Stealth stereocontrol: stereochemical reversal of a Michael addition reaction

Barrett, Anthony G. M.; Weipert, P. D.; Dhanak, Dashyant; Husa, R. K.; Lebold, Suzanne A.

doi:10.1021/ja00026a016

Cited by 26 publications

(3 citation statements)

References 3 publications

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“…Moreover, further examples can be found in the review of Barrett. [12] Barrett and co-workers [13] have investigated the stereochemical outcome of the reaction between different nucleophiles and 1-nitro-1-(phenylthio)alkenes 1 bearing a chiral sugar residue. The results show that the 1,4-addition followed by ozonolysis gave thioesters 2 in moderate to very good yields and with excellent diastereoselectivities.…”

Section: Substrate-controlled Diastereoselective Michael Addition To mentioning

confidence: 99%

Asymmetric Michael Additions to Nitroalkenes

2002

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The asymmetric conjugate addition of various carbon and heteroatom nucleophiles to nitroalkenes as a tool for the construction of highly functionalized synthetic building blocks is presented. Diastereoselective, substrate-controlled 1,4-additions are also included. Besides auxiliary controlled asymmetric Michael additions, external asymmetric versions employing enantiopure additives, addition-elimination processes with enantiopure leaving groups, and catalytic asym-

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Section: Substrate-controlled Diastereoselective Michael Addition To mentioning

confidence: 99%

Asymmetric Michael Additions to Nitroalkenes

2002

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“…Irrespective of the merits or otherwise of the above hypothesis (vide infra), we now report our preliminary observations in this area. Following on from an initial screening of several Brønsted acids, ureas, and solvent systems, p-toluenesulfonic acid (0.5 mol equiv) and the stealth nucleophile, 6 1,3-dimethylimidazolidin-2-one (2.0 mol equiv), were selected for further study in order to evaluate the scope and utility of this inexpensive system. 7 The results of this study are shown in Table 1 and reveal several features of interest.…”

Section: Methodsmentioning

confidence: 99%

Studies towards the Taming of the ‘Carbocation’ in the Regioselective Ring Opening of Epoxides to Allylic Alcohols

Chapman¹,

Herbal²,

Motherwell³

2010

Synlett

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Regioselective isomerisation of epoxides to allylic alcohols can be achieved using p-toluenesulfonic acid in the presence of 1,3-dimethylimidazolidin-2-one.The regioselective isomerisation of an unsymmetrically substituted epoxide into an allylic alcohol is a very valuable transformation for organic synthesis. 1 The Sharpless protocol 2 of ring opening with phenylselenide anion followed by hydrogen peroxide induced syn elimination of the resultant selenoxide is now a recognised method for introduction of the double bond at the less hindered terminus. For alkene formation at the more hindered site, the use of a stoichiometric amount of a strong chelating base possessing an oxophilic metal cation, such as lithium diethylamide or diethylaluminium 2,2,6,6-tetramethylpiperidide, has often proven to be the method of choice. 3 By way of contrast, efforts to control the evolution of an incipient carbocation intermediate generated by coordination of a Brønsted or Lewis acid to the oxygen lone pair can be fraught with danger, inasmuch as several competing pathways including hydride shift, alkyl shift, proton loss, and nucleophilic capture can all compete. 4 This situation is summarised in Scheme 1 for the simple case of 1-methylcyclohexene oxide and clearly indicates that careful selection of both cation and counterion is required to direct a desired pathway.In terms of allylic alcohol formation, our own studies were therefore inspired by a method developed by Noyori 5 involving sequential ring opening with trimethylsilyl triflate in the presence of 2,6-lutidine followed by anti elimination from the unstable diequatorial triflate using DBU (Scheme 1). The results of a simple preliminary experiment in which efforts were made to replace the silicon electrophile by a proton from p-toluenesulfonic acid and the nucleophile by DMF in order to create a positively charged leaving group were completely unsuccessful and merely served to illustrate the aforementioned complexity of the situation (Scheme 2). Scheme 1Scheme 2

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“…60 The potassium salts of cyclic imides give a stereochemical outcome which is opposite with respect to sodium alkoxides and TsNHK. 61 Cleavage of the obtained nitronate salts with ozone gives the thiol esters 112 -114 that can be finally converted into a methyl esters such as 115 using mercuric acetate in methanol (Scheme 33).…”

Section: Oxidative Methodsmentioning

confidence: 99%