Can I renounce my Singapore citizenship before obtaining another country citizenship?

Under Article 128 of the Singapore Constitution, a citizen of Singapore can only renounce his/her Singapore citizenship if he/she:

a) Is aged 21 and above;
b) Is of sound mind; and
c) Is also or is about to become a citizen of another country.

As such you may only renounce Singapore Citizenship if you have documents to show that you are about to become the citizen of another country.

Please note that the Government may withhold the renunciation of Singapore citizenship by a male citizen if he is subjected to the Enlistment Act (Cap. 93) unless he has:
a) discharged his liability for full-time service under section 12 of that Act; 
b) rendered at least three years of operationally ready national service under section 13 of that Act in lieu of such full-time service; or
c) complied with such conditions as may be determined by the Government

You may refer to Renunciation of Singapore citizenship for more information.

This information provided here is sourced from the ICA website.