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1. Either commercial absolute alcohol or the 95% grade may be used.
2. 1,1,3,3-Tetraethoxypropane is available from Kay-Fries Chemicals, Inc., New York 16, New York, or from Distillation Products Industries, Rochester 3, New York, and may be used without further purification.
3. Longer boiling does not affect the yield but causes the product to be somewhat dark-colored. A shorter heating period or lack of mechanical stirring decreases the yield.
4. Electrometric titration shows the purity to be at least 95%. The product does not melt below 300°.
5. If concentrated sulfuric acid is substituted for the hydrochloric acid in the procedure, 2-mercaptopyrimidine bisulfate is obtained in about 50% yield. Recrystallization from aqueous acetic acid provides the bisulfate as yellow needles, m.p. 186-
ORGANIC SYNTHESES, VOL. 43
186.5°. (Anal. Calcd. for C4H6N204S2: C, 22.9; H, 2.9. Found:
C, 23.1; H, 2.9.)
6. pH indicator paper may be used, or the solution may be made weakly basic to litmus. Excess base dissolves the product and should be avoided.
3. Methods of Preparation
The synthesis of 2-substituted pyrimidines from 1,3-dicarbonyl compounds and urea derivatives was first described by Evans2 and was later improved by Hunt, McOmie, and Sayer3 for the preparation of 2-mercapto-4,6-dimethylpyrimidine. Burness4 employed 3-ketobutyraldehyde acetal in this procedure to give
2-mercapto-4-methylpyrimidine. 2-Mercaptopyrimidine has been prepared from 1,1,3,3-tetraethoxypropane and thiourea by variations of this basic method3'6'6 as well as by the reaction of
2-chloropyrimidine with thiourea 7 or sodium hydrosulfide.8
4. Merits of the Preparation
This preparation describes a convenient and general method of synthesis of substituted pyrimidines from compounds containing a /3-dicarbonyl group, either intact or as the corresponding ketal. The usefulness of the 2-mercaptopyrimidines is enhanced by the ease of removal of the mercapto group by desulfurization 9 or oxidation 10 and its replacement by other functional groups.10
1 Research Department, Union Carbide Chemicals Company, South Charleston, West Virginia.
2 P. N. Evans, J. Prakl. Chem.,  48, 489 (1893).
s R. R. Hunt, J. F. W. McOmie, and E. R. Sayer J. Chem. Soc., 1959, 525.
4 D. M. Burness, J. Org. Chem., 81, 97 (1956).
6 Т. V. Protopopova and A. P. Skoldinov, J. Gen. Chem. (USSR), 27,1276 (1957).
6 J. W. Copenhaver and R. F. Kleinschmidt, Canadian Patent 534,307 (1956).
7 М. P. V. Boarland and J. F. W. McOmie, J. Chem. Soc., 1951, 1218.
8 R. 0. Roblin, Jr., and J. W. Clapp, J. Am. Chem. Soc., 72, 4890 (1950).
9 G. R. Pettit and E. E. van Tamelen, Org. Reactions, 12, 364 (1962).
10 G. W. Kenner and A. Todd in R. C. Elderfield, Heterocyclic Compounds, Vol. 6, pp. 283-287, John Wiley and Sons, New York, 1956.
METHYL BUTADIENOATE (Butadienoic acid, methyl ester)
[ Heat -> CH2=C—CH-CO2CH3 + CH2=CHC02CH3
Submitted by H. B. Stevenson and W. H. Sharkey.1*
Checked by R. D. Birkenmeyer, W. E. Russey, and F. Kagan.lb
A “Vycor” tube 550 mm. long and 25 mm. in outside diameter is packed for 500 mm. of its length with 6-mm. x 6-mm. quartz rings and mounted vertically so that the upper section is encased in a tube furnace 150 mm. long and the lower section is encased in a tube furnace 300 mm. long. The upper furnace, which serves as a preheater, is monitored by a thermocouple placed between the tube and the furnace heating elements. The temperature of the lower furnace is monitored by a thermocouple located in the center of the lower packed section. The upper end of the pyrolysis tube is fitted with a Y-tube carrying the thermocouple well and a graduated addition funnel with a pressure-equalizing arm. The lower end is attached through one 500-ml. trap and two 200-ml. traps, each immersed in a mixture of solid carbon dioxide and acetone, to a regulated vacuum source. One gram of hydro-quinone is placed in the first trap.
The pyrolysis tube is flushed with nitrogen, the lower section is heated to 600° and the upper section to 300° (Note 1), and the pressure is regulated at 25 mm. Then 184 g. (1.00 mole) (Note 2) of dimethyl 3-methylenecyclobutane-l,2-dicarboxylate2 is admitted over a period of 3.hours (Note 3). The product, which amounts to 172-177 g., collects in the traps. It is distilled through a 13-mm. x 1.2-m. Nester spinning-band still.3 First
ORGANIC SYNTHESES, VOL. 43
there is 41-47 g. (48-55%) of methyl acrylate, 1.4010, b.p. 34-36°/150 mm., then 39-43 g. (40^4%) of methyl butadienoate, 1.4635 (Note 4), b.p. 59-60°/52 mm. or 48-49°/26 mm. By continuing the distillation, 20-30 g. of starting material, b.p. 125-150°/25 mm., can be recovered.
1. The temperature of the lower section is quite critical and should be maintained within the range 590-610°. However, the preheater section is needed only to volatilize the ester, so any temperature between 200° and 400° is satisfactory.