Synthesis of delta-Lactones

pyranes

Silyl enol ethers and MeCN (at 60oC) do not undergo C­C coupling reactions with allyl ironcarbene complexes 1 or 2 like lithium enolates and enoxy borates (see path B), but act as bases and formally abstract a proton from the allyl terminus C-6. The main products formed in high yields (typically over 90%) are the corresponding novel 6-methoxy-2H-pyran tricarbonyliron complexes 21[2]. Treatment of 21 with CAN leads to demetalation and cleavage of the methoxy group in one step to give the corresponding alpha, beta-unsaturated delta-lactones 19 under mild conditions. Configurations at stereocenters present are completely retained.


Experimental data

Synthesis of 21 (General Procedure):
A solution of 1.00 g (2.94 mmol) 1 and 1.26 g (2.95 mmol) 1-Trimethylsilyloxy cyclopentene in 20 ml Me3CN was stirred at room temperature for 10 h. All volatile components were evaporated in vacuo and the residue was extracted several times with petroleum ether. The combined organic extracts were dried, concentrated and the residue was purified by column chromatography (ether/petroleum ether 1:10).

Tricarbonyl [(3-6-eta4)-(6-methoxy)-2H-pyran]eisen (0) (21a): R3=H IR (KBr, Film): 3080 , 2990, 2960, 2040, 1985, 1955,1470, 1425, 1380, 1245, 1225, 1165, 1070, 1020, 775 cm-1. - 1H NMR (400 MHz, CDCl3): 3.17 [ddt, 3J (2-H/3-H) = 3J (2-H/3-H) = 2.1 Hz, 3J (4-H/3-H) = 6.1 Hz, 4J (5-H/3-H) = 1.5 Hz, 1 H, 3-H], 3.56 [dd, 2J (2-H/2-H) = 11.3 Hz, 3J (2-H/3-H) = 2.1 Hz, 1 H, 2-H], 3.60 [dd, 2J (2-H/2-H) = 11.3 Hz, 3J (2-H/3-H) = 2.1 Hz, 1 H, 2-H], 3.67 (s, 3 H, OCH3), 4.63 [dd, 3J (4-H/5-H) = 4.0 Hz, 4J (5-H/3-H) = 1.5 Hz, 1 H/5-H], 5.20 [dd, 3J (4-H/5-H) = 4.0 Hz, 3J (3-H/4-H) = 6.1 Hz, 1 H, 4-H]. - 13C NMR (100.5 MHz, CDCl3): 55.4 (C-5), 58.1 (C-O), 60.5 (C-3), 65.9 (C-2), 77.3 (C-4), 140.4 (C-6), 211.8 (Fe=CO). - MS (70 eV), m/z (%): 252 (4) [M+], 224 (20) [M+ - CO], 196 (11) [M+ - 2 CO], 168 (35) [M+ - 3 CO], 138 (20) [{168} - CH2O], 110 (100) [Fe(C4H6)+], 56 (63) [Fe+], 28 (23) [CO+]. - C9H8FeO5 (251.94): ber. C 42.92, H 3.20; gef. C 43.04, H 3.22.


[2] W. Förtsch, F. Hampel and R. Schobert Chem. Ber. 1994, 127, 711.


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