Exo-endo amidine

FIGURE 15.32 (a) Replacement of ester ether oxygen by a fluoro-nitrogen (b) Exo-endo amidine in place of a car-... 

Among more exotic surrogates, the 3,4-diamino thiadiazole dioxide moiety was proposed as a weakly acidic urea equivalent (Figure 15.38). The similar thiatriazole dioxide is found in the H2 antagonist tuvatidine (HUK 978). Other bioisosteres are exo-endo amidinic heterocyles bearing an electron-attracting function in the a position (Figure 15.38). 

Fig. 13.16a) as mentioned by Lipinski. Other uncommon examples are found in the replacement of the ester function of acetylcholine by exo-endo amidinic functions of 3-aminopyridazines in muscarinic agonists (Fig. 13.16b) and of the carbomethoxy group of a-yohimbine (rauwolscine) by a A -methylsulfonamide function (Fig. 13.16c). ... 

Application of this one/pot protocol to the preparation of enantiopure amidine (215) derived from phenylglycine led to the formation of the desired cycloadducts (71). The reaction proceeded with good stereocontrol with an endo/exo ratio in the order of 10 1. In the case of the methyl ester derived product 216 (R = Me), a single isomer was isolated. The minor exo products 217 proved to have an unexpected stereochemistry at C(7), which was consistent with the energetically disfavored anti attack of the dipolarophUe, and it was therefore assumed that the product had... 

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