Protecting group dance

Scheme 7.4 Protecting group dance to block a less reactive hydroxyl group.

The principle of in situ protection is that the more reactive of two similar functional groups is (temporarily) transformed into a less reactive or unre-active moiety. However, the situation is much more complex if one wants to protect the less reactive of two functionalities. Usually one resorts to a (highly unsatisfactory) multistep protecting group dance [37] (Scheme 7.15). 

Scheme 7.15 Protecting group dance in order to protect the less reactive of two similar functional groups...

Scheme 7.16 Protecting group dance on shifting the protection between two pairs of hydroxyl groups...

This first synthesis of colchicine hy Eschenmoser was followed by 14 further syntheses, which have been comprehensively discussed recently [15]. Four of these syntheses, those by Woodward (Scheme 11.12) [16], Evans (Scheme 11.13) [17], Schmalz (Scheme 11.14) [15, 18, 19], and by Cha (Scheme 11.15) [20] will be presented below. The syntheses by Woodward, Evans, and Schmalz likewise do not offer a solution to the regioselectivity problem associated with colchiceine. That was finally attained by the synthesis of Cha, which is compromised, however, by a protecting group dance from Boc to acetyl and back to Boc at the amino group. 

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