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The ozonolysis of terpenoids, a Pandora’s box of by-products

Posted on 30. November, 2017.

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The ozonolysis of alkenes is a general reaction that has made a significant contribution to the degradation and elucidation of the structures of natural products. The cleavage of alkenes by ozone has not only provided information on their immediate structural environment but also opened up the structures of compounds for further degradation. 

When it was introduced in the early part of the twentieth century, the regioselectivity of ozonolysis provided a considerable advantage over other widely used oxidants, such as potassium permanganate. While the application of ozonolysis to structure determination has long been superseded by spectroscopic methods, there is a current interest in the ozonolysis of terpenoid alkenes not only because it provides a simple method for obtaining chiral intermediates for synthetic applications from readily available natural products but also because of the importance of the reactions of atmospheric ozone with the volatile terpenoid organic compounds such as limonene and α-pinene that are produced by plants. The applications of ozonolysis in organic chemistry
and in the context of organic peroxides, has been reviewed on a number of occasions.
Over the years, it has become clear that the ozonolysis of terpenoid alkenes can also produce a complex mixture of by-products. The reaction not only converts an alkene that is sensitive to reaction with electrophiles into two carbonyl groups in which the carbon atoms are sensitive to reaction with nucleophiles, but also may produce a range of other products.
The object of this review is to describe these by-products and account for their origin. In this context, it is worth noting that
ozone is a powerful oxidant that will not only cleave alkenes but also insert oxygen into a C–H bond to form an alcohol or a hydroperoxide and oxidise secondary alcohols to ketones. For example, the oxidative cleavage of acetals to esters can be highly regioselective. Thus the benzylidene acetal of cholestane-2β,3β-diol gave 92% of the axial 2β-benzoate ester. The allylic oxidation of a hindered triterpenoid 8,9-ene proceeded selectively at C7 to give the Δ8-7-ketone.

Read the full review, free, in Journal of Chemical Research, Volume 41, Number 10, October 2017, pp. 557-563

Author: James R. Hanson
Department of Chemistry, University of Sussex, Brighton, Sussex BN1 9QJ, UK