Ylids, nitrile

Similar cycloadditions between thiirene dioxides and 1,3-dipoles generated in situ give heterocycles which result from either loss of sulfur dioxide or from the three-membered ring opening of the initially formed adduct (e.g. 174). Such cycloadditions with nitrilium imides (173a) and nitrile ylids (173b) are illustrated in equation 69175. 

Diazoalkane" Nitrous oxide Nitrile imine Nitrile ylid Nitrile oxide ... 

Photolysis of the azirines 68 in the presence of DEAZD gives 1,2,4-triazolines (69, R = Et) via cycloaddition to the nitrile ylid.114 The nitrile ylid generated thermally from 70 gives 1,2,4-triazolines (69, R = Me, R1 = R2 = CF3) (Scheme 7).11S The cycloadditions proceed in good yield, and the triazolines 69 are readily converted into aromatic 1,2,4-triazoles. 

Conversion of cephalosporin 71 to the imidoyl chloride, followed by treatment with base in the presence of DEAZD gives a novel spiro 0-lactam (72), again via nitrile ylid cycloaddition.116... 

Independent work by Schmid93 and by Padwa94 on the photochemistry of 2H-azirines has shown that irradiation of such systems leads in the first instance to the formation of nitrile ylids (nitrilium betaines). Subsequent 1,3-addition to a variety of dipolarophiles affords five-membered heterocycles. These additions take place in a stereospecific and regioselective manner thus, irradiation of the diphenyl-2f/-azirine 117 in the presence of dimethyl maleate leads to the formation of the two isomeric 1-pyrrolines... 

The first example of a [ 6 + 4] cycloaddition of a nitrile ylid has recently been reported104 irradiation of 3-phenyl-2,2-dimethyl-2i/-azirine (129) in the presence of 6,6-dimethylfulvene (130) in cyclohexane gave the [n6 + 4] adduct 131 together with the [ 4 + 2] adduct 132. 

Nitrile ylids generated in this way can be trapped with methanol108 Thus, on irradiation in methanol, the azirine 137 was converted to the methoxy-imine (138). The intramolecular equivalent reaction leading to the oxazoline 139 has also been observed on irradiation of the 2//-azirine (140 R = OH),... 

Photoelimination of carbon dioxide from the 2-oxazolin-5-one 474 in the presence of methyl acrylate affords the cis- and frans-l-pyrrolines 475 and 476.394 A nitrile ylid is believed to be involved in this and other analogous transformations.395... 

Other 1,3-dipolar reagents show the same mode of reactivity towards cyclopropenones. Thus, the Munchnones 412 serving as potential azomethine ylides259-261 or the nitrile ylids 41 3262 effect expansion of the three-membered ring to the 4-pyridone systems 411/414 as a result of (2 + 3) cycloaddition to the C /C2 bond. 

Scheme 9. 1,3-Dipolar cycloaddition of the nitrile ylid 37 onto methyl 2-chloro-2-cyclopro-pylideneacetate (1-Me) [26 a]...

When 4,5-dihydro-1,3,5-A5-oxazaphospholes (ll)63 are heated, nitrile ylids (12) are obtained.84 The (3 + 1) cycloaddition of the latter to isocyanides leads to azetines (14), which, on photolysis in benzene, revert to the ylids (12) these intermediates have been trapped with DM AD85 giving compound 13. 

IV-silylmethyl a-cyanoimines have been studied as a precursor of nitrile ylids. They undergo facile cycloaddition with dipolarophiles. Thus reaction with DM AD in the presence of silver fluoride leads to the formation of the corresponding pyrrole derivative in good yield, but no reaction occurs with the non-activated n-bond of styrene.453... 

From a secondary amide, formation of a nitrile ylid because of plausible elimination of trimethylsilanol prior to cyclization might be expected. However, it is possible that this elimination occurs after cyclization. Thus, the true 1,3-dipole would be the imidate methylide. In any case, the final product will be a A -pyrroline.458... 

A mechanism accounting for these results was proposed that in fact excludes the intermediacy of a nitrile ylid.453... 

Condensation of an MSMA isonitrile with an aroylfluoride gives a salt which undergoes loss of trimethylsilyl fluoride to form a nitrile ylid. This transient species reacts with DMAD to form 2-aroyl pyrrole in high yield.162 Substitution of acyl chloride for acyl fluoride in the reaction affords only poor yields of adducts.465... 

The thermolysis of (358) also leads to aromatisation, in this case in a process believed to involve an intermediate nitrile ylid. Evidence for this is obtained by thermolysis of a series of cyclopropenyl-substituted oxazolinones such as (359) for which cycloreversion with elimination of C02 is known to lead to a nitrile ylid. In some cases the ylid could be trapped by addition to methyl propiolate. Substituent effects suggest that the nitrile ylids undergo stepwise addition to produce a bicyclobutyl zwitterion which can either collapse to an azabenzvalene or rearrange to a cyclobutenyl cation 286>. 

In the absence of traps, the azirines can undergo further transformations that can be rationalized in terms of vinylnitrene or nitrile ylid intermediates.185,200202 Thus, photolysis of 3,4,5-triphenylisoxazole gives some of... 

Photocyclisation of 8-alkoxy-l,2,3,4-tetrahydro-l-naphthalenones and 4-alkoxy-6,7,8,9-tetrahydro-5H-benzocyclohepten-5-ones gives naphtho[l, 8-bc]furans and cyclohepta[cd]benzofurans respectively, and conformational and substituent effects of 1,5-biradicals in the cyclisation process are discussed." " The same authors also describe substituent effects on the photocyclisation of ethyl 2-(8-oxo-5,6,7,8-tetrahydro-l-naphthyloxy)acetates and ethyl 2-(5-oxo-6,7,8,9-tet-rahydro-5H-benzocyclohepten-4-yloxy)acetates to give naphtho[l,8-bc]furans and cyclohepta[c,d]benzofurans respectively." Also reported are cyclisations involving photogenerated radical cations of unsaturated silyl enol ethers, fragmentation cyclisations of unsaturated ot-cyclopropyl ketones which occur by photoelectron transfer and give polycyclics, and kinetic and theoretical studies of [2+3] cycloadditions of nitrile ylids. These reactions have been studied mechanistically and their synthetic potential investigated. 

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