Fragmentation of a six-membered ring

This chemistry is quite different from the examples we have just seen. The starting material has a bridged bicyclic structure and was made by a Diels–Alder reaction. Fragmentation is initiated by formic acid (HCO₂H), which protonates the tertiary alcohol and creates a tertiary carbocation. The ether provides the push. More serious electronic interactions are needed in this fragmentation as the C–C bond being broken is not in a strained ring.

The yield of 50% may not seem wonderful, but there is obviously a lot of chemistry going on here so it is perfectly acceptable when so much is being achieved. The first stage is the fragmentation itself. Drawing the product first of all in the same shape as the starting material and then redrawing, to ensure that we don’t make a mistake, we discover that we are well on the way to nootkatone. Note that the stereochemistry of the two methyl groups comes directly from the stereochemistry of the starting materials and no new stereogenic centres are created in the fragmentation. Although one six-membered ring is fragmented, another remains.

The first-formed product now cyclizes to form the second six-membered ring. This recreates a carbocation at the tertiary centre like the one that set off the fragmentation as the more nucleophilic end of the isolated alkene attacks the end of the conjugate electrophile. This is a thermodynamically controlled reaction with the new stereogenic centre choosing to have an equatorial substituent.

The cation picks up the only nucleophile available—formic acid. This gives the product of the fragmentation, which contains two unstable functional groups—a tertiary formate ester and an enol ether—and this product is not isolated from the reaction mixture. In water it hydrolyses to the enone, which undergoes elimination of formate to give nootkatone on heating.

Yet after all this effort, none of the synthetic samples of nootkatone delivered that intense grapefruit smell—for the simple reason that nootkatone is not the flavour principle of grape fruit! The samples of nootkatone that had been isolated from grapefruit contained minute traces of the true flavour principle—a simple thiol. Humans can detect 2 × 10⁻⁵ ppb (parts per billion) of this compound, so even the tiniest trace is very powerful. Nonetheless, the syn theses allowed chemists to correct a misconception.

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المزيد

مواضيع ذات صلة

The Hammett relationship

Systematic structural variation

Crossover experiments

Labelling experiments reveal the fate of individual atoms

Be sure of the structure of the product

Variation in the structure of the aldehyde

formation of an ester intermediate

formation of an intermediate dimeric adduct

Be sure of the structure of the product

Variation in the structure of the aldehyde

Proposed mechanism C: formation of an ester intermediate

Proposed mechanism B: formation of an intermediate dimeric adduct