Kinetic resolution of 4-substituted-3,4-dihydrocoumarins via Pd-catalyzed asymmetric allylic alkylation reaction: enantioselective synthesis of trans-3,4-disubstituted-3,4-dihydrocoumarins

Abstract

The kinetic resolution of 4-substituted-3,4-dihydrocoumarins was realized via Pd-catalyzed allylic substitution reaction using Trost's chiral ligand L12, affording optically active mono- and trans-3,4-disubstituted dihydrocoumarin derivatives in high yields and with high enantioselectivities with an S factor up to 55.

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Dihydrocoumarin as a subunit has been found in a wide range of natural products as well as in biologically active molecules (Fig. 1).¹ In addition, it is also useful building blocks in the synthesis of some other important compounds.^2,5a Therefore, numerous procedures have been developed to approach mono- or disubstituted dihydrocoumarin derivatives,^3–6 however, only a few of them^5a,e are suitable to synthesise trans-3,4-disubstituted dihydrocoumarin derivatives in an asymmetric and catalytic way, all of which are involved in conjugate additions. The chiral center at the 3-position was installed using an aldol reaction which proceeds very often via the intermediates of conjugate additions.^5a,b Thus, the development of new asymmetric and catalytic strategies to access trans-3,4-disubstituted dihydrocoumarins remains to be explored.

Fig. 1 Some natural products.

In contrast, the palladium-catalyzed asymmetric allylic alkylation (AAA), one of the most powerful tools for the enantioselective formation of carbon–carbon and carbon–heteroatom bonds,⁷ has found a wide range of applications in organic synthesis. Different kinds of “hard” carbanions, including the ones derived from ketones, amides, acyl silanes, 2-methylpyridine, etc.,^8,9 have also been used successfully as nucleophiles in recent achievements. However, only one report appeared using esters as nucleophilic precursors in this reaction,^8j in spite of the fact that esters are a very useful class of compounds in organic synthesis.¹⁰ Kinetic resolution as a powerful tool has also been used in Pd-catalyzed AAA reactions,¹¹ whereby the nucleophiles, including “hard” carbanions, can also be resolved.¹² However, the examples are still rare. We have worked on the Pd-catalyzed AAA reaction for many years, developing high diastereo- and enantioselective reactions with different kinds of “hard” carbanion nucleophiles,⁹ as well as the resolution of ketones with high efficiencies.^12b In light of these results, we envisioned that the esters and lactones could be resolved in the Pd-catalyzed AAA reaction. Herein we would like to report our strategy to approach optically active mono- and trans-3,4-disubstituted dihydrocoumarin derivatives using kinetic resolution of 4-substituted-3,4-dihydrocoumarins via a Pd-catalyzed AAA reaction.

Initially, 6-methyl-4-phenyl-3,4-dihydrocoumarin 1a was subjected to the reaction with allyl carbonate 2a in the presence of catalytic amounts of [Pd(C₃H₅)Cl]₂ and L1, using LiHMDS as a base in THF at −78 °C, providing 3a in 51% yield with 62% ee and 39% recovery of 1a with 66% ee, the S-factor being 8.3 (Table 1, entry 1). These results encouraged us to further study the impact of the reaction parameters on the reaction (Table 1).

