselective fashion.⁷ Due to the broad range of bioactivity, the synthesis of tetrahydropyridine derivatives in a stereoselective manner is extremly useful. Hence, the application of organocascade sequences has been successfully extended for those compounds bearing two stereogenic centers.⁸ Recently, Sun, Lin, and co-workers were able generate three stereogenic centers on a tetrahydropyridine ring by sequentially employing two different chiral organocatalysts (Scheme 1).⁹ We envisaged that a tetrahydropyridine ring could be synthesized via a Michael/aza-Henry/cyclization domino sequence using merely one organocatalyst at lower catalyst loading without any other additives.

Scheme 1. Organocascade Strategies for the Synthesis of Tetrahydropyridine Derivatives.

By following Rawal’s¹⁰ and others¹¹ as well as our own findings¹² on the squaramide-catalyzed Michael addition of dicarbonyl compounds on nitroalkenes, we synthesized the Michael adduct 5a and carried out domino aza-Henry/half-aminalization reactions between 5a and various imines in the presence of squaramide A at the onset. The N-tosylimine, however, did not provide any products, even in the presence of an additional base catalyst (Table 1, entry 1–5). The desired product was obtained in the case of N-(p-methoxybenzyl) (PMB) and N-(p-methoxypenyl) (PMP) imines, although the yield was not satisfactory (Table 1, entries 6–13). By switching to an N-methylimine, the desired tetrahydropyridine was obtained in 79% yield, 1.9:1 dr, and 98% ee (Table 1, entry 14). In order to distinguish between beneficial steric or electronic effects, we also tried the tert-butylimine as substrate (Table 1, entry 17). The fact that no reaction was observed in this case suggests that the steric hindrance is the limiting factor for this reaction step.

Table 1. Screening of Various Imines and Conditions for the Optimization of the Aza-Henry/Condensation Reaction Sequence^a.

entry	R	baseb	temp (°C)	yieldc (%)	drd	eee (%)
1	Ts		–25	0
2	Ts	DBU (20)	–25	0
3	Ts	DBU (20)	25	0
4	Ts	K2CO3 (50)	–25	0
5	Ts	morpholine (20)	–25	0
6	PMP		–25	14	n.d.	n.d.
7	PMP	DBU (20)	–25	18	n.d.	n.d.
8	PMP	DBU (20)	25	0
9	PMP	K2CO3 (50)	–25	0
10	PMB		–25	13	n.d.	n.d.
11	PMB		25	11	n.d.	n.d.
12	PMB	DBU (20)	–25	41	1.4:1	n.d.
13	PMB	DBU (25)	25	5	n.d.	n.d.
14	Me		–25	79	1.9:1	98
15	Me		25	67	1:1.1	n.d.
16	Me	DBU (30)	25	21	1:1.3	n.d.
17	t-Bu		–25	0

^aReactions were carried out on a 0.25 mmol scale with 0.5 mmol (2.0 equiv) of imine and 5 mol % of squaramide catalyst A in 0.2 mL of CH₂Cl₂ (c = 1.25 M) for 1.5–4 days.

^bAmount of additive given in mol % in parentheses.

^cYield of the isolated product after flash chromatography.

^dDiastereomeric ratio of (4R,5R,6S) to (4R,5R,6R).

^eDetermined by HPLC with a chiral stationary phase.

With the best imine found, we went on to optimize the general reaction conditions by first screening different solvents. In contrast to the enantiomeric excess, which remained constant, the yield turned out to be strongly dependent on the nature of the solvent. Among the tested mediums, toluene, diethyl ether, acetonitrile, and CH₂Cl₂, the latter turned out to be the best one (Table 1, entry 14, and Table 2, entries 1–3). Second, we varied the catalyst loading. Interestingly, the catalyst promotes the reaction more efficiently at lower loadings. Thus, merely 0.5 mol % of catalyst A was found to be sufficient to obtain 4a in high yield of 87% in a slightly prolonged reaction time without any loss of enantioselectivity (Table 2, entry 6). Lastly, we expanded the optimization on the amount of imine to determine its impact on the yield and the selectivity. We found that the diastereomeric ratio slightly increased by varying the amount, whereas the overall yield decreased (Table 2, entries 7–10). Consequently, the best conditions were found with 0.5 mol % of A and 2 equiv of an N-methylimine in CH₂Cl₂ at −25 °C.

