Deoxidative C-Substitutions

Of these Meissenheimer-type reactions, classical C-chlorinations (Section 3.1.5), C-hydroxylations (Section 4.1.1), and C-aryloxylations (Section 4.4.1) have been covered previously. The remaining types are illustrated in the following examples. 

2(lf/)-Quinoxalmone 4-oxide (271) gave 3-nitromethyl-2(lf/)-quinoxalinone (272) (MeNOa, pyridine, EtOH, reflux, 1 h 52%).  

6-Fluoro-2(17/)-quinoxalinone 4-oxide (273) gave 6-fluoro-3-methyl-2(l//)-quinoxalinone (275), probably by degradation of the adduct (274) (ACCH2-COjEt, PhH, H2O, NaOH, 65°C, 1 h % several analogs likewise).  

2-Ethylquinoxaline 4-oxide (278) gave 3-ethyl-2-quinoxalinecarbonitrile (279) (MeaSiCN, l,8-diazabicyclo[5.4.0]undec-7-ene, THE, 20°C, 30 min 89%  

Quinoxaline 1,4-dioxide (282) gave 2-quinoxalinecarboxamide (283) (neat HCONH2, 200°C, 1 h 30%, after separation from other products) other simple quinoxalinecarboxamides were made similarly but all in 10% 

2-Phenylquinoxaline 1,4-dioxide (276) gave 2-phenacyl-3-phenylquinoxaline 4-oxide (277) (BzMe, KOH, MeOH, reflux, 5 min 45%).137 

---

Some aspects of the deoxidative C-substitution of quinoxaline N-oxides (see Section 4.6.2.2 for cross-references) and the photoinduced rearrangement of quinoxaline dioxides into quinoxalinediones (see Section 4.5.1) have been discussed already. The remaining reactions of quinoxaline A -oxides are covered in the following subsections. 

This example shows that azinium cations generated from N-oxides are not always attacked by a nucleophile at C-a or C-y positions. In the above-mentioned reaction, it is the C-(3 position that is the preferential site for nucleophilic attack. Reactions of this type are usually accompanied by loss of the N-oxide function. The presence of a substituent able to stabilize the positively charged species (e.g., 4) seems to be an important feature in determining the site selectivity for these deoxidative nucleophilic substitutions (Scheme 6) (78JQC2514). Similar substitution reactions at C-3 (C-0) of the pyridine ring have also been observed in pyridine N-oxides (86H161). 

---