Precursor generation

The direct goal of stereochemical strategies is the reduction of stereochemical complexity by the retrosynthetic elimination of the stereocenters in a target molecule. The greater the number and density of stereocenters in a TGT, the more influential such strategies will be. The selective removal of stereocenters depends on the availability of stereosimplifying transforms, the establishment of the required retrons (complete with defined stereocenter relationships), and the presence of a favorable spatial environment in the precursor generated by application of such a transform. The last factor, which is of crucial importance to stereoselectivity, mandates a bidirectional approach to stereosimplification which takes into account not only the TGT but also the retrosynthetic precursor, or reaction substrate. Thus both retrosynthetic and synthetic analyses are considered in the discussion which follows. 

The Pd-based methodology described by Ma et al. has scope for the synthesis of fused 1,3-oxazepines <06T9002>. Considerable value would be added to this methodology if it could be adapted to the simpler 1,3-benzazepine system, perhaps via alternative methods for carbinolamine precursor generation. 

Nodes 2 and 3 correspond to the two precursors generated during the sequence. A dotted line comes out of precursor 3. 

The option "Modify precursor" (Process menu) permits you to modify the position of the atoms in any precursor on the synthesis tree. It has been introduced because, when the program disconnects a molecule, partially overlapping atoms may appear, or the precursor generated may come up in an impossible conformation. 

Sequences of disconnections can be built by the CHAOSBASE program, analogous to those included in the CHAOS program. If CHAOS finds a sequence included in a data base, applicable to the target molecule, a dialog box will come up, which is very similar to the one in the last figure but also contains the buttons Last and Next. These buttons allow you to see the various intermediate precursors generated in the sequence. 

Figure 6, Precursor generation. Note that overall transforms may be encoded and applied without restrictions as to the actual mechanism.

Depending upon the particular precursors generated in the gas phase, etching, recombination, or film formation (i.e., polymerization) can occur. Also, gas-phase oxidant additives (O2, F2, CI2, etc.) can dissociate and react with unsaturate species. As an example, O2 can undergo the following reactions in a CF4 plasma ... 

Propargylic Ethers as Alkene Metathesis Initiator Precursors Generation of Alkenyl Alkylidene-Ruthenium Catalysts... 

As pointed out in Section 4.2.2, cationic polymerization processes are initiated by photoinitiators, which are essentially precursors generating Lewis and Bronsted acids. The mechanism of the process is ionic, and this chemistry does not function with the type of double bonds and unsaturation found in fhe monomers and oligomers reacting via free radical mechanism. 

A coozonolysis (two compounds in presence of ozone) is possible if one precursor generates the carbonyl oxide in situ that then reacts with the second compound - the carbonyl. JV-Methyl oximes have been found to be ideal precursors, because they readily react as dipolarophiles in a 1,3-dipolar cycloaddition with ozone. A retro- 1,3-dipolar cycloaddition then leads to the formation of the carbonyl oxide and methyl nitrite ... 

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