The preparation steps for synthesis of Viagra (sildenafil citrate) are as follows:

1. Methylation of 3-propylpyrazole-5-carboxylic acid ethyl ester with hot dimethyl sulfate.

2. Hydrolysis with aqueous NaOH to free acid.

3. Nitration with oleum/fuming nitric acid.

4. Carboxamide formation with refluxing thionyl chloride/NH4OH.

5. Reduction of nitro group to amino.

6. Acylation with 2-ethoxybenzoyl chloride.

7. Cyclization.

8. Sulfonation to the chlorosulfonyl derivative.

9. Condensation with 1-methylpiperazine.