Diastereomeric -sparteine-lithium

Kinetic resolution between diastereomeric (—)-sparteine-lithium complexes may also occur on a later step of a reaction sequence. Almost no stereodifferentiation between the enantiomers (R)- and (5)-116 takes place in the deprotonation of rac-116 (S,S)-117 and (7 ,5 )-117 are formed in essentially equal amounts (equation 26) . Only (R,S)-... 

Enantiomerically and diastereomerically enriched lithium-(-)-sparteine complexes of primary 2-alkenylcarbamates, which are configurationally stable as solids (Section 1.3.3.3.1.2.), are transmetalated stereospecifcally by tetraisopropoxytitanium. The resulting titanates are stable in solution and give rise to homoaldol adducts with enantiomeric purities up to 94 % ee107,107a. 

The problem can be solved by the transformation of the lithium carbanions into the more reactive trichlorotitanium intermediates via the stannanes. Finally, the (- )-sparteine complex of (5)-( )-l-methyl-2-butenyl diisopropylcarbamate105 (Section 1.3.3.3.1.2.) is apparently transmetalated by tetraisopropoxytitanium with inversion of configuration, leading to homoaldol products with moderate diastereomeric excess103. 

Equilibration of Configurationally Labile Organo-lithium Reagents. The equilibration of diastereomeric pairs of alkyllithium-(—)-sparteine complexes and trapping by achiral electrophiles gives enantioenriched products. Examples are a-(A/,JV-diisopropylcarbamoyloxy)benzyllithium in ether, not in THF, l-phenylethyllithium, and the dilithium salt of A/-methyl-3-phenylpropanoic acid amide (eq 2). ... 

This transformation has been extended recently by the development of methods for the enantiotopos selective lithiation of a prochiral 2-butenylcarbamate and the subsequent formation of chiral 2-butenyltitanium reagents from these species (Scheme 10-94) [180]. The reaction of -2-butenylcarbamate 279 with a solution of 5ec -butyllithium and (-)-sparteine generates a crystalline lithium complex 280-sparteine. Upon addition of 4 equivalents of Ti(Oi-Pr)4, a homogeneous titanium complex 283 (see below) is formed. The titanium reagent is then allowed to react with aldehydes to produce diastereomerically pure homoallylic alcohols 282 with good to high enantioselectivity. The high enantioselectivity observed in the... 

The (Z)- or (E)-phenylhexenyl carbamates 172 are smoothly deprotonated by s-BuLi/(-)-sparteine, and the lithium compound cyclizes during approximately 20 h at -78°C to form the (cyclopentyl)benzyllithium 173 which is in equilibrium with its epimer 174 [Eq. (45)] [110]. Trapping this mixture yields the essentially enantiomerically and diastereomerically pure side-chain substituted fra s-2-benzylcyclopentyl carbamates 175 in fair yields. Some of the intermediate 174 is lost due to 1,3-elimination resulting in formation of the achiral bicycle [3.1.0] hexane derivative 176 [111, 112]. Related results have been reported by Nakai et al., when allowing the ( )-6-phenylhex-5-enyl Md f-diisopropylcarb-amate to react under similar conditions [111]. 

A similar situation is given in the meso-dicarbamate 192 [see Eq. (61)] [120]. The pro-S proton at the pro-R branch exhibits the highest reactivity in the (-)-sparteine-mediated deprotonation to form the lithium compound 193 with a small amount of the diastereomer 195. By applying prolonged reaction times (4-5 h), it is found that 195 is decomposed more rapidly than 193, leading to a further enrichment. Trapping of the reaction mixture by different electrophiles leads to essentially enantiomerically and diastereomerically pure products 194a-c. Allylation and benzylation result in lower diastereomeric ratios, probably due to SET mechanisms in the substitution step. 

A reaction involving a chemically reactive stereogenic centre and an electrophile in the presence of a stoichiometric amount of chiral compound can often be accompanied by the DKR process. For example, as shown in Scheme 5.53, a sec-alkyUithium/(—l-sparteine/COj combination results in the formation of the corresponding acid with a high optical purity in high yield [135]. The rapidly interconverting enantiomeric lithium carbanions could both form complejKS with (—[-sparteine. One of the diastereomeric complexes, which undergoes rapid epimerization in situ, reacts with COj to yield the enantiomericaUy enriched product. 

Treatment of the a r/-tetrabromide (268) with methyl-lithium in the presence of (—)-sparteine gave two diastereomeric diallenes, (269) and (270). Their identity was shown by the fact that (269) is optically active whereas (270) is not, consistent with its we o-formulation. Both allenes were hydrolysed to their respective ketones, which were also optically active and inactive... 

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