Question

give the steps for the enantioselective synthesis of

(L)-tyrosine

and another for (D)-tyrosine

both starting from the corresponding (Z)-enamido acid and using a rhodium as a catalyst

Answer 1

Enantioselective Synthesis is the synthesis of a compound by a method that favors the formation of a specific enantiomer or diastereomer. Enantiomers are stereoisomers that have opposite configurations at every chiral center. Diastereomers are stereoisomers that differ at one or more chiral centers.

This is the generalised mechanism.

Catalyst used in our process is BINAP catalyst.

This catalyst is synthesized from a rhodium slat and a phosphorus compound. BINAP moiety (Two Naphthalene rings at right angles) make the catalyst chiral. (Technically has no tetrahedral sterotegenic centers).

Syn addition of H2 Gas To Z-Enamido acid, using Hr, or Ru Catalyst with Chiral Ligand.

1) (Z) Enamido Acid + H2/Rh (Chiral Auxillary) = S enantiomer ONLY

2) H2O Then acid (Neutralization)

OR

2) Acid Water heat.

NH3. (Racemic Mix of L/D)

1) HVZ reaction: Bromonation of an A-carbon of carboxilic acid.

1) Br2/PBr3
2)H2O

3) NH3 (xs Ammonia)

Displaces Br in Sn2 reaction. (This gives mixture of D & L)

The preparation of d-tyrosine from the readily available natural isomer has been accomplished by catalytic racemization with acetic anhydride in aqueous solution and resolution of the N-acetyl-dl compound with brucine.