Transposition s. Rearrangement

Transposition s. Rearrangement C-Transsilylation 26,653 Trap, nucleophilic, methanol, as - 26,158 N,N,N-Triacylamines... 

The so-called isoxazoline transposition or Angeli s rearrangement (Scheme 79) involves the conversion of methylfuroxans by treatment with alkoxides or alcoholic alkali hydroxides into the oximes of isoxazolidin-4-ones (531) (81G167). 

Sigmatropic rearrangements have also found use in allylic S — N transposition. The rearrangement of allyl thiocyanates to allyl isothiocyanates (equation 27) ° belongs to this class as does the rearrangement of 5-allyl thioimidates (equation 28). ° ... 

Harger has studied the rearrangement of A-substituted N-phosphinoylhydroxylamines in the presence of base . He proposed a concerted mechanism based on the observed retention of the configuration at the phosphorous center during the transposition , and on studies with 0-labelled compounds . Similar cyclic transition states 572 were proposed in the base-induced rearrangement of A,0-bis(diphenylphosphinoyl)hydroxylamines (571) (equation 254). However, in the rearrangement of O-benzoyl-A-(diphenylphosphino-thiol)hydroxylamine where a transposition of O and S atoms occurs, the proposed cyclic transition state has sulfur participation . 

Allylimidates underwent a clean Claisen-type rearrangement in the presence of 5 mol % palladium chloride (equation 28),40 as did allyl carbamates39 and S-allylthioimidates (equation 29).41 This S to N rearrangement has found application particularly in the synthesis of pyrimidines, systems for which thermal S to N allylic rearrangements were generally ineffective (equation 30).42 Finally, 0-allyI S-methyl dithiocarbonates cleanly underwent palladium(ll)-catalyzed O to S allylic transposition (equation 31 ).43... 

This chemistry forms the basis of a general method for 1,3-hydroxy transposition in allylic alcohols (equation 7). The starting alcohol is converted by 3,3-sigmatropic rearrangement of the 0-allyl-S-methyldithiocarbonate followed by hydrostannolysis to the allylic stannane, which is oxidized by MCPBA in a completely regiospecific manner. A similar sequence has been reported for allylsilanes. "... 

The rearrangement of the allylic acetate in the prostaglandin side chain (entries 3 and 4) demonstrates the influence of chirality and double-bond geometryl8- 24-25-39. Oxygen transposition in the (Z)-allylic acetate led to the (15.S (-acetate with the E configuration of the new double bond, whereas the ( )-acetate gave under similar conditions the (15R)-acetate, also with the E configuration of the double bond. 

Lanthanide Lewis acids catalyze many of the reactions catalyzed by other Lewis acids, for example, the Mukaiyama-aldol reaction [14], Diels-Alder reactions [15], epoxide opening by TMSCN and thiols [14,10], and the cyanosilylation of aldehydes and ketones [17]. For most of these reactions, however, lanthanide Lewis acids have no advantages over other Lewis acids. The enantioselective hetero Diels-Alder reactions reported by Danishefsky et al. exploited one of the characteristic properties of lanthanides—mild Lewis acidity. This mildness enables the use of substrates unstable to common Lewis acids, for example Danishefsky s diene. It was recently reported by Shull and Koreeda that Eu(fod)3 catalyzed the allylic 1,3-transposition of methoxyace-tates (Table 7) [18]. This rearrangement did not proceed with acetates or benzoates, and seemed selective to a-alkoxyacetates. This suggested that the methoxy group could act as an additional coordination site for the Eu catalyst, and that this stabilized the complex of the Eu catalyst and the ester. The reaction proceeded even when the substrate contained an alkynyl group (entry 7), or when proximal alkenyl carbons of the allylic acetate were fully substituted (entries 10, 11 and 13). In these cases, the Pd(II) catalyzed allylic 1,3-transposition of allylic acetates was not efficient. 

The synthetically most useful oxygen-sulfur transpositions (equation 16) are the allyl sulfenate-allyl sulfoxide and the propargyl sulfenate-allene sulfoxide rearrangements, the driving force of both being the formation of the strong S=0 bond at the cost of the weak O—S bond (Scheme 12). ... 

The only synthetic application of an allylic S S transposition (equation 25) seems to be in the synthesis of ( )-a,p-unsaturated aldehydes via a 3,3-sigmatropic rearrangement of allylic dithiocarbamates (Scheme 35). °°... 

Moliner, V., Castillo, R., Safont, V. S., Oliva, M., Bohn, S., Tunon, I., Andres, J. A theoretical study of the Favorskii rearrangement, calculation of gas-phase reaction paths and solvation effects on the molecular mechanism for the transposition of the a-chlorocyclobutanone. J. Am. Chem. Soc. 1997,119,1941-1947. 

For a related example of diaxial to diequatorial rearrangement which illustrates the expected transposition of the vicinal substituents see S. Hanessian and A. P. A. Staub, Carbohydr. Res., 1970,14, 424. 

Rearrangements of ylidic intermediates proceed with allylic transposition. This useful property is the key feature of one of the best preparations of 1, S-dienes (Scheme 9). Trost has used this approach to synthesize yomogi alcohol (165) in a particularly simple way by 5-alkylation of sulfide (31) and solvolysis of the resulting sulfonium ion (Scheme 37). ... 

Scheme 18.38 l.S-Sulfinyl transposition via sequential Mislow-Evans rearrangement provides an a-p-y-fi-dienyl aldehyde anion equivalent.—... 

Payne further investigated the scope and limitations of the epoxy alcohol rearrangement under the standard conditions depicted in Scheme 10.1. A major quantity of 3 was found to be readily transformed to the isomeric 4, attaining an equilibrium ratio of 3 4 = 8 92. The position of equilibrium for this structural transposition was further probed by using representative isomeric substrates thus, only a small amount of anti-6 was obtained from (E)-5, while the diastereomeric syn-6 was formed from (Z)-5 as a mixture of syn-6 (Z)-5 = 42 58 after 1 h at ambient temperature. The same situation was noticed for (E)-7, which afforded anti-S in a ratio of E)-7 anti-6 = 44 56, and (Z)-7, which was preferentially converted to syn-8, leaving only 5% of (Z)-7 in the mixture. 

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