Ethyl 4-(4-cyclopropyl-2-oxo-2H-pyran-6-yl)butanoate

Catalyst- and Additive-Free Baeyer−Villiger-type Oxidation of α-Iodocyclopentenones to α-Pyrones: Using Air as the Oxidant

Yuanyuan Zhou,  Xianxiao Chen,  Xiangxiang Ling  and  Weidong Rao 

Abstract

An efficient synthetic approach for the synthesis of α-pyrones via Baeyer−Villiger-type oxidation of α-iodocyclopentenones through a catalyst- and additive-free system using air as an environmentally benign oxidant is described. The reaction exhibits excellent functional group compatibility and provides a simple and efficient protocol for the construction of highly functionalized α-pyrones under mild reaction conditions.

Ethyl 4-(4-cyclopropyl-2-oxo-2H-pyran-6-yl)butanoate (2aa) Product 2aa was obtained as yellow oil in 50% yield (38 mg) following the general procedure; 1H NMR (600 MHz, CDCl3) δ 5.84 (s, 1H), 5.61 (s, 1H), 4.13-4.09 (m, 2H), 2.48 (t, J = 7.3 Hz, 2H), 2.33 (td, J = 7.3, 2.3 Hz, 2H), 1.97-1.94 (m, 2H), 1.66-1.63 (m, 1H), 1.26-1.22 (m, 3H), 1.07-1.05 (m, 2H), 0.80-0.79 (m, 2H); 13C NMR (150 MHz, CDCl3) δ 172.8, 163.9, 163.0, 162.8, 106.7, 102.1, 60.5, 33.2, 33.0, 22.1, 15.4, 14.2, 10.0; HRMS (ESI) calcd. for C14H18O4Na [M+Na]+ : 273.1097, found: 273.1101

 
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https://pubs.rsc.org/en/Content/ArticleLanding/2019/GC/C9GC02725D?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+rss%2FGC+%28RSC+-+Green+Chem.+latest+articles%29#!divAbstract

http://www.rsc.org/suppdata/c9/gc/c9gc02725d/c9gc02725d1.pdf

4-(trifluoromethoxy)benzyl trichloroacetimidate:

4-(Trifluoromethoxy)benzyl Trichloroacetimidate

To a stirred suspension of NaH (60%, 127 mg, 3.45 mmol) in t-BuOMe (100 mL) was added 4-(trifluoromethoxy)benzyl alcohol 11 (5 mL, 34.5 mmol) dropwise by syringe at room temperature. After H2 evolution had ceased (about 5 min), the clear mixture was cooled to 0 °C. Cl3CCN (3.46 mL, 34.5 mmol) was added dropwise by syringe, and the resulting solution was stirred for 1 h. After warming to room temperature, the solution was concentrated in vacuo, and the residue was resuspended in heptane (100 mL) containing MeOH (0.14 mL, 3.45 mmol). After 10 min of stirring, the suspension was filtered through a thin bed of silica and washed with heptane. Concentration in vacuo affords the title compound as a yellow-orange liquid (11.4 g, 98%) that was used directly without further purification. Rf 0.3 (5% EtOAc/hexanes);

 1H NMR (501 MHz, CDCl3) δ 8.43 (s, 1H), 7.48 (d, J = 8.2 Hz, 2H), 7.24 (d, J = 8.2 Hz, 2H), 5.35 (s, 2H); 

13C NMR (126 MHz, CDCl3) δ 162.6, 149.3, 134.3, 129.4, 121.6, 121.3, 119.6, 69.9; 

IR [CH2Cl2 solution] νmax (cm−1) 3345, 2954, 1667, 1511, 1261, 1221, 1166, 1077, 998, 828, 797, 649; 

HRMS (ESI-TOF) calcd for C10H7Cl3F3NO2 334.95, found C8H6F3O 174.03.

(R)-3-Chloro-2-hydroxypropyl-4-methoxybenzoate

(R)-3-Chloro-2-hydroxypropyl-4-methoxybenzoate 10

To a stirred solution of (R)-3-chloro-1,2-propanediol (6.61 g, 59.8 mmol) in CH2Cl2 (120 mL) was added imidazole (4.07 g, 59.8 mmol). After the reaction mixture had cooled to 0 °C, p-methoxybenzoyl chloride (10.2 g, 59.8 mmol) in CH2Cl2 (10 mL) was added dropwise via addition funnel. The resulting solution was allowed to warm to room temperature and stirred until complete consumption of starting material by thin layer chromatography (TLC). The mixture was poured into saturated aq NH4Cl (150 mL), and the aqueous layer was extracted with CH2Cl2 (3 × 100 mL). The combined organic extracts were dried (MgSO4), filtered, and concentrated in vacuo to provide chlorohydrin 10as a clear, viscous oil (11.43 g, 78%) that was used without further purification. Rf 0.3 (20% EtOAc/hexanes); ee >99% as determined by chiral SFC (see the Supporting Information); 

1H NMR (500 MHz, CDCl3) δ 8.07−7.94 (m, 2H), 7.00−6.88 (m, 2H), 4.46 (d, J = 5.1 Hz, 2H), 4.22 (dd, J = 10.6, 5.3 Hz, 1H), 3.88 (s, 3H), 3.78−3.64 (m, 2H), 2.73 (d, J = 5.6 Hz, 1H); 

13C NMR (126 MHz, CDCl3) δ 166.7, 163.9, 132.1, 121.9, 114.0, 70.1, 65.7, 55.7, 46.3; 

IR [CH2Cl2 solution] νmax (cm−1) 3454, 2959, 2889, 1713, 1606, 1512, 1259, 1170, 1104, 1028, 848, 770, 697, 614; 

HRMS (ESI-TOF) calcd for C11H13ClO4 (M)+244.0573, found 244.0501.

https://pubs.acs.org/doi/full/10.1021/jo1015807
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https://pubs.acs.org/doi/suppl/10.1021/jo1015807/suppl_file/jo1015807_si_001.pdf

Pretomanid

Pretomanid

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The combined organic extracts were washed with brine, dried (Na2SO4), filtered, and concentrated. Chromatography (75% EtOAc/hexanes) followed by recrystallization (i-PrOH/hexanes) affords PA-824 (1) (2.41 g, 62%) as a crystalline solid. Mp 150−151 °C (lit.(11a) mp 149−150); Rf 0.2 (75% EtOAc/hexanes); ee >99.9% as determined by chiral SFC (see the Supporting Information);

 1H NMR (500 MHz, d6-DMSO) δ 8.09 (s, 1H), 7.48 (d, J = 8.6 Hz, 2H), 7.39 (d, J = 8.2 Hz, 2H), 4.81−4.62 (m, 3H), 4.51 (d, J = 11.9 Hz, 1H), 4.39−4.19 (m, 3H);

 13C NMR (126 MHz, d6-DMSO) δ 148.7, 148.1, 143.0, 138.3, 130.4, 122.0, 120.0, 119.8, 69.7, 68.8, 67.51, 47.73;

IR [CH2Cl2 solution] νmax (cm−1) 2877, 1580, 1543, 1509, 1475, 1404, 1380, 1342, 1281, 1221, 1162, 1116, 1053, 991, 904, 831, 740;

HRMS (ESI-TOF) calcd for C14H12F3N3O5 359.0729, found 359.0728.

https://pubs.acs.org/doi/full/10.1021/jo1015807