Table 1 Optimization of the parameters for the reaction of 1a with 2^a

Entry	Base	Solvent	L	2	1a		3a			S
					Yield^b (%)	ee^c (%)	Yield^b (%)	ee^c (%)	dr^d
a Reaction was carried out in THF at −78 °C, molar ratio of 1a/2/[Pd(C3H5)Cl]2/L/base = 100 : 50 : 2.5 : 5 : 120. b Isolated yield. c Determined using chiral HPLC. d Determined using ^1H NMR. e Calculated using the method described by Kagan.^13 f Due to a reversed sequence of peaks using HPLC. g Reaction was carried out at −65 °C.
1	LiHMDS	THF	L1	2a	39	66	51	62	>30/1	8.3
2	NaHMDS	THF	L1	2a	59	37	36	69	>30/1	7.8
3	KHMDS	THF	L1	2a	35	6	57	4	>30/1	1.1
4	LDA	THF	L1	2a	27	−1^f	71	0	7/1	1
5	KO^tBu	THF	L1	2a	24	−5^f	38	2	16/1	1.1
6^g	LiHMDS	DME	L1	2a	31	67	55	50	>30/1	5.8
7	LiHMDS	Et2O	L1	2a	50	39	50	31	>30/1	2.7
8	LiHMDS	Toluene	L1	2a	36	39	48	42	14/1	3.5
9	LiHMDS	DCM	L1	2a	35	19	50	15	14/1	1.6
10	LiHMDS	Hexane	L1	2a	33	27	36	39/58	1/1	2.9
11	LiHMDS	THF	L1	2b	36	64	52	55	>30/1	6.5
12	LiHMDS	THF	L1	2c	61	24	28	70	>30/1	7.1
13	LiHMDS	THF	L1	2d	40	34	32	73	6/1	8.9
14	LiHMDS	THF	L1	2e	49	51	43	67	10/1	8.3
15	LiHMDS	THF	L1	2f	36	65	54	54	>30/1	6.4
16	LiHMDS	THF	L2	2a	19	—	47	0	>30/1	—
17	LiHMDS	THF	L5	2a	30	43	51	31	>30/1	2.8
18	LiHMDS	THF	L6	2a	35	63	47	60	>30/1	7.5
19	LiHMDS	THF	L7	2a	36	58	49	39	>30/1	3.9
20	LiHMDS	THF	L8	2a	36	65	54	58	>30/1	7.2
21	LiHMDS	THF	L9	2a	37	−43^f	55	−34^f	>30/1	3.0
22	LiHMDS	THF	L10	2a	43	61	47	60	>30/1	7.3
23	LiHMDS	THF	L11	2a	43	59	49	66	>30/1	8.7
24	LiHMDS	THF	L12	2a	40	66	45	86/81	2.5/1	26
25	LiHMDS	THF	L12	2f	49	75	46	89	>30/1	39

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First, some bases were examined (Table 1, entries 1–5). It was found that they had a great impact on the enantioselectivity, the ee values of both 1a and 3a were less than 10% when employing KHMDS, LDA and KOt-Bu as the base (entries 3–5), while a comparable S-factor was afforded if NaHMDS was used (entry 2 vs. entry 1) instead of LiHMDS. The screening of some common solvents showed that the S-factor decreased from 8.3 to 5.8 with DME as the solvent (entry 6 vs. entry 1). An even worse S-factor was afforded with Et₂O, toluene, DCM or hexane as the solvent (entries 7–10), and also no diastereoselectivity was observed in hexane (entry 10). The influence of the leaving group (LG) on the allyl reagent was also investigated (entries 11–15). The S-factor decreased when carbonate 2b, acetate 2c and phosphate 2f were used (entries 11, 12 and 15). The reactions of carboxylate 2d and phosphate 2e gave similar S-factors but lower dr values than that of 2a (entries 13 and 14 vs. entry 1). A series of chiral ligands with different electronic and steric factors were screened (Fig. 2). Racemic 3a was afforded in 47% yield and 1a was recovered in 19% yield by using L2 (entry 16), and the reaction was sluggish in the presence of L3 or L4 (not shown in Table 1). A worse S-factor was achieved when L5 with an o-tolyl group on the P atom was used (entry 17 vs. entry 1). The reaction using L6 with much more steric hindrance on the P atom, afforded a slightly lower S-factor (entry 18 vs. entry 1), and those using ligands L7, L8, L9 and L10 with a biphenylene backbone, afforded S-factors between 3.0–8.7 (entries 19–23). However, the S-factor increased significantly with Trost's ligand L12 though the diastereoselectivity was low (entry 24). Both the S-factor and the diastereoselectivity were improved if allyl phosphate 2f was used, the S-factor being 39 and dr being 30/1 (entry 25).

