Remote Stereocontrolled Construction of Vicinal Axially Chiral

Publication Date (Web): January 8, 2019 ... The direct diastereo- and enantioselective 1,8-conjugate additions of thiazolones and azlactones, respecti...
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Letter Cite This: Org. Lett. 2019, 21, 503−507

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Remote Stereocontrolled Construction of Vicinal Axially Chiral Tetrasubstituted Allenes and Heteroatom-Functionalized Quaternary Carbon Stereocenters Pei Zhang,†,§ Qiuhong Huang,†,§ Yuyu Cheng,‡ Rongshi Li,*,† Pengfei Li,*,‡ and Wenjun Li*,† †

Department of Medicinal Chemistry, School of Pharmacy, Qingdao University, Qingdao, 266021, China Department of Chemistry and Shenzhen Grubbs Institute, Southern University of Science and Technology, Shenzhen, 518055, China

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‡

S Supporting Information *

ABSTRACT: The direct diastereo- and enantioselective 1,8conjugate additions of thiazolones and azlactones, respectively, to para-quinone methides generated in situ from propargylic alcohols have been achieved in the presence of chiral phosphoric acids. The remote stereocontrolled activation protocol provides an efficient and facile approach for the construction of vicinal axially chiral tetrasubstituted allenes and heteroatom-functionalized quaternary carbon stereocenters, which expands the synthetic potential of chiral phosphoric acids.

A

Scheme 1. Catalytic Asymmetric Construction of Adjacent Axially Chirality and Central Chirality Framework

xially chiral allenes are surprisingly abundant and featured in hundreds of natural products with biological activity,1 functional materials,2 and useful synthetic building blocks in organic synthesis.3 In particular, tetrasubstituted axially chiral allenes have also been proven to be promising organocatalysts4 and ligands.5 Accordingly, much effort has been devoted to developing efficient methods for the synthesis of axially enantioenriched allenic compounds.6 However, the direct asymmetric transformations of achiral or racemic substrates by a chiral catalyst for the construction of axially chiral tetrasubstituted allenes are still not well-established, although this approach is highly attractive.7−9 Furthermore, efficient access to vicinal axially chiral allenes and central chiral carbon stereocenters remains a remarkable challenge. In 2013, Maruoka and co-workers reported the asymmetric functionalization of cumulenolates under phase-transfer catalysis, providing an efficient access to the construction of vicinal axially chiral tetrasubstituted allenes and nitrogen-containing tertiary carbon stereocenters (Scheme 1A).10 Similarly, Miller et al. developed a peptide-catalyzed alleno-Mannich reaction.11 However, a major limitation exists due to lacking starting material with the pre-established trisubstituted allene motifs. With the aid of chiral disulfonimide, List et al. established an alkynylogous Mukaiyama aldol reaction of silyl alkynyl ketene acetals for the construction of vicinal axially chiral tetrasubstituted allenes and oxygen-containing tertiary carbon stereocenters (Scheme 1B).12 The prepreparation of silyl alkynyl ketene acetals as starting materials is still an important issue. The landmark study reported by Lin and Sun et al. in 2017 presented N-triflylphosphoramides that were efficient catalysts for the synthesis of axially chiral tetrasubstituted allenes from racemic propargylic alcohols (Scheme 1C).13 Notably, diastereo- and enantioselective construction of vicinal axially chiral allene and quaternary carbon stereocenters was © 2019 American Chemical Society

accomplished via chiral ion-pair catalysis. Importantly, the synthetic strategy allows in situ generation of the reactive intermediates, thereby simplifying substrate preparation and expanding functional group compatibility. However, as a nontrivial extension to this elegant work, the catalytic Received: November 28, 2018 Published: January 8, 2019 503

