Alkenyl acetate epoxidation

Slightly more exotic carbon-centered nucleophiles can also participate in the ring-optening of epoxides. For example, the vinyl metallate 81, prepared by the treatment of the alkenyl acetal 86 with Schlosser s reagent, attacks mono-substituted epoxides 82 at the C-2 position to give the labile homoallyl alcohols 83 in fair to very good yields <96TET1433>. 

Organotin phosphates catalyse epoxide polymerisation, and triflates the hydrostannation of aldehydes and ketones,while triorganotin enolates, which result from the alkoxide and alkenyl acetates, are used to couple ciC-haloketones (to 1,4-diketones and... 

Acetals as Chiral Auxiliaries. There have been many applications of acetals of 2,4-pentanediol as chiral auxiliaries to control the diastereoselectivity of reactions on another functional group. Examples include cyclopropanation of alkenyl dioxanes, lithium amide-mediated isomerization of epoxides to allylic alcohols, and addition of dioxane-substituted Grignard reagents or organolithiums to aldehydes. 

Alkenylsilanes and -stannanes, and arylsilanes and -stannanes are useful reagents for transfer of an sp -carbon unit to electrophiles under titanium catalysis. Epoxides are opened by TiCE to generate cationic carbon, which is successfully trapped with bis(trimethylsilyl)propene as an aUcenylsilane (Eq. 122) [305]. Other Lewis acids, for example ZnCla, SnCU, and BF3 OEt2, proved less satisfactory. Cyclic epoxides such as cyclopentene and cyclohexene oxides gave poorer yields. An intramolecular version of this reaction proceeded differently (Eq. 123) [305]. Eqs (124) and (125) illustrate diastereoselective alkenylation and arylation of (A,0)-acetals that take advantage of the intramolecular delivery of alkenyl and aryl groups [306], Cyclic ethers... 

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. 

Allyiic halides, alcohols, ethers, acetates, lactones, phosphates, epoxides, sulfides, sulfonium salts, se-lenides and ammonium salts undergo transition metal catalyzed coupling reactions with C(sp )—Li, —Mg, —B, —Al, —Sn, —Zt, —Cd and — Hg reagents. Table 1 summarizes the allyiic leaving groups, alkenyl and aryl metallic reagents, catalytically active metals and references and Table 2 the regio- and stereo-chemical aspects. 

The substrates bearing both an active methyne or methylene group and an allylic moiety, such as allyl acetate 602 and alkenyl epoxide 603, readily undergo... 

Most tosylhydrazone sodium salts could be usually isolated from the reaction of the corresponding tosylhydrazones with sodium methoxide. However, a number of more functionalized hydrazone sodium salts, such as those derived from alkenyl aryl sulfonylhydrazones, ketone sulfonylhydrazones, and trimethylsi-lylacrolein sulfonylhydrazones, could not be isolated and an in situ salt generation procedure using NaHMDS was developed. A mixture of the desired hydrazones and NaHMDS was stirred at —78 °C then concentrated down before the addition of rhodium acetate, tetrahydrothiophene, BnEtsNCl, and the aldehyde to produce the required epoxide in high yields (eq 29). 

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