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

Xiao-Hui Li a, Ping Fang a, Di Chen a and Xue-Long Hou *ab
aState Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, China. E-mail: xlhou@sioc.ac.cn
bShanghai-Hong Kong Joint Laboratory in Chemical Synthesis, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, China

Received 27th June 2014 , Accepted 24th July 2014

First published on 25th July 2014


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.


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 them5a,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.
image file: c4qo00178h-f1.tif
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 2a

image file: c4qo00178h-u1.tif

Entry Base Solvent L 2 1a 3a S
Yieldb (%) eec (%) Yieldb (%) eec (%) drd
a Reaction was carried out in THF at −78 °C, molar ratio of 1a/2/[Pd(C3H5)Cl]2/L/base = 100[thin space (1/6-em)]:[thin space (1/6-em)]50[thin space (1/6-em)]:[thin space (1/6-em)]2.5[thin space (1/6-em)]:[thin space (1/6-em)]5[thin space (1/6-em)]:[thin space (1/6-em)]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 −1f 71 0 7/1 1
5 KOtBu THF L1 2a 24 −5f 38 2 16/1 1.1
6g 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 −43f 55 −34f >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


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).


image file: c4qo00178h-f2.tif
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 1a

image file: c4qo00178h-u2.tif

Entry Recovered substrate Product S
1 Yieldb (%) eec (%) 3 Yieldb (%) eec (%) drd
a Reaction was carried out in THF at −78 °C, molar ratio of 1/2f/[Pd(C3H5)Cl]2/L12/LiHMDS = 100[thin space (1/6-em)]:[thin space (1/6-em)]50[thin space (1/6-em)]:[thin space (1/6-em)]2.5[thin space (1/6-em)]:[thin space (1/6-em)]5[thin space (1/6-em)]:[thin space (1/6-em)]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


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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