Diastereoselective SmI₂-Mediated 3-exo-trig Cyclisation of δ-Oxo-Alkylidenemalonates to Cyclopropanols

Abstract

In the presence of samarium diiodide and a proton donor (tert-butanol or phenol), δ-oxo-γ,γ-disubstituted-alkylidenemalonates readily cyclise according to the 3-exo-trig mode to give, depending on the exact nature of the substrate, trans and cis cyclopropanols or lactones derived from the cis isomers. Total stereoselectivity towards the formation of trans cyclopropanols is observed with di-tert-butyl alkylidenemalonates when PhOH is used as the protonating agent.

samarium - radical reactions - cyclopropanol - cyclisations (3-exo-trig) - alkylidenemalonates

References

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Selected spectroscopic data.
Compound 9a: ¹H NMR (200 MHz, CDCl₃): δ = 3.74 (3 H, s), 3.72 (3 H, s), 2.94 (1 H, d, J = 12.0 Hz), 1.29 (3 H, s), 1.21 (3 H, s), 1.15 (1 H, d, J = 12.0 Hz), 0.93 (s, 3 H). IR (CHCl₃): 1734 cm^-1.
Compound 9d: ¹H NMR (400 MHz, CDCl₃): δ = 7.51-7.39 (5 H, m), 3.83 (3 H, s), 3.71 (3 H, s), 2.95 (1 H, d, J = 11.5 Hz), 1.64 (1 H, d, J = 11.5 Hz), 1.43 (3 H, s), 0.92 (3 H, s). IR (CHCl₃): 1734 cm^-1.
Compound 10d: ¹H NMR (250 MHz, CDCl₃): δ = 7.59-7.35 (5 H, m), 3.85 (3 H, s), 3.60 (1 H, s), 2.08 (1 H, s), 1.24 (3 H, s), 0.96 (3 H, s). IR (CHCl₃): 1781, 1739 cm^-1.

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10a

This at least is the case for olefinic carbinyl radicals with k_c values [10b] [c] of 0.8 × 10⁴ s^-1 and 2.3 × 10⁵ s^-1 for cyclisations of, respectively, the parent 3-butenyl and 5-hexenyl radicals at 25 °C. Two radical processes may lead to cyclopropanols and cyclopentanols, namely the 3-exo and 5-exo cyclisations of ketyl radicals onto olefinic bond or, conversely, the 3-exo and 5-exo-cyclisations of carbinyl radicals onto carbonyl groups. Unfortunately, to the best of our knowledge only k_c value of the 5-exo-trig cyclisation of 5-oxo-pentyl radical is known. [10d]

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13

Selected spectroscopic data.
Compound 14a: ¹H NMR (400 MHz, CDCl₃): δ = 2.72 (1 H, d, J = 11.5 Hz), 1.91 (1 H, br s, OH), 1.48 and 1.47 [(9 + 9) H, 2 s], 1.31 (3 H, s), 1.25 (3 H, s), 1.08 (1 H, d, J = 11.5 Hz), 0.97 (3 H, s). IR (CHCl₃): 1721 cm^-1.
Compound 15a: ¹H NMR (400 MHz, CDCl₃): δ = 3.13 (1 H, d, J = 11.0 Hz), 1.45 and 1.44 [(9 + 9) H, 2 s], 1.40 (3 H, s), 1.06 (6 H, br s), 0.71 (1 H, d, J = 11.0 Hz). IR (CHCl₃): 1722 cm^-1.
Compound 14b: ¹H NMR (400 MHz, CDCl₃): δ = 2.78 (1 H, d, J = 12.0 Hz), 2.73 (1 H, br s, OH), 1.57-1.49 (1 H, sept, J = 8.0 Hz), 1.39 (18 H, br s), 1.18 (3 H, s), 1.11 (1 H, d, J = 12.0 Hz), 0.94 (3 H, s), 0.93 (3 H, d, J = 8.0 Hz), 0.89 (3 H, d, J = 8.0 Hz). IR (CHCl₃): 1721 cm^-1.
Compound 14c: ¹H NMR (250 MHz, CDCl₃): δ = 3.15 (1 H, br s, OH), 3.06 (1 H, d, J = 11.5 Hz), 1.78-1.63 (1 H, m), 1.52 and 1.50 [(9 + 9) H, 2 s], 1.26 (3 H, s), 1.17 (1 H, d, J = 11.5 Hz), 1.13 (3 H, s), 0.66-0.49 (2 H, m), 0.48-0.41 (1 H, m), 0.40-0.32 (1 H, m). IR (CHCl₃): 1723 cm^-1.
Compound 15c: ¹H NMR (400 MHz, CDCl₃): δ = 3.10 (1 H, d, J = 11.0 Hz), 2.32 (1 H, br s, OH), 1.44 and 1.43 [(9 + 9) H, 2 s], 1.31-1.15 (1 H, m), 1.12 (3 H, s), 1.06 (3 H, s), 0.66 (1 H, d, J = 11.0 Hz), 0.55-0.46 (2 H, m), 0.45-0.36 (1 H, m), 0.35-0.26 (1 H, m). IR (CHCl₃): 1722 cm^-1.

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