II. CHEMISTRY 117 



acid, which iiuhca(os that tlio action of nitrite on tetrahychoptoricUnos is 

 one ot" o\i(hition i-atluM- than nitroHatiou. Furthormoro, since XXV is 

 hlocked in the S po-sition with an ethyl group and yet has properties which 

 are simihir to those of leucovorin, the possibiUty of a 5 ,8-linked ring system 

 in leucovorin can also be eliminated. 



Because of the unique stability of the forinyl group in 5-fonnyltetrahy- 

 dropteridines to dilute alkali, it seemed proba])le that there might be hy- 

 drogen bonding or even actual linkage of the formyl to the 4-hydroxyl 

 group. This (juestion was investigated by the preparation of 2-amino-4- 

 niethyl-().7-diphenylpteridine (XXVI), ^" followed by the usual hydro- 

 genation and formylation to give 2-amino-4-methyl-5-formyl-6,7-diphenyl- 

 o ,6 ,7 ,8-tetrahydropteridine (XXVII). As with the reduced and formylated 



H 



H I . 



CfiHs— 

 CeHo — 





-NHs 



N ^.n^y^^^^r 



CHa 



CH, 



HC=0 



XXVI XXVII 



(2-Amino-4-methyl-6,7-diphenylpteridine) (2-Amiiio-4-metliyl-5-formyl-6,7- 



diphenyl-5, 6, 7, 8-tetrahydropteridine) 



4:-hydrox3^3teridines, XXVII is stable to hot O.l N sodium hydroxide and 

 surjjrisingly stable to acid, requiring hot 2.5 N hydrochloric acid to remove 

 the formyl group. Such behavior virtually eliminates the possibility of 

 bonding between the 4-hydroxyl and the formyl group and also presents 

 evidence that the 4-oxygen of pteridines in acid solution exists mainly as 

 the ketone form, since the 5-formyl group is more labile in the 4-hydroxy- 

 pteridines than it is in XXVII. 



As additional evidence that the formyl group of leucovorin is not in- 

 volved in a new^ ring linking the 5 and 10 positions, reduced 10-methyl 

 PGA^* has been found to formylate under the usual conditions in the 

 j)resence of acetic anhydride to give 2-amino-4-hydroxy-o-formyl-10- 

 methyl-5,6,7,8-tetrahydro PGA (XXVIII). ^^ This analog has the same 

 type of alkali-stable, acid-labile formyl group as leucovorin and exhibits 

 the same behavior in the polarograph. When titrated immediately with 

 nitrous acid, XXVIII does not react, but after removal of the formyl group 

 by standing in dilute acid, nitrous acid is readily absorbed. The ])reparation 



9^ D. B. Cosulich and J. M. Smith, Jr., /. Am. Chem. Soc. 70, 1922 (1948). 



