(m, 1H), 1.91–1.82 (m, 1H), 1.58 (s, 3H), 1.46 (s, 18H), 1.36 (s, 3H), 0.91 (s, 9H), 0.90 (s, 9H), 0.10–0.05 (m, 12H). 13C NMR (125 MHz, CDCl3) δ 171.6, 155.8, 155.7,