Total Synthesis of Ustiloxin D and Considerations on the Origin of Selectivity of the Asymmetric Allylic Alkylation

As part of investigations into cell cycle checkpoint inhibitors, an asymmetric synthesis of the antimitotic natural product, ustiloxin D, has been completed. A salen−Al-catalyzed aldol reaction was employed to construct a chiral oxazoline 9 (99% yield, 98% ee) that served the dual purpose of installing the necessary 1,2-amino alcohol functionality as well as providing an efficient synthon for the requisite methylamino group at C9. The chiral aryl−alkyl ether was assembled using a Pd-catalyzed asymmetric allylic alkylation that notably delivered a product with stereochemistry opposite to that predicted by precedent. The linear tetrapeptide was subsequently cyclized to produce ustiloxin D. The mechanistic origin of the allylic alkylation selectivity was further investigated, and a working hypothesis for the origin of the observed stereoselectivity has been proposed.