Endo-exo isomerism is a special type of isomerism found in organic compounds with a substituent on a bridged ring system. The prefix endo is reserved for the isomer with the substituent located closest, or "syn," to the longest bridge. The prefix exo is reserved for the isomer with the substituent located farthest, or "anti", to the longest bridge. Here "longest" and "shortest" refer to the number of atoms that comprise the bridge. This type of molecular geometry is found in norbornane systems such as dicyclopentadiene.
The terms endo and exo are used in a similar sense in discussions of the stereoselectivity in Diels–Alder reactions.
Descriptors of the relative orientation of groups attached to non-bridgehead atoms in a bicyclo[x.y.z]alkane (x ≥ y > z > 0).
If the group is orientated towards the highest numbered bridge (z bridge, e.g. C-7 in example below) it is given the description exo; if it is orientated away from the highest numbered bridge it is given the description endo. If the group is attached to the highest numbered bridge and is orientated towards the lowest numbered bridge (x bridge, e.g. C-2 in example below) it is given the description syn; if the group is orientated away from the lowest numbered bridge it is given the description anti.
Cyclopentanes.
The conformational analysis of substituted cyclopentanes is much more complicated than that of cyclohexanes.
The energy differences between various envelope and twist conformations in five-membered rings are generally small, and there are as many as ten different envelope and ten different twist conformations, and each conformation has multiple dihedral angle relationships.
Several of the 20 possible conformations may be populated in an individual structure. Thus the vicinal couplings in 5-membered rings are highly variable.
For cyclopentanes in envelope conformations Jcis > Jtrans in the flat part part of the envelope, whereas in twist conformations the tendency is for Jtrans > Jcis.
In general, no firm assignments of stereochemistry can be made using the size of couplings alone unless a specific substitution pattern or heterocyclic system has been carefully investigated, or if substitution patterns allow prediction of the conformation.
Inspection of the double Karplus curves indicates a significant difference between the typical behavior of adjacent CH2 groups in cyclohexanes and cyclopentanes.
In a chair cyclohexane only one of the four vicinal couplings can be large (> 7 Hz), whereas in a cyclopentane it is common for 2 or even 3 of the 3J couplings to be large.
In most cyclopentanes, the C-C-C-C dihedral angles are significantly smaller than the 60° found in cyclohexanes. Cis protons will tend to have H-C-C-H dihedral angles close to 0°, andtrans near 120°.
The cis couplings (8-10 Hz) are usually larger than trans (2-9 Hz).
However the Karplus curves for cyclopentane have a region where the cis and trans lines cross (Figure above, at ca 20° dihedral angle), so there are cases where cis and trans couplings are identical (see below, where the allylic proton is a quartet of doublets, arising from accidental equivalence of three vicinal couplings), as well as a smaller region where Jtrans > Jcis .
If the ring puckering is strong enough, then Jtrans > Jcis. In bicyclo[2.2.1]heptanes the endo-endo and exo-exo 3J are always greater than endo-exo couplings.
Thus stereochemical relations among vicinal protons in 5-membered rings cannot be reliably determined by simply measuring coupling constants, except in cases where the substitution pattern of the specific ring system has been carefully investigated.
For example, in the benzodihydrofurans below, changing the size of the substituent R causes a reversal in the size of Jcis and Jtrans.
READ'
http://www.unk.edu/uploadedFiles/academics/gradstudies/ssrp/Myers.pdf
AND
http://pubs.acs.org/doi/abs/10.1021/ed068p426?journalCode=jceda8
The ENDO product is the one where the "outside" groups on the diene are on the SAME side of the 6-membered ring as the electron withdrawing group (EWG). The EXO product is the one where the "outside" groups are on the OPPOSITE side of the ring as the 6-membered ring.
Note how the Endo and Exo products are related - they're diastereomers
Here's some examples.
OVERLAPS
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