One-Pot Access to Functionalised Malamides via Organocatalytic Enantioselective Formation of Spirocyclic ß-Lactone-Oxindoles and Double Ring-Opening.

Malamides (diamide derivatives of malic acid) are prevalent in nature and of significant biological interest, yet only limited synthetic methods to access functionalised enantiopure derivatives have been established to date. Herein, an effective synthetic method to generate this molecular class is developed through in situ formation of spirocyclic ß-lactone-oxindoles (employing a known enantioselective isothiourea-catalysed formal [2+2] cycloaddition of C(1)-ammonium enolates and isatin derivatives) followed by a subsequent dual ring-opening protocol (of the ß-lactone and oxindole) with amine nucleophiles. The application of this protocol is demonstrated across twelve examples to give densely functionalised malamide derivatives with high enantio- and diastereo-selectivity (up to >95:5 dr and >99:1 er).

Isothiourea-Catalysed Acylative Kinetic Resolution of Tertiary Pyrazolone Alcohols.

The development of methods for the selective acylative kinetic resolution (KR) of tertiary alcohols is a recognised synthetic challenge with relatively few successful substrate classes reported to date. In this manuscript, a highly enantioselective isothiourea-catalysed acylative KR of tertiary pyrazolone alcohols is reported. The scope and limitations of this methodology have been developed, with high selectivity observed across a broad range of substrate derivatives incorporating varying substitution at N(2)-, C(4)- and C(5)-, as well as bicyclic constraints within the pyrazolone scaffold (30 examples, selectivity factors (s) typically >100) at generally low catalyst loadings (1 mol%). The application of this KR method to tertiary alcohols derived from a natural product (geraniol), alongside pharmaceutically relevant drug compounds (indomethacin, gemfibrozil and probenecid), with high efficiency (s > 100) is also described. The KR process is readily amenable to scale up, with effective resolution on a 50 g (0.22 mol) scale demonstrated. The key structural motif leading to excellent selectivity in this KR process has been probed through computation, with an NC=O•••isothiouronium interaction observed within the favoured transition state. Similarly, the effect of C(5)-aryl substitution that leads to reduced experimental selectivity is probed, with a competitive π-isothiouronium interaction identified as leading to reduced selectivity.