1,2-Epoxy-2-(4-methylphenyI)propane:
- To a solution of dimethyloxosulphonium methylide [prepared from sodium hydride (1.5g, 50% dispersion), finely powdered trimethyloxosulphonium iodide (6.6g) and dimethyl sulphoxide (30 ml) under a nitrogen atmosphere] is added with stirring, a solution of p-methylacetophenone (3.4g) in dimethyl sulphoxide (10 ml).
- The reaction mixture is left overnight at room temperature and then heated to 50 °C for 1 hour.
- After cooling and adding water (60 ml), the mixture is extracted with ether, the combined ethereal extracts washed with water and then dried over anhydrous sodium sulphate.
- Evaporation of the solvent followed by chromatography over neutral alumina using light petroleum (b.p. 40-60 °C) as eluant, affords the epoxide (3.3g, 90%).
2-(4-methyIphenyI)prop-2-en-l-oI:
- To a solution of diisopropylamine (3g) in ether is added with stirring butyllithium (15 ml, 2.1 m in ether) under a nitrogen atmosphere.
- After 30 minutes, the solution of the foregoing epoxide (3g) in ether (40 ml) is added slowly with stirring and the mixture left overnight at room temperature.
- The mixture is refiuxed for 4 hours and the disappearance of epoxide is monitored by t.l.c.
- The solution, after cooling, is partitioned between ether and water, the organic phase washed successively with water and brine and then dried over anhydrous sodium sulphate.
- Solvent evaporation followed by chromatography over neutral alumina affords 2-(4-methyl-phenyl)prop-2-en-l-ol (2.6g, 85%), pure by t.l.c. analysis; b.p. 105-107 °C/5-7mmHg; i.r. (film) 3380, 1050 (OH), 3070, 1650, 890 (>C=CH₂), and 820 cm⁻¹ (para-substituted benzene).
Conversion into 6-methyI-2-(4-methylphenyI)hepta-l,5-diene:2-(4-methyl-phenyl)prop-2-enyl vinyl ether
- A mixture of the foregoing allylic alcohol (2.5g), ethyl vinyl ether (75 ml) and freshly crystallised mercury(n) acetate (600 mg) is refiuxed continuously for 12 hours on a water bath.
- The reaction mixture is chilled in ice and mixed with 10 per cent aqueous sodium carbonate solution (25g) and stirred well for 30 minutes at °C.
- The organic layer is separated, washed with water and dried over anhydrous potassium carbonate.
- Evaporation of the solvent and chromatography of the residue on neutral alumina (50g) furnished, on elution with light petroleum (b.p. 40-60 °C), the allyl vinyl ether which is further purified by distillation under reduced pressure, b.p. 90-92 °C/5-7mmHg; yield 2.0g (80%); 1630, 1610 (C=C), 1200 (vinyl ether), 900 (>C=H 2 ) and 820 cm⁻¹ (para-substituted benzene).
Claisen-Cope rearrangement of the allyl vinyl ether:
- The ether (1.1g) is heated for 30 minutes at 180-185 °C under nitrogen in a fully immersed half-filled sealed Pyrex glass tube (CAUTION).
- After cooling, the product is distilled under reduced pressure to furnish 4-(4-methylphenyl)pent-4-enal (850 mg, 87%), b.p. 100-102 °C/5-7mmHg; i.r. (film) 2700, 1710 (CHO), 3070, 1650, 890 (>C=CH₂), and 825 cm⁻¹ (para-substituted benzene).
Wittig reaction:
- To the phosphorane [prepared from isopropyltriphenyl-phosphonium iodide (2.6g) in dimethyl sulphoxide (6 ml) and sodium hydride (300 mg, 50% dispersion) under a nitrogen atmosphere], is added the foregoing aldehyde (700 mg) in tetrahydrofuran (1ml).
- The contents are stirred for 2 hours and then left overnight at room temperature and warmed to 50 °C for 2 hours.
- The cooled reaction mixture is poured on to crushed ice and extracted with light petroleum (b.p. 40-60 °C).
- The organic extract is dried over anhydrous sodium sulphate, the solvent removed and the residue chromatographed over neutral alumina (20g) when elution with light petroleum (b.p. 40-60 °C) gives 6-methyl-2-(4-methylphenyl)hepta-1, 5-diene (450 mg, 66%).
- This is further purified by distillation under reduced pressure, b.p. 110-113 °C/5-7mmHg; p.m.r. (CCl₄ , TMS) 𝜹 1.52 (s, 6H, C=C(Me)₂), 2.34 (s, 3H, Ar.Me), 5.05 (t, 1H, — CH=C— ), 5.24 (broad s, 2H,>C=CH₂), and 7.23 (4H, aromatic protons).
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