When you feel at ease with structure drawing and calculations, you will be ready to tackle the set problems. These are based on the stereochemistry and conformational analysis of decalin derivatives. The structures of both cis- and trans-decalin can be retrieved from the fragment library.
2a. A useful approach is to examine the model of trans-decalin (1) and decide how many isomers exist (for example consider 1a-d)- can these be interconverted by a cyclohexane flip.
In building the models of decalones, start with trans-decalin and introduce a C=O double bond adjacent to CH - does it matter which of the four atoms adjacent to the ring junction this is? If you can satisfy yourself that there is a unique isomer of 1, carry out the Molecular Mechanics MM2 minimisation routine. When this is complete, record the energy, and return to the structure builder and examine the resulting geometry; note any close contacts which might be energetically unfavourable.
2b. Carry out the same routine in building
cis-1-decalone (2), where you may find that there are two distinct
isomers - verify this by measuring the dihedral angles O=C-C-H (to the ring
junction) in 2a and 2b to be sure that they are different. If
there are two isomers how would they interconvert?
2c. Having made your conclusions, carry out Molecular Mechanics MM2 calculations to discover what is the energy difference between cis- (2) and trans -1-decalones (1).
2d.The comparison between trans-1 (1) and trans-2-decalones (3) should be straightforward, and you should suggest an explanation for any difference observed.
an online
chemistry lecture course © Karl
Harrison,
Department of Chemistry, University
of Oxford 1998