Table 2. Optimization of the Aza-Henry/Cyclization Reaction Sequence^a.

entry	solvent	catalyst loading (mol %)	3a (equiv)	time (d)	yieldb (%)	drc	eed (%)
1	toluene	5	2	2.5	33	1.2:1	94
2	Et2O	5	2	5	58	2.1:1	97
3	MeCN	5	2	4	83	1.9:1	97
4	CH2Cl2	10	2	0.5	69	1.7:1	98
5	CH2Cl2	1	2	1.5	82	1.7:1	97
6	CH2Cl2	0.5	2	1.5	87	1.8:1	98
7	CH2Cl2	0.5	1.1	1.5	54	2.4:1	98
8	CH2Cl2	0.5	1.5	1.5	78	1.7:1	98
9	CH2Cl2	0.5	4	1.5	79	2.1:1	98
10	CH2Cl2	0.5	10	1.5	60	6.4:1	99

^aReactions were carried out on a 0.25 mmol scale with the indicated amount of imine and squaramide catalyst A in 0.2 mL of solvent (c = 1.25 M) for the stated time at −25 °C.

^bYield of the isolated product after flash chromatography.

^cDiastereomeric ratio of (4R,5R,6S) to (4R,5R,6R).

^dDetermined by HPLC with a chiral stationary phase.

With the optimized reaction conditions in hand, we started to examine the scope of this organocatalytic asymmetric Michael/aza-Henry/cyclization cascade starting from the Michael addition of the dicarbonyl compound to the nitroalkene. β-Nitroolefins bearing various substituents on the aryl ring provided the corresponding tetrahydropyridines 4a–d in 69–88% yield and high enantioselectivities. Furthermore, the nitroalkene substituted with a heteroaromatic ring was also tolerated under the standard reaction conditions, hence yielding 4e in an excellent yield of 91% with high enantiomeric excess. To our delight, even an aliphatic nitroalkene provided 4f in 32% yield with excellent dr of >20:1 and 93% ee. Interestingly, the thienyl- as well as the cyclohexyl-substituted nitroolefins gave a higher dr. For the next set of reactions, we varied the substituents of the 1,3-dicarbonyl compound. Both β-ketoesters tested gave high yields and enantiomeric excesses of the corresponding products 4g and 4h. Moreover, 4i was obtained in 89% yield and high enantiomeric excess by using a phenyl-substituted diketone. Although β-ketoamides and various cyclic dicarbonyl compounds could be transformed to the corresponding Michael adducts, they did not react in the second step. Finally, we employed aldimines with different substituents. As expected, the nature with regard to steric hindrance of the imine carbon and the bulkiness of the R⁴ moiety is crucial for the outcome of the reaction. Whereas several ortho- and meta-substituted imines did not react, the para-substituted imines led to the desired tetrahydropyridines 4j and 4k. Furthermore, different heterocyclic substituted imines, e.g., 2-furanyl and 3-Boc-indolyl, have proven to be suitable substrates for this transformation (Table 3, 4l,m). Imines bearing an alkyne group also worked well to provide synthetically interesting 6-ethynyl-substituted N-heterocycle 4n in 62% yield, 1.5:1 dr and 99% ee. The alkyne-substituted N-(p-methoxybenzyl)imine gave 22% yield of 4o in excellent dr and enantiomeric excess.

Table 3. Scope of the One-Pot Reaction to Tetrahydropyridines^a.

