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Asymmetric [3+2] Cycloaddition of 2,2#-Diester Aziridines to Synthesize Pyrrolidine Derivatives Yuting Liao, Baixin Zhou, Yong Xia, Xiaohua Liu, Lili Lin, and Xiaoming Feng ACS Catal., Just Accepted Manuscript • Publication Date (Web): 04 May 2017 Downloaded from http://pubs.acs.org on May 4, 2017
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Asymmetric [3+2] Cycloaddition of 2,2′-Diester Aziridines to Synthesize Pyrrolidine Derivatives Yuting Liao, Baixin Zhou, Yong Xia, Xiaohua Liu,* Lili Lin and Xiaoming Feng* Key Laboratory of Green Chemistry & Technology, Ministry of Education, College of Chemistry, Sichuan University, Chengdu 610064 (China). ABSTRACT: A highly diastereo- and enantioselective [3+2] cycloaddition of 2,2'-diester aziridines with 3,4-dihydropyran derivatives and acyclic enol ethers has been established. Various optically active octahydropyrano[2,3-c]pyrrole and 3-methoxypyrrolidine derivatives were generated in moderate to high yields (up to 94%) and good stereoselectivities (>19:1 dr, up to 95.5:4.5 er). The methodology was also applied in the highly diastereoselective synthesis of D-galactal derivatives. The absolute configuration of the octahydropyrano[2,3-c]pyrroles showed that the reactions using 3,4-dihydropyran and 6-alkyl substituted ones as substrates gave reversed diastereoselection in the final cyclization step. KEYWORDS: asymmetric cycloaddition, 2,2'-diester aziridines, 3,4-dihydropyrans, dysprosium, octahydropyrano[2,3c]pyrroles. Asymmetric 1,3-dipolar cycloaddition of aziridines provides a direct access to five-membered nitrogen-containing rings which serve as important structural units in many bioactive compounds.1 The ring-opening reaction of aziridines can generate 1,3-dipoles via either C−N bond cleavage or C−C bond cleavage, depending on the electronic nature of the strained aza-ring and reaction conditions. In the presence of Lewis acids, 2,2'-diester aziridines can undergo C−C bond heterolytic cleavage to generate azomethine ylide intermediates, participating in cycloadditions with various dipolarophiles, such as aldehydes, imines, alkenes, alkynes, 2,3-disubstituted indoles, donor-acceptor cyclopropanes, isocyanides and heterocumulenes to generate the corresponding heterocycles.2 Nevertheless, most of these reactions are reported in racemic version, and it is desirable to find proper chiral Lewis acid catalyst systems for highly enantioselective [3+2] cycloadditions of 2,2'-diester aziridines. Chiral pyrrolidines and tetrahydropyrans are important structural units for their massive appearance in bioactive molecules, such as octahydropyrano[2,3-c]pyrrole derivatives.3 In 2004, Johnson and coworkers attempted to synthesize these compounds through the reaction of aziridines with 3,4-dihydropyran in the presence of stoichiometric amount of zinc chloride. However, an unexpected [4+2] cycloaddition was observed when N-phenyl substituted 2,2'-diester aziridines were used (Scheme 1a).2a Subsequently, the Zhang group utilized N-tosyl substituted 2,2'-diester aziridines instead, and accomplished the desired [3+2] cycloaddition to obtain the pyrrolidine derivatives (Scheme 1b). Nevertheless, the substrate scope was limited to 3,4-dihydropyran as the only one cyclic dipolarophile, and the asymmetric example using a chiral Y(OTf)3/Pybox catalyst gave the product in 79.5:20.5 er.2b In light of the excellent performance of chiral N,N'-dioxide/metal salt catalyst in asymmetric reactions,4 we attempted to utilize this kind of privileged catalyst to realize the asymmetric [3+2] cycloaddition of 2,2'-diester aziridines with enol ethers. In comparison with the
Scheme 1. Asymmetric cycloadditions between 3,4-dihydropyrans and 2,2'-diester aziridines asymmetric [3+2] cycloaddition to synthesize 1,3- oxazolidines that we have recently achieved,5 the asymmetric reaction between 2,2'-diester aziridines and enol ethers seems more challenging due to the construction of three consecutive stereocenters. We found that the optimized Dy(OTf)3/L1/LiNTf2 catalyst system enabled an efficient asymmetric [3+2] cycloaddition. A wide range of substituted 3,4-dihydropyrans and other acyclic enol ethers, as well as aryl or alkyl substituted aziridines were tolerated in this reaction, providing the desired octahydropyrano[2,3-c]pyrroles and 3-methoxypyrrolidines in high diastereo- and enantioselectivities. Moreover, the chiral
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catalyst was also applied in the synthesis of highly enantioenriched D-galactal derivatives. It was noteworthy that the reaction of 2,2'-diester aziridines with 3,4-dihydropyran and 6-substituted 3,4-dihydropyrans showed reversed diastereoselection in the final ring-closure step.
