Letter pubs.acs.org/OrgLett
Cite This: Org. Lett. 2017, 19, 5932-5935
Palladium-Catalyzed Asymmetric Allylic Alkylation of AlkylSubstituted Allyl Reagents with Acyclic Amides Yang-Jie Jiang,† Gao-Peng Zhang,† Jian-Qiang Huang,† Di Chen,† Chang-Hua Ding,*,† and Xue-Long Hou*,†,‡ †
State Key Laboratory of Organometallic Chemistry and ‡Shanghai−Hong Kong Joint Laboratory in Chemical Synthesis, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, China S Supporting Information *
ABSTRACT: A wide range of alkyl-substituted allyl reagents, as well as nonstabilized carbon nucleophiles, was successfully used for the first time in the palladium-catalyzed asymmetric allylic alkylation reaction, affording the corresponding allylic alkylated products in high yields with high enantioselectivities. The usefulness of the protocol has been demonstrated by the enantioselective synthesis of an important chiral building block and enantiomer of Dubiusamine A.
P
use of nonstabilized carbon nucleophiles in the reactions with alkyl-substituted allyl reagents. These limitations have severely hampered the applications of the Pd-catalyzed AAA reaction. Obviously, the use of alkyl substituted allyl reagents in Pdcatalyzed AAA, especially with nonstabilized carbanion, is a great challenge that remains to be addressed. During our studies on Pd-catalyzed AAA reactions, we realized regio- and enantioselectivities in the reaction using aryl-substituted allyl reagents and successfully utilized nonstabilized carbanions as nucleophiles.10 Further studies showed that a wide range of alkyl-substituted allyl reagents could also be used in Pd-catalyzed AAA reaction with nonstabilized carbon nucleophiles. In this Letter, we report our preliminary investigations on the Pd-catalyzed AAA reaction of alkyl-substituted allyl reagents with acyclic aliphatic amides to afford corresponding products in high yields with high enantioselectivity. The utility of the method was demonstrated in a formal catalytic asymmetric synthesis of an enantiomer of Dubiusamine A. Initially, phenylethyl allyl carbonate 2a was used as a reagent to react with three nucleophiles, such as propiophenone, ethyl 2methyl-3-oxobutanoate, and N,N-diphenylpropionamide, using Pd(OAc)2 and (R)-BINAP (L1) as the catalyst with LDA as the base in THF (Scheme 2). Only β-H elimination of 2a was observed for the reaction with propiophenone, while the corresponding product, in 80% yield but with very low er (54:46), was afforded only if ethyl 2-methyl-3-oxobutanoate was the nucleophile. Pleasingly, the reaction of N,N-diphenylpropionamide (1a) and allyl reagent 2a provided allylic alkylation product 3a in 66% yield with 63.5:36.5 er. This preliminary result
alladium-catalyzed asymmetric allylic alkylation (AAA) reaction, one of the most important protocols to form carbon−carbon bond and carbon−heteroatom bond enantioselectively in organic synthesis, has attracted great attention for decades.1 Tremendous efforts have been made to address the challenging issues of the reaction, such as the use of nonstabilized carbon nucleophiles and alkyl-substituted allyl reagents, since its discovery about 40 years ago.2 So far, enormous achievements have been made in many aspects of those challenging issues, including the use of nonstabilized carbon nucleophiles3−6 and regio- and enantioselectivities in the reaction of monosubstituted allyl reagents and 1,3-unsymmetric substituted ones.7,8 However, the allyl reagents have been limited to those with aryl and methyl as substituents in most cases, in addition to the cyclic allyl ones such as cyclohex-2-en-1-yl acetate.1 Only a few examples used alkyl substituted allyl reagents in Pd-catalyzed AAA reaction with the limitation of employing stabilized carbanions and oxygen as nucleophiles (Scheme 1).9 No report has appeared regarding the Scheme 1. Palladium-Catalyzed AAA Reaction of AlkylSubstituted Allyl Reagents
Received: September 19, 2017 Published: October 18, 2017 © 2017 American Chemical Society
