1.3- Oxazine ring

There are oxazine disperse dyes reported (4) and several reactive dyes containing the oxazine ring and a few containing thiazine (5) (see Dyes, reactive). 

Additional examples have been reported (5). Disperse dyes which contain an oxazine ring include the yeUow 1,3-dinitro-lOhd-phenoxazine... 

Karrer thought that the dihydro-1,3-oxazine skeleton can possibly be formed in proteins during their transformation to polypeptides. Fodor - and Fiescr explained the stereospecific migration of acyl groups from N to 0 and vice versa in tropine alkaloids by an intermediate formation of a 1,3-oxazine ring. 

The compounds with one double bond are dihydro-1,3-oxazines, and the structures shown in Fig. 1 are possible. The conformations of dihydro-1,3-oxazine rings arc based on analogy with cyclohexcne and also on the conformational analysis of several benzo-1,3-oxazine derivatives. 

In addition to the compounds just tabulated, a number of less typical methods can also be used to form 1,3-oxazine rings. 

Infrared absorption spectra gave more information regarding the structure of 1,3-oxazine derivatives. These studies were mainly concerned with tetrahydro-ljS-oxazine " and dihydro-1,3-oxazine derivatives. - " According to Eckstein et al. a number of bands of frequencies 1150-1050, 955-925, and 855-800 cm characterize the hemiacetal bond C—0—C. Lukes et expressed the view that a triplet of bands at 1143, 1129, and 1083 cm characterized the tetrahydro-1,3-oxazine ring. However, Bergmann and Kaluszyner assigned these bands to the N—C—O system in the tetrahy dro-1,3-oxazines. 

A number of drugs with a 1,3-oxazine ring have been suggested in connection with chemical work in the field. Lately such interest has increased since a natural product—geissospermine—was found to contain this ring. However, the work in this field is still experimental, based on working hypotheses. 

Die Reduktion muB unter kontrollierten Bedingungen durchgefiihrt werden, da sonst der Tetrahydro-oxazin-Ring reduktiv zum Amino-alkohol aufgespalten wird1 2. 

Oxazines During the hydrogenolysis of an unsaturated oxazine ring the double bond was also hydrogenated (Scheme 4.146). With sodium amalgam, the double bond was not saturated.545... 

Hydrolytic ring opening of the 1,3-oxazine ring of the tetrahydro-[l,3]oxazino[3,4- ][l,2]oxazin-8-one 389 and subsequent protection of the carboxyl group has been performed (Scheme 61) <1999TL4391>. 

The l,3-oxazin-2-ones and 1,3-oxazine-2-thiones previously synthesized were used to prepare various N- and O-heterocyclic systems fused with 1,3-oxazine rings.172 For example, furan-l,3-oxazin-2-one or furan-l,3-oxazine-2-thiones (190a,b) and pyran-l,3-oxazin-2-ones or pyran-l,3-oxazine-2-thiones (190c,d) were prepared in very good yields, ranging from 83% to 90%, by montmorillonite K-10 clay-catalyzed cyclodehydration of 189a,b and 189c,d, respectively (Scheme 35). 

It is very likely, that this reaction occurs due to the equilibrium between trimethylsilyl halide and a nitrogen-containing nucleophile, which increases the electrophilicity of silyl Lewis acids. It should be noted that the configuration of stereocenters at the carbon atoms of the oxazine ring is partially distorted. Hence, it is assumed that the reaction proceeds through the intermediate cation B, which is partially isomerized into the stereoisomeric cation B through the open chain cation B". 

Keywords Aminonaphthol, biological activity, naphth-l,3-oxazines, ring-closure reactions... 

