Azidoformate photolysis

Surprisingly, in view of the use of hexafluorobenzene as an inert solvent for (ethoxycar-bonyljnitrene insertions into alicyclic C —H bonds,149 the thermolysis, or photolysis, of ethyl azidoformate in an excess of hexafluorobenzene yields hexafluoro-1//-azepine 7.150... 

The pyranofurooxazoline 109 can be prepared by a nitrene insertion reaction of the corresponding furan 110 upon treatment with ethyl azidoformate at — 50 °C under photolysis conditions. Compound 109 is moisture sensitive, and upon treatment with wet acidic THF was converted quantitatively to the more polar furanopyran 111. The structure and stereochemistry of 109 were proved unambiguously by X-ray diffraction, showing that the nitrene inserted anti to the bridgehead methyl group <1999JOC736> (Scheme 30). 

The conversion of azidoformates to fused-ring aziridines via the thermal generation of a nitrene has previously been reported. More recently, the photolytic conversion of a sugar-derived azidoformate has been used to prepare fused-ring aziridines <06JOC8059>. Photolysis of azidoformate 66 at 254 nm provides aziridine 67 in excellent yield. The resulting bicyclic aziridine was reduced to provide oxazolidinone 68 in 95% yield. Oxazolidinone 68 was subsequently converted to L-daunosamine. 

The ring expansion of arenes by electron-deficient singlet nitrenes is by far the most versatile synthetic route to 1H -azepines. The first l//-azepines were prepared independently in 1963 by Hafner, and by Lwowski, and their coworkers. They found that ethoxycarbonyl-nitrene (Scheme 26, path a R=C02Et), generated by photolysis of ethyl azidoformate, adds to benzene to give initially the unstable azanorcaradiene (227), electrocyclic ring... 

The asymmetric addition of ethyl azidoformate (N3C02Et) to an optically active enam-ine, prepared from cyclohexanone and (S )-2-pyrrolidinemethyl methyl ether, followed by photolysis produces 2-(ethoxycarbonylamino)cyclohexanone with modest enantiomeric excess (18 %ee) in 40% yield159. Use of (S -pyrrolidinemethyl trimethylsilyl ether as a chiral auxiliary increases the steric hindrance and provides the same product with the highest value of %ee (35%) in 51% yield. 

The standard preparation of 2-oxazolidinones is by treatment of /3-amino alcohols with phosgene or its synthetic equivalents ethyl chloroformate, dialkyl carbonates or even urea (equation 174). Isocyanates react with oxiranes in the presence of amines to yield 2-oxazolidinones (equation 175) (79LA200). The action of isocyanates on cyclic carbonates results in 2-oxazolidinones (equation 176) and photolysis of alkyl azidoformates affords oxazolidinones via intermediate nitrenes (equation 177). 

Behavior of Carbethoxynitrene toward Polyisoprene Structures. After studying the reaction of a number of divalent carbon derivatives with polyenes (17, 18), attention was directed to monovalent nitrogen derivatives. Of these, carbethoxynitrene, N—C02Et, gives addition reactions with olefins (12, 13). It can be produced either by photolysis of ethyl azidoformate (12), or by a-elimination (13) (Figure 5). 

Ethoxycarbonyl nitrene is readily formed by thermolysis or photolysis of cthoxycarbonyl azide (ethyl azidoformate), as well as by the base-induced a-elimination of (4-nitrophenylsulfonyloxy)-carbamate9 13> 158> 159. 

A single example of the formation of an oxaziridine by nitrene addition to a carbonyl group was found in the photolysis of ethyl azidoformate in the presence of excess acetone (for about 40 hr), producing oxaziridine 13. The reaction failed to give the corresponding oxaziridine when acetone was substituted by cyclohexanone. ... 

This reaction should be seen in marked contrast to the more difficult photolysis of ethyl azidoformate, which requires low-wavelength light and quartz equipment, suggesting that the ylid is likely to prove a valuable precursor of the nitrene. The photolysis of 60 (R = C02Ph or S02CgH4Me ) proved them to be less efficient nitrene sources (87JCS(P1)1553). 

Cycloaddition of phenyl azide (106, R = Ph) with methylenecyclopropanes (105) followed by photolysis of the primary products (107) yielded spiro compounds (108), in which the aziridine ring could easily be cleaved by acids leading to 109 and (equation 22). Alternatively, compound 108 (R = COOMe) was formed directly upon irradiation of methyl azidoformate (106) in the presence of excess 105 ... 

Other types of carbonyl azides, such as azidoformates and carbamoyl azides, previously believed not to undergo the rearrangement, can also be induced to rearrange by photolysis in alcohols . Diaryl-carbamoyl azides have been found to rearrange even upon heating in /-butanol and this reaction was used for the synthesis of 1,1-diaryl-hydrazines, thus JV-aminocarbazole (41) was prepared in 80% yield from 40 . 

