As 1999 is with us we at Advanced Chemistry Development are in the process
of upgrading our assigned structure databases. At present our databases
include assigned structures numbering 82,000 H-1, 67,000 C-13, 18,500 P-31
and 11,500 F-19, and therefore has offered to us a significant source of
information which could expose "errors in the literature". We have a
combination of other assigned structures presently being screened through
our quality process (about 50,000 NEW structures) and it is this process
that I wish to gain advice from the list about.
Shortly we will be publishing a short article regarding a particular class
of chemical structures for which a significant number of assignments exist
in the literature, published over a period of years, in a series of
different journals in submitted papers from different authors. For this
particular class of compounds there exists within the structure a
para-substituted aromatic moiety. In each publication the quaternary
carbons are assigned.=20
In our C-13 NMR prediction program it is possible, post prediction, to
click on each individual nucleus and see all related structures in the
database that were used for the prediction. For the case of the two
aromatic quaternary carbons, observation of the assigned structures used
for prediction gives two distinct columns of shifts for BOTH carbons, 15ppm
apart. Close examination of the data within the literature suggests that
the assignments of the quaternary carbons have commonly been confused and
misassigned. In order to resolve this issue we ran a series of 2D
experiments on an example structure from this chemical class and have
distinguished the appropriate assignment. Now for the question to the=
list...
Since we can now re-assign the correct shifts for the quaternary carbons
for this particular series of molecules, we could do this but these would
no longer be "true literature shifts" so:
1) Should the misassigned shifts simply be left absent from these entries
in the DB?
2) Should we reassign the shifts and simply note in the DB that these
shifts were reassigned by us but that the original literature reference is
valid for all other shifts.
WE CANNOT SIMPLY ENTER THE BAD SHIFTS INTO THE DATABASE AS THE DB SHIFTS
SERVE AS ONE OF THE CONTRIBUTIONS TO THE PREDICTION MADE BY THE PREDICTION
ALGORITHM
This particular class of compounds is just one example of this problem and
we have identified MANY such issues with literature data while entering
100,000s of shifts into our databases.=20
Your suggestions will be representative of what people think we should do
in such situations and I look forward to your feedback. Best wishes!
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