Early picosecond studies were carried out by Schneider et al, [63] on the parent spiro-oxazine (NOSH in Scheme 8) and similar derivatives. In a back-to-back work, they also described a complimentary CARS (coherent anti-Stokes Raman spectroscopy) investigation [69], Simply put, these authors found that the closed spiro-oxazine ring opened in 2-12 psec after laser excitation. The reaction was slower in more viscous solvents. An intermediate state formed within the excitation pulse and preceded the formation of merocyanine forms. This transient was named X in deference to the X transient named by Heiligman-Rim et al. for the spiropyran primary photoproduct [8], (See also the previous section.) The name X has since been adopted by other workers for the spiro-oxazines [26,65],... 

Spiro-oxazine ring closure takes the opposite trend with respect to solvent polarity compared to the spiropyrans [4,25,68]. Again, we can rationalize this with the... 

The easily accessible aroylaminoimidazole-4-carboxylates (228) give imi-dazo[4,5-d][l,3]oxazin-7-ones (229) in a novel cyclization with dichlorotri-phenylphosphorane (Scheme 85) (93S107). Heterocyclic )3-enamino esters, such as 5-amino-2-(methylthio)thiazole-4-carboxylates and 4-amino-2-(methylthio)thiazole-5-carboxylates can also be used in both cases 1,3-oxazine rings are smoothly condensed to the existing heterocyclic ring (93S107). 

The linear regression analysis data are listed in Table VIII. For ease of comparison, the data on the parent 2-aryltetrahydro-l,3-oxazine 389 and the corresponding 1,3- and 3,1-benzoxazine isomers 392 and 397 are also given. The data in Table VIII show that p is practically the same for all the 1,3-oxazine ring systems investigated, whereas the intercept values are characteristic of the substituents. 

Synthesis of the 1,2-oxazine ring by intramolecnlar cyclization of y-hydroxyoximes using Mitsunobu reaction was described. ... 

The nomenclature of ring systems containing the partially or fully hydrogenated monocyclic, bicyclic, and tricyclic fused 1,2-oxazine ring skeleton are shown in Figure 1. 

A previous review has highlighted the following methods of ring synthesis intramolecular cyclization of oximes, nitro alkenes, and nitrones, and [4+2] cycloaddition reactions <1996CHEC-II(6)279>. In addition to that, this review includes the intramolecular cyclization of hydroxylamines, hydroxamates, hetero-Diels-Alder [4+2], 1,3-dipolar cycloaddition of nitrile oxides to alkenes, and [3+3] cycloaddition reactions. This review does not cover cycloaddition reactions of the [4+2] [3+2] and [4+2] [3+2] [3+2] types which primarily led to heterocycle-fused oxazine ring systems. 

To characterize the electronic and steric effects of the substituents at positions 4-6 on the stability of the 2-aryl-l,3-oxazine ring forms exhibiting ring-chain tautomerism, a relative ring stability parameter (c) has been introduced. The value of c is calculated as the difference between the value of log Xx = h for a 2-aryl-l,3-oxazine derivative bearing substituents at positions 4-6 and the intercept value (log Xo = —0.15) for the parent unsubstituted 2-arylperhydro-l,3-oxazine (c = log Xx = h log Xq). 

Aryl substituents on the 1,3-oxazine ring give rise to a characteristic effect on the two- or three-component ring-chain tautomeric equilibria of 2,3-dihydro-l//-naphth[l,2-< ][l,3]oxazines 35-44 and the regioisomeric 3,4-dihydro-2//-naphth[2,l-H[l,3]oxazines 45-51 (Scbeme 6). The major ring forms of the equilibria 36-51 contain the 1,3- or... 

Hydrolytic reactions can also be applied in the synthesis of aldehydes or ketones via the corresponding 1,3-oxazine derivatives. The anion formed from 3-methyl-2-(4-pyridyl)tetrahydro-l,3-oxazine 155 on treatment with BuLi proved to react with various electrophiles (alkyl halides, carboxylic esters, acid chlorides, or aldehydes) exclusively at position 2 of the 1,3-oxazine ring and not at the pyridine nitrogen atom. The readily formed 2,2-disubstituted-l,3-oxazine... 

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