Isothiourea-Catalysed Acylative Kinetic Resolution of Tertiary Pyrazolone Alcohols.

ABSTRACT

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.