Diisopropyl nitrones

If desired, the nitrone can be recrystallized from diisopropyl ether (200 mL) (Notes 12, 13), affording 9.0 g (84%) of a white solid (Note 14). Concentration of the mother liquor and recrystallization of the residue from 40 mL of diisopropyl ether provides 0.600 g of additional nitrone. Total yield 9.6 g (89%) of the nitrone as a white solid (Note 15). 

Asymmetric additions of Reformatsky-type reagents to nitrones 258a and 258b have also been reported (Scheme 139). The reagents were prepared in situ from ZnEt2 and the corresponding iodoacetic acid ester. Diisopropyl (R,R)-tartrate 262 was employed as a chiral inductor. Enantioselectivities varied significantly the best results were obtained at 0 °C when a nitrone was added to the reaction mixture over a 2 h period. 

The asymmetric 1,3-dipolar cycloaddition of nitrones (515), possessing an electron-withdrawing group, to allylic alcohols was achieved by using diisopropyl (/ ,/ )-tartrate [(R,R-DIPT)] as a chiral auxiliary. The isoxazolidines (516) and... 

On the other hand, approaches to the use of catalytic amounts of chiral ligands have been developed. Thus, the use of a sub-stoichiometric amount (50 mol%) of DBNE (1) affords A,A-diphenylphosphinylamine with 85% in 69% yield121a. Similarly, 25 mol% of chiral aziridinyl alcohol 56 (R = - ) affords (V,(V-diphenylphosphinylamine with 65% in 60% yield123. In the enantioselective addition reaction of diethylzinc to a nitrone, 20 mol% of the metal alkoxide of diisopropyl tartrate 62 catalyzed the formation of a... 

Most of the approaches outlined in Figure 15.10 have been successfully realized on insoluble supports, either with the alkene or alkyne linked to the support, or with support-bound 1,3-dipoles (Table 15.16). Nitrile oxides are highly reactive 1,3-dipoles and react smoothly with both electron-poor and electron-rich alkenes, including enol ethers [200]. The addition of resin-bound nitrile oxides to alkenes (Entries 5 and 6, Table 15.16) has also been accomplished enantioselectively under catalysis by diisopropyl tartrate and EtMgBr [201], The diastereoselectivity of the addition of nitrile oxides and nitrones to resin-bound chiral acrylates has been investigated [202], Intramolecular 1,3-dipolar cycloadditions of nitrile oxides and nitrones to alkenes have been used to prepare polycyclic isoxazolidines on solid phase (Entries 7 and 9, Table 15.16). 

Under photo-stimulation, isoindolyloxyl radical (5) abstracts primary, secondary, or tertiary hydrogens from unactivated hydrocarbons including cyclohexane, isobutane, or n-butane (Scheme l).23 The nitroxide (5) traps the resultant carbon-centred radical (R ) and so afford the A -aI koxyisoindo les (6). Blank photolysis experiments with no added hydrocarbon have shown some unprecedented / -fragmentation of (5) to afford the nitrone (7). A number of C60 nitroxide derivatives have been synthesized and characterized by ESR spectroscopy which show features common to nitroxide radicals.24 Reaction of nitroxide and thionitroxide radicals with thiyl radicals have been observed, from which sulfinyl, sulfonyl, and sulfonyloxy radicals were generated.25 The diisopropyl nitroxide radical was generated in the reaction of lithium diisopropylamide with a-fluoroacetate esters.26... 

Highly porous silica gel served as a support for the TADDOL moiety derived from inexpensive and readily available i-tartaric acid, which provided access to htanium-based Lewis acid catalysts (Heckel, 2000). Such entihes are employed successfully for enantioselective reactions. TADDOLs were covalently attached to the trimethyl-silyl-hydrophobized silica gel, controlled-pore glass (CPG) at about 300 m2 g-1, at a loading of 0.3-0.4 mmol gl (Heckel, 2002). In a carefully monitored mulh-step immobilization procedure, the TADDOLs were titanated to yield dichloro-, diisopropyl-, or ditosyl-TADDOLates. These catalysts were employed in dialkylzinc addihon to benzaldehydes and diphenyl nitrone addihon to 3-crotonyloxazolidinone, a [3+2] cycloaddition. 

A catalytic asymmetric 1,3-DC of nitrone 99 with y-substituted allylic alcohols was achieved by using diisopropyl tartrate as chiral ligand. Isoxazolidines 100 were formed with high ee <02CL302>. 

Scheme 16.1 Solid-phase synthesis of isoxazolidines according to the split-and-combine method, (a) Distribution of the resin into two equal portions, coupling of bromocarboxylic acids with N,N -diisopropyl-carbodiimide (D1C), combination of the resin, substitution with hydroxylamine. (b) Distribution of the resin into three equal portions, condensation with three different aromatic aldehydes to the corresponding nitrones, combination of the resin, (c) Distribution of the resin into three equal portions, cyclo-addition with three different dipolarophiles to isoxazolidines, combination of the resin.

Dipolar cycloadditions. In the reaction of allylic alcohols with nitrile oxides cocomplexation of the alcohol and diisopropyl tartrate to Zn directs the steric course in the formation of 2-isoxazolines. Bonding of the nitrones that participate in cycloadditions to the boron atom of a chiral oxazaborolidine (4) through their oxygen atoms is important to determine the transition states leading to isoxazolidine products. 

TL4419). The exo adduct 16 is formed in 95% ee from diphenylnitrone and 3-crotonyl-oxozolidin-2-one in the presence of titanium catalysts generated in situ from diisopropyl-titanium dichloride and chiral diols (94JOC5687). Intramolecular cycloaddition of the nitrone 17 yields the bicyclic isoxazoli-dine 18 stereoselectively (94JCS1661). The oxadiazabicyclo[3.3.0]octane 21 is formed when the oxime 19 is heated. The reaction proceeds by an intramolecular 1,3-dipolar... 

Di-r rr-butyl-4-nitrophenol is methylated by CH2N2 and silylated by ClSiMcj to give nitronate esters. Apparently steric crowding around the phenolic group forces the electrophiles to react with the alternative hard basic center. The less hindered 2,6-diisopropyl-4-nitrophenol affords a mixture of methyl ether and nitronate ester (185,186). 

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