Fig. 2 The structure of the chiral ligands L1–L12.

With the optimized conditions, the substrate scope was examined (Table 2). It can be seen that all reactions provided the allylated products 3 in high yields with an excellent trans-stereoselectivity, a dr over 30/1 and high yields of the recovered starting materials 1, the S-factor being 8–55. The substituent on the phenyl ring of the 3,4-dihydrocoumarins’ core had a smaller effect on the enantioselectivity and yield for both the recovered starting materials and allylated products (entries 1–2). The reaction also proceeded well for the substrates with a 4-aryl substituent having either an electron-withdrawing or donating group at different positions of the phenyl ring (entries 3–8). It is worthwhile to note that the dihydrocoumarins with aliphatic substituents at the 4-position were also suitable substrates for this kinetic resolution, affording the corresponding allylated products 3i–k, accompanied by recovered 1i–k, in high efficiency, the S-factor being 13–43 (entries 9–11). Only for substrate 1l with a pent-4-en-1-yl group the S-factor decreased to 8 (entry 12). Hence, when R¹ was an alkyl group, the results were uncertain. The S-factor was excellent for the reaction of the dihydrocoumarin with an ethyl group on the 4-position (entry 9), while the S-factor dropped greatly for that with a pent-4-en-1-yl group on the 4-position (entry 12) and comparative S-factors were afforded for the dihydrocoumarins with an isopropyl or cyclohexyl group on the 4-position (entries 10 and 11).

Table 2 Kinetic resolution of 4-substituted-3,4-dihydrocoumarins 1^a

Entry	Recovered substrate			Product				S
	1	Yield^b (%)	ee^c (%)	3	Yield^b (%)	ee^c (%)	dr^d
a Reaction was carried out in THF at −78 °C, molar ratio of 1/2f/[Pd(C3H5)Cl]2/L12/LiHMDS = 100 : 50 : 2.5 : 5 : 120. b Isolated yield. c Determined using chiral HPLC. d Determined using ^1H NMR. e Calculated using the method described by Kagan.^13
1	1a	49	75	3a	46	89	>30/1	39
2	1b	43	76	3b	46	92	>30/1	55
3	1c	45	79	3c	50	87	>30/1	35
4	1d	33	43	3d	46	90	>30/1	29
5	1e	48	76	3e	43	87	>30/1	33
6	1f	42	91	3f	51	76	>30/1	23
7	1g	42	94	3g	48	80	>30/1	31
8	1h	46	90	3h	44	82	>30/1	31
9	1i	51	75	3i	47	90	>30/1	43
10	1j	44	64	3j	44	76	>30/1	14
11	1k	52	67	3k	48	74	>30/1	13
12	1l	44	70	3l	50	60	>30/1	8

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The absolute configuration of 1i was determined to be S by comparing its optical rotation with that reported in the literature.¹⁴ The NOE data of 3i showed its 3,4-trans-configuration. Thus, the absolute configuration of 3i was assigned to be (3R,4R).

In summary, the present work successfully realized the kinetic resolution of lactones reacting as carbon nucleophiles in the Pd-catalyzed AAA, providing optically active mono- and trans-3,4-disubstituted dihydrocoumarin derivatives in high yields with high ee values, the S-factor being 8–55. It also provided a new approach for applying the Pd-catalyzed AAA in organic synthesis. Studies on the extension of the protocol to other carbon nucleophiles and applications of the aforementioned procedure in organic synthesis are in progress.

Acknowledgements

This work was financially supported by the Major Basic Research Development Program (2010CB833300), the National Natural Science Foundation of China (21272251, 21121062, 21032007), the Chinese Academy of Sciences, the Technology Commission of Shanghai Municipality, and the Croucher Foundation of Hong Kong.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedures and analysis data for new compounds, ¹H, ¹³C NMR and HPLC spectra of compounds 1a–1l, 3a–3l. See DOI: 10.1039/c4qo00178h

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