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yield was obtained when 2-nitrobenzenesulfonyl substituted 1g was used, albeit the er value increased slightly (entry 7). Replacing the N-protecting group from sulfonyl to diphenylphosphoryl group eroded the result (entry 8). The absolute configuration of N-Ts substituted product 3da was determined to be (4aR,5R,7aS) by X-ray crystallographic analysis.8 It was noteworthy that only one diastereomer was detected in these cases. Table 2. Substrate scope for cyclic enol ethersa
The optimization of the reaction conditions was carried out with racemic 2,2'-diester aziridine 1a and 3,4-dihydropyran 2a as the model substrates (eq. 1; see SI in details). Initial studies revealed that the chiral catalytic system of Nd(OTf)3/L2/LiNTf2, which has been successfully applied in the asymmetric synthesis of 1,3-oxazolidines,5 was able to promote this reaction in CH2Cl2, giving the desired octahydropyrano[2,3-c]pyrrole 3aa in 88% yield, >19:1 dr and 87:13 er. After the exploration of the reaction parameters, such as a series of lanthanide metal salts,6 the subunits of chiral N,N'-dioxides, solvents, and additives, etc., we established the optimized reaction condition as follows: Dy(OTf)3/L1/LiNTf2 (1:1:3, 5 mol %) as the chiral catalyst; Cl3CMe containing 1.25% 1,4-dioxane as the reaction solvent;7 3 Å MS as the additive and 35 oC as the temperature. Under this optimal reaction condition, the product 3aa was given in 94% yield, >19:1 dr and 93:7 er. Table 1. Investigation on the diester moiety and Nsubstituents of 2,2′-diester aziridinesa
entry
2
yield (%)b
erc
1 2 3 4 5 6 7 8
2b 2c 2d 2e 2f 2g 2h 2i
9
2j
10e 11e
2k 2l
55 (3ab) 75 (3ac) 78 (3ad) 77 (3ae) 74 (3af) 70 (3ag) 77 (73)d (3ah) 75 (3ai) 52 (3aj) 30 (3aj′) 44 (3ak) 71 (3al)
93:7 95:5 95:5 95:5 95:5 94.5:5.5 94:6 (93.5:6.5)d 95:5 92.5:7.5 71.5:28.5 72:28 82:18
Unless otherwise noted, the reactions were performed with Dy(OTf)3/L1/LiNTf2 (5 mol %, 1:1:3), 3 Å MS (100 mg), aziridine 1a (0.10 mmol) and 3,4-dihydropyran 2 (1.5 equiv) in solvent (1.25% 1,4-dioxane in 1.5 mL 1,1,1-trichloroethane) under nitrogen at 35 oC for 17 h. b Isolated yield by silica gel chromatography. c The dr value was >19:1 determined by 1H NMR spectroscopy and ee values were determined by chiral HPLC analysis. d Aziridine (3.0 mmol) was used. e 3,4-Dihydropyrans 2 (2.0 equiv) was used. a
entry
R1
R2
1
yield (%)b
erc
1 2 3 4 5 6 7 8
Et Me iPr Et Et Et Et Et
Ms Ms Ms Ts 4-ClC6H4SO2 PhSO2 2-O2NC6H4SO2 Ph2PO
1a 1b 1c 1d 1e 1f 1g 1h
94 (3aa) 75 (3ba) 91 (3ca) 85 (3da) 90 (3ea) 89 (3fa) 77 (3ga) 44 (3ha)
93:7 91.5:8.5 83.5:16. 91.5:8.5 92:8 92.5:7.5 94:6 86:14
The reactions were performed with Dy(OTf)3/L1/LiNTf2 (5 mol %, 1:1:3), 3 Å MS (100 mg), aziridine 1 (0.10 mmol) and 3,4-dihydropyran 2a (2.0 equiv) in solvent (1.25% 1,4-dioxane in 1.5 mL 1,1,1-trichloroethane) under nitrogen at 35 oC for 17 h. b Isolated yield by silica gel chromatography. c The dr value was >19:1 determined by 1H NMR spectroscopy and ee values were determined by chiral HPLC analysis. a
