91Ó°ÊÓ

When a substitution is applied to the re or si face of the molecule (pronounced "ray" and "sigh"), a trigonal planar sp2 hybridised atom can be turned to a chiral centre in stereochemistry.

When looking at the face labelled re if, the substituents at the trigonal atom are arranged in decreasing cahn-ingold-prelog priority order in a clockwise order, and si if the priorities decrease in a counter clockwise order; it should be noted that the designation of the resulting chiral centre as S or R is dependent on the priority of the incoming group.

Prochirality is a word that describes how compounds with ${\mathrm{sp}}^2$ hybridisedcentresare not chiral but have diverse faces/sides.

1. The trigonal planar atom Re/Si.
2. Pro-R/pro-s: Tetrahedral attacked carbon is pro R or from pro-S which side the molecule is attacked, especially in enzyme catalysis.

In step 6, the aceto acetyl ACP is reduced on the si face.