1 (237)

Prévia do material em texto

Solutions to Problems 189 (b) 1. NaBH₄. Reduces ketone to alcohol. 2. NaOH. Makes alkoxide, leading to internal displacement of Br⁻ and oxacyclopropane formation. (c) The first step must form a ß-OH, with the new hydroxy group trans to the original Br. Otherwise the second step will not give an oxacyclopropane, for the reason given in (a). Notice that the ß-OH and α-Br groups are automatically trans, diaxial as a result of the natural shape of the steroid rings. 73. I₂ I equiv. H₂O₂ allicin 74. Four diastereomers can be drawn based on the general structure given in the problem: OH OH OH OH R R Br S Br Br S Br S R S R C₆H₅ C₆H₅ C₆H₅ C₆H₅ Their most stable conformations are as follows. OH OH OH OH C₆H₅ Br C₆H₅ C₆H₅ Br C₆H₅ Br Br (a) Upon treatment with base, the reaction that occurs will be governed principally by the configura- tion of the stereocenters in the substrate and the conformation in which the compound is most sta- ble. The main options are E2 reaction and deprotonation of the -OH group followed by internal displacement. For internal (backside, displacement, the -Br and -OH groups must be trans and both axial: H OH O O C₆H₅ C₆H₅ C₆H₅ Br Br The substrate for this process is therefore identified as compound A. Next, we look for a substrate that can easily eliminate to furnish the enol pictured in the problem. H OH OH O OH C₆H₅ Br C₆H₅ C₆H₅