5-acetyl-2,5-dimethyl-2-hydroxy-tetrahydrofuran-3-one (diacetyl dimer of formula (I)) in the form of one of its stereoisomers or in the form of a mixture consisting of or containing two, three or four of its stereoisomers
The stereoisomers of diacetyl of formula (I) are:
- A racemic mixture of diacetyl dimer of formula (I) (comprising all 4 isomeres (Ia), (Ib), (Ic) and (Id)) which was prepared in accordance with the abovementioned instructions of Diels et al. in Chem. Ber. 1914, 47, 2355-2365and optionally purified according to the procedure described by Birch et al. in J. Chem. Soc. 1957, 412-414 was used in the following examples:
- Diacetyl (5 g) was mixed with water (20 g) and CaCO3 (50 mg) was added. The mixture was stored in a stoppered vessel at 5 °C over night. The solution was acidified with diluted HCl and extracted with tert.-butylmethylether. The organic phases were washed with saturated sodium chloride solution, dried over Na2SO4 and the solvent and remaining diacetyl were evaporated in vacuo to yield 2.5 g diacetyl dimer of formula (I)..
- According to the NMR the main isomer (80 % of all isomers) was the enantiomeric couple (Ia) and (Ib) (denoted *), the minor isomer (20 % of all isomers) the enantiomeric couple; (Ic) and (Id) (denoted **): 1H-NMR (400 MHz, CDCl3): δ = 3.37 (1H*, d, 19.1 Hz, H-4a), 2.94 (1H**, d, 19.1 Hz, H-4a), 2.6 (1H**, d, 19.1 Hz, H-4b), 2.34 (3H*, s, H-7), 2.29 (3H*, s, H-7), 2.25 (1H*, d, 19.0 Hz, H-4b), 1.61 (3H**, s, H-8 or 9), 1.52 (3H*, s, H-8 or 9), 1.51 (3H*, s, H-8 or 9), 1.48 (3H**, s, H-8 or 9) ppm.
13C-NMR (100 MHz, CDCl3): δ = 210.8 (C, C-6), 208.5 (C, C-3), 98.7 (C, C-2), 84.1 (C, C-5), 40.9 (CH2, C-4), 24.6 (CH3, C-7), 24.2 (CH3, C-9), 21.7 (CH3, C-8) ppm.
- In the 13C-NMR spectra only the main isomer, the enantiomeric couple (Ia) and (Ib) was detected.
- A racemic mixture which was prepared in accordance with the instructions of Poje and Perina in Tetrahedron 195, 41, 1985 - 1987 was used in the following examples. The mixture contained the following stereoisomers:
MS (EI, 70 eV): m/z = 215 (6%), 155 (7%), 129 (16%), 111 (6%), 87 (13 %), 85 (14 %), 43 (100 %).
1H-NMR (400 MHz, CDCl3): δ = 4.67 (1H, bs, OH), 3.23 (1H, d, J = 13.6 Hz, H-6), 2.35 (3H, s, H-12), 2.25 (3H, s, H-9), 1.92 (1H, d, J = 13.6 Hz, H-6), 1.46 (3H, s, H-7), 1.40 (3H, s, H-10), 1.35 (3H, s, H-13) ppm.
13C-NMR (100 MHz, CDCl3): δ = 210.42 (C, C-11), 208.05 (C, C-8), 114.49 (C, C6a), 112.50 (C, C-3a), 108.36 (C, C-2), 87.51 (C, C-5), 44.53 (CH2, C-6), 25.10 (CH3, C-9), 24.89 (CH3, C-13), 24.87 (CH3, C-12), 21.28 (CH3, C-10), 20.90 (CH3, C-7) ppm.