4.5- Dihydro-3- -oxazol-2-ones

Maleic anhydride adds to androst-14,16-diene to give the 14/8,17/8 Diels-Alder adduct. A pregn-16-en-20-one undergoes cycloadditions with the nitronic esters (113) to give the tetrahydro-oxazoles (114), which lose methanol with a trace of boron trifluoride to give the dihydro-oxazoles (115). The remarkable feature of the... 

This kind of simple proton abstraction is, however, not the sole and not even the most common mechanism of racemization. The most frequently invoked pathway involves cyclic intermediates, 4,5-dihydro-oxazole-5-ones or azlac-tones ... 

Similar alkylations may be effected on oxygen. l-(2-Chloroethyl)imidazolidin-2-one (312) when treated with potassium hydroxide or sodium hydride underwent ring closure to the tetrahydroimidazo[2,l-6]oxazole (313) (57JA5276). This approach can be used for the preparation of bicyclic hydantoins and the corresponding dihydro derivatives of (313) using the mesylate of (312) and NaH (77JHC5U, 79JMC1030). 

Thieno[3,4-d]oxazole-3a(4H)-carboxylic acid, dihydro-2-methyl-synthesis, 6, 1020 Thieno[2,3-d Joxazoles synthesis, 6, 990 Thieno[3,2-g]pteridine structure, 3, 284 lH-Thieno[3,4-c]pyran-2-ones synthesis, 4, 1032 Thienopyrazines synthesis, 4, 1022-1024 Thieno[2,3-6]pyrazines, 4, 1023 electrophilic substitution, 4, 1024 Thieno[3,4-6]pyrazines, 4, 1024 Thieno[3,4-c]pyrazole, 4,6-dihydro-3-hydroxy-carbamates... 

Acid-catalyzed ring transformation of the 2-aminomethyl-2,3-dihydro-77/-pyrimido[2,TA [l,3]oxazol-7-one 193 resulted in the pyrimido[l,2- ]pyrimidine 194 (Equation 19) <2000NN1381>. 

Aromatic pyrrolo[l,2-r(]oxazoles cannot be drawn, and the only structure that one can find in the literature is the l //,3/7-derivativcs. The derivatives 377,5/7 and 3 //.7// are unknown. All compounds described are dihydro and mainly oxo perhydro derivatives. 

Ring cleavage of the bicyclic dihydro-oxazolo [3,4-c]oxazol-3-one 111 derived from D-serine was realized by action of boron trifluoride-acetic acid complex leading to the oxazolidinone 112, a useful building block for the synthesis of... 

A new heterocyclic system, 3 ,4-dihydro-3//-benzo[4,5]imidazo[l,2-f]oxazol-l-one 462, was synthesized by reaction of 4,4-dimethyl-5-methylene-l,3-dioxolan-2-one with o-phenylenediamine in the presence of copper bromide as catalyst in carbone dioxide at 60-80 °C under high pressure (Equation 217) <1999CHC216>. 

Anhydro-acylation of l,2-dihydro-2-oxo-l-pyridylacetic acid with acid anhydrides yields bicyclic meso-ionic 4-acyl-l,3-oxazol-5-ones (67). Huisgen and his co-workers have reported extensive studies of the monocyclic meso-ionic l,3-oxazol-5-ones (66), which they have lightheartedly named munchnones. Cyclodehydration of N-benzoyl-A -methyl-C-phenylglycine using acetic anhydride at 55° gives the 3-methyl-2,4-diphenyl derivative (66, R = = Ph R = Me) as... 

Each desired enantiomer of 3-substituted dihydro-2(3.//)-furanones and 3-substituted tetrahydro-2//-pyran-2-ones was prepared starting either from 4,5-dihydro-2-(m-trimethylsilyloxy)oxazoles and alkylating with the corresponding alkyl halide, or by reversing the order of alkyl group introduction. Upon hydrolysis with dilute hydrochloric or sulfuric acid, 2-substituted cyclic lactones were obtained in 40-75% overall yield and 60-86% enantiomeric excess20. 

Intramolecular cyclization of 4,5-dihydro-3-(2-thienyl)-oxazol-2-ones 39 gave 1 -hydroxyethylthieno[2,3-d]pyrimidine-2,4(3// )-diones 40... 

Olefins react with benzenetellurinyl trifluoroacetate in acetonitrile in the presense of boron trifluoride etherate to give 2-acetaminoalkyl phenyl tellurium oxides that are converted, upon treatment with triethylamine in tetrahydrofuran at 30° for 4 h, to 4,5-dihydro-l,3-oxazoles4. These highly regio- and stereoselective reactions allow the one-pot, high-yield conversion of olefins to 1,3-oxazoles. Ethyl cyanide and phenyl cyanide can be used instead of acetonitrile. 

Various other [3 + 2] cycloadditions, affording chiral, anellated C6o derivatives with stereogenic centers in the addends are reported in literature. The products were generally obtained as racemates and resulted from reaction of buckminsterfullerene with species like 2,3-disubstituted 2//-azirincs (via nitrile ylides [under direct irradiation] or via 2-azaallenyl radical cations [sensitization by photoinduced electron transfer]),365 1-substituted 5-diazopentane-1,4-diones (via cyclic carbonyl ylides),366 7-alkylidene-2,3-diazabicyclo[2.2.1] hept-2-ene (via a diradicaloid trimethylenemethane derivative),367 1-benzylpy-razolidine-3-ones in the presence of aldehydes (via pyrazolidinium ylides),368 2-trifluoromethyl-2,5-dihydro-l,3-oxazol-5-ones (via nitrile ylides),369 nitro-alkanes in the presence of triethylamine and trimethylsilyl chloride (via N-silyloxynitrones),370 or dv-HOCH2 CH=C H C H 2 OCO 2 H( in the presence of... 

Trifluoromethyl-l, 3-oxazoles are formed on heating 2-trifluoromethyl-2-acyl-2//-oxazol-5-ones (71CB1408) as well as 3-trifluoromethyl-3-alkox-ycarbonyl-2,2,2-trimethoxy-5-phenyl-2,3-dihydro-l,4,2-oxazaphospholes (89CZ243). Both reaction sequences include a thermally induced [3 + 2] cycloreversion reaction and a 1,5-electrocyclization of the conjugated 1,3-dipolar species initially formed (Scheme 93). 

FIGURE 10.1 Pharmacophore model representation derived from the cycUn-dependent kinase 2 (CDK2) inhibitor 3- [(2,2-Dioxido-l,3-dihydro-2-benzothien-5-yl)amino]-methylene -5-(l,3-oxazol-5-yl)-l,3"dihydro-2H-indol-2-one in its binding site, PDB entry Ike processed using LigandScout ... 

---