[njord_b]

Hi everyone, i have a problem with naming a molecule in Fischer projection, this is an example of my problem, why it has to be like the picture 1 and not like 2?

would be the second one wrong?
why hydroxymethyle can´t be the substituent on third carbon? is there a rule for this or is this some trivial thing? i just don´t get it, and i am only guessing and i am wrong all the time on exams, please help

[discodermolide]

What about the phenyl group on the carbon you labelled 2? Give that a priority and see what you get.

[njord_b]

i did not get it, can u be more specific about this? thanks in advance

[discodermolide]

There are 4 groups attached to the carbon labelled with the red number 2, you need to assign each group a priority number. So on that carbon you have a COOH, a phenyl ring, a CH₂Cl and a carbon chain (which contains other functional groups). Then assign the (R) or (S) label. Do the same for the second chiral carbon.
http://en.wikipedia.org/wiki/Cahn–Ingold–Prelog_priority_rules

As for Fischer projections see:
http://www.chem.ucalgary.ca/courses/351/Carey5th/Ch07/ch7-7.html

[njord_b]

i understand the priority system and R/S, but the thing i don´t know is whether this compound is 3-amine-4-hydroxyde-3-methyl ... acid OR 3-amine-3-hydroxymethyle-4-methyl ... acid,

i will take the longest carbon chain, but which carbon will be the fourth? that on the picture 1 or 2 and how to determine it?

[discodermolide]

I'm sorry I thought the confusion was about the R/S system, my mistake.

[discodermolide]

The compound is (2R,3S)-3-amino-2-(chloromethyl)-4-hydroxy-3-methyl-2-phenylbutanoic acid.

[njord_b]

and why it can not be 3-amino-2-(chloromethyl)-3-hydroxymethyle-2-phenylbutanoic acid?

[discodermolide]

see
http://www.acdlabs.com/iupac/nomenclature/79/r79_921.htm