4	R1	R2	R3	R4	R5	timeb (d)	yieldc (%)	drd	eee (%)
a	Me	Me	Ph	Ph	Me	1 + 2	81	1.7:1	98 (99)
b	Me	Me	p-MeC6H4	Ph	Me	1 + 2	80	2.1:1	97 (99)
c	Me	Me	p-FC6H4	Ph	Me	1 + 2	69	2.1:1	99
d	Me	Me	o-ClC6H4	Ph	Me	1 + 2	88	2.3:1	98
e	Me	Me	2-thienyl	Ph	Me	1 + 0.5	91	13.3:1	98 (96)
f	Me	Me	Cy	Ph	Me	1 + 3	32	>20:1	93
g	OMe	Me	Ph	Ph	Me	1 + 2	80	1.5:1	98
h	OEt	Me	Ph	Ph	Me	1 + 2	88	1.5:1	99
if	Ph	Me	Ph	Ph	Me	1 + 2	89	1.1:1	99g
j	Me	Me	Ph	p-BrC6H4	Me	1 + 2	37	1.5:1	99
k	Me	Me	Ph	p-MeC6H4	Me	1 + 2	77	1.7:1	99
l	Me	Me	Ph	2-furanyl	Me	1 + 2	78	1.2:1	96g
m	Me	Me	Ph	2-(N-Boc-indolyl)	Me	1 + 2	45	1:1.4	99
nh	Me	Me	Ph	PhC≡C	Me	1 + 4	62	1.5:1	99
oh	Me	Me	Ph	PhC≡C	PMB	1 + 4	22	>20:1	95

^aReactions were carried out with 0.25 mmol of 1 and 2 combined with 0.5 mol % of A in 0.2 mL (c = 1.25 M) of CH₂Cl₂. After 1 day, 0.5 mmol (2.0 equiv) of 4 was added and the reaction was continued at −25 °C for the indicated time.

^bReaction time of first + second step.

^cYield of the isolated product after flash chromatography.

^dDiastereomeric ratio of (4R,5R,6S) to (4R,5R,6R).

^eEnantiomeric excess of the main diastereomer determined by HPLC with a chiral stationary phase. Values for the recrystallized samples are given in parentheses.

^fA small amount of the (4R,5S,6S) isomer was also obtained.

^gEnantiomeric excess of the (4R,5R,6R) diastereomer.

^hReactions were carried out with 0.5 mmol of 1 and 2 combined with 0.5 mol % A in 0.4 mL of CH₂Cl₂ (c = 1.25 M) and 1.0 mmol of 3 after 1 day.

Finally, we demonstrated that a domino reaction proceeds smoothly without any loss of efficiency when all the substrates as well as the catalyst A were added together at the beginning (Scheme 2). Furthermore, a gram-scale reaction was successfully performed without much alternation of the product yield and stereoselectivity, which shows the practical and preparative utility of this domino process (Scheme 3).

Scheme 2. Synthesis of Tetrahydropyridines under Domino Conditions.

Scheme 3. Gram-Scale Synthesis of Tetrahydropyridines.

The relative configuration was determined by NOE contacts between the concerning hydrogen atoms, whereas the absolute configuration was assigned by the X-ray crystal structure (Figure 2).^13,14

Figure 2.

X-ray crystal structure of 4a.¹⁴

In conclusion, we have developed a squaramide-catalyzed asymmetric multicomponent Michael/aza-Henry/cyclization triple domino reaction of 1,3-dicarbonyl compounds, β-nitroolefins, and imines to provide potentially bioactive 1,4,5,6-tetrahydropyridines in very good yields and up to high diastereo- and excellent enantioselectivities. Additionally, this protocol required only one catalyst at an extremely low catalyst loading of 0.5 mol %. The new procedure can be successfully scaled up to gram amounts without losing the reaction efficiency. Further applications of squaramide-catalyzed organocascade sequences for the synthesis of valuable chiral heterocycles are under investigation, and the results will be reported in due course.

Supporting Information Available

Experimental procedures and characterization data including copies of the NMR and HPLC spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

The authors declare no competing financial interest.