Reaction Steps
In a round bottomed flask, at −78° C., under argon atmosphere, a solution of DIPA (1.6 ml, 9.3 mmol) in dry THF (40 ml) was treated with n-BuLi (2.5 M in n-hexane, 3.4 ml, 8.5 mmol). After 30 min a solution of ethyl 2-(dibenzylamino)-acetate [prepared as described in Example 47, step 1] (2.2 g, 7.8 mmol) in dry THF (40 ml) was added dropwise via cannula. After 15 min, 2-methyl-propanoyl chloride (2.4 mL, 23.3 mmol) was added dropwise at −78° C. and the mixture stirred for 10 min at rt. The reaction was then quenched with H2O, and Et2O was subsequently added. The organic layer was washed with brine, dried over Na2SO4, filtered, and concentrated in vacuo to give a crude product, as an oil. Purification by column chromatography using a Teledyne ISCO apparatus, eluting with Cy:AcOEt (98:2) gave a crude compound (3.2 g) as a mixture of two tautomers (ketone:enol=ca. 65:35), as a colorless oil, which was used without further purification in the following step. Ketone isomer: MS (ESI) m/z: 354 [M−H]+; (ESI) m/z: 352 [M−H]−. 1H NMR (DMSO-d6) δ 0.86 (d, J=6.73 Hz, 3H), 0.91 (d, J=7.01 Hz, 3H), 1.22 (t, J=7.11 Hz, 3H), 2.99 (hept, J=6.90 Hz, 1H), 3.78 (d, J=14.11 Hz, 2H), 3.84 (d, J=14.13 Hz, 2H), 4.10-4.24 (m, 2H), 4.29 (s, 1H), 7.17-7.48 (m, 10H). Enol isomer: MS (ESI) m/z: 354 [M−H]+; (ESI) m/z: 352 [M−H]−. 1H NMR (DMSO-d6) δ 0.48 (s, 3H), 0.50 (s, 3H), 1.43 (t, J=7.09 Hz, 3H), 3.16-3.28 (m, 1H), 3.90 (d, J=12.66 Hz, 2H), 3.96 (d, J=12.66 Hz, 2H), 4.38 (q, J=7.08 Hz, 2H), 7.10-7.46 (m, 10H), 12.32 (s, 1H).
