in PYRO2 points to the significance of positional disorder. It is
the ‘‘freezing out’’ of these independent conformations whose
signature is present in the asymmetric unit and hence gives rise to
the presence of conformational polymorphism. The nucleating
units contain this information and the molecular recognition
events eventually giving rise to crystal formation. In all these
crystal structures it is the formation of N–H/O hydrogen bonds
giving rise to different structural motifs, namely dimers, chains
and ribbon motifs along with the presence of weak C–H/O and
van der Waals interactions which contribute towards the overall
crystal packing.
Fig. 21 Packing diagram in REWDUS depicting N–H/O]C
hydrogen bonds generating an octameric unit. The two six-membered
rings are in staggered conformation.
Acknowledgements
Susanta K. Nayak thanks CSIR for research fellowship. We all
thank IISc, Bangalore X-ray facility under the DST, India
scheme. The authors also thank Mr. M. R. Vippila for recording
1
all the H-NMR of the compounds under study.
Notes and references
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Fig. 22 Packing diagram in YAMQAF depicting N–H/O and O–H/O
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series). ILAHEI (ortho chloro in oxo series) is isostructural with
PYR1 (para F) in oxo series. The compound also exists in a screw
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(meta-methoxy) from the thioxo series. The molecular confor-
mation of these oxo compounds does not indicate the presence of
intra-molecular C–H/p interaction (a significant feature
observed in the corresponding thioxo series) indicating that the
aryl ring is free to rotate with respect to the six-membered tet-
rahydopyrimidine ring.
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Conclusions
In this article, we report the synthesis and characterization of
eight differently substituted tetrahydropyrimidine derivatives
(varying functional group substitution on the phenyl ring). The
ester moiety exists in an s-cis orientation in all compounds.
However the ethyl group of the ester moiety exists in different
molecular orientations in differently substituted derivatives. In
some cases, it also exhibits positional disorder, due to the greater
thermal motion of the ethyl group. It is noteworthy that
PYRO2B, 3, 5 and 6 crystallizes with more than one molecule in
the asymmetric unit. The existence of concomitant polymorphism
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604 | CrystEngComm, 2011, 13, 591–605
This journal is ª The Royal Society of Chemistry 2011