Discussion
Absolute configuration of the chiral centres (C-2, C-4) and prochirality descriptors (HA and HB) in the following compound is :
(Fischer projection, top to bottom:)
| COOH | ||
| H | C-2 | OH |
| HA | C-3 | HB |
| H | C-4 | OH |
| COOH |
Answer: A
The descriptors are 2R, 4S; HA pro-s; HB pro-r.
C-2: priorities OH > COOH > C-3 > H. With H on a horizontal bond, OH (right) to COOH (top) to C-3 (bottom) runs anticlockwise, which reads S; because the lowest priority group points towards the viewer, the true descriptor is reversed: R.
C-4: priorities OH > COOH (bottom) > C-3 (top) > H. OH to COOH to C-3 runs clockwise, read as R and reversed to S. Both OH groups on the same side make this the meso (2R,4S) diacid.
HA: replace it by D. At C-3 the R branch ranks above the S branch, so C-2 > C-4 > D > HB. C-2 (top) to C-4 (bottom) to D (left) runs clockwise, read as r and reversed (HB is horizontal) to s. So HA is pro-s and HB pro-r, which only the first option gives with 2R, 4S.
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