Transforms and Retrons

Given structure 1 as a target and the recognition that it contains the retron for the Diels-Alder transform, the application of that transform to 1 to generate synthetic precursor 2 is straightforward. The problem of synthesis of 1 is then reduced retrosynthetically to the simpler 

Additional keying information can come from certain other structural features which are present in a retron- or partial-retron-containing substructure. These ancillary keying elements can consist of functional groups, stereocenters, rings or appendages. Consider target structure 5 

On the basis of the preceding discussion the reader should be able to derive retrosynthetic schemes for the construction of targets 6, 7, and 8 based on the Diels-Alder transform. 

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From now on, the terms "transform" or "disconnection and "retron" or "substructure" will be used indistinctly. 

Once a particular 6-membered ring is selected as a site for applying the Diels-Alder transform, six possible [4 + 2] disconnections can be examined, i.e. there are six possible locations of the Jt-bond of the basic Diels-Alder retron. With ring numbering as shown in 36, and... 

Estrone (54, Chart 6) contains a full retron for the o-quinonemethide-Diels-Alder transform which can be directly applied to give 55. This situation, in which the Diels-Alder transform is used early in the retrosynthetic analysis, contrasts with the case of ibogamine (above), or, for example, gibberellic acid (section 6.4), and a Diels-Alder pathway is relatively easy to find and to evaluate. As indicated in Chart 6, retrosynthetic conversion of estrone to 55 produces an intermediate which is subject to further rapid simplification. This general synthetic approach has successfully been applied to estrone and various analogs. ... 

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