Carbon-13 NMR spectra: 1 = C, 2 = A, 3 = B. Approximate shifts, read from the printed axes, are shown in the annotated structures.
Figure 1.
Derived carbon assignments for the three isomers, excluding the solvent signal near 77 ppm
. Details.
  • C: has five carbon environments: acetal CH near ; substituted/terminal alkyne carbons near and ; OCH near ; and methyl near . One alkyne signal is close to the CDCl solvent region, which must not be counted as a new molecular environment.
  • A: has ester carbonyl near , substituted alkene C near , vinyl CH near , OCH near , two nonequivalent alkene methyls near and , and ethoxy methyl near .
  • B: has carbonyl near , vinyl CH near , terminal vinyl CH near , OCH near , acyl-alpha CH near , the next CH near , and terminal methyl near .
The two methyls in A are chemically distinct because one is cis and the other trans to each substituent at the other alkene carbon. Their two measured shifts are secure; assigning an individual methyl to a particular side of the drawn double bond requires an additional stereospecific shift argument or experiment, so their spatial interchange is left explicit.

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