Oxa-aromatics, cycloadditions and reactions

Ohkata, K., Akiba, K-A., Cycloadditions and Reactions of Oxa-Aromatics with Nucleophiles, 65, 283. 

Cycloadditions and reactions of oxa-aromatics with nucleophiles , Ohkata, K. and Akiba, K.-Y., Adv. Heterocycl. Chem., 1996, 65, 283. 

Cycloadditions of oxa-aromatics and their reactions with nucleophiles 96AHC(65)283. 

In this Diels-Alder reaction with inverse electron demand the overlap of the LUMO of the 1-oxa-l,3-butadiene with the HOMO of the dienophile is dominant. Since the electron-withdrawing group of the oxabutadiene at the 3-position lowers its LUMO dramatically, both the cycloaddition and the condensation usually take place at room temperature. The reaction can be performed as a two-, three- or four-component transformation. There is actually no restriction on the aldehydes thus, aromatic, hetero-aromatic, saturated aliphatic and unsaturated aliphatic aldehydes may be used. In addition, ketones such as a-oxocarbocylic esters can also be employed. As 1,3-dicarbonyl compounds cyclic substances such as Meldrum s acid, barbituric acid and derivates, coumarines, any type of cycloalkane-1,3-dione and / -ketoesters, as well as their phosphorus, nitrogen or sulfur analogues and acyclic... 

Ketones and aldehydes can undergo photochemical [2-1-2] cycloaddition reactions with alkenes to give oxetanes. This is called the Paterno-Buchi reaction. For alkyl carbonyl compounds both singlet and triplet excited states seem to be involved, but for aromatic compounds the reaction occurs through the triplet state.The regiochemistry can usually be accounted for on the basis of formation of the most stable 2-oxa-1,4-diradical. For example, styrene and benzaldehyde give 2,3- not 2,4-diphenyloxetane. ... 

Ibata was the first to show that the masked carbonyl ylide embedded within the isomiinchnone framework would readily undergo 1,3-dipolar cycloaddition with various dipolarophiles [34], The isolable isomiinchnone 4 was observed to react with dimethyl fumarate to produce cycloadduct 7 which possesses the 7-oxa-2-azabicyclo[2.2.1]heptane skeleton. When the reaction of 1 was carried out using catalytic amounts of Cu(acac)2 in the presence of various dipolarophiles, smooth dipolar cycloaddition was observed to occur giving mixtures of endo and exo isomers. In most cases, the exo isomers were favored. All of Ibata s isomiinchnone cycloadditions contain aromatic substituent groups, presumably selected to facilitate dipole formation. The synthetic utility of the cycloaddition reaction is diminished, however, because of the low reactivity of the aromatic substituents toward further manipulation. 

In more detailed accounts, a range of aromatic and aliphatic nitrile A -oxides, prepared in situ by reaction of the corresponding hydroxamoyl chloride with triethylamine, underwent cycloaddition to 1,3,3-trimethylcylopropene to give 2-oxa-3-azabicyclo[3.1.0]hex-3-enes... 

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