“…Having shown that the dihydrooxazonium salt 27 can be generated from hydroxamic acid 26 , and further that this intermediate transforms directly to azadiene 4a , we returned to the preparation of synthon 12 in order to determine whether it could be successfully N -alkylated. A new route to this compound was devised based upon both the reported use of Pb(OAc) 4 to oxidize tetrahydrooxazine to the parent dihydro system 33 ( 11 ; R 1 = H) and our own experience in the conversion of α-aminonitriles to imodyl cyanides using the same oxidant ) .…”
In connection with the development of the intramolecular Diels-Alder reaction (IMDA) of 1-azadienes, the 5,6-dihydro-4H-1,2-oxazine 12has been evaluated as a synthon equivalent of the 2-cyano-1-azadiene system. It was found that the dihydrooxazonium salt 27, generated in situ from the cyclic hydroxamic acid derivative 26, is converted directly to azadiene 4a via tautomerization to the corresponding enamine and a particularly facile Eschenmoser type cycloreversion process. Conditions were subsequently found for the preparation of synthon 12. N-Alkylation of this intermediate with alkyl bromides in the presence of Ag(+) ion also resulted in direct formation of the 2-cyano-1-azadiene products 38a-dand 4a. Microwave irradiation of a benzene solution of azadiene 4a proved to be a convenient means to effect its IMDA conversion to indolizidine 5a. To avoid decomposition of azadiene 38c, its intramolecular cycloaddition giving 40 (60%) was achieved by flash vacuum thermolysis.
“…Having shown that the dihydrooxazonium salt 27 can be generated from hydroxamic acid 26 , and further that this intermediate transforms directly to azadiene 4a , we returned to the preparation of synthon 12 in order to determine whether it could be successfully N -alkylated. A new route to this compound was devised based upon both the reported use of Pb(OAc) 4 to oxidize tetrahydrooxazine to the parent dihydro system 33 ( 11 ; R 1 = H) and our own experience in the conversion of α-aminonitriles to imodyl cyanides using the same oxidant ) .…”
In connection with the development of the intramolecular Diels-Alder reaction (IMDA) of 1-azadienes, the 5,6-dihydro-4H-1,2-oxazine 12has been evaluated as a synthon equivalent of the 2-cyano-1-azadiene system. It was found that the dihydrooxazonium salt 27, generated in situ from the cyclic hydroxamic acid derivative 26, is converted directly to azadiene 4a via tautomerization to the corresponding enamine and a particularly facile Eschenmoser type cycloreversion process. Conditions were subsequently found for the preparation of synthon 12. N-Alkylation of this intermediate with alkyl bromides in the presence of Ag(+) ion also resulted in direct formation of the 2-cyano-1-azadiene products 38a-dand 4a. Microwave irradiation of a benzene solution of azadiene 4a proved to be a convenient means to effect its IMDA conversion to indolizidine 5a. To avoid decomposition of azadiene 38c, its intramolecular cycloaddition giving 40 (60%) was achieved by flash vacuum thermolysis.
“…Although the parent oxynitrene (HON) is an important intermediate in combustion and atmospheric chemistry, , only relatively limited experimental investigations of oxynitrenes have been reported, − in contrast to the more thoroughly studied carbon- and nitrogen-substituted nitrenes. − (Limited reports of sulfur-substituted nitrenes have also appeared. − ) In fact, direct observations of oxynitrene intermediates (RON, R = H, Cl, Br, or CN) via low-temperature matrix IR spectroscopy have only recently been reported. − We encountered oxynitrene intermediates in our study of the photochemistry of O 2 -substituted diazeniumdiolates during the course of our development of novel photochemical precursors to nitric oxide (NO). − …”
Section: Introductionmentioning
confidence: 92%
“…Oxynitrenes have been generated by the oxidation of O -alkylhydroxylamines 3-11,36 and by base-catalyzed decomposition of N -sulfonyl- O -alkylhydroxylamines. , (Analogous oxidation reactions of the corresponding amino compounds have been used to generate aminonitrenes 19,37-40 and sulfenylnitrenes. , ) In the absence of trapping reagents, oxynitrenes undergo rearrangement reactions to form nitroso compounds that tautomerize (when possible) to oximes. Although evidence for oxynitrene intermediates in the above studies has been inferred by alkene trapping to form N -alkoxyaziridines, the intermediacy of oxynitrenes in these reactions has been questioned. , 1 …”
New sulfoximine- and phenanthrene-based photochemical precursors to oxynitrenes have been developed. These precursors have been used to examine the chemistry and spectroscopy of oxynitrenes. The first EPR spectra of oxynitrenes are reported and are consistent with their triplet ground states. Additional support for the triplet ground state of oxynitrenes is provided by trapping and reactivity studies, nanosecond time-resolved IR investigations, and computational studies.
“…Alkoxyamidyls generated under a variety of conditions are well known to dimerise to hydrazines, which decompose to esters. 35,36,38,47,49,50 Therefore 20 can dimerise to give the symmetrical hydrazines 21 and ultimately 17 (Scheme 6, path iv). We and others have more recently shown this decomposition to involve a double HERON rearrangement to two molecules of ester with liberation of nitrogen, as 21 are NNO anomeric amides.…”
N,N-dialkoxyamides 1c, a virtually unstudied member of the new class of anomeric amides, amides bearing two electronegative atoms at nitrogen, have been synthesised in useful yields directly from hydroxamic esters using phenyliodine(III)bis(trifluoroacetate) (PIFA). Infrared carbonyl stretch frequencies and carbonyl (13)C NMR properties have been reported, which support strong inhibition of amide resonance in these amides. Their thermal decomposition reactions in mesitylene at 155 °C proceed by homolysis to form alkoxyamidyl and alkoxyl free radicals in preference to HERON rearrangements to esters. The reactions follow first-order kinetics and for a series of N,N-dimethoxy-4-substituted benzamides, activation energies of 125-135 kJ mol(-1) have been determined together with weakly negative entropies of activation.
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