Sigmatropic rearrangements analysis

Schmidt reaction of ketones, 7, 530 from thienylnitrenes, 4, 820 tautomers, 7, 492 thermal reactions, 7, 503 transition metal complexes reactivity, 7, 28 tungsten complexes, 7, 523 UV spectra, 7, 501 X-ray analysis, 7, 494 1 H-Azepines conformation, 7, 492 cycloaddition reactions, 7, 520, 522 dimerization, 7, 508 H NMR, 7, 495 isomerization, 7, 519 metal complexes, 7, 512 photoaddition reactions with oxygen, 7, 523 protonation, 7, 509 ring contractions, 7, 506 sigmatropic rearrangements, 7, 506 stability, 7, 492 N-substituted mass spectra, 7, 501 rearrangements, 7, 504 synthesis, 7, 536-537... 

When an alkyl group migrates, there is an additional stereochemical feature to consider. The shift can occur with retention or inversion at the migrating center. The analysis of sigmatropic shifts of alkyl groups is illustrated in Fig. 11.7. The allowed processes include the suprafacial 1,3-shift with inversion and the suprafacial 1,5-shift with retention. Sigmatropic rearrangements of order [3,3] are very common ... 

A similar analysis of [1,5] sigmatropic rearrangements shows that in this... 

The predictions of the reactivities by the geminal bond participation have been confirmed by the bond model analysis [103-105] of the transition states and the calculations of the enthalpies of activation AH of the Diels-Alder reaction [94], the Cope rearrangement [95], the sigmatropic rearrangement [96], the Alder ene reaction [100], and the aldol reaction [101] as are illustrated by the reactions of the methyl silyl derivatives in Scheme 38 [102], The bond is more electron donating than the bond. A silyl group at the Z-position enhances the reactivity. 

A similar analysis of [1,5] sigmatropic rearrangements shows that in this case the thermal reaction must be suprafacial and the photochemical process antarafacial. For the general case, with odd-numbered /, we can say that [1,/] suprafacial migrations are allowed thermally when j is of the form 4n + 1, and photochemically when j has the form An - 1 the opposite is true for antarafacial migrations. 

The aromaticities of symmetry-allowed and -forbidden transition states for electrocyclic reactions and sigmatropic rearrangements involving two, four, and six r-electrons, and Diels-Alder cycloadditions, have been investigated by ab initio CASSCF calculations and analysis based on an index of deviation from aromaticity. The order of the aromaticity levels was found to correspond to the energy barriers for some of the reactions studied, and also to the allowed or forbidden nature of the transition states.2 The uses of catalytic metal vinylidene complexes in electrocycliza-tion, [l,5]-hydrogen shift reactions, and 2 + 2-cycloadditions, and the mechanisms of these transformations, have been reviewed.3... 

For analysis of the photochemical reaction, the interaction of the hydrogen Is orbital with -tt3 of the allyl system is used. The interaction is bonding at both the migration origin and terminus, so the [ 1,3] sigmatropic rearrangement is photochemically allowed. 

A single report on 1,4-oxathiocines has been found in the literature published since 1995. The 2-styryl-l,3-oxathilane 215 with the methyl diazoacetate, catalyzed by Rh2(OAc)4, produced a complex mixture of products. 1,4-Oxathiocine 217 and 1,3-oxathiolane 218 were isolated from the mixture in low yield and their stereochemistry was assigned by NMR analysis (Scheme 25) <2006T3610>. The proposed mechanism of this transformation may involve formation of the ylide 216, which presumably underwent [2,3]-sigmatropic rearrangement into 217, or Stevens rearrangement into 218 (cf. Scheme 24 (Section 14.06.6.6) and Scheme 26 (Section 14.06.8)). 

For sigmatropic rearrangement of order (i,j) in which both i and j are greater than 1, the migrating group of the above analysis consists of more than one atom, and appropriate... 

The Chemistry of the Tetracyclic Diterpenoids.—The reaction of cnt-kaur-16-ene with thallium(lli) nitrate affords cnt-kaur-16-en-15j8-ol nitrate which undergoes a ready [3,3] sigmatropic rearrangement to cnt-kaur-15-en-17-ol nitrate. The reactions of phyllocladene and of labda-8(17)-en-13-ol with sodium azide and iodine chloride have been examined. ° The synthesis of 13-hydroxylated cnt-kaur-16-ene derivatives such as steviol using an acyloin-like cyclization of keto-esters has been developed. A detailed analysis was made of the products arising from the use of sodium in liquid ammonia in this reaction. 

Scheme 4 [ls,3a]hydrogen, [la,3s]methyl and [la,3s]fluorine sigmatropic rearrangements in the allyl system. ELF analysis reveals a concerted interaction in the first case, a biradical interaction in the second one and a ion-pair interaction in the last one... 

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