Conclusions
The above route to the enantiomers of (methyl 6-oxo-2,4-hexadienoate)Fe(CO)3 from hexadienal is both shorter (4 steps) and higher yielding [26% (+)-1 and 25% (–)-1] then our previous route4 from glyceraldehyde acetonide [5 steps, 15% (+)-1 and 21% (–)-1].
Experimental Section
General. Spectrograde solvents were used without further purification with the exception of anhydrous methylene chloride, which was purchased from Aldrich. (Methoxycarbonylmethylene)triphenylphosphorane (95%) and 2,4-hexadienal were purchased from Avocado Research Chemicals/Alfa Aesar and Acros Organics, respectively. Column chromatography was performed on silica gel (Silicycle, P60, 40-63 µm).
Melting points were obtained on a Mel-Temp melting point apparatus. All 1H and 13C NMR spectra were recorded on Varian spectrometers at the indicated frequency. Elemental analyses were obtained from Midwest Microlabs, Indianapolis, IN, and high-resolution mass spectra were obtained from the University of Nebraska-Center for Mass Spectrometry.
Methyl (2E,4E,6E)-octa-2,4,6-trienoate (6). To a solution of (methoxycarbonylmethylene)- triphenylphosphorane (37.5 g, 0.112 mol) in dry THF (275 mL) at room temperature was added (2E,4E)-hexa-2,4-dienal (5; 7.00 g, 72.8 mmol). The mixture was stirred at room temperature, under nitrogen, for 36 h, after which the solvent was evaporated and the residue was dissolved in a minimal amount of dichloromethane. Petroleum ether (600 mL) was added to the stirred solution to precipitate triphenylphosphine oxide. The precipitate was separated by filtration and the filtrate was concentrated. The residue was purified by column chromatography (SiO2, hexanes/ethyl acetate 97.5:2.5) to afford the known trienoate 6 (8.55 g, 77 %) as a colorless solid. The NMR spectral data for this compound was consistent with the literature values.13
Methyl (2E,4E,6R,7R)-6,7-dihydroxyocta-2,4-dienoate (7). To a solution of methyl octatrieno- ate 6 (5.00 g, 32.9 mmol) in t-BuOH (165 mL) and H2O (165 mL) at 0 °C was added AD-mix-ß (46.5 g), followed by MeSO2NH2 (3.15 g, 32.9 mmol). The mixture was stirred at 0 °C for 24 h, at which time ethyl acetate (100 mL) was added, and the reaction was quenched with saturated aqueous sodium sulfite (50 mL). The layers were separated and the aqueous layer was extracted several times with ethyl acetate. The organic layers were combined, washed with 2N KOH, followed by brine, dried (MgSO4), and concentrated. The residue was purified by column chromatography (SiO2, hexanes/ethyl acetate 3:2) to afford 7 (4.90 g, 80%) as a colorless gum, which solidified in the freezer; mp 52–55 °C [lit.6a 53–55 °C]. [a]D + 68.5 (c 1.3, EtOH) [lit.6a [a]D + 73.7 (c 1.3, EtOH)]. The NMR spectral data was identical to the literature values.6a
Bis-(R)-MTPA ester of methyl (2E,4E,6R,7R)-6,7-dihydroxyocta-2,4-dienoate (8). To a solution of methyl 6,7-dihydroxyocta-2,4-dienoate 7 (18.6 mg, 0.100 mmol) in CH2Cl2 was