Photolysis of j -methoxy and /i-phenyl phenyl zizides afforded the corresponding azo derivatives in 94% and 81% respectively. A mixture of these two azides yielded the three possible azo products in about equal amounts The tendency towards dimerization on photolysis is strengthened by addition of sensitizers. 2-Azidobiphenyl yielded in the presence of acetophenone mainly the azo derivative (112), with only minor amounts of carbazole . Azidoformates can also form... 

As the two components do not react in the dark, a triazoline intermediate is not probable. Other A -carbethoxyaziridines were formed via carbethoxy nitrene when ethyl azidoformate was irradiated in dihydropyran and in enolacetates . The stereospecificity of the addition is high. The photolysis of cis and trans 2-butene at — 20° yielded mainly the cis (130) and trans (131) aziridines respectively This stereospecificity has been found to diminish upon dilution... 

Azidoformates, containing a C—H bond at the right distance from the azido group, cyclize upon thermolysis or photolysis to yield 2-oxazolidones. 5,5-Dimethyl-2-oxazolidone (198) was obtained in this manner in 60-75% yield from <-butyl aT idoformate . Heating... 

The 1,3,4-oxadiazole system is formed on cycloaddition of carbo-alkoxy nitrenes to nitriles. The photolysis of ethyl azidoformate in acetonitrile yielded 2-ethoxy-5-methyl-l,3,4-oxadiazole (210) (yields 52-60%) With benzonitriles yields were much lower . 

Azidoformate esters, N3CO2R, also can show the Curtius rearrangement. A low yield (ca. 10%) of a trimer of methoxyisocyanate was isolated after irradiation of methylazidoformate in aprotic media , but generally both photolysis and thermolysis lead to similar products of secondary reactions that are characteristic of the formation of nitrenes with the rearrangement product either not present or in very minor yield. In aromatic solvents insertions can lead to... 

Proof for the triplet character of the hydrogen abstracting species is obtained in sensitization experiments. Thus, direct photolysis of ethyl azidoformate in cyclohexene results in the formation of only 3% of the hydrogen abstraction product (urethane) and a trace of 6w-cyclo-hexene -. In the sensitized photoreaction, where triplet nitrene is produced directly, the urethane yield increases to 74%, the yield of Aw-cyclohexene to 63%. 

Irradiation of ethyl azidoformate (66) at 2537 A in cyclohexene results in five identifiable products 7-ethoxycarbonyl-7-azabicyclo [4.1.0]-heptane (67) (50%), iV-2-cyclohexenylurethane (68) (9%), JV-3-cyclohexenylurethane (69) (3%), urethane (70) (3%) and bi(cyclohex-2-enyl) (71) (1-7%). Photolysis of ethyl azidoformate... 

All these products are compatible with the formation of a nitrene, yet on their own they do not constitute conclusive proof for the real existence of a discrete intermediate. It was only when Lwowski was able to show that the nitrene produced by a-elimination from N- -nitrobenzenesulphonoxyurethane (74) led to the same products as the photolysis of ethyl azidoformate that the evidence for a photolytic nitrene mechanism became entirely convincing, the more... 

The photolysis of ethyl azidoformate (66) in aromatic solvents leads to the formation of azepines C75). Toluene, xylene, mesitylene... 

In the photolysis of neat ethyl azidoformate (66) diethyl azodiformate (76) is produced in the early stages of the process, to be later replaced by the triethyl nitrilotriformate (77) which is produced in 58% yield A detailed investigation showed that the nitrilo compound (77) is apparently not formed by the action of a nitrene, and the reaction between an azide and an excited azodiformate was suggested as a possible pathway. Hancock has proposed an alternative mechanism which involves triazacyclopropene (see, however, Huisgen ). 

Photolysis of ethyl azidoformate (66) in alcohols leads to urethanes as the main products Thus JV-f-butoxyurethane (80) is produced... 

Sheinker has studied the u.v. and i.r. spectra of azidoformates. The 285 nm band, seen in alkyl azides, was not found perhaps it disappears into the short wavelength band due to a blue shift. The u.v. spectrum of ethyl azidoformate extends to nearly 300 nm (see Table 2) but its photolysis is most conveniently carried out with the mercury resonance line near 254 nm, for which efficient lamps are available . Ethyl azidoformate shows i.r. absorption bands at 2185 and 2137 cm"i (Ng), 1759 and 1730 (C=0) and 1242 (0-0) cm i 235 -pjje y spectrum is given in Table 2. The i.r. and... 

Thermolysis of lanostanol 3iS-azidoformate (20) provides a method for functionalization of the 4q -methyl group.Two compounds (21) and (22) were produced in addition to lanostanone. N.m.r. data identified the involvement of the 4a-methyl group in the formation of (21). A recent report indicates that nitroxide photolysis results in functionalization of the 4/8-methyl group. 

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