Addressing the Whale in the Room: Sustainable Scents beyond Ambergris

Every so often I hear an interesting news story about a clued-up beach goer discovering thousands of pounds worth of ambergris washed up on the beach.  Ambergris is a weird substance.  It is produced in the intestines of Sperm whales where the indigestible beaks and pens of squids accumulate and bind together.  Even more weirdly, ambergris is prized in perfumery for its unique, musky scent but poses ethical and environmental challenges due to its extraction from endangered sperm whales. 

Sustainable alternatives to ambergris include labdanum, ambrette oil and ambroxide.  Labdanum is a resin from rockrose plant and ambrette oil is produced from ambrette seed, which is produced by a variety of hibiscus plants.  

Ambroxide is synthesized from sclareol, a component of the essential oil of clary sage.  Sclareol is oxidatively degraded to sclareolide, which is hydrogenated to ambradiol.  This is then dehydrated to form ambroxide.  These compounds are all chiral compounds, and the chiral centres within the compounds must each have one of two specific orientations in space.  This gives rise to challenges in the synthesis of ambroxide because it is difficult to purely produce compounds with the correct chiral orientation.  To do so, asymmetric catalysts can be used, which favour the formation of one chiral centre over another, or mild catalysts can be used, which retain the chirality of the starting material through to the product.  

Ruthenium catalysts are known to be very effective in hydrogenating sclareolide to ambradiol.  However, ruthenium is rare and highly expensive.  There are also toxicity concerns around ruthenium, making it more difficult to work with and dispose of.  In contrast with ruthenium, manganese is highly abundant, and is non-toxic.  Manganese catalysts produced by Matt Clarke's research group at the University of St Andrews have been found to successfully hydrogenate sclareolide to ambradiol under mild conditions, retaining the chiral centres as desired.  Manganese catalysts could be used as a more sustainable alternative to produce chiral molecules, such as ambroxide.  

Ambroxide is used in many high-end perfumes.  By using ambroxide in place of ambergris, the fragrance industry has shown that it can maintain luxury standards while reducing its environmental impact.