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N-({(2S,6R,8R,10S)-10-(benzyloxy)-8-[(2E)-penta-2,4-dien-1-yl]-1,7-dioxaspiro[5.5]undecan-2-yl}methyl)acetamide

I was asked to identify the functional groups in this molecule above.

So far I have found an amide ($\ce{-NHAc}$), an ether ($\ce{-OBn}$), and a conjugated diene, but I was also told that there is an acetal ($\ce{RHC(OR')2}$).

However, in the only part of the molecule where I can see a carbon atom attached to two other oxygen atoms, the carbon atom is not attached to a hydrogen atom.

Where is the acetal group in this molecule?

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According to the IUPAC definition, an acetal is any compound with a formula $\ce{R2C(OR')2}$ (R' ≠ H). In the past, "acetal" referred only to compounds of this type, which were derived from aldehydes (one R = H), however, it now broadly encompasses all compounds of this type, including those derived from ketones, which were formerly only referred to as ketals. Interestingly, there was a period of time in which use of the term "ketal" was discouraged (IUPAC says 'abandoned', but many chemists kept using it anyway), but it has now been re-embraced as a term, for the subset of acetals that are derived from ketones.

One last note - the molecule shown in this question is a "spiroacetal" - a type of acetal where the acetal carbon is the only atom joining two rings.

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  • $\begingroup$ IUPAC notwithstanding, the acetal/ketal distinction is more meaningful than acetal itself. $\endgroup$ – user55119 Jan 2 at 21:13

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