Stereochemistry of Hydroboration

Directing Effects in the Hydroboration of Steroidal Alkenes

“…In polycyclic systems exemplified by the steroids, there is a balance between the steric effects exerted by the carbon skeleton and its substituents such as the angular C-10 (β) methyl group, 6,7 and the stereoelectronic effects exerted by vinyl and allylic substituents.…”

“…Comparison of these results emphasises the transannular stereochemical directing effect of the 10-methyl group on the facial selectivity of hydroboration of a 4-ene which was described in the earlier work. 6,7 However, both 17β-acetoxyandrost-4-en-3β-ol 13 and 19-norandrost-4-ene-3β,17β-diol 14, gave the corresponding 5α-androstane and 19-nor-5α-androstane-3α,4α,17β-triols whilst 17β-acetoxyandrost-4-en-3α-ol 15 gave entirely products, such as 5β-androstane-3α,4β,17β-triol, arising from β-face attack. This was despite the steric hindrance arising from the C-10β methyl group.…”

“…Studies on the hydroboration:oxidation of simple cyclic olefins such as 1-methylcyclopentene, 1-methylcyclohexene, norbornene and α-pinene 5 led to the conclusion that the reaction proceeded preferentially from the less-hindered face of the alkene. These studies were extended to steroidal alkenes 6,7 in which it was shown that hydroboration took place on the less-hindered α-face of the alkene opposite to the 10βmethyl group. Thus cholest-1-ene 1 gave 5α-cholestan-1α-ol 2 (35%) and 5α-cholestan-2α-ol 3 (40%), 5α-cholest-2-ene gave 5α-cholestan-2α-ol (35%) and 5α-cholestan-3α-ol (45%) and 5α-cholest-3-ene gave 5α-cholestan-3α-ol (40%) and 5α-cholestan-4α-ol (45%).…”