Chemoenzymatic preparation of musky macrolactones Tancredi Fortunati, Mariantonietta D’Acunto, Tonino Caruso, Aldo Spinella * Dipartimento di Chimica e Biologia, Universit a di Salerno, Via Giovanni Paolo II, 132, 84084 Fisciano, Salerno, Italy article info Article history: Received 20 November 2014 Received in revised form 18 February 2015 Accepted 3 March 2015 Available online 7 March 2015 Keywords: Musks Macrolactones Enzyme-catalyzed Non-aqueous solvent CALB abstract A chemoenzymatic approach to some musk macrolactones has been explored by the optimization of macrolactonization catalyzed by Candida antarctica lipase B (Novozym 435). This fast and high yield op- timized methodology represents a large improvement to previously reported results. The methodology was applied to the preparation of 16-hexadecanolide, exaltolide, ambrettolide and (15R)-15-hexadecanolide. Ó 2015 Elsevier Ltd. All rights reserved. 1. Introduction Macrocyclic musks are an important group of natural com- pounds used in perfumery. Actually, due to the expensive recovery from natural source, they are produced industrially by chemical synthesis. 1 From the structural point of view the main macrocyclic musks are C15e17 ketones and C15, C16 lactones. In this paper we have explored a chemoenzymatic approach to some musk macro- lactones, in order to study a green approach to their preparation. In recent years, biocatalytic methodologies have attracted organic chemists as a green alternative to conventional chemical syn- thesis. 2e4 In this context, enzymatic processes in organic solvents 5 are particularly appealing and several studies have been conducted in order to optimize the application of biocatalysis to organic syn- thesis. In particular, lipases in organic solvents catalyze esterifica- tion and transesterification reactions and this ability has been widely used for the kinetic resolution of racemates. 6 Trans- esterification catalyzed by lipase can also be used for ring closure of hydroxyesters to macrolactones. In 1984, Gatfield was the first to apply this approach to the synthesis of pentadecanolide. 7 Then, in 1987, Yamada et al. described the successful lipase catalyzed preparation of macrocyclic lactones from several u-hydroxyacid methyl esters. In particular, 16-hexadecanolide (2) was prepared in 80% yield using lipases from Pseudomonas sp. in dry benzene for 72 h. 8 However, in spite of the advantages offered by this approach some drawbacks such as the long reaction time required for maximal yields and the cost of the enzyme made this methodology scarcely used and few synthetic applications have been reported in literature. Hence, we decided to reinvestigate the enzymatic mac- rolactonization process, examining different types of lipase and reaction conditions, in order to improve its application. 2. Results and discussion We started with the preparation of 16-hexadecanolide (2), an important macrocyclic musk lactone isolated from the aroma of orchids Epidendrum aromaticum and Cattleya aurantiaca. 9 We first considered the reaction environment because this is one of the most important factor limiting enzyme activity in or- ganic solvents. It is well known that hydrophobic solvents are the best choice for enzymes in organic media (in our work all reactions were conducted in cyclohexane), however, they should be nearly but not completely anhydrous because some water is needed for the catalytic function of the enzyme. 10 In fact, water plays a key role in biocatalysis in organic environments, since it is involved in noncovalent interactions essential for maintaining the active con- formation of the enzyme. However, while the addition of water to solid enzyme preparations in organic solvents can enhance the enzyme activity by increasing the flexibility of the active site, excess of water may facilitate the aggregation of the enzyme causing a decrease in its activity. Usually the water in the system is de- scribed in terms of water activity (a w ) where the a w of a ‘sealed’ system is often given by the ratio of its vapor pressure (P w ) and vapor pressure of pure water (P  w ): a w ¼P w /P  w . Conditions, with a fixed value of a w , are generally set by adjusting a w value through the use of salt hydrates that facilitate the exchange of water with * Corresponding author. Fax: þ39 089969603; e-mail address: spinella@unisa.it (A. Spinella). Contents lists available at ScienceDirect Tetrahedron journal homepage: www.elsevier.com/locate/tet http://dx.doi.org/10.1016/j.tet.2015.03.007 0040-4020/Ó 2015 Elsevier Ltd. All rights reserved. Tetrahedron 71 (2015) 2357e2362