They may be legal tender, but I'd suspect that most vendors would balk at accepting them -- it's been 20 years since they were taken out of circulation! I'm very surprised you got those in Singapore (which gets lots of Japanese visitors), I suspect the money changer just managed to get rid of some notes nobody else wants... You may have to make a trip to a bank, or perhaps even the Bank of Japan (which has branches in Tokyo and Osaka among other places), to get rid of them.