neo-Inositol: Assigning Descriptors to

Chirotopic and Achirotopic Carbons

How to Manipulate JSmol Structures

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Chirotopic C1: In the discussion of neo-inositol in the main document the term "look back" has its origins in the digraph on the right. C1 is a stereogenic, chirotopic atom which requires an R/S descriptor. To evaluate C1 a digraph is constructed to assign temporary descriptors (Ro/So).

For any carbon in an inositol, oxygen will always be the top priority group and hydrogen the lowest. How does one assign priorities to the two carbon chains? Using C4 as an example, the light red path (3'-->2'-->1'-->0) leads to phantom atoms faster than the bold blue-red bold-light blue path. Therefore, comparing atoms in sphere 4 C1' has (0,0,0) vs. C2(O,C,H). (0,0,0) loses all the time!

An alternative argument is as follows. There are two paths to C4 from C1, the bold blue and red paths. Upon reaching C4 the path back to duplicate C1' is completed in both paths. C1 and C1' both have an atomic number of 6, but C1' is a duplicate atom attached to three phantom atoms of atomic number zero. At C4 and all other carbons, oxygen has the highest priority while hydrogen has the lowest. The bold blue path from C4 to C1 takes priority over the alternative light red path (3'-->2'-->1') because the latter terminates in phantom groups. The bold blue path dominates over the bold red path because C6/C4 (SoSo) has priority over C2/C4 (SoRo) [CIP Rule 4b]. The order of priority is O>SoSo>SoRo>H. C1 has the S-configuration.

The temporary descriptors for C2, C3, C5 and C6 can also be determined with this diagram. Using C5 as an example, the path leading to C1' on the right side of the digraph is longer than the path to C1' on the left. Therefore, the longer path will always take precedent over the shorter path because the phantom atoms in shorter path are accessed sooner than the ones in the longer path. Consequently, C5 is assigned So.


Achirotopic C2: C2 and C5 lie in a mirror plane and, accordingly, they are pseudoasymmetric. Since it is impossible to assign a higher priority chain by the method applied above, the default is to use the temporary assignments of the C2 vicinal carbons, C1 and C3. CIP Rule 5 states R>S (Ro>So). The priorities are O>Ro>So> H. C2 is of the s-configuration.