DOI: 10.1021/acs.orglett.8b03801 Org. Lett. 2019, 21, 503−507

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Organic Letters

(entry 1). To obtain the enantioenriched 3aa, the screening of chiral phosphoric acids was carried out (entries 2−5). Pleasingly, an essential improvement was achieved when the sterically more congested CPA-5 was employed and 3aa was obtained in 89% yield with 34% ee and >20:1 dr (entry 5). Notably, the enantioselectivity increased significantly as a result of better steric discrimination when a sterically more congested thiazolone was employed (entries 6−9). Particularly, 78% ee was obtained from the reaction between 1a and 2,5-diphenylthiazol-4(5H)-one 2d (entry 8). After optimizing other reaction parameters, we identified that the desired product 3ad could be obtained in 89% yield with 97% ee and >20:1 dr when the reaction was run in PhCF3 for 48 h in the presence of CPA-5 with a loading of 1 mol % (entry 10; see Supporting Information (SI) for details). Under the standard conditions, we then investigated the scope of thiazolones 2 (Scheme 2). It was found that this

construction of vicinal axially chiral allene and heteroatomfunctionalized quaternary carbon stereocenters remains unknown. Several challenges exist in the design of a catalytic asymmetric construction of vicinal axially chiral allene and heteroatom-functionalized quaternary carbon stereocenters. (1) In view of propargylic alcohol acting as the precursor of the allene motif, a suitable reaction partner is indispensable to construct the heteroatom-functionalized quaternary carbon stereocenter. (2) The selectivity is an important issue in the construction of adjacent axially chirality and central chirality, which requires remarkable remote regio- and stereocontrol. In fact, it is indeed an asymmetric 1,8-conjugate addition, which is still in its infancy.14 (3) The presence of heteroatoms can bring new synthetic and biological values, but the strong coordinating and adsorptive properties of heteroatoms should be solved.15 Based on our previous studies16 in the catalytic asymmetric reaction of para-quinone methides17 and the construction of heteroatom-functionalized quaternary carbon stereocenters,18 we became interested in the construction of vicinal axially chiral tetrasubstituted allenes and heteroatomfunctionalized quaternary carbon stereocenters from propargylic alcohols (Scheme 1D). We began our investigations by using the reaction between propargylic alcohol 1a and thiazolones 219 as a model reaction (Table 1). In the presence of chiral phosphoric acid CPA-1, 1a reacted smoothly with 5-methyl-2-phenylthiazol-4(5H)-one 2a to afford the racemic product 3aa in 68% yield with >20:1 dr

Scheme 2. Scope of Reactions between 1 and 2a,b,c

Table 1. Optimization of Reaction Conditionsa

entry

CPAs

R

solvent

yield (%)b

ee (%)c

1 2 3 4 5 6 7 8 9 10d

CPA-1 CPA-2 CPA-3 CPA-4 CPA-5 CPA-5 CPA-5 CPA-5 CPA-5 CPA-5

Me Me Me Me Me Et i-Pr Ph Bn Ph

CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 PhCF3

3aa, 68 3aa, 83 3aa, 83 3aa, 96 3aa, 89 3ab, 85 3ac, 82 3ad, 85 3ae, 93 3ad, 89

0 0 0 −14 34 35 46 78 66 97

a Unless noted, 1 (0.05 mmol), 2 (0.06 mmol), CPA-5 (1 mol %) in PhCF3 (0.3 mL) at room temperature for 48 h. All diastereoselectivity ratio (dr) values > 20:1, determined by 1H NMR. bIsolated yield. c Determined by HPLC analysis using a chiral stationary phase. d−20 °C, 72 h. ePhCF3 (0.6 mL), 72 h. fdr = 14:1. gdr = 10:1.

strategy was applicable to various thiazolones bearing different types of substituents. Notably, 3ac was obtained in 75% yield with 92% ee and >20:1 dr from the reaction of 5-isopropyl-2phenylthiazol-4(5H)-one 2c with 1a. The reaction of 5-benzyl2-phenylthiazol-4(5H)-one 2e also resulted in the formation of 3ae in 86% yield with 84% ee and >20:1 dr. Importantly, both electron-withdrawing and -donating groups could be introduced into the aromatic ring of 2 to afford the corresponding 3af−al in 76−89% yields with 81−94% ee and >20:1 dr. Both the electronic nature and position of the substituents on the aromatic ring of 2f−l had a small effect on the efficiency and

a Unless noted, 1a (0.05 mmol), 2 (0.06 mmol), CPAs (2 mol %) in the solvent (0.3 mL) at room temperature for 12 h. All diastereoselectivity ratio (dr) values > 20:1, determined by 1H NMR. bIsolated yield. cDetermined by HPLC analysis using a chiral stationary phase. dCPA-5 (1 mol %), 48 h.

