DEVELOPMENT OF BIOCATALYSTS AND BIOCATALYTIC METHODS FOR THE STEREOSELECTIVE CONSTRUCTION OF C-S BONDS

Chiral sulfur compounds bearing a stereocentre at the C(sp3)–S bond find broad applications in chemistry, and in particular in the pharmaceutical and flavours & fragrances sectors. Diltiazem, clindamycin, penicillins and cephalosporins are examples of drugs containing a stereodefined C-S bond, where the stereochemical configuration at the sulfur atom plays a crucial role in modulating the pharmacological activity and selectivity. Similarly, chiral sulfur compounds find application in the flavour & fragrance industry, where molecules like bor g-hydroxysulfides often owe their organoleptic properties to the presence of a stereodefined C(sp3)-S bond. The development of efficient and selective synthetic methods to generate a stereocentre at the C–S bond remains a key challenge in modern organic synthesis. Current approaches include transition-metal asymmetric catalysis, chiral auxiliary-based strategies, or organocatalysis. However, such approaches often show limitations and drawbacks especially in terms of sustainability and green chemistry. While biocatalysis has been widely employed for the generation of stereocentres at C-C, C-O and C-N bonds, yet its application in the construction of chiral C(sp3)–S bonds remains poorly explored. Two distinct biocatalytic approaches for the synthesis of chiral sulfur compounds bearing C(sp3)-S bonds have been recently developed by our research group. First, chiral sulfides bearing stereodefined C(sp3)–S bonds have been synthesised through ene-reductase (ENE) biocatalysed reduction of prochiral vinyl sulfides bearing a C(sp2 )–S bond. ENE biocatalysts have been successfully employed in cooperative sequential/concurrent chemoenzymatic and biocatalytic cascades to afford chiral sulfides with good to excellent yields (up to 96%) and enantioselectivities (up to >99% ee). Moreover, a series of one-pot chemoenzymatic and biocatalytic cascades have been developed to access valuable chiral b-hydrosulfides with excellent diastereo- and enantioselectivities. Second, a lactonase biocatalyst has been developed through rational engineering of the reconstructed ancestral PON enzyme N9, and it was found able to catalyse the dynamic kinetic resolution of chiral a-thiosubstituted-g-thiolactones with excellent conversions and ees (up to >99%). Computational and site-directed mutagenesis studies were carried out to provide insights on the mechanism and the stereoselectivity of the lactonase biocatalyst.