Next, the steric and electronic nature of diester moiety and N-substituents on the 2,2'-diester aziridine 1 were tested (Table 1). The reaction of aziridine tethering diethyl esters (1a) worked better than methyl (1b) and isopropyl (1c) substituted ones (entry 1 vs entries 2 and 3). The yield of the reaction was more sensitively affected by the steric hindrance rather than the electronic nature of the substituents on the N-sulfonyl group of 1 (entries 4−7), and 77%
Subsequently, other challenging substituted 3,4-dihydropyrans were investigated. Compared with 3,4-dihydropyran 2a, 6-substituted 3,4-dihydropyrans were less reactive. This might be due to the generation of one chiral quaternary carbon center in the ring-closure step. As shown in Table 2, excellent diastereoselectivities (>19:1 dr) were attained when using 6-alkyl substituted 3,4-dihydropyrans as substrates. The reaction of 6-methyl-3,4-dihydropyran 2b afforded the product 3ab in moderate yield (55%) and 93:7 er (entry 1). The absolute configuration of the product 3ab was determined to be (4aR,5R,7aR).9 It is interesting that the diastereoselectivity for 6-methyl dihydropyran 2b is opposite to that for dihydropyran 2a.10 Notably, 3,4-dihydropyran 2c and 2d which contained an elongating carbon chain such as n-hexyl and n-decyl group were suitable substrates, delivering the corresponding product 3ac and 3ad in 75−78% yield and 95:5 er (entries 2 and 3). Moreover, the installation of various functional groups into the terminal position of the carbon chain in the 6-alkyl-3,4-dihydropyrans, such as iPr-, TBSO-, vinyl and phenyl had no impact on the outcomes
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(70−77% yield, 94:6-95:5 er for 3ae−3ai) (entries 4−8). The preparation of the product 3ah was carried out in a gram scale with the result maintained (entry 7). When benzyloxyl group was introduced at the allylic position, the cycloadduct 3aj was generated in 52% yield and 92.5:7.5 er. To our surprise, this reaction was accompanied with an unexpected generation of a Mannich-type intermediate 3aj' in 30% yield and 71.5:28.5 er (entry 9).11 Subsequently, other enol ether derivatives were tested for the reaction. 1H-Isochromene 2k,12 a useful synthetic precursor of isochromans and five-membered ring enol ether 2,3-dihydrofuran 2l were tolerated under the reaction condition (entries 10 and 11).
Scheme 2. Substrate scope for acyclic enol ethers As for acyclic enol ethers, only one asymmetric example on 2-methoxy-1-propene has been reported so far, and 78.5:21.5 er represented the best result.2b Using the current catalyst system, the reaction of 2-methoxy-1-propene 2m with aziridine 1a performed well, offering the desired product 3am in 61% yield, >19:1 dr and 85:15 er (Scheme 2a). In addition, the utilization of Tb(OTf)3/L3/LiNTf2 catalyst system could promote the asymmetric [3+2] cycloaddition of (1-methoxyvinyl)benzene 2n with aziridine 1a smoothly, giving 2,4-diphenylpyrrolidine derivative 3an in 61% yield, >19:1 dr and 90.5:9.5 er (Scheme 2b). Table 3. Substrate scope for D-galactal derivativesa
a Unless otherwise noted, the reactions were performed with Dy(OTf)3/N,N′-dioxide/LiNTf2 (5 mol %, 1:1:3), 3 Å MS (100 mg), aziridine 1a (0.10 mmol) and 3,4-dihydropyran 2 (2.0 equiv) in solvent (1.25% 1,4-dioxane in 1.5 mL 1,1,1-trichloroethane) under nitrogen at 35 oC for 60 h. b The reaction was performed with 5 mol %
Y(OTf)3, 4 Å MS (37.5 mg), 1a (0.10 mmol) and 2o (2.0 equiv) in CH2Cl2 (1.0 mL) at rt.
