2,4-Diamino-5-benzylpyrimidines as antibacterial agents. 8. The 3,4,5-triethyl isostere of trimethoprim. A study of specificity.

3,4,5-Triethylacetophenone was synthesized in 60% yield by a Friedel-Crafts reaction from 4-ethylacetophenone and converted to 2,4-diamino-5-(3,4,5-triethylbenzyl)pyrimidine (2), a trimethoprim (1) isostere, by standard techniques. This compound is more lipophilic than 1 by three log units (log P, octanol/water). Compound 2 was approximately equipotent with 1 in inhibiting Escherichia coli dihydrofolate reductase (DHFR), 2-fold more potent against P. berghei and N. gonorrhoeae DHFR, and 10 and 25 times better an inhibitor of rat and chicken liver DHFR, respectively. Although the 3,4-dimethoxy analogue 19 was 10-fold less inhibitory to E. coli DHFR than 1, it was 3-4 times more potent on the vertebrate isozymes, whereas the diethyl congener 10 followed 19 in its E. coli DHFR binding but was less active on rat and chicken DHFR. Therefore, a significant portion of the selectivity of 1 for bacterial, as opposed to vertebrate, DHFR, involves the methoxy functions. An analysis of the X-ray data on 1 and 2 complexed with chicken DHFR, coupled with kinetic data, led to the conclusion that the difference in binding energies of the methoxy and ethyl compounds probably involve desolvation factors, as well as direct energies of interaction with protein atoms. Thus, one cannot invoke lipophilicity or shape alone in explaining the relationship in properties of 1 and 2.

2,4-diamino-5-benzylpyrimidines and analogues as antibacterial agents. 6. A one-step synthesis of new trimethoprim derivatives and activity analysis by molecular modeling.

A new route to 2,4-diamino-5-(4-hydroxybenzyl)pyrimidines has been developed that involves the condensation of 2,4-diamino-5-(hydroxymethyl)pyrimidine with phenols in acidic medium. The use of phenol and its 2,6-dialkyl derivatives produces 5-(4-hydroxybenzyl)pyrimidines exclusively. However, 2,6-dimethoxyphenol produces a mixture of 5-(3-hydroxy-2,4-dimethoxybenzyl)- and 5-(4-hydroxy-3,5-dimethoxybenzyl)pyrimidines. The phenolic condensation has been used to prepare a series of alkyl-substituted 5-(4-hydroxybenzyl)- and 5-(4-alkoxybenzyl)pyrimidines. The use of 1,2,3-trimethoxybenzene in place of a phenol produces 2,4-diamino-5-(2,3,4-trimethoxybenzyl)pyrimidine, a trimethoprim isomer with low antibacterial activity. The use of molecular models of several of the new ortho-substituted derivatives in the active site of dihydrofolate reductase has provided a rational explanation for their activities relative to trimethoprim.

2,4-Diamino-5-benzylpyrimidines and analogues as antibacterial agents. 5. 3',5'-Dimethoxy-4'-substituted-benzyl analogues of trimethoprim.

Forty trimethoprim analogues in which the para substituent in the benzene ring was varied were prepared for antibacterial evaluation. All were very potent inhibitors of Escherichia coli dihydrofolate reductase. The similarity of their inhibitory activities strongly suggested that the side chains beyond the first two atoms were not in contact with the enzyme. However, among 38 ether derivatives which varied widely in their bulk and lipophilicity, very few approached trimethoprim in their broad-spectrum in vitro antibacterial activity. The 4'-methyl and 4'-ethyl analogues and the allyloxy and gamma-chloropropoxy ethers had activities fairly close to that of trimethoprim. The two ethers were chosen for further evaluation in vivo. Neither compound quite matched trimethoprim in efficacy in mice, and their half-lives, as well as that of the beta-methoxyethoxy analogue, were found to be shorter in dogs.

2,4-Diamino-5-benzylpyrimidines as antibacterial agents. 4. 6-Substituted trimethoprim derivatives from phenolic Mannich intermediates. Application to the synthesis of trimethoprim and 3,5-dialkylbenzyl analogues.

The preparation of a wide variety of 6-substituted trimethoprim analogues was readily accomplished by the reaction of 2,4-diamino-6-substituted-pyrimidines with 2,6-dimethoxy-4-[(N,N-dimethylamino)methyl]phenol at 120--160 degrees C. The less reactive 2,6-dialkyl-4-[(N,N-dimethylamino)methyl]phenols reacted successfully with 2,4-diamino-6-(alkylthio)pyrimidines to give 5-(substituted benzyl)pyrimidines. The phenolic groups of the products were alkylated in high yield when a nonreactive 6-substituent was present in the pyrimidine ring. 6-(Alkylthio) groups were easily removed with Raney nickel. Trimethoprim was thus obtained in high yield from its 6-(methylthio) counterpart. The 6-substituted trimethoprim analogues all had low activity as inhibitors of Escherichia coli dihydrofolate reductase and as antibacterial agents.