[(±)-23]: Rf = 0.32 (silica, 20% EtOAc in hexanes); IR (film): νmax = 3068, 2938, 2867, 2713, 1714, 1651, 1469, 1429, 1405, 1355, 1233, 1180, 1161, 1135, 880, 712 cm–1; 1H NMR (500 MHz, CDCl3) δ =
9.41 (s, 1 H), 4.84 (s, 1 H), 4.73 (s, 1 H), 2.56–2.46 (m, 3 H), 2.36 (dd, J = 17.9, 10.5 Hz, 1 H), 2.29–2.26 (m, 2 H), 2.11 (s, 3 H), 2.08–2.05 (m, 1 H), 1.84–1.77 (m, 1 H), 1.65–1.62 (m, 2 H), 1.57–1.51 (m, 1 H), 1.13 (ddd, J = 13.3, 5.1, 2.4 Hz, 1 H) ppm; 13C NMR (125 MHz, CDCl3) δ = 207.32, 205.20, 147.50, 107.13, 48.36, 47.40, 36.43, 36.00, 34.38, 31.23, 30.33, 25.51, 20.21 ppm; HRMS (m/z): [M + H]+ calcd for CHO+ 207.1379, found 207.1377.