Moreover, some interesting compounds based on 3,4dihydropyran cores were examined (Table 3). D-Galactal derived 2o,13 an easily accessible and bioactive starting material was tested under the standard condition, very low yield and dr value were obtained. Zhang’s racemic method led to a moderate result of 43% yield and 4:1 dr.2b We supposed that there might be a diastereo-discrimination between the chiral substrate and chiral catalyst. By utilizing the enantiomer of L1 as the ligand, a satisfying result of 66% yield and 14:1 dr was achieved. Under the same condition, TIPS-substituted D-galactal 2p could be smoothly transformed into the corresponding cycloadduct 3ap in 63% yield and >19:1 dr. The absolute configuration of 3ap was determined to be (2R,3R,4R,4aS,5S,7aR) by X-ray crystallographic analysis.14 Table 4. Substrate scope for 3-substituted 2,2'diester aziridinesa
entry
R1
R2
1
yield (%)b
erc
1 4-ClC6H4 Ms 1i 64 (3id) 95:5 2 4-BrC6H4 Ms 1j 67 (3jd) 95.5:4.5 3 4-FC6H4 Ms 1k 70 (3kd) 95:5 4 4-NCC6H4 Ms 1l 50 (3ld) 91.5:8.5 Ms 1m 56 (3md) 93:7 5 4-F3CC6H4 6 4-PhC6H4 Ms 1n 68 (3nd) 94.5:5.5 7 4-MeC6H4 Ms 1o 58 (3od) 93.5:6.5 8 3-ClC6H4 Ms 1p 56 (3pd) 95:5 9 2-ClC6H4 Ms 1q 41 (3qd) 83:17 10d 3-ClC6H4 Ms 1p 70 (3pa) 93:7 npentyl 11d Ts 1r 20 (3ra) 71.5:28.5 a Unless otherwise noted, the reactions were performed with Dy(OTf)3/L1/LiNTf2 (5 mol %, 1:1:3), 3 Å MS (100 mg), aziridine 1 (0.10 mmol) and 3,4-dihydropyran 2d (1.5 equiv) in solvent (1.25% 1,4-dioxane in 1.5 mL 1,1,1-trichloroethane) under nitrogen at 35 oC for 17 h. b Isolated yield by silica gel chromatography. c The dr value was >19:1 determined by 1H NMR spectroscopy and ee values were determined by chiral HPLC analysis. d 3,4-Dihydropyran 2a (2.0 equiv) was used.
Then the scope of racemic 3-substituted 2,2'-diesters aziridines was examined for the asymmetric [3+2] cycloaddition with enol ether 2d. As shown in Table 4, varying the electronic nature of substituents at the para-position of 3-aryl moiety had little influence on the results, providing the corresponding products (3id−3od) in 5070% yield, >19:1 dr, 91.5:8.5-95.5:4.5 er (entries 1−7). Ortho-chloro-substituted aziridine 1q underwent the reaction in lower yield and enantioselectivity (3qd; 41% yield, >19:1 dr and 83:17 er) (entry 9). Comparatively, the asymmetric [3+2] annulation of 3,4-dihydropyran 2a with aziridine 1p could afford the cycloadduct 3pa in 70% yield, >19:1 dr, 93:7 er (entry 10). It was noteworthy that 3-aliphatic substituted aziridine 1r was also compatible under the reaction condition, although lower yield and moderate enantioselectivity were obtained (entry 11).
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ee of 3ad
100
(>19:1 dr, up to 95.5:4.5 er) using Dy(OTf)3/N,N'-dioxides/LiNTf2 catalyst system. Both the substrate scope and the enantioselectivity were greatly improved for this asymmetric [3+2] cycloaddition. It was also found the steric hindrance of the dipolarophiles would affect the reaction pathway and diastereoselection. The catalyst system was also applied in the highly diastereoselective synthesis of D-galactal derivatives. Further investigations on the other type of asymmetric cycloadditions of 2,2'-diester aziridines are underway.
ee of 1a
83
80 ee (%)
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60 40 20 0
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ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website.
0.5 1.0 1.5 2.0 2.5 3.0 Substrate Ratio (1a/2d)
Figure 1. Enantioselectivity information for this reaction The enantiomeric excess of the aziridine 1a and the product 3ad were detected (Figure 1). Varying the ratio of the two reactants have no influence on the enantioselectivity of the product 3ad, and the unreacted aziridine was recovered in very low ee value. This indicates the cycloaddition of aziridine occurs through the formation of an azomethine ylide intermediate via C−C bond cleavage.5 Based on aforementioned results and our previous study on the asymmetric cycloaddition of 2,2'-diester aziridines with aldehydes,5 we proposed a plausible catalytic cycle (Scheme 3). Firstly, with the assistance of LiNTf2, the carbon-carbon bond of 2,2'-diester aziridines cleaves to form the dipole intermediate. It is caught by the chiral Dy(III)/L1 complex due to the strong bidentate coordination of the two ester groups to the metal center. A concert [3+2] cycloaddition with 6-substituted 3,4-dihydropyrans occurs enantioselectively, preferably giving the (4aR,5R,7aR)-octahydropyrano[2,3-c]pyrrole. However, if 3,4-dihydropyran participated in the reaction, it would follow an anomeric epimerization to the thermodynamic trans-products (path I).10 Nevertheless, a different stepwise pathway could not be excluded (path II).