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DOI: 10.1021/acs.orglett.8b03801 Org. Lett. 2019, 21, 503−507

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Organic Letters Scheme 4. Scope of Reactions between 1 and 4a,b,c

stereoselectivity. The heteroaromatic 2m was also compatible under standard reaction conditions to afford the desired 3am in 91% yield with 92% ee and >20:1 dr. The absolute configuration of 3ai was unambiguously confirmed by X-ray crystallography (CCDC 1877263). Subsequently, the substrate scope of propargylic alcohols 1 was examined by reactions with 2d. A series of propargylic alcohols 1b−i bearing electronically distinct substituents at different positions of the aromatic ring (R2) participated in the asymmetric 1,8-conjugate addition smoothly to afford 3bd−id in 81−97% yield with 90 → 99% ee and >20:1 dr. In addition, the heteroaromatic 1j also afforded the corresponding 3jd in 81% yield with 87% ee and 14:1 dr. Notably, the group R1 of 1 could be substituted aromatic groups and the corresponding 3kd−ld were obtained in excellent yields with excellent stereoselectivities. In particular, alkyl substituted propargylic alcohol 1m (R1 = Me) led to the formation of 3md in 65% yield with 76% ee and 10:1 dr. Taken altogether, our results demonstrated that catalyst CPA-5 mediated asymmetric 1,8conjugate addition not only furnished the axially chiral tetrasubstituted allene unit but also simultaneously established an adjacent sulfur-containing quaternary carbon stereocenters, which is a significant challenge in asymmetric synthesis.20 Based on these encouraging data, we have a high motivation to extend the scope of nucleophiles to azlactones,21 which are structurally related to thiazolones. Notably, such reaction for the construction of vicinal axial chirality and central chirality has remained unknown. The initial investigation indicated that the desired product 5aa was obtained in 75% yield with 30% ee and >20:1 dr (Scheme 3). The careful screening of CPA and Scheme 3. Reaction between 1a and 4a

a Unless noted, 1 (0.05 mmol), 4 (0.06 mmol), CPA-4 (1 mol %) in CH 2 CH 2 (0.3 mL) at room temperature for 36 h. All diastereoselectivity ratio (dr) values > 20:1, determined by 1H NMR. bIsolated yield. cDetermined by HPLC analysis using a chiral stationary phase. ddr = 15:1.

modifying reaction parameters revealed that CPA-4 enabled the formation of 5aa in 93% yield with 89% ee and >20:1 dr in CH2Cl2 with a catalyst loading of 1 mol % at room temperature for 36 h (see SI for details). With the optimized reaction conditions in hand, we proceeded to establish the substrate scope for the asymmetric 1,8-conjugate additions between para-quinone methide generated in situ from propargylic alcohol 1a and azlactones 4 (Scheme 4). With 1a as the partner, the asymmetric reaction was amenable to a wide scope of different substituted azlactones 4a−i, affording the corresponding products 5aa− ai in 65−94% yields with 82−90% ee and >20:1 dr. The electronic nature and position of substituents on the aromatic ring (R4) did not impose an evident effect on the stereoselectivity of the reaction. The reaction of 4-ethyl-2(naphthalen-3-yl)oxazol-5(4H)-one 4j afforded product 5aj in 76% yield with 50% ee and >20:1 dr. Using 2-(4methoxyphenyl)-4-methyloxazol-5(4H)-one 4k led to the formation of product 5ak in 82% yield with 76% ee and >20:1 dr. Furthermore, the Bn group substituted azlactone 4l (R3 = Bn) reacted smoothly with 1a to furnish the product 5al in 71% yield with 91% ee and >20:1 dr. It should be noted that

