# Which of the following isomers of 2,3‐dihydroxy‐4‐methoxy‐4‐oxobutanoic acid are identical?

Which of the following are identical?

1. A and B are identical
2. A and B are diastereomers
3. A and C are enantiomers
4. A and B are enantiomers

What I know is that when we rotate a Fisher projection by 180°, then we get identical compounds (I might be wrong). But how to tell if the compounds are enantiomers of each other or not?

For that how to get the same group, i.e. $$\ce{-COOH}$$ in this case at the top to compare the compounds?

Also, is there any trick to tell that if they are diastereomers, enantiomers or identical by the hydroxyl group positions in Fisher projections?

• It is so because A and B are enantiomers. Tell me why. – user55119 Nov 21 '19 at 3:22
• Rotating a Fischer projection by 180 deg. does not always give an identical compound. – user55119 Nov 21 '19 at 3:47