Urea, protonation

The last reaction perhaps involves an intermediate such as 33a which expells a proton and dimethyl sulfide. Formation of the Schiff s base with t-butylamine, reduction with sodium borohydride and hydrogenolysis of the benzyl ether produces sulfonterol (28). Despite the fact that the methylene hydrogen of sulfonterol must be much less acidic than of the corresponding urea proton on carbuterol or the sulfonamide proton on soterenol, good bioactivity is retained. 

The binding of the guest molecule to the periphery of 33 was studied by NMR spectroscopy, and this shows a shift of the urea protons of the dendrimer from 6.20 and 5.43 ppm to 6.34 and 5.64 ppm, respectively, resulting after the addition of 32 equiv of acid 32 to 33. This shift is similar to that previously reported for other guest molecules [108]. In a H-NOE NMR experiment of the dendrimer with 32 equiv of acid 32 the aromatic... 

As part of a study of host-guest complexation, the hydrogen-bonded dimerization of the substituted urea N-phenyl-N -(2-pyridyl)urea (U) in 1 1 CH2CI2/toluene was studied. The chemical shift of the high-field urea proton is especially sensitive to concentration (see Figure 22-20). 

Chemical shift d of hiqh field urea proton) ... 

Dimerization af N-phenyl-N -(2-pyridyl)urea (II) Studied by NMR (Chemical shift a of high fiold urea proton) K= 39.1 ... 

When two different calixarene tetraureas are mixed together. heterodimers form along with the corresponding homodimers. From a mixture of tetraurea and tetrasul-fonyl urea, heterodimers form exclusively. " The increased acidity of the — SO2NH urea proton may be the reason for sueh selection. The preference of the aryl urea/ sulfonyl urea eould possibly be the basis of a binary molecular code. 

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