Question

n structure I was converted to quinine (A), other compounds were formed, including compound D. Horo a) There are carbon stereocenters in structure D. Assign the R and S configuration at each stereocenter of D. b) The quinine isolated from this reaction was found to have a specific rotation of [a] =+35° (c = 1, CHC13). Following a chiral resolution the specific rotation was found to be [«]17 = -108° (c = 1, CHCl). The specific rotation of the desired enantiomer is all? = -117° ( C 1, CHC13). Calculate the enantiomeric composition of quinine before and after the chiral resolution. Show your work. HINT: Found specific rotation -[a]. (X..)+ [a] (X_), where (X) = (1 - (X2) % (+)-enantiomer and % (-)-enantiomer before chiral resolution- % (+)-enantiomer and % (-)-enantiomer after chiral resolution-

Answer 1

4.

(a) There are   4 carbon stereocenters in structure D.

M HARK

R, S configuration have been assigned for structure D:

Absolute Configuration of carbon stereocenters on D is 3R,4S,8S,9S (Numbering of side chain, starting from N)

(b) Specific rotation, [$\alpha$], is the observed rotation at a specific cell length and sample concentration.

carbon stereocenters on quinine (A) has absolute configuration = 3R,4S,8S,9R

Specific rotation of desired enantiomer 'A' = - 117^o

So, Specific rotation of its enantiomer = +117^o

Found specific rotation :[$\alpha$] = [$\alpha$]₊*X₊ +  [$\alpha$]_-*X_- = [$\alpha$]₊*X₊ +  [$\alpha$]_-* (1- X₊ )

X_- = % of (- ) Enantiomer   and X₊= % of (+) Enantiomer

Enantiomeric composition of quinine :

• Before chiral resolution :

[$\alpha$] = (+117*X₊ ) + ( -117*X_- )  = (+117*X₊ ) + ( -117*(1- X₊ ) ) = + 35

(+117*X₊ ) + ( -117 ) + (+ 117* X₊ ) ) = + 35

or 2 (+117*X₊ ) = 152

or (+117*X₊ ) = 76 or X₊ = 0.649 = 64.9 %  

(Desired enantimer ) or X_- ( (- ) Enantiomer ) = 35.1%

and X₊ ( (+) Enantiomer ) = 64.9 %

• After chiral resolution :

[$\alpha$] = (+117*X₊ ) + ( -117*X_- )  = (+117*X₊ ) + ( -117*(1- X₊ ) ) = -108

(+117*X₊ ) + ( -117 ) + (+ 117* X₊ ) ) = -108

or 2 (+117*X₊ ) = +9

or (+117*X₊ ) = 4.5 or X₊ = 0.0385 = 3.85 %  

(Desired enantimer ) or X_- ( (- ) Enantiomer ) = 96.15 %

and X₊ ( (+) Enantiomer ) = 3.85 %