Trifluoroacetic acid → Benzyl 6-methyl-7-oxo-1-azabicyclo[3,2,0]hept-2-ene-2-carboxylate

Participants

Reaction Steps

A solution of 4-allyl-3-methyl-1-(1-benzyloxycarbonyl-1-triphenylphosphoranylidenemethyl)azetidine-2-one (38) (0.29 g) in ethyl acetate (25 ml) was treated with trifluoroacetic acid (0.4 ml) and then cooled to -78°. Ozone was bubbled through the solution until it just became pale blue in colour. The excess ozone was blown off in a stream of argon and an ethyl acetate solution of triphenylphosphine (0.14 g) was then added. After a further 10 minutes the reaction flask was transferred to an ice bath and saturated aqueous sodium bicarbonate (45 ml) was added with vigorous stirring. This was continued for 30 minutes and the ethyl acetate solution was then separated, washed with brine and dried over sodium sulphate. The solution was concentrated and chromatographed on silica gel 60 (<230 mesh) eluting with ethyl acetate/60°-80° petroleum ether mixtures grading from 3:7 to 6:4. This gave the trans-isomer of benzyl 6-methyl-7-oxo-1-azabicyclo[3,2,0]hept-2-ene- 2-carboxylate (39) (0.08 g, 55%); νmax (CHCl3) 3000, 1780, 1725 and 1615 cm-1 ; δppm ((CD3)2CO) 1.34 (3H, d, J 7 Hz, CH3), 2.85 (2H, dd, J 9 and 2 Hz, C4--H2), 3.24 (1H, dq, J 3 and 7 Hz, C6--H), 3.91 (1H, dt, J 3 and 9 Hz, C5--H), 5.20 (2H, s, benzyl CH2), 6.45 (1H, t, J 2 Hz, C3--H) and 7.2-7.5 (5H, m, phenyl); (M+ at m/e 257.1052 C15H15NO3 requires 257.1051).

Provenance & Evidence