Disconnection Versus Interconversion of the Functional Group

When disconnection of a C-C bond is completed in a way to obtain synthons with a charge stabilized by the neighboring group, we talk about logical disconnection or disconnection that follows the correct mechanism. As to the electronic properties, synthons resulting from correct disconnection accommodate a stabilized charge, negative or positive, or an electronic sextet in carbene. 

In the second group we consider interconversions of functional groups with the exchange of hetereoatoms, breaking of old and formation of new C-heteroatom bonds. Examples of these transformations are interconversion of an amide to ester, thioketone to ketone or alkylhalide to alcohol. They are related to S5mthetic reactions formation of amide from ester, thioketone from ketone or haloalkane from alcohol. Characteristic of all the above interconversions is the disconnection (imaginative process ) of the C-heteroatom bond, C-N or C-O. In the synthetic direction C-N, C-S and C-Hal bonds are formed. Therefore, such FGIs are also denoted as DIS-C-X, where X stands for heteroatom. 

There are three main transformations of the functional group interconversion (FGI), elimination (FGE) and addition (FGA), assigned by double arrows (Table 1.1). Another important retrosynthetic tool represents reconnection (RCN) of acyclic to cyclic structures. Reversal to a synthetic direction is the formation of an acyclic structure by ring opening. A typical example is reconnection of an a, co-dicarbonyl compound to cycloalkene, e.g., 1,6-hexanedialdehyde to cyclohexene. In the synthetic direction ozonolysis of the double bond in cyclohexene affords 1,6-dialdehyde. 

Most rearrangements well known to synthetic organic chemists as the Beckmann or Claisen rearrangement can be analyzed retrosynthetically as retro-rearrangements. They are characterized by multiple disconnections-reconnections. Detailed retrosynthetic consideration of some rearrangements is given in Chap. 8. 

Concluding this section, it should be pointed out that the disconnection procedure is primarily governed by the number, type and relative position of the functional groups in the target molecule. 

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