Disconnections important bonds

There are, of course, many other ways of disconnecting double bonds you are about to meet an important disconnection of double bonds conjugated with carbonyl groups. Chapter 31 is devoted to the alternative methods available for making double bonds and controlling their stereochemistry. 

Below are summarized some important guidelines for choosing disconnections of bonds. Thus, the initial stage of the retrosynthetic analysis key fragments are recognized, which then can be recombined in the forward synthetic step in an efficient way. ... 

A logical extension of this strategy, particularly important for bridged polycyclic compounds, is the common atom approach. Atoms common to two or more rings (the common atoms) are marked disconnections of bonds to these atoms must inevitably reduce the number of rings. 

How is the synthetic analysis just presented related to a study of chemical reactions In all of these disconnections, we converted the disconnect products to actual compounds so we could make predictions about which chemical reactions might work. In all cases, one of two bonds was disconnected the bond connected to the functional group or the one next to it. We can use these observations to understand what reaction types are important for making C—C bonds. From Figure 1.1, it is clear that many functional groups are chemically related. We can use this information to understand chemical reactions that convert one functional group into another. 

This information is then used to disconnect bonds in the target, simplifying the structure along reasonable chemical reaction pathways. Wipke and Howe- defined the important bonds that are disconnected (as approached by the computer program LHASA, sec. 10.4.B, developed by Corey for analysis of retrosynthetic pathways). 

Why is the relative size of the disconnection fragments important Remember from Section 25.1 that the first disconnection gives fragments that are combined in the last chemical step of the synthesis. For disconnection of bond b, coupling the synthetic equivalents for 53 and 54 will give 52. Likewise, coupling the synthetic equivalents for 10 and 55 will give 52. Assume that 53, 54, and 55 all require further disconnection. Disconnection... 

The strategy of using intramolecular reactions to set up the correct relationslup between two groups is of more general importance. We obviously want to disconnect bonds a and b in TM 328 so that we add a four carbon fragment to PhOMe in the synthesis. 

The material in the succeeding chapters describes both the synthesis of the indole ring and means of substituent modification which are especially important in indole chemistry. The first seven chapters describe the preparation of indoles from benzenoid precursors. Chapter 8 describes preparation of indoles from pyrroles by annelation reactions. These syntheses can be categorized by using the concept of bond disconnection to specify the bond(s) formed in the synthesis. The categories are indicated by the number and identity of the bond(s) formed. This classification is given in Scheme 1.1. 

The category of 2-Gp-keyed transforms which disconnect C-C bonds is among the most important of all transform types. These transforms, especially in their stereoselective versions, are workhorses of retrosynthetic planning as their names alone attest aldol, Michael, Claisen,... 

Not all of these bonds would be disconnected in any one synthesis but it is important that you learn to see the possibilities quickly,... 

Remember, however that CHAOS does not intend to be a database of "synthetic methods" presently available to organic chemists. CHAOS is aimed at finding the intermediate precursors of the "synthesis tree" by means of selected basic disconnections (either "consonant" or "dissonant"). In the author s view, it is of minor importance, for instance, how a "carbene" -resulting from a -(2 + 1) cycloelimination- is generated, or whether a double bond is the direct result of a Wittig reaction or the dehydration of an alcohol formed in a Grignard reaction. In... 

With this dialog box you can select the restrictions on disconnection which you want. Observe too that prevention of disconnection of core bonds comes up on screen automatically, as it is very often important that this option is active. If you want, you can disactivate it by clicking it. In the lower part there are two lines with the names "Comments 1" and "Comments 2," where notes, bibliographic references, etc., relevant to the disconnection being introduced can be written. 

This group represents the most important group of the four-bond-disconnections class. 

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