Stereoselectivity enone formation

Introduction and stereochemical control syn,anti and E,Z Relationship between enolate geometry and aldol stereochemistry The Zimmerman-Traxler transition state Anti-selective aldols of lithium enolates of hindered aryl esters Syn-selective aldols of boron enolates of PhS-esters Stereochemistry of aldols from enols and enolates of ketones Silyl enol ethers and the open transition state Syn selective aldols with zirconium enolates The synthesis of enones E,Z selectivity in enone formation from aldols Recent developments in stereoselective aldol reactions Stereoselectivity outside the Aldol Relationship A Synthesis ofJuvabione A Note on Stereochemical Nomenclature... 

Chamberlin and Reich investigated hydride additions to a,3-unsaturated ketones and the correlation of conformational preferences in enones with the ( )/(Z) stereoselectivity in formation of the corresponding enolates. They found that in acyclic systems s-trans enones gave enolates A and s-cis enones gave enolates B (Scheme 15). The reduction of a,3-unsaturated amides with L-selectride gave the same stereochemical results (Scheme 16). ... 

The Meyer-Schuster rearrangement of propargyUc alcohols is used for generating enones under 1,3-allyhc transposition of the C-O-functionahty [116]. It has been shown that the (B) /(Z) -selectivity of enone formation is drastically influenced by the catalyst, if hetero-polymolybdates are employed (Scheme 3.72). Thus, the acidic species leads to the (B)-olefin 375, whereas the silver salt generates the (Z)-enone 374 stereoselectively [117]. The reason may be that the acidic molybdate acts as a bulky proton donor. 

The stereoselectivity obtained in hydrogenation of an enone can be due to the formation of an intermediate in which alkene, hydrogen, and alcohol groups bind simultaneously to the metal (Equation (5)). This kind of stereoselectivity is typical in catalytic reactions where a polar group resides near to a C=C bond. 

Few applications of cyclizations to form fused ring 8-lactones or tetrahydropyrans are found. Two consecutive bromolactonizations were used to effect stereoselective dihydroxylation of a cyclohexadi-enone system in a total synthesis of erythronolide B (Scheme S).64 Iodolactonization of an NJV-di-ethylbenzamide derivative to form a ds-fused benzolactone was a key step in a recent synthesis of pancratistatin.641 A di-fused tetrahydropyran was produced in good yield by intramolecular oxymercura-tion as shown in equation (17),59 although attempts to cyclize a more highly functionalized system have been reported to fail.65 Formation of a fused ring tetrahydropyran via an anti-Markovnikov 6-endo sel-enoetherification has been reported in cases where steric and stereoelectronic factors disfavor a 5-exo cyclization to a spirocyclic structure.38... 

The structure of the cyclic enone affects the stereoselectivity in the formation of the four-membered ring, as could be seen in the photoaddition of testosterone derivatives 159 and 162 to cyclopentene. Rubin and colloborators88 reported favored cis -fused product... 

Also, psudo-P-D-mannopyranose (115) has been synthesized from 99 by the following reactions [28], Halogenation of 99 with triphenylphosphine, imidazole and iodine gave 1 L-4-0-benzyl-3-deoxy-3-iodo-1,2 5,6-di-0-isopropylidene-a//o-inositol (106), m.p. 77.4 °C, [oc]p° —30.1° (chloroform). Treatment of 106 with lithium aluminium hydride-resulted in a formation of two endocyclic olefins (107) and (108) in an approximately 1 2 ratio. Oxidation of 108with dimethyl sulfoxide and oxalyl chloride gave the enone (109) as a syrup, [a] 0 —68.11° (chloroform). Stereoselective... 

Besides simple enones and enals, less reactive Michael acceptors like /3,/3-disubstituted enones, as well as a,/3-unsaturated esters, thioesters, and nitriles, can also be transformed into the 1,4-addition products by this procedure.44,44a,46,46a The conjugate addition of a-aminoalkylcuprates to allenic or acetylenic Michael acceptors has been utilized extensively in the synthesis of heterocyclic products.46-49 For instance, addition of the cuprate, formed from cyclic carbamate 53 by deprotonation and transmetallation, to alkyl-substituted allenic esters proceeded with high stereoselectivity to afford the adducts 54 with good yield (Scheme 12).46,46a 47 Treatment with phenol and chlorotrimethylsilane effected a smooth Boc deprotection and lactam formation. In contrast, the corresponding reaction with acetylenic esters46,46a or ketones48 invariably produced an E Z-mixture of addition products 56. This poor stereoselectivity could be circumvented by the use of (E)- or (Z)-3-iodo-2-enoates instead of acetylenic esters,49 but turned out to be irrelevant for the subsequent deprotection/cyclization to the pyrroles 57 since this step took place with concomitant E/Z-isomerization. 

Intramolecular conjugate addition naturally results in ring formation. The body of examples of this type of reaction is subdivided into the Sections 4.7.1.1.2.1. Acyclic Acceptors and 4.7.1.1.2.2. Cyclic Acceptors , depending on whether or not the accepting enone/enoate moiety is part of a ring system. In the latter case, the cyclization affords a bi- or oligocyclic product. The steric restrictions thus imposed reduce the number of thermodynamically feasible products. As a consquence, product structures can usually be predicted easily, and high stereoselectivities are observed. 

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