Structure basic center

The presence in the heterocycle of additional basic centers or those open to alkylation can lead to a change in reaction directions. It essentially limits the application of this method in the formation of a-methoxy nitrones. In such cases, it is reasonable to use diazomethane and, depending on the structure of hydroxamic acid (198-201) the yields of a-methoxy nitrones (197), (202-204) can rise from 17% up to 62% (Scheme 2.70) (353). 

The substituents at the nitrogen atoms are invariably tert-butyl groups (see objective 3) in combination with silicon, which simultaneously functions as an electron donor as well as a bridging atom to other intramolecular basic centers X (nitrogen or oxygen atoms). Formula 1 shows the general structural framework of the compounds. 

As structure (165) indicates, N-1 is the most basic center in (13). This was further supported (69JHC559) upon methylation of (13) to give exclusively (167). However, it is... 

Protonation of quinazoline in anhydrous medium probably gives the cation 7 since N3 is the basic center however, water addition at the 3,4-positions will afford the resonance-stabilized hydrated cation 8, irrespective of the site of protonation in the anhydrous species. Quinazolines with 0X0. thioxo, amino and, less frequently, with other substituents at position(s) 2 or/and 4 are capable of tautomerism. Quinazolin-2-ones, quinazolin-4-ones, and quina-zoIine-2,4-diones have been shown to exist as oxo tautomers in solution and in the solid state. In quinazolin-4-ones the 3//-tautomer 9a is present to a greater extent than the 1 //-tautomer 10a. Quiiiazolin-2-oiie prefers the 1//-tautomeric form 11a which is more stable than the 3//-quinoid structure na. Similarly, in quinazoline-2-thione and -4-thione as well as quinazoline-2,4-dithiones the 3//-, 1//-, and 1//,3//-tautomeric structures 9b, 11b, and 13b, respectively, are predominant. 2-Thioxo-2,3-dihydroquinazolin-4(l//)-one adopts the preferred tautomer 13a... 

Phosphazene bases represent a class of non-ionic materials which exhibit exceptionally high degrees of basicity, this being dependent upon the structural composition of the molecules [1, 2]. The core structural unit of non-ionic phosphazene bases is derived from a nitrogen basic center double-bonded to a pentavalent phosphorus (PI unit). This peralkylated triaminoiminophosphorane, through oligomerization, can result in a structurally diverse range of phosphazene bases (Fig. 1). 

A flat aromatic structure, cuplanar with the basic center, allowing van der Waals bonding to a flat surface on the receptor site to reinforce the ionic bond... 

A suitably positioned projecting hydrocarbon moiety funning a ibtcc-diinensional geometric pattern with the basic center and the flat aromatic structure... 

The 5 to 7 A distant compounds are readily explained by an interaction of the basic center with asp301, and its position is confirmed in the crystal structure... 

In addition to the attachment of a variety of different bases to the material structure, it is also possible to control the environment surrounding the basic center. Grafted MTS materials can be silylated to cap the silanols. This prevents the SiOH function hydrogen-bonding with the basic center, reducing its activity, and modifying the transport properties of the system to enable adsorption and desorption to occur more readily. 

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