Strategically carboxylic acid group

When the Woodward-Eschenmoser synthesis began, it was known from the work of Bernhauer et al.5 that cobyric acid (4), a naturally occurring substance, could be converted directly into vitamin B12. Thus, the synthetic problem was reduced to the preparation of cobyric acid, a molecule whose seventh side chain terminates in a carboxylic acid group and is different from the other side chains. Two strategically distinct and elegant syntheses of the cobyric acid molecule evolved from the combined efforts of the Woodward and Eschenmoser groups and both will be presented. Although there is naturally some overlap, the two variants differ principally in the way in which the corrin nucleus is assembled. 

Fluorenone derived linker 17 prepared in two steps was coupled to aminomethyl-PS via DIPCDI [21]. Due to the presence of an electron-withdrawing carboxamide group, the release of carboxylic acids from this support requires strong acids, such as trifluoromethanesulfonic acid (TFMSA) (Scheme 1). Insertion of an oxygen adjacent to the biphenyl rings to the fluorenone scaffold provides xanthene 18 handle [22]. The oxygen is strategically located to decrease the acid concentration required... 

Such substitutions in saturated compounds can be carried out by a variety of strategics involving different nucleophiles and leaving groups, but the oxidation states remain the same. Acyl substitutions are analogous. For this reason carboxylic acid derivatives are treated as a common family of compounds. All have the same oxidation level and all can be converted from one to another by substitution reactions not requiring oxidation or reduction. 

Answer The final product is an alcohol, and in the process of converting the carboxylic acid into an alcohol, an ethyl group must be installed. It might be hard to see, at first glance, how this transformation can be achieved. But don t get discouraged. You are not expected to know how to solve problems like this instantly. Synthesis problems require thought and strategizing. Remember that you always want to try to go backwards as much as possible (retrosynthetic analysis). So, let s work our way backwards. 

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