tert-Butyl-3-methoxy-5,5-dimethyl-7-oxo-heptanoate

Hydroboration of tert-butyl-3-methoxy-5,5-dimethyl-6-heptenoate [1] with either BH₃…THF or BH₃…Me₂S provided the terminal alcohol tert-butyl-3-methoxy-5,5-dimethyl-7-hydroxy-heptanoate, in unoptimized, nonreproducable and disappointingly low 30% yield [2]. The poor yield is presumably due to isolation problems. Typically, this alcohol was not isolated but was directly oxidized as follows without purification.

To a room temperature solution of pyridine (6.2 ml, 77 mmol) and CrO₃ (3.9g, 39 mmol) in CH₂Cl₂ (60 ml), the title compound (1.65g, 6.4 mmol) in CH₂Cl₂ (15 ml) was added. After stirring for 30 minutes, the reaction was poured into ether (400 ml), then filtered through Florisil and concentrated to give, after flash chromatography (4 : 1 pet ether : ether) the title aldehyde as a colorless oil, 1.12g, in 75 percent yield. This aldehyde was unstable and typically used immediately and without purification.

¹H NMR (CDCl₃): δ: 9.87 (t, J = 2.8Hz, 1H), 3.75 (m, 1H), 3.23 (s, 3H), 2.68 (dd, J = 14.7, 4.1Hz, 1H), 2.52 (dd, J = 12.7, 2.8Hz, 1H), 2.25 - 2.21 (m, 2H), 1.78 (dd, J = 12.3, 7.4Hz, 1H), 1.42 (s, 9H), 1.5 - 1.3 (m, 1H), 1.04 (s, 6H).