Question

Challenge Problem 18.38 The following is an example of a reaction sequence developed by Derin C. D'Amico and Michael E. Jung (UCLA) that results in enantiospecific formation of two new chirality centers and a carbon-carbon bond. The sequence includes a Horner-Wadsworth-Emmons reaction, a Sharpless asymmetric epoxidation, and a novel rearrangement that ultimately leads to the product. Propose a mechanism for rearrangement of the epoxy alcohol under the conditions shown to form the aldol product. (Hint: The rearrangement can also be accomplished by preparing a trialkylsilyl ether from the epoxy alcohol in a separate reaction first and then treating the resulting silyl ether with a Lewis acid catalyst (e.g., BF3).] OH عہ (1) CH.CHCO,CH, PO(OCH3)2, NaH (2) DIBAL-H H CH3 t-BuOOH TI(O-1-Pr)4 D-(-)-diisopropyl tartrate TBSO H ΓΗ TBSOT OH (t-butyldimethylsily! triflate), 1.3 equiv. N,N-ethyldiisopropylamine, 1.35 equiv. CH3 molecular sieves, 94% -42°C (95% enantiomeric excess) CH3 Using the atom labels (1 - 8) in the image below, complete the description of the four mechanism arrows. To indicate a bond, write the atom label on both ends of the bond with a slash between them (e.g. "3/5" indicates the bond between atoms labeled 3 and 5).