Flexible Routes to the 5‐Hydroxy Acid Fragment of the Cryptophycins

Two solutions to establishing the anti stereochemistry of the vicinal stereocenters in the 5‐hydroxy acid subunit of cryptophycin, based on initial Evans syn aldol reactions between an N‐(propionyl)oxazolidinone 4 and a C3 aldehyde, were developed. In the first route, the secondary hydroxy group was inverted by use of Mitsunobu reaction conditions, whereas the second route features an inversion of the methyl‐bearing stereocenter, achieved by reductive removal of the chiral auxiliary, elimination to afford the terminal alkene, and anti‐selective hydroboration. The aryl part can be attached either by Wittig−Horner olefination or by a modified Julia coupling. Both routes provide the hydroxy acid 16 in a very efficient manner. The substrate for the hydroboration, alkene 22, could also be obtained from (S)‐malic acid. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)