Experimental procedures, full spectroscopic data for all new compounds, and copies of 1H, 13C NMR, and HPLC spectra (PDF) X-ray crystallographic data for 3da (CIF) X-ray crystallographic data for 3ab (CIF) X-ray crystallographic data for 3ap (CIF)
AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected] [email protected] ORCID Yuting Liao: 0000-0002-2181-8501 Baixin Zhou: 0000-0002-3241-1440 Yong Xia: 0000-0002-9091-5372 Xiaohua Liu: 0000-0001-9555-0555 Lili Lin: 0000-0001-8723-6793 Xiaoming Feng: 0000-0003-4507-0478
Notes The authors declare no competing financial interest.
ACKNOWLEDGMENT We appreciate the National Natural Science Foundation of China (Nos. 21290182 and 21572136) for financial support.
REFERENCES
SCHEME 3. A plausible catalytic cycle In summary, we have successfully realized the asymmetric [3+2] cycloaddition of 2,2'-diester aziridines with a wide range of substituted 3,4-dihydropyrans and other acyclic enol ethers. Various octahydropyrano[2,3-c]pyrroles and 3-methoxypyrrolidines were attained in moderate to high yields (up to 94%) and good stereoselectivities
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C.; You, S.-L. Chin. J. Chem. 2014, 32, 709-714; (i) Ghosh, A.; Pandey, A. K.; Banerjee, P. J. Org. Chem. 2015, 80, 7235-7242; (j) Soeta, T.; Miyamoto, Y.; Fujinami, S.; Ukaji, Y. Tetrahedron 2014, 70, 6623-6629; (k) Craig, R. A., II; O’Connor, N. R.; Goldberg, A. F. G.; Stoltz, B. M. Chem. - Eur. J. 2014, 20, 4806-4813; (l) Wang, B.; Liang, M.; Tang, J.; Deng, Y.; Zhao, J.; Sun, H.; Tung, C.-H.; Jia, J.; Xu, Z. Org. Lett. 2016, 18, 4614-4617. (3) For selected reports on the synthesis and bioactivity study of octahydropyrano[2,3-c]pyrroles: (a) Vaněčková, N.; Hošt′álková, A.; Šafratová, M.; Kuneš, J.; Hulcová, D.; Hrabinová, M.; Doskočil, I.; Štěpánková, Š.; Opletal, L.; Nováková, L.; Jun, D.; Chlebek, J.; Cahlíková, L. RSC Adv. 2016, 6, 80114-80120; (b) Ma, D.; Zhao, C.; Li, H.; Qi, J.; Zhang, L.; Xu, S.; Xie, X.; She, X. Chem. - Asian J. 2013, 8, 364-368; (c) Anthony, G. S. J.; Takao, K.; Dhirubhai, M. A.; Christopher, R. P.; Grant, W. Int. Pat. Appl. WO 2009/037220, 2009; US 20100210680, 2010. (4) For reviews on chiral N,N′-dioxides: (a) Liu, X. H.; Lin, L. L.; Feng, X. M. Acc. Chem. Res. 2011, 44, 574-587; (b) Liu, X. H.; Lin, L. L.; Feng, X. M. Org. Chem. Front. 2014, 1, 298-302; For recent examples: (c) Yao, L.; Zhu, Q.; Wei, L.; Wang, Z.-F.; Wang, C.