Rule:You may not use First class and Business class of certain aircraft which are A380, B777-300ER and A345 (code 380, 77W and 345) of Singapore Airlines (SQ) as flight awards.So Singapore is unfortunatly out. What a pity, esspecially since they are starting with a380 from March from Zrh.
Unless you get CO to do a manual sell...http://www.flyertalk.com/forum/united-mileage-plus/1014570-reason-why-co-people-getting-singapore-f-awards-you-not.html