Appendix A 250
Appendix A
Preparation of the key starting material for article 3
General method for the synthesis of 1,3-dialkyl-5,6-diaminouracil derivatives (10a, b)
In the investigation of article 3 (Chapter 8), 5,6-diaminouracil derivatives were used as key starting materials for the preparation of the target xanthine derivatives (Scheme 1). The 5,6-diaminouracil derivatives may be prepared in good yields from the corresponding N-N’-disubstituted urea by methods previously described (Blicke & Godt, 1954).
In short, the appropriate N-N’-dimethyl urea or N-N’-diethyl urea (A) was condensed with cyanoacetic acid in the presence of acetic anhydride (i) to form the cyanoacetylurea intermediate (B). Hereafter, a 10% sodium hydroxide solution (ii) was added in order to obtain the corresponding substituted 6-aminouracils (C). Nitrosouracil (D) was obtained by subsequent nitrosation of the obtained 6-aminouracil with sodium nitrite in the presence of glacial acetic acid (iii). Finally, the desired 5,6-diaminouracils (10a, b) were obtained by a reduction of the nitrosouracil with sodium hydrosulfite in concentrated ammonium hydroxide (iv).
Scheme A1. Reagents and conditions: Synthetic pathway substituted 5,6-diaminouracil derivatives (10a, b). Key: (i) cyanoacetic acid, acetic anhydride; (ii) NaOH (aq); (iii) NaNO2, CH3CO2H; (iv) Na2S2O4,
NH4OH. NH O N R1 O CN R3 (ii) (B) (C) N N O R1 O R3 NH2 NO (i) (iii) (D) (iv) N N O R1 O R3 NH2 NH2 N N O R1 O NH2 R3 NH NH R1 O R3 (A) 10a: R1 = R3 = CH 3 10b: R1 = R3 = C 2H5
Appendix A 251
The synthesis of the 1,3-dialkyl-5,6-diaminouracil derivatives (10a, b)
The appropriate N-N’-dimethyl urea or N-N’-diethyl urea (50 mmol) and cyanoacetic acid (50 mmol) were placed in a round bottom flask. Hereafter, acetic anhydride (6.25 ml) was added and the reaction was heated (60 ºC) for 3 hours, with a CaCl2 trap attached. A light yellow
solution was obtained. The reaction was cooled on ice and the pH was adjusted to 11, by adding 10% sodium hydroxide solution (60 mL) to the above, yielding a white suspension.
Figure A1. A photo of the white suspension obtained after addition of the sodium hydroxide solution.
Stirring was continued for another 30 minutes at room temperature. Subsequently, a solution of sodium nitrite (60 mmol) was added, followed by glacial acetic acid (8 mL). At this stage the reaction turned pink and the pH tested 5. Over the next hour, a further 7 mL glacial acetic acid was added drop wise to the reaction. Stirring was continued for another hour.
Figure A2. A photo taken after the reaction turned pink to demonstrate how the colour intensified to a darker pink over time and to finally result in the purple product.
The appropriately substituted 1,3-dialkyl-5-nitroso-6-aminouracil was obtained as a purple product after it was collected via filtration and washed with 20 mL diethyl ether.
Appendix A 252 Figure A3. Chemical structure of 1,3-dialkyl-5-nitroso-6-aminouracil and a photo of 1,3-diethyl-5-nitroso-6-aminouracil obtained as a purple product.
Ammonia water (20 mL, 33%) was added to powdered 1,3-dialkyl-5-nitroso-6-aminouracil (20 mmol), yielding an orange suspension. The suspension was then heated to 40 ºC. This was followed by adding freshly prepared sodium hydrosulfite (11 g in 50 mL H2O) over a period of 15
min. Approximately 47 mL sodium hydrosulfite was added. The suspension became a red solution which later turned green. Over the next 4 hours, the reaction was cooled on ice. Finally, the 1,3-dialkyl-5,6-diaminouracil was obtained as light yellow crystals after it were collected via filtration and washed with 20 mL water. The product was left overnight to dry in a fume hood.
Figure A4. Chemical structure of 1,3-dialkyl-5,6-diaminouracil and a photo of 1,3-diethyl-5-nitroso-6-aminouracil obtained after filtration as light yellow crystals.
R1 = R3 = CH 3 or C2H5 N N O R1 O R3 NH2 NO N N O R1 O R3 NH2 NH2 R1 = R3 = CH 3 or C2H5
Appendix B 253
