Rearrangements, oxazole reactions

In studies on l-diazo-2-ketosulfones, Shioiri et at. found that the thermal decomposition of benzoyl(sulfonyl)diazomethanes 6 with benzyl alcohol in acetonitrile also gave two products.<82CPB526> One is the 4-sulfonyloxazole 7 whereas the other product 8 results from rearrangement and reaction with the alcohol. The ratio of products varies with the nature of the sulfone substituent with the benzyl group giving highest yields of oxazole (Scheme 5). 

The reaction of cyclohexene with the diazopyruvate 25 gives unexpectedly ethyl 3-cyclohexenyl malonate (26), involving Wolff rearrangement. No cyclo-propanation takes place[28]. 1,3-Dipolar cycloaddition takes place by the reaction of acrylonitrile with diazoacetate to afford the oxazole derivative 27[29]. Bis(trimethylstannyl)diazomethane (28) undergoes Pd(0)-catalyzed rearrangement to give the A -stannylcarbodiimide 29 under mild conditions[30]. 

The behaviour of /S-oxovinylazides is quite similar to those above. The Z isomer (556), formed from the /S-halo carbonyl compound and sodium azide, is unstable losing N2 and forming the isoxazole (557) in an anchimerically assisted concerted reaction (75AG(E)775, 78H(9)1207). At moderate temperatures (50-80 °C) the E isomer formed acylazirines which at higher temperatures rearranged to oxazoles and isoxazoles. 

Oxazol-5(2H)-one, 2-benzylidene-4-methyl-tautomerism, 6, 186 Oxazol-5(2ff)-one, 2-methylene-isomerization, 6, 226 Oxazol-5(2H)-one, 2-trifluoromethyl-acylation, 6, 201 Oxazol-5(4ff)-one, 4-allyl-thermal rearrangements, 6, 199 Oxazol-5(4H)-one, 4(arylmethylene)-Friedel-Crafts reactions, 6, 205 geometrical isomerism, 6, 185 Oxazol-5(4ff)-one, 4-benzylidene-2-phenyl-configuration, 6, 185 photorearrangement, 6, 201 Oxazol-5(4ff)-one, 4-benzyl-2-methyl-Friedel-Crafts reactions, 6, 205 Oxazol-5(4ff)-one, 4-methylene-in amino acid synthesis, 6, 203 Oxazol-5(4ff) -one. 2-trifluoromethyl-hydrolysis, 6, 206 Oxazolones... 

H,3H- Pyrrolo[l, 2-c]oxazole-l, 3-dione, 5,6,7,8-tetrahydro-IR spectra, 6, 978 [2.2](2,5)Pyrrolophane, N-aryl-rearrangements, 4, 209 Pyrrolophanes natural products, 7, 764 synthesis, 7, 771 Pyrrolophanes, N-aryl-synthesis, 7, 774 (2,4)Pyrrolophanes synthesis, 7, 771 Pyrrolo[3,4-c]pyran-4-ones synthesis, 4, 288 Pyrrolopyrans synthesis, 4, 525, 526 Pyrrolopyrazines synthesis, 4, 526 Pyrrolo[l, 2-a]pyrazines synthesis, 4, 516 Pyrrolo[2,3-6]pyrazines Mannich reaction, 4, 504 Vilsmeier reaction, 4, 505 Pyrrolo[3,4-c]pyrazole, 1,3a,6,6a-tetrahydro-structure, 6, 976 synthesis, 6, 1019 Pyrrolopyrazoles synthesis, 5, 164 Pyrrolo[l,2-6]pyrazoles synthesis, 6, 1002, 1006 Pyrrolo[3,4-c]pyrazoles reactions, 6, 1034 synthesis, 6, 989, 1043 Pyrrolo[3,4-c]pyrazolones synthesis, 6, 989 Pyrfolopyridazines synthesis, 4, 517 Pyrrolo[l, 2-6]pyridazines synthesis, 4, 297 6/7-Pyrrolo[2,3-d]pyridazines synthesis, 4, 291 2/f-Pyrrolo[3,4-d]pyridazines synthesis, 4, 291 6/7-Pyrrolo[3,4-d]pyridazines synthesis, 4, 291... 

The Cornforth rearrangement involves the thermal interconversion of 4-carbonyl substituted oxazoles, with exchange between the C-C-O side-chain and the C-C-O fragment of the oxazole ring. These reactions generally involve compounds where a heteroatom (-OR, -SR, -Cl) is attached to the 5-position (R2) of the starting oxazole. 

Williams and McClymont have observed that acylation reactions of the dianion of 2-(5-oxazolyl)-l,3-dithiane (15) lead to formation of 4,5-disubstituted oxazole products through a Comforth rearrangement pathway under base-induced, low-temperature conditions. For example, deprotonation of 15 with LiHMDS (3.0 equivalents) at -78°C, followed by addition of benzoyl chloride or p-chlorobenzoyl chloride and warming to 0°C, provided 16 in 74% and 47% yield, respectively. 

Although 2-acyl-2//-azirines are known to give oxazoles upon irradiation, the reaction is wavelength dependent, and isoxazoles are formed at some wavelengths, as they are in the thermal rearrangement of 2-acyl-2//-azirines.<74TL29,75JA4682> Since the thermal reaction of diazocarbonyl compounds with nitriles leads to oxazole formation, it would seem that mechanistic path C is unlikely in these reactions. 

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