Question

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What is the sterochemistry at the fusion points ?

please explain more because I did not understand please !

Answer 1

So the stereochemistry at the fusion point refers to the spacial arrangement of the atoms around an atom or bond. In the case of the fusion of two cyclohexane rings we could have a cis or trans stereochemistry.

cis relationship of the rings flexible structure trans relationship of the rings axial positions in blue equatorial positions in red The change in conformation is not possible because there is no way for the ring on right to be axial

According the conformation analysis cis is the preferred stereochemistry for the fusion of the rings.

Moreover the electrocyclization that the molecule suffers to be converted in the final product is a pericyclic rearrangement. The system has 6 pi electrons and for the orbitals to be allowed to react under thermal conditions, the only type of fusion is cis stereochemistry.

If you want further details on electrocyclization please let me know. I´ll be glad to help with that :)