-J. Angew. Chem., Int. Ed. 2016, 55, 5829-5833; (d) Yao, Q.; Liao, Y. T.; Lin, L. L.; Lin, X. B.; Ji, J.; Liu, X. H.; Feng, X. M. Angew. Chem., Int. Ed. 2016, 55, 1859-1863; (e) Lian, X. J.; Lin, L. L.; Fu, K.; Ma, B. W.; Liu, X. H.; Feng, X. M. Chem. Sci. 2017, 8, 1238-1242. (5) Liao, Y. T.; Liu, X. H.; Zhang, Y.; Xu, Y. L.; Xia, Y.; Lin, L. L.; Feng, X. M. Chem. Sci. 2016, 7, 3775-3779. (6) For recent reviews on the application of lanthanide metal salts in asymmetric catalysis: (a) Chen, W.; Yang, D. Chin. J. Org. Chem. 2016, 36, 2075-2090; (b) Pellissier, H. Coord. Chem. Rev. 2017, 336, 96-151. (7) 1.25% = n(1,4-dioxane) : n(Cl3CMe). See the supporting information on the screening of this ratio in detail. (8) CCDC 1529324 ((4aR,5R,7aS)-3da). (9) CCDC 1534134 ((4aR,5R,7aR)-3ab). (10) No anomeric epimer was observed during the reaction. For similar report on the anomeric epimerization of fused bicyclic
compounds: Gordon, H. L.; Freeman, S.; Hudlicky, T. Synlett 2005, 19, 2911-2914. (11) For reports on the Mannich reactions of 3,4-dihydropyrans: (a) Cakir, S. P.; Mead, K. T. Synthesis 2008, 871-874; (b) Li, G. L.; Kaplan, M. J.; Wojtas, L.; Antilla, J. C. Org. Lett. 2010, 12, 1960-1963. (12) For recent examples: (a) Wenderski, T. A.; Marsini, M. A.; Pettus, T. R. R. Org. Lett. 2011, 13, 118-121; (b) Dörrich, S.; Bauer, J. B.; Lorenzen, S.; Mahler, C.; Schweeberg, S.; Burschka, C.; Baus, J. A.; Tacke, R.; Kraft, P. Chem. - Eur. J. 2013, 19, 11396-11408; (c) Kawai, J.; Chikkade, P. K.; Shimizu, Y.; Kanai, M. Angew. Chem., Int. Ed. 2013, 52, 7177-7180; (d) Gharpure, S. J.; Shelke, Y. G.; Reddy, S. R. B. RSC Adv. 2014, 4, 4696246965; (e) Wang, X.; Dong, S.; Yao, Z.; Feng, L.; Daka, P.; Wang, H.; Xu, Z. Org. Lett. 2014, 16, 22-25; (f) Haidzinskaya, T.; Kerchner, H. A.; Liu, J.; Watson, M. P. Org. Lett. 2015, 17, 38573859; (g) Parhj, B.; Gurjar, J.; Pramanik, S.; Midya, A.; Ghorai, P. J. Org. Chem. 2016, 81, 4654-4663; (h) Li, W.; Wiesenfeldt, M. P.; Glorius, F. J. Am. Chem. Soc. 2017, 139, 2585-2588. (13) For selected examples on the transformations of D-Galactal derivatives: (a) Balmond, E. I.; Coe, D. M.; Galan, M. C.; McGarrigle, E. M. Angew. Chem., Int. Ed. 2012, 51, 9152-9155; (b) Das, S.; Pekel, D.; Neudörfl, J.-M.; Berkessel, A. Angew. Chem., Int. Ed. 2015, 54, 12479-12483; (c) Wang, B.; Xiong, D.C.; Ye, X.-S. Org. Lett. 2015, 17, 5698-5701; (d) Yu, Y.; Xiong, D.-C.; Ye, X.-S. Org. Biomol. Chem. 2016, 14, 6403-6406; (e) Thombal, R. S.; Jadhav, V. H. RSC Adv. 2016, 6, 30846-30851; (f) Medina, S.; Harper, M. J.; Balmond, E. I.; Miranda, S.; Crisenza, G. E. M.; Coe, D. M.; McGarrigle, E. M.; Galan, M. C. Org. Lett. 2016, 18, 4222-4225; (g) Mirabella, S.; Cardona, F.; Goti, A. Org. Biomol. Chem. 2016, 14, 5186-5204; (h) Nieto, C. P.; Sau, A.; Williams, R.; Galan, M. C. J. Org. Chem. 2017, 82, 407-414. (i) Sau, A.; Williams, R.; Palo-Nieto, C.; Franconetti, A.; Medina, S.; Galan, M. C. Angew. Chem., Int. Ed. 2017, 56, 36403644. (14) CCDC 1529327 ((2R,3R,4R,4aS,5S,7aR)-3ap).
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