Geminal coupling in six-membered rings

As a general guide, CH2 groups close to a stereogenic centre are more likely to be revealed as diastereotopic than those further away. Those in part of a structure with a fixed conformation are more likely to be revealed as diastereotopic than those in a flexible, freely rotating part of the molecule. 

In this molecule, all three marked CH2 groups are diastereotopic, but it is more likely that the ones next to the stereogenic centre, whether in the ring or in the open chain, will show up as A13 systems in the NMR. The remote CH2 group at the end of the chain is more likely to be A2 in the NMR, but one cannot be sure. You must be able to recognize diastereotopic CH2 groups and to interpret AB and ABX systems in the NMR. You must also not be surprised when a diastereotopic CH2 group appears in the NMR spectrum as an A2 or A2X system. 

For another example, look back at thienamycin on p. 832. Compare the two 0CH2Ph groups both have a diastereotopic CH2 pair, but one appears as a singlet and one as an AB system. 

There is just the one axial/axial coupling and so the left-hand side chain must occupy an axial position. This is perhaps a bit surprising—it s large and branched—but the molecule has no choice but to place one of the two side chains axial. 

ceritr For another example, look back at thienamycin on p. 832. Compare the 

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