5 - keto - l - ascorbic acid - 5 , 5 - dimethyl ketal , alkali metal and alkaline earth metal salts thereof ; process for the preparation thereof ; its use as an antioxidant and intermediate for production of 5 - keto - l - ascorbic acid and l - ascorbic acid . 5 - keto - l - ascorbic acid and its 5 , 5 - dimethyl ketal exhibit antiscorbutic activity .