Oxazolin-4-ones cycloadditions

Reports of [ 2 + 2] cycloaddition of nitrogen containing heterocycles to alkenes are so numerous that attention can be drawn here to only a few. Recent examples include the acetone-sensitized photoaddition of 4-oxazolin-2-one (248) to ethylene to give the cis-adduct 249,203 the photocycloadditions of N-substituted imidazoles to acrylonitrile204 and of N-methyl-4-hydroxy-quinol-2-one to cyclohexene,205 and the photoaddition of pentafluoro-pyridine to ethylene to give the 1 1- and 1 2-adducts 250 and 251,... 

Dipolar cycloaddition of 1,1-dimethoxyethene to the mesoionic pyrido[2,l-6][l,3]oxazoline 59 gave the adduct 60.The similar reaction with 61 afforded quinazolin-4-one (62). When 1-dimethylamino-l-methoxyeth-ylene was applied in the latter reaction, a mixture of quinolizin-4-one (63) and the ring-opened product 64 was obtained (88CB951). 

The first example of an enantioselective thiadiene cycloaddition involved the reaction of 2,-4-diphenyl- 1-thiabuta-1,3-diene with l-propenoyl-l,3-oxazolidin-2-one. Stoichiometric quantities of a copper triflate bis-imine complex catalyst 428 and 4 A molecular sieves are necessary to achieve the highest enantioselectivity and the best endojexo ratio. The absolute configuration of the major endo isomer was determined by reduction of the acyloxazolidine side chain to the known (3/ ,4/ )-5-hydroxymethyl derivative (Scheme 137) <1997J(P1)2957>. The process is improved using a homochiral Cu triflate or Ni perchlorate bis(oxazoline) complex when catalytic amounts are adequate for a range of thiabutadienes <1999CC1001>. 

Dipolar cycloaddition of nitrile oxides to unsaturated five-membered carbocyclic and heterocyclic systems gives fused 1,2-oxazolines. Steric effects are apparently of far greater importance than electronic ones, especially with regard to the orientation of the dipolarophile towards the nitrile oxide in the cycloaddition reaction. Originally, only one... 

Irradiation of phenyl-2//-azirines in the presence of carbon dioxide leads to the formation of the 3-oxazoline-5-one system121-123 and, in some cases, to the isomeric 2-oxazolin-5-one122 [Eq. (24)1. The azirines serve as incipient nitrile ylides, whose 1,3-dipolar structure permits cycloaddition to the dipolarophile C02123 [Eq. (25)1. The reverse reaction, photolytic extrusion of C02 from pseudoxazolones, is synthetically useful, since the dipolar nitrile ylide thus formed can be trapped with a variety of dipolarophiles. Thus, 2,2,4-triphenyl-3-oxazolin-5-one (48) is readily converted into the stabilized ylide (49)124 [Eq. (26)1, and the use of methyl acrylate,122 acrylonitrile,122 and dimethylacetylene dicarboxy-... 

Irradiation of a solution of 51 in benzene causes an interesting isomerization to 4-phenyl-4-oxazolin-2-one (56), via an epoxy isocyanate141 lEq. (33)1. In the presence of 1,1-dimethoxyethylene, a 2 + 2 cycloaddition occurs in competition with a-cleavage to give 57. Compound 51 is also reported to react with dimethylacetylene dicarboxylate... 

Pfaltz s semicorrin copper complexes - a real breakthrough more then ten years ago -can also be employed for intramolecular [2-1-1]-cycloadditions (Scheme 2) and achieve between 14 and 95 % ee depending on the substrate. [4] The related, otherwise very succes-ful, bis(oxazoline) complexes [5] (see above) do not appear to be as effective here though only one example has been investigated. 

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