No, this is not allowed. According to the Romanian Ministry of Foreign Affairs:
Starting July 11, 2014, in keeping with the provisions of Decision No 565/2014/EU, Romania acknowledges as being equivalent to its own national visas, for transit and for stays up to 90 days per period of 180 days the following:
- The two or multiple entry short-stay visa issued by a Schengen member state;
Attention! The number of entries as well as the right of stay established as per the Schengen visas must not have been exhausted.
As your visa is a single-entry visa, it is not usable for entry to Romania. You will have to apply for a separate Romanian visa.