Hydroxyls halogenation

New polar (Cyelie) Amines Amino aleohols (a-Hydroxyl/halogen) aeids... 

New polar (Cyclic) Amines Amino alcohols (a-Hydroxyl/halogen) acids... 

Nature utilizes the shikimate pathway for the biosynthesis of amino acids with aryl side chains. These nonprotein amino acids are often synthesized through intermediates found in the shikimate pathway. In many cases, L-a-amino acids are functionalized at different sites to yield nonprotein amino acids. These modifications include oxidation, hydroxylation, halogenation, methylation, and thiolation. In addition to these modifications, nature also utilizes modified biosynthetic pathways to produce compounds that are structurally more complex. When analyzing the structures of these nonprotein amino acids, one can generally identify the structural similarities to one of the L-a-amino acids with aromatic side chains. 

The organoboranes have proven to be very useful intermediates in organic synthesis. In this section, we will discuss methods by which the boron atom can efficiently be replaced by hydroxyl, halogen, or amino groups. There are also important processes which use alkyboranes in the formation of new carbon-carbon bonds. These reactions will be discussed in Section 9.1. 

Side-chain aromatic or heteroaromatic moieties may be hydroxylated, halogenated or N-methylated. 

In steroids the 6a-position (e.g. prednisolone, Figure 20.11) is a position that is normally hydroxylated. Grafting a methyl in this place prevents its hydroxylation. Halogens (particularly fluorine) suit even better because they are not sensitive at all to oxidative attacks. 

Polydienes can be hydroxylated, halogenated, cyanomethylated, and y-oxy-alkylated by hydroboration coupled with other reactions, although precise control seems difficult. A typical reaction sequence is shown in Scheme 8. 

Hydroxylation Halogenation Halohydrin foimation Cycloadditions Introduction of fimctional groups... 

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