Sildenafil alternate routes

SCHEME 16.5 Alternative routes to sildenafil via the halo derivatives 13 and 14. 

More than 15 different routes to sildenafil have been reported in the chemical or patent literature. During the development program, the two main alternatives examined by Pfizer were synthesis via the aldehyde 11 to give dihydrosildenafil 12 followed by subsequent oxidation (Scheme 16.4) or synthesis via the halo derivatives such as 13 or 14 (Scheme 16.5). 

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