Pteridines, substituted synthesis

Pachter, I.J. (1963) Pteridines. II. Synthesis of 6-substituted 7-aminopteri-dines from aldehydes. Journal of Organic Chemistry, 28,1191-1196. 

The first are competitors of PABA (p-aminobenzoic acid) and thus intermpt host de novo formation of the tetrahydrofoUc acid required for nucleic acid synthesis. Examples of dmgs that fall into this group are the sulfones and sulfonamides. The most weU-known of the sulfones is dapsone (70, 4,4 -diaminodiphenyl sulfone, DDS), whose toxicity has discouraged its use. Production of foHc acid, which consists of PABA, a pteridine unit, and glutamate, is disturbed by the substitution of a sulfonamide (stmcturally similar to PABA). The antimalarial sulfonamides include sulfadoxine (71, Fanasd [2447-57-6]) sulfadiazine (25), and sulfalene (72, sulfamethoxypyrazine [152-47-6] Kelfizina). Compounds of this group are rapidly absorbed but are cleared slowly. 

The cleavage of fused pyrazines represents an important method of synthesis of substituted pyrazines, particularly pyrazinecarboxylic acids. Pyrazine-2,3-dicarboxylic acid is usually prepared by the permanganate oxidation of either quinoxalines or phenazines. The pyrazine ring resembles the pyridine ring in its stability rather than the other diazines, pyridazine and pyrimidine. Fused systems such as pteridines may easily be converted under either acidic or basic conditions into pyrazine derivatives (Scheme 75). 

Examination of the pyrazino[2,3-rf]pyrimidine structure of pteridines reveals two principal pathways for the synthesis of this ring system, namely fusion of a pyrazine ring to a pyrimidine derivative, and annelation of a pyrimidine ring to a suitably substituted pyrazine derivative (equation 76). Since pyrimidines are more easily accessible the former pathway is of major importance. Less important methods include degradations of more complex substances and ring transformations of structurally related bicyclic nitrogen heterocycles. 

A new, versatile and selective synthesis of 6- and 7-substituted pteridines was reported by Rosowsky (73JOC2073). /3-Keto sulfoxides, which can be viewed as latent a keto aldehydes, react with (251) to give 6-substituted pterins, and the use of a-keto aldehyde hemithioacetals leads in a regiospecific synthesis to the isomeric 7-substituted pterins (equation 85). 

Since the structures of the Gabriel-Isay condensation products of 5,6-diaminopyrimidines with unsymmetrical 1,2-dicarbonyl or a-substituted monocarbonyl compounds are always ambiguous, the synthesis of 6- and 7-substituted pteridines by an unambiguous approach was and still is a necessity and an important challenge. 

Pteridin-4-one, 7-(p-substituted phenyl)-hydroxylation, 3, 287 Pteridin-6-one, 7-amino-synthesis, 3, 314... 

Thieno[3,4-d]oxazole-3a(4H)-carboxylic acid, dihydro-2-methyl-synthesis, 6, 1020 Thieno[2,3-d Joxazoles synthesis, 6, 990 Thieno[3,2-g]pteridine structure, 3, 284 lH-Thieno[3,4-c]pyran-2-ones synthesis, 4, 1032 Thienopyrazines synthesis, 4, 1022-1024 Thieno[2,3-6]pyrazines, 4, 1023 electrophilic substitution, 4, 1024 Thieno[3,4-6]pyrazines, 4, 1024 Thieno[3,4-c]pyrazole, 4,6-dihydro-3-hydroxy-carbamates... 

An acid-catalyzed substitution of a 6-oxo group on 2-aminopteridine-4,6-dione with hydrogen chloride in alcohols (65-100°, 3 hr, 80% yield) represents a convenient synthesis of the 6-alkoxy analogs. The reaction proceeds also with pteridine-2,4,6-trione and its 1-methyl and 1,3-dimethyl derivatives. While methoxylation of 2,4,7-trichloro-quinoline gives about equal amounts of 2- and 4-substitution, acid-catalyzed hydrolysis gives specific reaction at the 2-position only. ... 

Recent advances in selective synthesis of 6- and 7-substituted pteridines 98H(48)1255. 

The synthesis of deoxysepiapterin (82) has been recently achieved by homo-lytic nucleophilic substitution of the pteridine nucleus by acyl radicals (505). Since this substitution arises preferentially at the most electron-deficient 7 position, protection at 7 position is necessary for nucleophilic attack at the 6 position. 2,4-Diamino-7-methylthiopteridine (597) and 2-amino-4- -pentyloxy-7-n-pro-pylthiopteridine (600), protected by the thio function, can be used as starting materials. Homolytic acylation of 597 with the system propionalde-hyde/Fe2+//ert-butylhydroperoxide afforded 6-propionylpteridine (598) in good yields, which could be transformed to deoxysepiapterin (82) by selective hydrolysis followed by deprotection of the thio function (Scheme 75). Deoxysepiapterin (82) can also be prepared by a similar procedure from 600. 

A major recent growth point in substitution reactions has been the synthesis of pteridine glycosides, especially ribosides for study as probes in DNA chemistry taking advantage of the fluorescent properties of pteridines (see Section 10.18.12.4). Typically these reactions are developments of standard methods of glycosylation used with purines and pyrimidines as nucleophiles. In these and in other cases, the ambident nucleophiles within the pterin... 

The substitution of pteridines at positions adjacent to the pyridine-like nitrogen atoms in either the pyrimidine or the pyrazine is a well-established synthetic procedure and remains an important contributor to the synthesis of complex substituted pteridines. Significant extensions of these methods have been described at both the pyrimidine and pyrazine rings. 

With carefully selected aliphatic precursors, the synthesis of single stereoisomers of side-chain-substituted pteridines has been achieved (Schemes 20-22). The synthesis of L-biopterin 106 requires 5-deoxy-L-arabinose 102 as a key intermediate preparable from the expensive sugars L-rhamnose and L-arabinose. Alternatively, the readily... 

In modern medicinal chemistry, the creation of diversity on a structural framework is important. In principle, diversity at positions 2, 4, 6, 7, and 8 of pteridines can be achieved using such solid-phase chemistry. This prototype solid-phase synthesis involved nitrosation of the resin-bound pyrimidine, reduction of nitroso group with sodium dithionite, and subsequent cyclization with biacetyl to afford pteridines 114 and 115. Cleavage from the resin by nucleophilic substitution of the oxidized sulfur linker using w-chloroperbenzoic acid or DMDO led to the pteridine products 116 and 117 (Scheme 23). 

Folate analogues continue to have importance in chemotherapy, especially heterocyclic analogues other than pteridines which are covered in Chapters 10.15-10.17 and 10.19. 1,3-Dimethyllumazine analogues of folates for use as model compounds have been prepared by side-chain elaboration of 6-bromomethyl-l,3-dimethyllumazine (Scheme 34) <1996JHC341>. More notable in this work, however, was the synthesis of the bromomethyl precursor itself in addition to routine bromination of the 6-methyllumazine 175 prepared by condensation of dihydroxyacetone with 5,6-diamino-l,3-dimethyluracil, a cycloaddition reaction between trimethylsilyl enol ethers and the pyrimidyl bisimine 177, via cycloadducts such as 176, afforded substituted pteridines in moderate to good yields. 

A one-pot procedure for the synthesis of amino and phenyl substituted pteridines using A,A -dimethyldichloromethy-leneiminium chloride has been described <2006H(68)933>. 

---