Rearrangement 1,2-disubstituted compounds

Rearrangement of 1,2-disubstituted compounds Diethoxytriphenylphosphorane, 109 epoxides to allylic alcohols Methylmagnesium N-cyclohexylisopro-pylamide, 189... 

Cope rearrangements are required to be suprafacial with respect to both components, but multiple stereochemical results are still possible. Cope rearrangements have a six-membered ring in the TS. The ring can be in one of four conformations (two chair and two boat). These different conformers can lead to different stereoiso-meric products. Consider the following possible conformations of the disubstituted compound. Cope rearrangement via one chair conformation leads to a product in... 

The mode of rearrangement of 1,8-bishomocubanes (1) catalysed by Rh and Pd complexes depends, not only on the metal and the ligands attached, but also on the level of substitution at the C-4 and C-5 positions of the carbocyclic species. Both snoutanes and the exo- and e Steric effects are significant and since the endo- som r (8) of the diene is formed preferentially in reactions of 9,10-disubstituted compounds, attack by the metal from the direction syn to the 9,10-substituted position must occur in most cases. 

From the addition reaetions of acrolein- to aldehyde-derived enamines, aminotetrahydropyrans have been obtained. On heating, these products rearranged to enaminoaldehydes in examples where the initial enamine was disubstituted (320,321). The addition of acrolein to ketone derived enamines has been applied to syntheses of heterocyclic and bridged bieyclic compounds (301,321-323). 

Regarding the series of hetero aromatic pentacyclic compounds with three heteroatoms, an accelerated synthesis of 3,5-disubstituted 4-amino-1,2,4-triazoles 66 under microwave irradiation has been reported by thermic rearrangement of dihydro-1,2,4,5 tetrazine 65 (Scheme 22). This product was obtained by reaction of aromatic nitriles with hydrazine under microwave irradiation [53]. The main limitation of the method is that exclusively symmetrically 3,5-disubstituted (aromatic) triazoles can be obtained. 

Some care must be taken in drawing conclusions from the E/Z or syn/anti selectivity of a given catalyst/alkene combination. The intrinsic stereoselectivity may be altered in some cases by subsequent isomerizations initiated by the catalyst. For example, epimerization of disubstituted vinylcyclopropanes is effectively catalyzed by palladium compounds the cis - trans rearrangement of ethyl chrysanthemate or of chrysanthemic acid occurs already at room temperature in the presence of PdCl2 L2 (L = MeCN, EtCN, PhCN)96 Oxycyclopropane carboxylic esters undergo metal-... 

P-Lactams have been used as a synthon for the preparation of a vast array of compounds. It has been reported that the reduction of 4-(haloalkyl)azetidin-2-ones with LiAlhL is a powerful method for the synthesis of stereodefined aziridines and azetidines <06OL1101>. However, reduction of 4-(haloalkyl)azetidin-2-ones with chloroalane afforded 2-(haloalkyl)azetidines, which were rearranged to 3,4-cw-disubstituted pyrrolidines and piperidines 32 <060L1105>. During these rearrangements, bicyclic azetidinium intermediates were formed which were ring opened by halides. The synthesis of a peptide-... 

Cyclohexadienylidenes, disubstituted at the 4-position are expected to be kinetically more stable than the parent carbene, however, the rearrangement to benzene derivatives is still very exothermic. The gas phase chemistry of 4,4-dimethyl-2,5-cyclohexadienylidene Is was investigated by Jones et al.100,101 The gas phase pyrolysis of the diazo compound 2s produces a mixture of p-xylene and toluene, and by crossover experiments it was demonstrated that the methyl group transfer occurs intermolecularly via free radicals. Thus, the pyrolysis of a mixture of the dimethyl and the diethyl derivative 2s and 2t... 

The reaction of ketoximes 235 with dimethyl carbonate in the presence of K2CO3, carried out in an autoclave at 180-190 °C, afforded 3-methyl-4,5-disubstituted 4-oxazolin-2-ones 236 (equation 102). The formation of compounds 236 occurred via [3,3]sigma-tropic rearrangement of intermediates of the oxime methylated with dimethyl carbonate. 

An unusual rearrangement following cyclization using a masked a-carbonyl equivalent, 2-anilino-2-ethoxy-3-oxothiobutanoic acid anilide 118, to form 6,7-disubstituted pteridine 119 by simply refluxing in ethanol for 3h was reported (Scheme 24) <2002JOC4526>. The synthesis of pyridopyrazine and related compounds using 39 was also discussed. 

As mentioned on p. 90, Markl has found that compounds having the 1.2-dihydro-X -phosphorin structure 777 thermally rearrange to 1.1-disubstituted X -phosphorins 178. On prolonged heating 178 splits off the two benzyl groups and yields 2.4.6-triphenyl-X -phosphorin 22. This sequence corresponds to thermodynamic stabilities. 

The final, critical oxidative spirocyclization of the 2,3-disubstituted indole to the spiro oxindole was effected by treatment of 124 with tert-butyl hypochlorite in pyridine to provide the labile 125 [Fig. (34)]. The Pinacol-type rearrangement was conducted by treating compound 125 with p-toluenesulfonic acid in THF/water. It is assumed that the chlorination of 124 proceeds from the least hindered face of the indole, to give the a-chloroindolene 125. The hydration of the imine functionality must also occur from the same a-face that is syn to the relatively large chlorine atom furnishing the syn-chlorohydrin 126, that subsequently rearranges stereospecifically to the desired spiro oxindole 127. 

Similar oxidative rearrangements to produce carbonyl compounds have been carried out with thallium(III) nitrate557 or peroxytrifluoroacetic acid-BF3.558 The Jones reagent (Cr03—H2S04) in the presence of a catalytic amount of Hg(II) is also effective.559 Chromyl chloride oxidation, when combined with reductive workup, is a simple, convenient one-step method to convert 2,2-disubstituted 1-alkenes to aldehydes.560,561... 

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