1.3‐Dipolare Cycloadditionen, XLI. Anlagerung der Nitrone an Styrol; Orientierung und räumlicher Ablauf

Abstract

Die Cycloaddition von 8 Nitronen an Styrol fiihrt zu 5-Phenyl-isoxazolidinen. Die Addukte des N-Methyl-C-phenyl-und des C.N-Diphenyl-nitrons werden durch hydrierende Ringoffnung und durch NMR-Spektren strukturell geklart. Das Verhaltnis der diastereomeren Addukte erlaubt Riickschliisse auf die Natur der Orientierungskomplexe der Komponenten.

Stereoselective addition of benzonitrile oxide and N-benzyl-C-phenylnitrone to (5RS,6SR)-5,6-dihydro-6-ethyl-5-methylpyran-2(2H)-one. Crystal structure of (1RS,4RS,5RS,6RS,9SR)-8-benzyl-1,5-dimethyl-4-ethyl-9-phenyl-3,7-dioxa-8-azabicyclo [4.3.0]nonan-2-one.

“…Methylation of the nitrone adducts at C-1 using lithium diisopropylamide and methyl iodide helped to establish their stereochemistry at C-9. It was found that methylation of adduct (7) gave compound (9) with retention of configuration, and that methylation of adduct (8) gave mainly (lo), together with a small amount of isomer (ll), formed presumably by ringopening of the intermediate isoxazolidine enolate (12), followed by ring-closing to give enolate (13). This would then be methylated to give the cis-fused product.…”

“…The endo-5-trig processes involved in this ring-opening and closing are usually unfavourable. The adduct (8) is somewhat strained with the 9phenyl substituent lying under the lactone ring. Despite this strain only a small amount of ring-opening is observed due to the stereoelectronic barrier.…”

The Effects of Lewis Acid on the 1,3-Dipolar Cycloaddition Reaction of C-Arylaldonitrones with Alkenes.

“…The structure of the cycloadducts was established as below. C-Aryl-N-methylnitrone has been known to react with styrene or the other monosubstituted alkenes to give 5-substituted isoxazolidines regioselectively 43,51) (see Chart 4 51) ). The structures of major and minor products of the reaction have been known to be cis-and trans-3,5-disubstituted isoxazolidines (3 and 3), respectively, on the basis of the analyses of the 1 H-NMR spectra 51) : though, in cis-isomer, the magnetic environment of one proton (Ha) at 4-position of the isoxazolidines is quite different from that of the other proton (Hb) at the same position of the ring, those of the corresponding two protons (Hc, Hd) of trans-isomer resemble each other.…”

“…These results are consistent with previous studies on 1,3-dipolar cycloaddition reaction of nitrone. [1][2][3][42][43][44][45][46][47][48][49][50] The formation and the ratio of two stereoisomers can be explained on the basis of both electronic and steric factors as follows. Aldonitrones such as 1a have been known to be stable in Z-configuration.…”

“…Aldonitrones such as 1a have been known to be stable in Z-configuration. [1][2][3][42][43][44][45][46][47][48][49][50]52) Monosubstituted alkenes such as 2C-M have been known to have relatively high HOMO and to undergo exclusively a dipole-LUMO/dipolarophile-HOMO controlled reaction with the Z-aldonitrone (1a) (see Chart 7). [1][2][3] Though the reaction proceeds with high regioselectivity, the stereoselectivity of the reaction was poor one.…”