1.3- Oxazines salts

It has often been necessary to isolate oxazine dyes as insoluble zinc salts. Addition of urea [57-13-6] CH4N2O, or thiourea [62-56-6] CH4N2S, to the reaction mixture is reported to give good yields without using zinc (12), eg, the salt (25) [33203-82-6] is formed. 

Treatment of 7V-benzyl-l,2-benzisoxazolin-3-one with base produced a benzoxazine-4-one (see Scheme 88). The base catalyzed rearrangement of the 2-methyl-3-phenyl-l,2-benzoisoxolium salt to an oxazine is believed to proceed via a similar intermediate (67AHC(8)277). A number of other decompositions could possibly proced via this proposed route (74HCA376, 67AHC(8)277), which has also been postulated for the rearrangement of a variety of isoxazolium salts, e.g. the conversion of (200) into (202) (Section 4.16.3.3.2(i)(b)). 

Oxazines can form salts with strong acids. The salts vary with regard to their solubility and stability. Some of the hydrochlorides are hydrolyzed by water. Some less common salts are known e.g., dichromates and ferrocyanates, " chloroplatinates and chloroaurates. Sometimes the picrates can also be used to identify the 1,3-oxazine derivatives. " In some instances a special procedure is required to form the picrates, as the heterocyclic ring can open under the action of picric acid. " ... 

In a basic medium, 5-nitro-5-hydroxymethyltetrahydro-l,3-oxazine derivatives can be coupled with aryl diazonium salts to form aryl-azo derivatives (54) with the elimination of a molecule of formaldehyde. ... 

King and Durst have recently found the ring opening of trisubsti-tuted isoxazole quaternary salts to result in the formation of 1,3-(2 )-oxazine derivatives. ... 

N-Substituted 5,6-dihydro-2//-1,2-oxazines were found to be significantly more stable than their N-unsubstituted analogs and could be distinguished from the corresponding 4H isomers using H NMR spectroscopy. Thus, it was shown that oxazinium salt 80 isomerizes on treatment with sodium carbonate to tricyclic... 

Synthesis of 5,6-Dihydro-4H-Oxazines Containing Functionalized Substituents at the C-3 Atom Unlike BENAs, six-membered cyclic nitroso acetals do not form quaternary ammonium salts in the reactions with StX/Nu. ... 

The only salt of this type was prepared by refluxing of tV-methylimidazole with 3-bromomethyl-6,6-dimethyl-4-phenyl-5,6-dihydro-4//-oxazine in toluene for 5h (473). 

The triazoloxazine 294 containing the saturated oxazine moiety was synthesized by Knight and Leeper <1998J(P1)1891> treatment of the hydrazinium salt 293 with trimethyl orthoformate yielded the product 294 in quantitative yield. 

Among the 1,3-oxazinium derivatives, the 3-azapyrylium salts are the most interesting [they are also called 1 -1,3-oxazinium (72S333), 1,3-oxazin-1-ium (83MI1), 1,3-oxazinium (78AHC1 84MI1), and 1,3-oxazinylium salts (81BCJ2387)]. The absence of a universally accepted name for these compounds indicates the novelty of this field of heterocyclic cation chemistry. 

The first 3-azapyrylium salts 48 and 50 were obtained from other heterocycles by oxidative dehydrogenation (Section III,C) and by 5-alkylation of 4-thiono-l,3-oxazine (49) (62CB937 72S333). Until recently only two... 

Deprotonation of the a-position of a carbenium center is one of the most typical properties of carbocations. All the alkyl-substituted heterocyclic ions possess an appreciable CH-acidity [82AHC(S)1] the 3-azapyrylium salts are no exceptions to this rule. The formation of anhydro bases, i.e., methylene-1,3-oxazines (e.g., 69), from methyl-3-azapyrylium salts is well known (72S333). 

The 1,3,5-oxadiazinium derivatives are heteroanalogs of the 1,3-oxazin-ium salts described above. They merit consideration because they can be... 

Information concerning the spectral characteristics of 1,3-oxazinium derivatives discussed above is scant. Systematic investigations have not been carried out. In work devoted to the synthesis of 5,6-dihydro-l,3-oxazinium, 1,3-oxazinium, and benzo-l,3-oxazinium salts, these products underwent deprotonation without identification or even isolation. Such articles contain only the spectra of the corresponding uncharged 1,3-oxazines. A few recent publications describe these salts as individual compounds. 

The 0X0 structures of l,3-oxazin-4-onium and benzo-l,3-oxazin-4-onium salts, but not their 4-hydroxy forms, have been confirmed by H-NMR spectra in which the chemical shifts of methyl groups in position 2 were almost identical for both NH- and NMe-substituted salts 106 (75KGS460 77KGS328, 77KGS704, 77ZOR2459). 

In the cyclization of the tram isomers 121 ( = 1, 2 R = H, Me), no difference was found in the reactions. The cyclizations proceeded in good yields (72-84%), resulting in 122 in all four cases (n = 1, 2 R = H, Me) (85T5981). The cyclizations to 1,3-oxazines took place with retention, which is in agreement with the suggested mechanism formation of the thiuronium salt and subsequent nucleophilic attack of the oxygen on the partially positive carbon. 

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