RESUMEN
This paper describes a convenient protocol for the regioselective sulfonylation of alpha-chelatable alcohols. Typically, the reaction of alpha-heterosubstituted alcohols with 1 equiv of p-TsCl and 1 equiv of Et(3)N in the presence of 2 mol % of Bu(2)SnO leads to rapid, regioselective, and exclusive monotosylation. The pK(a) of the amine was correlated to the reaction rate. A plausible mechanism for this reaction has been proposed on the basis of (119)Sn NMR studies.
RESUMEN
The synthesis of unit A of the cryptophycins from (S)-trans-3-penten-2-ol and from (S)-trans-4-hexen-3-ol has been completed. The key stereodetermining step is a [2,3]-Wittig rearrangement of a propargyl ether. Elaboration of the rearrangement product was accomplished by means of a selective hydroboration-oxidation of a terminal alkyne, Horner-Emmons homologation of the derived aldehyde, followed by selective ozonolytic cleavage and Wittig olefination. This synthesis provides easy access to the series of cryptophycin analogues that incorporate a modified aromatic ring in unit A.
