Homochiral enone

Diels-Alder reaction of the furan derivative 148 with homochiral bicyclic enone 149 is the key step [56] in the total synthesis of the diterpenes jatropho-lone A and B, 151 and 152, respectively, isolated from Jatropha gossypiifolia L [57], Initial efforts to carry out the cycloaddition between 148 and 149 under thermal or Lewis-acid conditions failed due to diene instability. Application of 5kbar of pressure to a neat 1 1 mixture of diene and dienophile afforded crystalline 150 with the desired regiochemistry (Scheme 5.23). Subsequent aromatization, introduction of the methylene group, oxidation and methylation afforded (-l-)-jatropholones 151 and 152. 

A very remote secondary H/D isotope effect has been measured for the 2 + 2-cycloaddition of TCNE to 2,7-dimethylocta-2,fran -4,6-triene. The reaction of nitric oxide with iV-benzylidene-4-methoxyaniline to produce 4-methoxybenzenediazonium nitrate and benzaldehyde is thought to proceed via a 2 + 2-cycloaddition between nitric oxide and the imine double bond. A novel mechanism for the stepwise dimerization of the parent silaethylene to 1,3-disilacyclobutane involves a low-barrier [1,2]-sigmatropic shift. Density functional, correlated ab initio calculations, and frontier MO analysis support a concerted 2 + 2-pathway for the addition of SO3 to alkenes. " The enone cycloaddition reactions of dienones and quinones have been reviewed. The 2 + 2-photocycloadditions of homochiral 2(5H)-furanones to vinylene carbonate are highly diastereoisomeric. ... 

Diastereoselective cyclopropanation of enones.1 The homochiral ketals of cyclic a,p-enones obtained by reaction with 1,4-di-O-benzyl-L- or D-threitol2 undergo diastereoselective cyclopropanation on reaction with the Simmons-Smith reagent (8-20 1). The diastereoselectivity is less in the case of ketals of cyclic p, 7-enones (2 1). 

The copper-catalysed asymmetric conjugate addition of dialkylzinc to enones (174 n = 1 or 2) generates homochiral zinc enolates (175), which can be trapped in situ with activated allylic electrophiles to produce (176). High trans selectivity (85 15 to... 

The first issue confronted by Myers was preparation of homochiral epoxide 7, the key intermediate needed for his intended nucleophilic addition reaction to enone 6. Its synthesis began with the addition of lithium trimethylsilylacetylide to (R)-glyceraldehyde acetonide (Scheme 8.6).8 This afforded a mixture of propargylic alcohols that underwent oxidation to alkynone 10 with pyridinium dichromate (PDC). A Wittig reaction next ensued to complete installation of the enediyne unit within 11. A 3 1 level of selectivity was observed in favour of the desired olefin isomer. After selective desilylation of the more labile trimethylsilyl group from the product mixture, deacetalation with IN HC1 in tetrahydrofuran (THF) enabled both alkene components to be separated, and compound 12 isolated pure. 

The reaction of homochiral allylsilanes with a-enones and a-enals is very valuable for the asymmetric synthesis of multisubstituted cyclopentanes (Equation (40)).161,162 The [3 + 2]-cycloaddition of allylsilanes is applicable to />-quinones163 and />-quinoneimines.164 3-Butyn-2-one undergoes a double cycloaddition with an excess amount of allyltriisopropylsilane to give a bicyclo[3.3.0]octane in good yield (Equation (41)).165... 

The formation of nonseparable mixtures of diastereomers in the asymmetric cyclopropanation of homochiral cycloalk-2-enone 1,4-0-benzyl-L-threitol acetals with the Simmons-Smith re-agent has been circumvented by acetalizing with ( —)-(5, S)-hydrobenzoin. Treatment of (5,5)-hydrobenzoin acetals with Simmons-Smith reagent in refluxing diethyl ether provided good yields of cyclopropane acetals with a diastereoselectivity of > 13 1 (Table 2). The products were recrystallized from anhydrous diethyl ether to give diastereochemically pure products. Hydrolysis of the acetals afforded enantiomerically pure cyclopropyl ketones. Since hydrobenzoin is available in both enantiomeric forms, either enantiomer of a particular cyclopropyl ketone can be prepared via this methodology. ... 

The reaction of homochiral crotylsilanes wifh a-enones and a-enals is very valu-... 

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