General principles of substitution

The following diagram illustrates the positions from which attack at the angular methyl groups has been realized. [Pg.238]

The oxygen atoms portrayed above can, in principle, be replaced by nitrogen atoms, although this has been verified by experiment only with 20-amino steroids. In view of the difficult access to 8/5-hydroxy steroids, these compounds have not been used for substitution reactions, but they fulfill all requirements for suecessful functionalizations at both C-18 and C-19. [Pg.238]

The actual substitution reaction can be divided into 3 steps [Pg.238]

The hydrogen transfer (from carbon to the heteroatom). Reactions of the earbon radical. [Pg.238]

The radical X is formed by homolysis of the X—R bond either thermally or photolytically. In the reactions of alcohols with lead tetraacetate evidence suggests that the X—R bond (X = 0, R = Pb(OAc)3) has ionic character. In this case the oxy radical is formed by a one electron transfer (thermally or photochemically induced) from oxygen to lead. [Pg.238]

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