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er was acquired (entry 3). The screening of solvents including Et2O and toluene and bases including NaHMDS indicated that dimethoxyethane (DME) and LiHMDS were the best choices, providing 3a in 94% yield with 73:27 er (entry 5, Table 1) (for detail, see Supporting Information (SI)). Then the effect of the chiral ligand on the reaction was investigated. The palladium complexes of several chiral ligands that show high catalytic activity in Pd-catalyzed AAA including SIOCPhox-L2,10a (S)-iPrPHOX-L3,7b and Trost’s ligand (R,R)-DACH-phenyl L44a could not catalyze the reaction since no conversion of 2a was observed (entries 6−8). The utilization of chiral bisphosphine ligands L5L7 with C2-symmetry and different scaffolds did not improve the enantioselectivity (entries 9−11 vs entry 5). Pleasingly, the use of a commercially available ligand L8 containing bis(3,4,5-trimethoxyphenyl)phosphine group led to higher er (83.5:16.5), although the yield decreased to 57% due to lower conversion of the amide 1a (entry 12). Further investigation revealed that when LiCl was added as an additive 99% yield with 93.5:6.5 er of product 3a could be reached (entry 13). The importance of both lithium cation and chloride anion was confirmed by using NaCl, LiBr, or LiI as the additive (entries 14−16 vs entry 13).10f Replacing LiHMDS with NaHMDS as base afforded 3a in 75% yield with low er (63:37), further confirming the importance of lithium cation (entry 17). Finally, allylic alkylation product 3a in >90% yield with 95:5 er was provided by changing the leaving group of allyl substrate 2 to OAc or OPO(OEt)2 (entries 18 and 19 vs entry 13). It was found also that the presence of two phenyl groups was important.10e Only 63% yield with 68:32 er of product was obtained if Nmethyl-N-phenylpropionamide instead of amide 1a was the substrate (for detail, see SI). With the above optimum reaction conditions established, a survey of the substrate scope was conducted (Table 2). In general, the reaction tolerated a variety of alkyl substituents on
Scheme 2. Palladium-Catalyzed AAA Reaction of Allyl Reagent 2a with Different Nucleophiles
prompted us to study the influence of the reaction parameters on the reaction to improve its efficiency (Table 1). Lowering the reaction temperature was beneficial for the yield of the reaction (entries 2−4). When the reaction was conducted at a temperature of −25 °C, 97% yield of product 3a in 66.5:33.5 Table 1. Optimization of Reaction Conditions for the PdCatalyzed Asymmetric Allylic Alkylation of Amide 1a with Allyl Reagents 2aa
entry
L
t (°C)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17d 18e 19f
L1 L1 L1 L1 L1 L2 L3 L4 L5 L6 L7 L8 L8 L8 L8 L8 L8 L8 L8
rt 0 −25 −40 −25 −25 −25 −25 −25 −25 −25 −25 −25 −25 −25 −25 −25 −25 −25
additive
LiCl NaCl LiBr LiI LiCl LiCl
yield (%)b
erc
66 91 97 83 94 nr nr nr 99 92 79 57 99 86 63 27 75 92 99 (95)
63.5:36.5 62:38 66.5:33.5 51.5:48.5 73:27
Table 2. Substrate Scope for the Pd-Catalyzed Asymmetric Allylic Alkylation of Amides 1 with Alkyl-Substituted Allyl Reagents 2a
72:28 69.5:30.5 67:33 83.5:16.5 93.5:6.5 72.5:27.5 88.5:11.5 73:27 63:37 95:5 95:5
a
Molar ratio of 1a/2a/base/[Pd]/ligand/additive = 130/100/130/5/ 5/100; entries 1−4: LDA as base and THF as solvent. bThe yield was determined by 1H NMR using mesitylene as internal standard. The number in parentheses of entry 18 being isolated yield. cDetermined by chiral HPLC. dNaHMDS used as base. eLeaving group of 2a being OAc. fLeaving group of 2a being OPO(OEt)2.
entry
R1 (1)
R2 (2)
yield (3, %)b
erc
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Me (1a) Me (1a) Me (1a) Me (1a) Me (1a) Me (1a) Me (1a)) Me (1a) Me (1a) Et (1b) nPr (1c) Et (1b) Et (1b) Et (1b) OMe (1e)
Ph(CH2)2 (2a) Et (2b) nPr (2c) nPentyl (2d) Bn (2e) iBu (2f) iPr (2g) BnO(CH2)3 (2h) TBSO(CH2)3 (2i) Ph(CH2)2 (2a) Ph(CH2)2 (2a) Et (2b) nPr (2c) iBu (2f) Ph(CH2)2 (2a)
95 (3a) 84 (3b) 92 (3c) 82 (3d) 83 (3e) 89 (3f) 90 (3g) 91 (3h) 88 (3i) 91 (3j) 97 (3k) 74 (3l) 80 (3m) 72 (3n) 79 (3o)
95:5 95.7:4.3 96:4 94.6:5.4 95.5:4.5 96.4:3.6 96:4 96:4 92.8:7.2 94.7:5.3 95:5 92.4:7.6 91.3:8.7 92.9:7.1 94.4:5.6
a Molar ratio of 1/2/LiHMDS/Pd(OAc)2/L8/LiCl = 130/100/130/ 5/5/100. The leaving group of 2 is OBoc for entries 2, 8, and 12−14. b Isolated yield. cDetermined by chiral HPLC.
