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Synthesis of 4-Iodo-2,5-Dimethoxybenzaldehyde

And its corresponding phenyl-2-nitropropene and phenylacetone

T Sargent III, AT Shulgin, CA Mathis
J. Med. Chem. 27, 1071-1077 (1984)

ASCII by GC_MS, HTML by Rhodium

Experimental

2,5-diiodo-1,4-dimethoxybenzene (2a)

To a solution of 6.9 g p-dimethoxybenzene (50 mmol) in 50 mL acetic acid there was added over the course of 3 min a solution of 20 g of ICl in 20 mL of acetic acid. The mixture was heated on a steam bath for 2 h and then cooled with external ice-water, resulting in the formation of a heavy steel-gray crystalline mass. This was removed by filtration, washed sparingly with cold acetic acid, and suspended in 200 mL of water. With good stirring, there was added small portions of Na2S2O4 until the color of the suspended solids had changed from gray to white. This product was removed by filtration, washed with water, and when reasonably dry, recrystallized from 50 mL of boiling acetonitrile. There was thus obtained 7.1 g (36% yield) of a white product, mp 161-165C. An analytical sample that was recrystallized again from acetonitrile had mp 167-168C (lit mp 171C).

4-iodo-2,5-dimethoxybenzaldehyde (3a)

To a stirred suspension of 19.6 g (50 mmol) of 2a in 500 mL of anhydrous ether (cooled externally with ice-water) there was added 34 mL of 1.6 N BuLi in hexane (54 mmol). After 10 min of stirring there was added 8.2 mL of N-methylformanilide, converting the loose white precipitate to a thick suspension. This was stirred for 10 min, then held at reflux for about 10 min, and then added to 800 mL of water acidified with HCl. Yellow solids were removed by filtration and washed with ether and acetonitrile (see below). The resulting clear mother liquors were separated, and the organic fraction was stripped in vacuo to yield, after air-drying, 17 g of a pale-yellow solid. This product was triturated under 20 mL of MeOH, filtered free of solvent, air-dried (7.5 g), and finally recrystallized from 60 mL of boiling 95% EtOH to yield 5.2 g of pale-yellow crystals of 3a: mp 136-137C.

The yellow insolubles obtained above (4.8 g) were ground under CH3CN, filtered free of solvent, and recrystallized from 35 mL of boiling CH3CN to yield 0.6 g of the yellow bisaldehyde 3c, mp 196-200C (lit mp 207C).

Attempts to iodinate 2,5-dimethoxybenzaldehyde in the 4-position with ICl employing the procedure in ref 13 were unsuccessful.

1-(4-iodo-2,5-dimethoxyphenyl)-2-nitropropene (4)

To a solution of 4.8 g of 3a (16.4 mmol) in 100 mL of nitroethane was added 0.3 g of ammonium acetate, and the mixture was heated on a steam bath for 6 h. The orange-red solution was decanted from a small amount of insolubles and stripped of nitroethane in vacuo, and the residue was treated with 20 mL of boiling MeOH. Crystals of 4 formed spontaneously, removed by filtration, and recrystallized from 65 mL of boiling MeOH to yield 2.3 g of product: gold crystals with mp 117-118C; yield 40%. An analytical sample obtained by recrystallization from methanol melted at 119-120C.

1-(4-iodo-2,5-dimethoxyphenyl)-2-propanone (5a)

A solution of 2.1 g of 4 (6.6 mmol) in 10 mL of hot acetic acid was added to a suspension of 4 g of electrolytic iron in 20 mL of acetic acid. Upon heating on a steam bath, the reaction mixture became very dark, and finally there was the deposition of white solids beneath a heavy black oil. Heating was continued for 0.5 h, the reaction mixture was poured into 500 mL of water, a small amount of unreacted iron and an insoluble brown residue was removed by filtration and washed with DCM, and the mother liquors were separated with the aqueous phase being extracted with an additional 2 x 75 mL DCM. The organic phases were pooled, the solvent was removed in vacuo, and the residue (3.9 g of a pale amber oil) was distilled at 0.4 mmHg to yield a fraction, bp 120-130C, of a colorless oil (1.1 g, yield 52%) that spontaneously crystallized. Recrystallization of an analytical sample from methylcyclopentane gave white crystals: mp 62-63C.