clash with the axial methyl group. Then reduction of the enone to allyl alcohol 4 with tri-tert-butoxyaluminium hydride in tetrahydrofuran followed by hydroboration with borane in THF yields the borane 5 (only one substituent displayed for clarity). The diastereoselectivity of the hydroboration is a result of two factors: avoidance of the axial methyl group as well as axial hydride addition to avoid a twist-boat conformation in the transition state. The Grob fragmentation to 6 takes place with sodium methoxide in methanol at reflux. A methoxide group attacks the boron atom giving a borate complex which fragments. As each boron atom can hold three substrate molecules (R), the ultimate boron byproduct is trimethyl borate. As seen in 6, the mesylate being in the equatorial position allows its sigma star orbital to align ideally with the sigma bond drawn, allowing for the correct olefin geometry seen in 7. Another example is an epoxy alcohol fragmentation reaction as part of the Holton Taxol total synthesis. aza-Grob fragmentation 3-aza-Grob fragmentation is variation which takes place when an electrofuge and nucleofuge are situated at positions 1 and 5 on a secondary or tertiary amine chain with the nitrogen at the 3 position. The reaction products are an electrofugal fragment, an imine, and a nucleofugal fragment (such as an alcohol). 3-aza-Grob fragmentation can proceed with several different nucleofuges. The reaction mechanism has been reported to begin with the reduction of an ether protected amide to form a secondary alcohol. Fragmentation then takes place in a concerted step to form the reaction products. The scope of the reaction has been found to cover THF and tetrahydrothiophene protecting groups using various hydride agents. See also Eschenmoser fragmentation Wharton reaction References Elimination reactions Name reactions