Trifluoroacetic acid → (+)-12-Oxocalanolide A

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A solution containing chromene 4 (344 mg, 1.0 mmol), acetaldehyde diethylacetal (473 mg, 4.0 mmol), trifluoroacetic acid (1.5 mL, 19.4 mmol) and anhydrous pryidine (0.7 mL) was heated at 140° C. under N2. The reaction was monitored by TLC analysis. After 4 hours, the reaction mixture was cooled to room temperature, diluted with ethyl acetate and washed several times with 10% aqueous NaHCO3 and brine. The organic layer was separated and dried over Na2SO4. The solvent was removed in vacuo and the crude product was purified by silica gel column chromatography eluting with ethyl acetate/hexane (2:3). Chromanone 7 (10,11-trans-dihydro-6,6,10,11-tetramethyl-4-propyl-2H,6H,12H-benzo[1,2-b:3,4-b':5,6-b"]-tripyran-2,12-dione) (110 mg, 30% yield) was obtained m.p. 176°~177° C. (Lit.5 130°~132° C.). 1H-NMR5 (CDCl3) δ 1.02 (3H, t, J=7.5 Hz, CH3); 1.21 (3H, d, J=6.8 Hz, CH3); 1.51 (3H, d, J=7.0 Hz, CH3); 1.55 (6H, 2s, 2 CH3); 1.63 (2H, sextet, J=7.0 Hz, CH2); 2.55 (1H, dq, J=6.9 Hz, J=11.0 Hz, H11); 2.88 (2H, t, J=7.6 Hz, CH2); 4.28 (1H, dq, J=6.3 Hz, J=11.0 Hz, H10); 5.60 (1H, d, J=9.9 Hz, H8); 6.04 (1H, s, H3); 6.65 (1H, d, J=11.8 Hz, H7); MS (CI): 369 (100, M+1).

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