5.6- Dihydro-4/7-1,3-oxazinium salts

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... 

The earliest syntheses of 5,6-dihydro-4//-1,3-oxazinium salts 1 were two-component cyclization reactions three-carbon 1,3-bifunctional fragments... 

It should be noted that the 1,3-bifunctional starting compounds 4-7 are quite difficult to obtain. Moreover, the direction of acid-catalyzed cycliza-tion of 3-acylaminopropanols depends considerably on their structure. Thus, N-acylaminoalcohols 8 can be cyclized to give either 5,6-dihydro-4//-1,3-oxazinium salts 10 or 2-oxazoline derivatives 12 as well as mixtures thereof (82MI1, 82MI2 see also 78AHC1). 

The situation was changed by the discovery of a novel approach to the synthesis of 5,6-dihydro-4//-l,3-oxazinium salts consisting in a polar 1,4-... 

As mentioned above, the polar 1,4-cycloaddition of N-acyliminium ions 14 and 18 to olefins gives 5,6-dihydro-4H-l,3-oxazinium salts 16 (Section fI,A,l), or forms the 1,3-oxazinium salts with acetylenes (Section III, A). The same ions 89 and 91 add nitriles to furnish 1,3,5-oxadiazinium salts 90 and 92, little investigated until now (65CB334 88ZOR230). Salts 92 are unstable and can dissociate at elevated temperatures with nitrile liberation. Therefore, they may serve as original reservoirs for active jV-acyliminium cations. 

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. 

Intense bands appear between 1620 and 1630 cm in vibrational spectra of 5,6-dihydro-47/-l,3-oxazinium salts 1. The fragment O —C—N may be responsible for this vibration. N-Protonated salts 1 have IR spectra with a wide band at 3305-3380 cm (NH") (89ZOR2416 92ZOR2569). 

Alkyl-5,6-dihydro-4H-l,2-oxazinium salts are formed by the alkylation of the free bases and these products are easily reduced by sodium borohydride to 2-alkyltetrahydro-l,2-oxazines (76HCA2765). 

Dihydro-4if- 1,2-oxazines are conveniently prepared by the cycloaddition of nitrosoalkenes and alkenes. Thus, 3-nitrosobut-3-en-2-one (142) reacts with trans-stilbene to give (143) (78CC847). Similarly, a-nitrosostyrene combines with cyclopentadiene to yield the oxazine (144) <79JCS(Pi)249). Chloronitrones (145) and alkenes in liquid sulfur dioxide containing silver tetrafluoroborate afford oxazinium salts (146) (77JOC4213). 

The mercapto group of 3-mercapto-3,4-dihydro-l//-pyrido[2,l-c][l,4]-oxazinium salt reacted with p-nitrobenzyl (55,65)-2-(diphenylposphono) oxy-6-[l(/ )-hydroxyethyl]carbapen-2-em-3-carboxylates in the presence of N,jV-diisopropylethylamine (86EUP168707, 86EUP169410). 3,4-Dihydro-l//,8//-pyrido[2,l-c][l,4]oxazine-8-thione was alkylated with diphenyl-methyl 3-iodomethyl-7-acylamino-3-cephem-4-carboxylates [89JAP(K) 89/290683],... 

The assumption that the azomethine 34 is formed by an intermolecular hydride transfer from aldehyde 30 to the nitrilium salt 31 is not confirmed because the different Schiff bases 34 obtained carry the group R which are originated from the aldehyde 30. In this way, the process described in equation 14 can be reversed and applied as an enamide synthesis by acylation of imines2,3. However, the 7V-ethylbenzonitrilium salt 35 reacts with benzaldehyde to give the more stable Af-benzoyl-7V-ethyliminium ions 3647,49 which add to trimethylethylene to form 5,6-dihydro-4i/-l,3-oxazinium salt 37. On the other hand, the reaction of ions 36 with phenylacetylene leads to another type of 1,3-oxazinium heterocycle, namely to 4i/-l,3-oxazinium hexachloroantimonate 38 (equation 15). 

The C chemical shift values for the tetramethyl-1,2-oxazinium salt (3) and for some 677-1,2-oxazines and dihydro-1,2-oxazines (4)-(8) are given in Table 1. 

C—C—C—N—C—O, C- —N—C—O—C, C—N—C—O—C—C This classification will be used here, although segregation into the various subgroups of each type is not attempted. Where there are appropriate examples within type, the order of discussion will be oxazinium salts, oxazines, oxazinones and oxazinethiones, dihydro-1,3-oxazines, dihydro-1,3-oxazinones and thiones, perhydro-1,3-oxazines, and finally perhydro-l,3-oxazinones and thiones. 

Electrochemical oxidation of N-subst. carboxylic acid amides 5,6-Dihydro-l,3-oxazinium salts from ethylene derivs. 

There are three principal series of 1,3-oxazinium derivatives with different unsaturation namely, dihydro-1,3-oxazinium 1,1,3-oxazinium 2a,b, and aromatic 3-azapyrylium salts 3 (see Scheme 1). 

Monocyclic 1,3-oxazepines (325) with aryl substituents at the 2-, 4- and 7-positions can be prepared in moderate yield (20-40%) by the reaction of aliphatic diazo compounds with 1,3-oxazinium perchlorates (324) (74S187). Tetra- and penta-phenyl-l,3-oxazepines (328 R = H or Ph) have been obtained via the reaction of azide with pyrylium salts (326) (78H(l 1)331). This principle had earlier been applied to the preparation of 1,3-benzoxazepines (74CR(C)(278)1389> and more recently to 3,1-benzoxazepines (81JHC847). The preparation of 2-phenyl-1,3-oxazepine.(331) by the UV irradiation of (329) is mechanistically interesting in that it apparently involves an intermediate (330) of the same type as (327) (73TL1835), but the method has only been used in this one case. One of the few examples of a dihydro-1,3-oxazepine (333) has been prepared by the thermolysis of the aziridine (332) (68JOC4547). 

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