Ozonolysis nopinone

There are some interesting examples of selective ozonolysis in the terpene field. Limonene is ozonized at the 8,9- double bond in preference to the 1- double bond. This is indicated by the fact that the amount of formaldehyde found is almost equal to the amount of ozone introduced, up to 1 mole. In like manner, terpinolene yields acetone in an amount nearly equal to the ozone passed, up to 1 mole. -Pinene should add ozone readily and form formaldehyde and nopinone on ozonolysis. Practically none of these products can be obtained by ordinary ozonolysis techniques. The hydrogens alpha to the double bond, with probable additional activation from the general strain of the system, are so active to peroxidation by oxygen that little ozonide is formed, because of the large excess of oxygen present. 

Yoshikoshi s synthesis15 of nootkatone (then supposed to be the flavouring principle of grapefruit) uses an optically active enone 52 prepared from P-pinene 48 by ozonolysis to (+)-nopinone 49 and a chemo- and regioselective aldol condensation using the silyl enol ether 50. Though the aldol reaction produces a mixture of diastereoisomers of 51, all dehydrate to the same enone E-52. 

Figure 6.40 Formal reaction scheme for flow ozonolysis of p-pinene (83) to nopinone (84).

Chamber (Caltech Indoor Chamber Facility) experiments have also been carried out by Lee et al. to monitor the time evolution and yields of gas-phase VOCs produced by the ozonolysis of ten different terpenes [197] six monoterpenes (a-pinene, (3-pinene, 3-carene, terpinolene, a-terpinene and myrcene), two sesquiterpenes (a-humulene and p-caryophyllene) and two oxygenated terpenes (methyl chavicol and linalool). Several gas-phase oxidation products were identified including formaldehyde, acetaldehyde, formic acid, acetone, acetic acid, nopinone and pinonaldehyde. 

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