Systematic organic chemistry;: Modern methods of preparation by W M et al Cumming

By W M et al Cumming

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O O N R 90 (R = H, NO2, 4-, 3-nitrophenyl) 1,3-Dipolar cycloaddition reactions of 2,6-dichlorobenzonitrile oxide with 2 ,3 -didehydro-2 ,3 -dideoxythymidine 91 (R = H, Me3 CMe2 Si), at its 2,5dihydrofuran double bond, gave nucleosides 92 and 93 in 67% yield and 3:2 ratio and 96% yield and 3:1 ratio, respectively (260). O Me NH N O RO 91 O RO O RO O O N 92 R1 R1 N O 93 1,3-Dipolar cycloadditions of benzonitrile oxide, its substituted derivatives as well as 9-anthro- and 2-furonitrile oxides to 5-alkoxy- and 5-hydroxy-2(5H )furanones afforded regiospeciÞcally furoisoxazoles 94.

Cl Cl NOH C NOAlCl3 Cl 30 C Ph Cl 31 Nitrile oxide–BF3 complexes can also be used as electrophilic moieties with aromatic systems. Introducing BF3 into a mixture of 2,6-dichlorobenzonitrile oxide and mesitylene in hexane, gave 88% Z -2 ,6 -dichloro-2,4,6-trimethylbenzophenone oxime (128). Nitrile oxides react in situ with formaldehyde dimethylhydrazone (129) to give oxime-hydrazones RC(:NOH)CH:NNMe2 (R = 4-O2 NC6 H4 , MeCO, MeC (:NOH)). The reaction is performed on treatment of oximes with CH2 :NNMe2 in the presence of Et3 N without isolation of the intermediate nitrile oxides.

4S )-p-Hydroxybenzyl-1,3-oxazolidin-2-one was used as a solid-supported chiral auxiliary in asymmetric 1,3-dipolar cycloadditions (145). It was also shown that Mg(II) cation (from magnesium perchlorate) catalyzes asymmetric 1,3-dipolar cycloaddition reactions using solid-supported oxazolidinone chiral auxiliaries (146). The results obtained support a reaction mechanism, which proposes the coordination of the Mg(II) to the dicarbonyl fragment of the chiral auxiliary. The resin-bound chiral auxiliaries could be recycled once, with little loss in regio- or stereoselectivity, but a second recycle gave products with signiÞcantly decreased regio- and stereoselectivities.

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