in animals the liver and adrenals are the main sites of 
synthesis. 
the bio synthesis of ascorbic acid is as follows.
d-glucose - d-glucuronic acid - l-gulonic acid -l-
gulonolactone - 2-keto-l-gulonolactone - l-ascorbic acid.
in this way  vitamin "c" is synthesised.
but in man l-gulono-oxidase enzyme is absent.
that's why man cannot synthesise vitamin"c".