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of the crude product). The pure major isomer of 7 was isolated in 61% yield. Deiodination of 7 with nBu3SnH/Et3B led to lactone 8 in 94% yield and 93.8:6.2 er. Debenzylation of lactone 8 by hydrogenation furnished alcohol 9 in 99% yield, the enantiomer of an advanced intermediate in the total synthesis of Dubiusamine A.12 Our enantioselective synthesis of alcohol 9 was realized in 56% overall yield for three steps, constituting a substantial improvement over reported synthesis (nine steps, 40%;12a five steps, 33%12b) (Scheme 4). The absolute configuration of the reaction products was determined by conversion of alcohol 5 to its ester 6, the er of which was determined to be 93.7:6.3, thus indicating the optical purity remained unchanged during the transformations of 3d to 6. The absolute configuration of the alcohol 5 was assigned as S by comparing its sign of optical rotation with that reported in the literature.13 Accordingly, the absolute configuration of the allylated product 3d was assigned as S (Scheme 3). In conclusion, the enantioselective reaction of a variety of alkyl-substituted allyl reagents with acyclic amides as nonstabilized carbon nucleophile was accomplished under Pdcatalysis for the first time, affording allylic alkylation products in high yields with high enantioselectivities. The usefulness of the protocol has been demonstrated by conversion of the products to the important chiral building block and enantiomer of Dubiusamine A.
allyl reagents 2, leading to the corresponding allylic alkylation products 3 in high yields with high enantioselectivities (entries 1−9). The length of linear alkyl substituents of the allyl reagents 2 had little effect on enantioselectivity, though the yield varied to some extent when the alkyl chain was changed (entries 1−5). The allyl reagent 2 with a steric substituent such as iso-butyl and iso-propyl also led to high enantioselectivities in 96.4:3.6 er and 96:4 er, respectively (entries 6 and 7). Notably, the reaction of allyl reagent 2 containing a BnO- or TBSO-ether functional group, which can be readily deprotected, provided also the corresponding 3h and 3i in 96:4 er and 92.8:7.2 er, respectively (entries 8 an 9). In terms of amides 1, when R1 group is a linear alkyl substituent, the yield and enantioselectivity remained high (entries 10−14). Interestingly, amide 1f with an α-methoxy group is tolerated, leading to product 3o in 79% yield with 94.4:5.6 er (entry 15). However, no desired product was observed if an amide with α-chloride was used (for detail, see SI). The synthetic utility of the protocol is demonstrated in the preparation of alcohol 5 (Scheme 3) as well as the efficient enantioselective formal total synthesis of an enantiomer of Dubiusamine A (Scheme 4). Scheme 3. Transformation of the Allylated Product 3d and Its Determination of Absolute Configuration
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.7b02927. Screening of reaction conditions, experimental procedures, NMR, and HPLC spectra (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected].
Scheme 4. Enantioselective Formal Synthesis of an Enantiomer of Dubiusamine A
ORCID
Xue-Long Hou: 0000-0003-4396-3184 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Financial support by National Natural Science Foundation of China (NSFC) (21532010, 21372242, 21472214, 21421091), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB20030100), the NSFC and the Research Grants Council of Hong Kong Joint Research Scheme (21361162001), the Technology Commission of Shanghai Municipality, and the Croucher Foundation of Hong Kong is acknowledged. This paper is dedicated to Professor Chi Ming Che of the University of Hong Kong on the occasion of his 60th birthday.
Alcohol 5 was the key intermediate in the synthesis of the pheromone of Neodipirion sertifer, a pest on Scandinavian pine trees.11 The alkylated product 3d was efficiently converted to alcohol 5 after amide reduction with LiAlH4 followed by Pd/Ccatalyzed hydrogenation of the double bond in 83% yield for two steps (Scheme 3). Dubiusamine A is an alkaloid isolated from the crude base of P. dubius.12a The compound 3h underwent an iodolactonization to afford butyrolactone 7 with 5/1 dr (determined by the 1H NMR
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