1,2-addition stereoselective

Me3Al-Cp2ZrCl2 or R2CuLi). In such cases of cis addition, stereoselectivity originates from a dominant cycloaddition mechanism. 

In addition, stereoselective synthesis of solenopsin A has been reported by four research groups. An approach utilizing the stereoselective reductive de-cyanation (596) starts with aminonitrile 229, prepared from 2-picoline. It was selectively hydrogenated in the presence of Pd-C, followed by alkylation with undecyl bromide, affording 231. Reductive decyanation of 231 with NaBH4 in MeOH led to predominant (8 2) formation of the trans isomer (232) which was then debenzylated to ( )-solenopsin A (Id). The cis product (Ic) was in turn prepared by treatment of 231 with sodium in liquid ammonia followed by de-benzylation (Scheme 10). 

In addition, stereoselective syntheses of pipecolic acid derivatives, such as the stereoisomers of 4-hydroxypipecolic acid, 223-225 3-hydroxypipecolic acid, 2261 and variously ring-substituted analogues, 227-234 have been reported. 

The success of the reaction, as well as its stereoselectivity, is dependent on both the disilane and the bis-diene employed. For example, the ethoxycarbonyl-substituted bis-diene reacts with either hexamethyldisilane or 1,2-diphenyltetramethyldisilane to give high yields of isomeric mixtures of the carbocycle products. In contrast, hexamethyldisilane does not have sufficient activity in reactions with the other bis-dienes studied. In addition, stereoselective formation of only one isomer is observed in some cases. 

Mukaiyama addition stereoselectivity Me3Si < Et3Si < (7-Bu)Me2Si < (i-Pr)3Si 15... 

Stereoselective total syntheses of (-)-deoxoprosopinine (20) and (-)-deoxoprosophylline (21), derived respectively from the natural bases prosopinine and prosophylline, have been described the route adopted should be applicable to other related piperidine bases.20 Prosafrinine (22), a piperidine alkaloid of Prosopis africana, has been synthesized stereoselectively, along with pseudocar-pamic acid (23), as racemic varieties.21 Additional stereoselective syntheses of ( )-carpamic and ( )-azimic acids have been described.22... 

Although some significant advances have been made, the problem of conjugated addition stereoselectivity is still far from being completely clarified. For an illustrative example, see ref.116. 

Additional stereoselective syntheses of acorone (147) and isoacorone (148) have been recorded (Scheme 26). ... 

In the second example, the electrophile is an anhydride with two reactive carbonyl groups tha nearly but not quite the same. This is a recipe for bad selectivity - reaction will occur at carbonyls. In addition, stereoselectivity will not be so good as the alkenes are trisubstituted there is not as much difference in stability between cis and trans as there was in the disubstitc enal. 

The Claisen rearrangement32 converts an allyl vinyl ether 126 into a y,8-unsaturated carbonyl compound 128. This is already useful simply because it is a good way to make y,8-unsaturated carbonyl compounds but we are interested in the additional stereoselective aspects of the reaction. This is a [3,3]-sigmatropic reaction 127. 

Grignard additions, stereoselectivity in 139-41 Grignard chemistry 165 industrial 175-81 Grignard radicals 248-9 Grignard reactions 185 activation 189 apparatus for studies 192 carbanion iniemiediaies in 235 chain mechanisms in 235 hypotheses 190 induction period 189 inhibition 189 initiation 171-5, 189... 

Fig. 2. Cram, Cornforth, Karabatsos, and Felkin models for nucleophilic addition stereoselectivity (S, A/, L, Z - small, medium, large, electronegative)...

Addition of Bromine and Chlorine (Section 6.3D) Clj or Brj is used to convert an alkene into a vicinal dihalide. The mechanism involves attack by the alkene tt bond on one atom of Xj to give a bridged halonium ion intermediate (a cation) that is, in turn, attacked by X from the backside to give the vicinal dihalide. Rearrangements do not occur.The reaction displays anti addition stereoselectivity because of the halonium ion intermediate.The reaction is stereospecific because Z alkenes give different products than do Ealkenes. 

Mukaiyaina addition stereoselectivity MesSi < EtsSi < (/-Bu)Me2Si < PhMe2Si < Ph2MeSi 16... 

Another inventive total synthesis of ( )-a- and ( )- lycorane is completed by sequential chemoseleclive conjugate addition-stereoselective nitro-Michael cyclization of a to-nitro-a,p,< ),a)-unsaturated ester [32]. This approach is based on the following retrosynthelic analysis (Scheme 9.21). 

---