Regioselectivity nitronate structures

For alkenes substituted with longer fluoroalkyl chains, yields and regioselectivities are not so dependent on nitrone structure. ... 

Formation of nitrones can be achieved in the first stage of a Krohnke type reaction in which p-n trosodi methy 1 an dine reacts with 2-oo-bromoacetylphenoxathiin in alkaline medium (336). The synthesis of a series of cyclic nitrones of structure (182) has been achieved by regioselective, and by an unusual [3 + 2] cycloaddition of a-nitrosostyrenes (181) to 1,3-diazabuta-l,3-dienes (180) (Scheme 2.64) (337a). Theoretical studies of the substitution effect at the imine nitrogen on the competitive [3 + 2] and [4 + 2] mechanisms of cycloaddition of simple acyclic imines with nitrosoalkenes have been reported (337b). 

The high selectivities found in the protonation experiments of the nitronate ions 44 suggested that also allyl anions 54 can be regioselectively protonated by a general acid protonation. Therefore, some lithium allyl compounds (Structures 6) were generated by deprotonation of alkenes with n-butyl lithium. 

Intramolecular cycloaddition of fV-benzyl-substituted 3-O-allylhexose nitrones furnishes chiral oxepane derivatives. The regioselectivity of the cycloaddition depends on several factors such as (1) the structural nature of the nitrone, (2) substitution and stereochemistry at 3-C of the carbohydrate backbone, and (3) substitution at the terminus of the O-allyl moiety. A mixture of an oxepane and a pyran is formed in the intramolecular oxime olefin cycloaddition of a 3-O-allyl carbohydrate-derived oxime <2003T4623>. The highly stereoselective synthesis of oxepanes proceeds by intramolecular nitrone cycloaddition reactions on sugar-derived methallyl ethers <2003TA3899>. 

Yield, regioselectivity and stereoselectivity depend on the structure of the nitrone. 

Yield, regioselectivity and stereoselectivity depend on the structure of the nitrone.74-76-77 For example, although 3,3,3-trifluoro-l-(phenylsulfonyl)propene provides high yields of cyclo-... 

Compounds 120 can also be oxidized regioselectively to afford aminonitrones 122. The X-ray structural analysis of 122 (Ar =/)-MeC6H4) shows an unexpected tautomeric fixation of the hydrogen atom and indicated that the resonance structures 122 and 122" participate in the stabilization of the nitrone 122. Thus, compounds 122 are unusually stable due to the contributions of anionic as well as cationic delocalized mesomeric structures (Scheme 26) <2000JPR245, 1999H(51)763>. 

The cycloaddition of nitrones lOj or 10k with various cyclic olefins is also stereoselective, and the ratio of exo (Vc)- and endo (Vt)-adducts depends on the structure of olefins, and to some extent on the nature of the ring-substituent, althouth generalization is difficult.69 The reaction of nitrone 10j with dimethyl tricyclo[4.2.2.02, ]deca-3,7,9-triene-7,8-di-carboxylate is also stereoselective. The endo-(12c)/e.vo(12d) adduct ratio is 0.5.53 The cycloaddition of nitrone lOj with some complex aji-unsaturated ketones is reported.70 The reaction with lumisantonin is regioselective to give a mixture of Vic (24%) and Vc (8%)71 (the carbonyl side chain is taken as X). The reaction with dihydrofuran is regio- and stereospecific. 6... 

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