Synthesis of the C(21)-C(26) fragment of superstolide A: concerning the stereochemistry of (E)-crotylboration reactions of alaninal derivatives

A stereoselective synthesis of the C(21)-C(26) fragment of superstolide A has been completed. The absolute and relative stereochemistry of intermediate 14 has been conclusively proven by NMR and X-ray diffraction methods. In the course of this work, it was found that the stereochemistry of 3 had been misassigned in our previously reported synthesis of the C(18)-C(26) segment. This error stems from the unexpected diastereoselectivity in the double asymmetric reaction of N-acetyl-d-alaninal 1 and the tartrate ester modified (E)-crotylboronate (R,R)-2.