A heteroatom of difference: investigation of thiazolidine- and oxazolidine-4-carboxylates as hydrolytically cleavable profragrances

Thiazolidine- and oxazolidine-4-carboxylates derived from L-cysteine and L-serine, respectively, as well as their structural analogues with different substitution patterns at the 1,3-heterocycle, were prepared and evaluated as hydrolytically cleavable profragrances for practical applications in functional perfumery. Hydrolysis experiments at acidic and alkaline pH and dynamic headspace measurements on cotton in a typical fabric softener or liquid detergent application showed that oxazolidine-4-carboxylates rapidly hydrolysed, whereas thiazolidine-4-carboxylates were sufficiently stable to successfully release fragrances from the target surface over several days. Substitution of the heterocycle at the N-atom or at the 5-position had a strong impact on fragrance release and on side reactions, such as ester hydrolysis at alkaline pH. The compounds did not form rapidly equilibrating reversible systems in aqueous media. Unsubstituted thiazolidine-4-carboxylates successfully released fragrances from acidic and alkaline product formulations, which is unusual for hydrolytically labile pH-dependent delivery systems. They provided a long-lasting fragrance effect in the two laundry care applications tested and thus represent a simple and straightforward, universally applicable solution for efficient fragrance delivery under realistic everyday life conditions.