Oxidative cycloaddition with

Phosphorus heterocycles 246 derived from hydroxyphenyl benzimidazole and aminophenyl benzimidazole undergo oxidative cycloaddition with 3,5-di-/fV/-butyl-l, 2-bcnzoquinonc to give a mixture of 247 and 248 (Equation 60) <2004HAC307, 2004HAC321>. Oxidative addition of ort o-chloranil to bis- and tetrakis-cyclodiphos-phazene compounds gives bis- and tetrakis-spirophosphoranes, for example, 41 (Equation 61). 

Nitrile Oxide Cycloadditions with Chiral Dipolarophiles. 386... 

Face Selectivity of Nitrile Oxide Cycloadditions with... 

Yamamoto and co-workers (135,135-137) recently reported a new method for stereocontrol in nitrile oxide cycloadditions. Metal ion-catalyzed diastereoselective asymmetric reactions using chiral electron-deficient dipolarophiles have remained unreported except for reactions using a-methylene-p-hydroxy esters, which were described in Section 11.2.2.6. Although synthetically very useful and, hence, attractive as an entry to the asymmetric synthesis of 2-isoxazohnes, the application of Lewis acid catalysis to nitrile oxide cycloadditions with 4-chiral 3-(2-aIkenoyl)-2-oxazolidinones has been unsuccessful, even when > 1 equiv of Lewis acids are employed. However, as shown in the Scheme 11.37, diastereoselectivities in favor of the ffc-cycloadducts are improved (diastereomer ratio = 96 4) when the reactions are performed in dichloromethane in the presence of 1 equiv of MgBr2 at higher than normal concentrations (0.25 vs 0.083 M) (140). The Lewis acid... 

Aliphatic nitro compounds are versatile building blocks and intermediates in organic synthesis,14 15 cf. the overview given in the Organic Syntheses preparation of nitroacetaldehyde diethyl acetal.16 For example, Henry and Michael additions, respectively, lead to 1,2- and 1,4-difunctionalized derivatives.14 18 1,3-Difunctional compounds, such as amino alcohols or aldols are accessible from primary nitroalkanes by dehydration/1,3-dipolar nitrile oxide cycloaddition with olefins (Mukaiyama reaction),19 followed by ring cleavage of intermediate isoxazolines by reduction or reduction/hydrolysis.20 21... 

An intramolecular double nitrile oxide cycloaddition with alkyne and olefin moieties was exploited in the synthesis of novel unsymmetrical hybrid spiro (isoxazole-isoxazoline) ligands 14 which were effective in promoting a Pd-catalyzed tandem cyclization <07TA919>. 

Houk s model has been used to account for diastereoselectivity observed in nitrile oxide cycloadditions with the (a-oxyallyl)silanes (46) (88T3945). The direction and magnitude of asymmetric induction was... 

Nitrile oxide cycloaddition with mono- and trisubstituted alkenes affords almost exclusively 5-mono- and 4,5,5-trisubstituted isoxazolines, respectively, the regioselectivity being determined by steric effects. Reverse regioselectivity in nitrile oxide cycloaddition to terminal alkenes has been reported <1997CC1517> for example, 4-A t/-butylbenzoni-trile oxide was forced to reverse alignment for the cycloaddition by formation of the inclusion complex 470 with /3-cyclodextrin. Under these conditions, 90% of the 3,4-disubstituted cycloadducts were obtained, whereas in the absence of cyclodextrin the aromatic nitrile oxide afforded only the 5-substituted isoxazoline. 

The adduct (112) from phenyldichlorophosphine and stilbene undergoes a highly stereo selective oxidative addition reaction with o-aminophenol to form the cis-spirophosphorane (113) whose structure was confirmed (as the benzyl derivative) by X-ray crystallography and found to possess almost perfect tbp geometry. On the other hand, the reaction of (112) with catechol led stereoselectively to the trans configuration of the spirophosphorane (114)35. Cyclisation of (115) with PhPCl2 yields (116) which undergoes oxidative cycloaddition with 2,3-butanedione (117) to form the cis and trans forms of the - diphosphaspiro [4,4] -nonane (118).36... 

Oxo-2-alkenylphosphonates. A synthesis of this class of apparently useful compounds involves nitrile oxide cycloaddition with allylic phosphonates, conversion of the resulting isoxazolines to the conjugated oximes, and hence to the enones. The last step is readily achieved by the use of TiCl,. 

Wentland has conununicated the synthesis of the isoxazolobenzazonine (258) <89TL1477>. Compound (259), prepared by nitrile oxide cycloaddition with the corresponding enamine, was first quaternized at the sj nitrogen, then this salt was allowed to react with sodium methoxide to give (258) in excellent yield (Equation (30)). 

Scheme 5 A proposed reaction mechanism for the Ni-catalyzed oxidative cycloaddition with alkynes...

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