aromatic moieties (R2) in propargylic alcohols 1 were welltolerated. Substituents such as halogen atoms, those with different electronic features, as well as different substitution patterns were all applicable to furnish the corresponding products 5bg−ig in good to high yields with high to excellent enantioselectivities and almost >20:1 dr. Moreover, the substituent could be introduced to the aromatic ring of R1 without compromising the yield and the asymmetric induction. The absolute configurations of products were assigned on the basis of X-ray crystallographic analysis of product 5gd (CCDC 1877259), which was obtained from the CPA-4 catalyzed reaction between propargylic alcohol 1g and 2-(4-chlorophenyl)-4-ethyloxazol-5(4H)-one 4d. Particularly, the heteroaromatic propargylic alcohol 1n was found to be compatible under these reaction conditions and was successfully transformed to the desired product 5ng in 91% yield with 88% ee and >20:1 dr. It should be noted that the catalytic asymmetric 1,8-conjugate addition has been successfully extended to a variety of azlactones and provided an efficient and facile access to vicinal chiral tetrasubstituted allenes and nitrogencontaining quaternary carbon stereocenters. 505

DOI: 10.1021/acs.orglett.8b03801 Org. Lett. 2019, 21, 503−507

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Then, we performed further investigations to demonstrate the utility of this synthetic strategy (Scheme 5). The CPA-5

Letter

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.8b03801.

Scheme 5. Further Investigations

Experimental section, characterization details, and X-ray data for 3ai and 5gd (PDF) Accession Codes

CCDC 1877259 and 1877263 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected], fl[email protected]. ORCID

Pengfei Li: 0000-0001-5836-1069 Wenjun Li: 0000-0001-9045-7845 Author Contributions §

P.Z. and Q.H. contributed equally.

Notes

The authors declare no competing financial interest.

mediated asymmetric reaction of 1a at 1.50 mmol with 2d proceeded well under the standard conditions to generate 3ad in 89% yield with 97% ee and >20:1 dr. And the CPA-4 catalyzed reaction of 1a at 1.50 mmol with 4a also worked well to furnish 5aa in 88% yield with 88% ee and >20:1 dr. In addition, the K2CO3 mediated ring-opening reaction of 5aa proceeded very well and the unnatural enantiopure α,αdisubstituted α-amino acid ester featuring axially chiral allene motif 6aa was obtained in 89% yield without compromising the enantioselectivity (Scheme 5B). To gain insight into the reaction mechanism, several control experiments were carried out (Scheme 5C). The propargylic alcohol 1a was transformed to para-quinone methide (p-QM) smoothly under the acidic conditions, and subjecting the fully characterized pure p-QM to the standard conditions led to successful formation of the corresponding products 3ad and 5aa in essentially the same yields and stereoselectivities as the standard protocol from 2d and 4a, respectively. Based on these observations and closely related report,13 the chiral phosphoric acid mediated reaction between propargylic alcohols 1 and nucleophiles (2 or 4) was proposed to proceed via 1,8-conjugate addition (Scheme 5D). In conclusion, we have established a chiral phosphoric acid mediated 1,8-conjugate addition of thiazolones and azlactones, respectively, to p-QMs generated in situ from propargylic alcohols. The reaction mechanism was confirmed by the control experiments. Notably, the synthetic strategy not only allows direct use of racemic propargylic alcohols instead of propargylic derivatives for catalytic asymmetric allene synthesis but also ensures excellent remote stereocontrol to construct vicinal axially chiral tetrasubstituted allenes and heteroatomfunctionalized quaternary carbon stereocenters. Furthermore, this strategy also expands the synthetic potential of chiral phosphoric acids.

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ACKNOWLEDGMENTS The authors acknowledge the financial support from National Natural Science Foundation of China (21502043, 21871128), the Natural Science Foundation of Shandong Province (ZR2017JL011), and the Shenzhen Nobel Prize Scientists Laboratory Project (C17783101).

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DEDICATION Dedicated to Professor Albert Sun Chi Chan. REFERENCES

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DOI: 10.1021/acs.orglett.8b03801 Org. Lett. 2019, 21, 503−507