The direct catalytic reaction between an enolizable carbonyl compound and an electrophile under proton-transfer conditions has emerged as a challenging versatile transformation in organic synthesis.[1] Over the last years several chiral Brønsted bases have been developed to promote this transformation diastereo-and enantioselectively.[2] However, successful examples are mostly limited to 1,3-dicarbonyl compounds and acidic carbon analogues as the pronucleophilic reaction partners. 5H-Thiazol-4-ones, in contrast, have been well known for a long time and have found several applications in pharmaceutical and medicinal chemistry.[3] Although structurally related to 5H-oxazol-4-ones [4] and 4H-oxazol-5-ones (azlactones), [5] 5H-thiazol-4-ones have, as far as we know, been never been used in asymmetric synthesis in spite of the fact that they may be easily deprotonated [6] and in spite of the importance of thiols and organosulfur compounds in organic synthesis [7] and chemical biology. [8] In this context, whilst chiral secondary thiol derivatives have been the subject of most investigations, tertiary thiols have remained mostly unexplored owing to the insufficient catalytic enantioselective methodology for their preparation in optically pure form. [9] The most general synthesis of organosulfur compounds involves reaction of a sulfur nucleophile with an electrondeficient p-olefin acceptor.[10] By using this approach Zhang and co-workers [11] reported an efficient catalytic asymmetric synthesis of tertiary thiols using chiral Brønsted bases and bsubstituted b-ethoxycarbonyl nitroalkene acceptors. Conversely, tertiary thiols may be produced through conjugate additions of sulfur-based carbon pronucleophiles.[12] For instance, using rhodanines as carbon pronucleophiles and iminium catalysis, Ye and co-workers [13] have recently reported the conjugate addition and the Diels-Alder reaction to a,b-unsaturated ketones and 2,4-dienals, respectively. Tertiary thiols have also been accessed through enantioselective a-sulfenylation of aldehydes, [14a] 1,3-dicarbonyl compounds, [14b] b-keto phosphonates, [14c] and 3-substituted oxindoles.[14d-g] Other methods include thiofunctionalization of unactivated alkenes,[15a] amination of 3-thiooxindoles, [15b] and the aldol [15c] and Mannich [15d] reactions of a-sulfanyl lactones. Accordingly, whilst many methodologies for the enantioselective synthesis of secondary thiols exist, approaches for the asymmetric synthesis of tertiary thiols are clearly necessary to help fill this important gap in organic chemistry. The inherent difficulty associated with the stereoselective construction of quaternary stereogenic centers is probably the reason for the limited number of studies. [16] In connection with our efforts directed towards the asymmetric synthesis of organosulfur compounds, that is, b,b-disubstituted b-mercapto carboxylic acids [17a,b] and thiiranes, [17c] we focused on the enantioselective generation of a tetrasubstituted carbon atom at the a position of a-mercapto carboxylic ...
