Remarkably Chemoselective Reduction of Unmodified Baylis-Hillman Adducts by InCl₃/NaBH₄: Application to the Stereoselective Synthesis of Trisubstituted Alkenones Including Two Alarm Pheromones [1]

Abstract

A novel, convenient and solely stereoselective synthesis of trisubstituted E-alkenones has been achieved by InCl₃/NaBH₄ mediated chemoselective reduction of unmodified Baylis-Hillman adducts derived from vinyl ketones and cycloalkenones for the first time. The efficiency of this methodology in the practical synthesis of (S)-(+)-manicone and (S)-(+)-normanicone, two alarm pheromones of Manica ants, has been demonstrated.

Key words

unmodified Baylis-Hillman adduct - chemoselectivity - stereoselectivity - α,β-unsaturated ketones - insects’ pheromones

1

IICT communication No. 060622. Part 80 in the series ‘Studies on Novel Synthetic Methodologies’.

References and Notes

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1

IICT communication No. 060622. Part 80 in the series ‘Studies on Novel Synthetic Methodologies’.

23

General Procedure for the Reduction of Adducts.
To a stirred solution of InCl₃ (90 mg, 0.4 mmol) and NaBH₄ (168 mg, 4.5 mmol) in dry MeCN (10 mL) was added a solution of Baylis-Hillman adduct 1 (3 mmol) in MeCN (4 mL) at r.t. under nitrogen atmosphere. Stirring was continued and the reaction was monitored by TLC. After completion (3-3.5 h), the reaction mixture was quenched with Et₂O (25 mL). After the mixture settled, the supernatant organic layer was decanted and the residual semi-solid mass (inorganic part) was further extracted with Et₂O. The combined ether extract was washed with brine, dried (Na₂SO₄), and concentrated to obtain the crude product, which was purified by column chromatography over silica gel to furnish the pure E-alkenone 2. The spectral (IR, ¹H NMR and ¹³C NMR and MS) data of some repesentative adducts and E-alkenones are given bellow.
Compound 1d: IR (KBr): 3474, 1707, 1678, 1442, 1032 cm^-1. ¹H NMR (300 MHz, CDCl₃): δ = 7.61 (1 H, dd, J = 8.0, 2.0 Hz), 7.38-7.17 (3 H, m), 6.12 (1 H, s), 5.94 (1 H, d, J = 3.5 Hz), 5.56 (1 H, s), 3.48 (1 H, d, J = 3.5 Hz), 2.75 (2 H, q, J = 7.0 Hz), 1.11 (3 H, t, J = 7.0 Hz). ¹³C NMR (75 MHz, CDCl₃): δ = 203.1, 150.0, 141.8, 128.5, 127.5, 126.8, 125.0, 72.7, 32.0, 8.2. LSIMS (FAB): m/z = 249, 247 [M⁺ + Na].
Compound 1g: syn:anti = 70:30. IR (KBr): 3418, 1708, 1682, 1463, 1379 cm^-1. ¹H NMR (300 MHz, CDCl₃): δ
(syn) = 6.10 (1 H, s), 5.91 (1 H, s), 4.31 (1 H, t, J = 5.5 Hz), 2.78-2.65 (3 H, m), 1.77-1.52 (3 H, m), 1.12 (3 H, t, J = 7.0 Hz), 0.90 (3 H, t, J = 7.0 Hz), 0.82 (3 H, d, J = 7.0 Hz); δ (anti) = 6.08 (1 H, s), 5.87 (1 H, s), 4.05 (1 H, t, J = 7.0 Hz), 2.78-2.65 (1 H, merged with the signals for anti), 2.38 (2 H, q, J = 7.0 Hz), 1.49-1.33 (3 H, m), 1.12 (3 H, t, J = 7.0 Hz), 0.90 (3 H, t, J = 7.0 Hz), 0.78 (3 H, d, J = 7.0 Hz). ¹³C NMR (75 MHz, CDCl₃): δ (syn) = 202.8, 149.8, 124.7, 75.1, 39.4, 32.0, 26.8, 13.0, 12.1, 8.1; δ (anti) = 203.7, 149.0, 125.0, 77.4, 39.8, 32.0, 24.9, 16.3, 11.9, 8.1. LSIMS (FAB): m/z = 193 [M⁺ + Na].
Compound 1h: syn:anti = 60:40, IR (KBr): 3418, 1714, 1675, 1460, 1367 cm^-1. ¹H NMR (300 MHz, CDCl₃): δ
(syn) = 6.12 (1 H, s), 5.97 (1 H, s), 4.32 (1 H, d, J = 5.5 Hz), 2.36 (3 H, s), 2.19 (1 H, d, J = 3.5 Hz), 1.72-1.54 (2 H, m), 1.22-1.16 (1 H, m), 0.99-0.87 (6 H, m); δ (anti) = 6.11 (1 H, s), 5.92 (1 H, s), 4.08 (1 H, d, J = 7.0 Hz), 2.36 (3 H, s), 2.19 (1 H, br s), 1.46-1.32 (2 H, m), 1.18-1.07 (1 H, m), 0.85-0.77 (6 H, m). ¹³C NMR (75 MHz, CDCl₃): δ (syn) = 201.0, 150.0, 126.0, 75.1, 39.2, 30.0, 26.9, 14.0, 12.2; δ (anti) = 201.4, 149.8, 126.5, 77.5, 39.8, 30.0, 25.0, 16.5, 12.0. LSIMS (FAB): m/z = 179 [M⁺ + Na].
Compound 2b: IR (KBr): 1718, 1672, 1462, 1373, 1219 cm^-1. ¹H NMR (300 MHz, CDCl₃): δ = 6.36 (1 H, d, J = 7.0 Hz), 2.70 (1 H, m), 2.68 (2 H, q, J = 7.0 Hz), 1.78 (3 H, s), 1.10-1.02 (9 H, m) Hz). ¹³C NMR (75 MHz, CDCl₃): δ = 202.5, 148.8, 134.7, 30.2, 28.6, 22.4, 11.6, 8.9. LSI-MS (FAB): m/z = 163 [M⁺ + Na].
Compound 2d: IR (KBr): 1718, 1675, 1630, 1468, 1438, 1363 cm^-1. ¹H NMR (300 MHz, CDCl₃): δ = 7.59 (1 H, s), 7.42 (1 H, m), 7.36-7.27 (3 H, m), 2.85 (2 H, q, J = 7.0 Hz), 1.92 (3 H, s), 1.19 (3 H, t, J = 7.0 Hz). ¹³C NMR (75 MHz, CDCl₃): δ = 202.6, 137.8, 135.6, 134.98, 134.92, 130.5, 129.9, 128.2, 126.6, 30.1, 13.8, 8.7. LSIMS (FAB): m/z = 209 [M⁺ + 1], 211.