Alkenes chiral—

Among many other methods for epoxidation of disubstituted E-alkenes, chiral dioxiranes generated in situ from potassium peroxomonosulfate and chiral ketones have appeared to be one of the most efficient. Recently, Wang et /. 2J reported a highly enantioselective epoxidation for disubstituted E-alkenes and trisubstituted alkenes using a d- or L-fructose derived ketone as catalyst and oxone as oxidant (Figure 6.3). 

Chiral dioxiranes, generated in situ from chiral ketones and Oxone , are promising reagents for the asymmetric epoxidation of unfunctionalized alkenes. Chiral ketone catalysts that are easily accessible in both enantiomers are targets for development. 

C2.7.6.2 CHIRAL HYDROGENATION OF ALKENES CATALYSED BY A RHODIUM COMPLEX... 

The general syntheses of alkenes (p. 28 — 44) and 1,2-dihydroxy compounds (p. 50—54 and 123 — 132) are not repeated here. But there is an important chiral pool" for chiral 1,2-disubstituted compounds, namely a-amino acids. 

Efficient acetalization of alkenes bearing various EWG with an optically active 1.3-diol 72 proceeds smoothly utilizing PdCN, CuCI. and O2 in DME to give the 1,3-dioxane 73[113], Methacrylamide bearing 4-t-butyloxazolidin-2-one 74 as a chiral auxiliary reacts with MeOH in the presence of PdCE catalyst... 

In this example addition to the double bond of an alkene converted an achiral mol ecule to a chiral one The general term for a structural feature the alteration of which introduces a chirality center m a molecule is prochiral A chirality center is introduced when the double bond of propene reacts with a peroxy acid The double bond is a prochi ral structural unit and we speak of the top and bottom faces of the double bond as prochiral faces Because attack at one prochiral face gives the enantiomer of the com pound formed by attack at the other face we classify the relationship between the two faces as enantiotopic... 

Once we grasp the idea of stereoisomerism m molecules with two or more chirality cen ters we can explore further details of addition reactions of alkenes... 

The double bond m 2 methyl(methylene)cyclohexane is prochiral The two faces however are not enantiotopic as they were for the alkenes we discussed m Section 7 9 In those earlier examples when addition to the double bond created a new chirality cen ter attack at one face gave one enantiomer attack at the other gave the other enantiomer In the case of 2 methyl(methylene)cyclohexane which already has one chirality center attack at opposite faces of the double bond gives two products that are diastereomers of each other Prochiral faces of this type are called diastereotopic... 

Section 7 13 Addition reactions of alkenes may generate one (Section 7 9) or two (Sec tion 7 13) chirality centers When two chirality centers are produced then-relative stereochemistry depends on the configuration (E or Z) of the alkene and whether the addition is syn or anti... 

Among chiral dialkylboranes, diisopinocampheylborane (8) is the most important and best-studied asymmetric hydroborating agent. It is obtained in both enantiomeric forms from naturally occurring a-pinene. Several procedures for its synthesis have been developed (151—153). The most convenient one, providing product of essentially 100% ee, involves the hydroboration of a-pinene with borane—dimethyl sulfide in tetrahydrofuran (154). Other chiral dialkylboranes derived from terpenes, eg, 2- and 3-carene (155), limonene (156), and longifolene (157,158), can also be prepared by controlled hydroboration. A more tedious approach to chiral dialkylboranes is based on the resolution of racemates. /n j -2,5-Dimethylborolane, which shows excellent enantioselectivity in the hydroboration of all principal classes of prochiral alkenes except 1,1-disubstituted terminal double bonds, has been... 

An efficient general synthesis of a-chiral (Z)- and (H)-a1kenes ia high enantiomeric purity is based on the hydroboration of alkynes and 1-bromoaIkynes, respectively, with enantiomericaHy pure IpcR BH readily available by the hydroboration of prochiral alkenes with monoisopiaocampheylborane, followed by crystallization (519). 

In the earlier, longer approach to (Z)-and (E)-alkenes, ThxR BH was used iastead of IpcR BH. It is also possible to prepare a-chiral acetylenes and alkanes by this method (76,520). In a shorter synthesis of a-chiral alkynes, a prochiral disubstituted (Z)-a1kene is hydroborated with... 

Chiral nitrones react with alkenes to produce 3,5-disubstituted isoxazolidines that are nonracemic diastereomeric mixtures and are oriented predominantly cis (equation 53) (77CC303, 79JOC1212). 

Aziridines have been prepared stereospecifically by the nucleophilic addition of the nitrogen residue to alkenes <80T73). Introduction of the nitrene is accomplished readily via a Michael-type addition with free diphenylsulfilimine (Scheme 12), and where a chiral sulfilimine is used the chirality is transferred to the aziridine with optical yields in excess of 25%. 

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