propanones from allylbenzenes through propanols
(Rated as: excellent)
Although 1-aryl-2-propanones are commonly prepared from phenols by formylation, modified Knoevenagel condensation with nitroethane and subsequent reduction and hydrolysis, we employed the Claisen rearrangement to introduce the three-carbon unit both for the efficiency and simplicity of operation. Hydration of the double bond was then accomplished in high yield by oxymercuration-reduction after due protection of the phenolic group. Oxidation of the resulting alcohol to the corresponding ketone was unsatisfactory with commonly used reagents. However, it could be successfully carried out using Na2Cr2O7-H2SO4 in DMSO, analogous to that of 2-phenyl ethanol. This procedure thus offers a convenient alternative to the preparation of 1-aryl-2-propanones, the synthetically useful intermediates.
A mixture of o-allyl-p-cresol (13g. 87 mmol), dimethyl sulphate (18.6g 148 mmol) and 10% aq. NaOH (50 ml) was stirred at 100°C for 4h. Usual work up and distillation in vacuo gave 12g (84% of the product bp 82°C 5mm Hg)
3-allyl-4-methoxy-toluene (9.7 g 60 mmol) was added dropwise into a stirred solution of Hg (OAc)2 (18.6 g 60 mmol) in aq. THF (1:1, 120 ml) at room temperature. After 20 min 3 M NaOH (60 ml) was added slowly with cooling followed by a solution of 0.5M NaBH4 in 3 M NaOH (60 ml). Filtration, usual work up and distillation afforded the product 2 as a colourless liquid (8.5 g, 79%), bp 123°C (10 mm Hg), which solidified on standing.
Sodium dichromate (6.6 g, 16.2 mmol) was dissolved by warming in 60g (54.5 ml) of DMSO containing 5.4g (30 mmol) of 1-(2-Methoxy-5-methyl)-phenyl propan-2-ol. The solution was cooled, conc. H2SO4 (4.5 ml) was added dropwise and the mixture heated at 70°C for 45 min. The resulting green solution was cooled, poured into crushed ice and extracted with ether. Work up and distillation yielded 4.67g (87%) of 1-(2-Methoxy-5-methyl)-phenyl propan-2-one, bp 130°C (10 mm Hg)
Same procedure on 2,5-Dimethoxyallylbenzene ../rhodium/chemistry /25.meo.
source:Tet. vol 41 p 107-110 1985