Isomerism diastereomeric

FIGURE 7 8 Stereo isomeric 2 3 dihydroxybu tanoic acids Stereoisomers I and II are enantiomers Stereoisomers III and IV are enantiomers All other rela tionships are diastereomeric (see text)... 

Racemic l-methyl-2-butenylboronates (E)- and (Z)-3 may be prepared selectively via reactions of the l-methyl-2-butenyl Grignard reagent with the appropriate borate ester. Use of triisopropyl borate provides a 96 4 mixture of (E)-3l(Z)-3 on a 0.36 mol scale15. Use of a bulkier borylating agent, such as 2-isopropyloxy-4,4,5,5-tetramethyl-l,3,2-dioxaborolane, reverses the selectivity, enabling a 91 9 mixture of (Z)-3/( )-3 to be obtained on a 0.5 mol scale. The diastereomeric purity of this mixture may be enhanced to 95 5 by treatment with 0.15 equivalents of benzaldehyde, since ( )-l-mcthyl-2-butenylboronatc ( )-3 is more reactive than (Z)-3. Repetition of this process provides (Z)-3 that is 98% isomerically pure. 

As mentioned above (Scheme 3), condensation of triallylborane and 3-methoxybut-l-yne led, after treatment with methanol, to 7-(l-methoxymethyl)-3-methoxy-3-borabicyclo[3.3.1]non-6-ene. Hydroboration-isomerization of the latter with a THF solution of diborane gave a THF complex of 2-methyl-1-boraadamantane 15 in 85% yield. Treatment of the latter with (S)-(—)-phenylethylamine gave a mixture of diastereomeric complexes ( ) 57 isolated as white, well-shaped crystals (Scheme 19) <2003MC121, B-2003MI97>. 

Reaction mixtures of isomeric cycloadducts from furans 87d and 87e gave, after purification by column chromatography on both silica gel and neutral alumina, mixtures of diastereomeric hydrolysed products 90 and 91 (Scheme 17) [16]. 

A site-inversion mechanism (the key feature of which is that isomerization between diastereomeric and A configurations is rapid on the propylene-insertion time scale) based on theoretical calculations was proposed by Cavallo and coworkers in order to explain the ligand-directed chain-end controlled polymerizations (Fig. 35) [42]. The site-inversion mechanism allows chain-end control to work in concert with the site control effects. Our experimental results and the expected catalytic behavior resulting from the site-inversion mechanism concur with each other very well. 

At atmospheric pressure the Diels-Alder adducts of 1,4-benzoquinones are often not stable under the conditions of reaction and undergo an isomerization leading to the corresponding hydroquinones (Scheme 4). Due to the acceleration at high pressure the temperature of reaction can be lowered so that the secondary isomerization does not proceed and the primary Diels-Alder adduct can be isolated in good yields. The diastereoselectivity at high pressure induced by a chiral auxiliary, however, is with a diastereomeric excess of d.e. = 36%, only moderate. 

As to most chiral atropisomeric ligand, resolution or asymmetric synthesis is requisite. Mikami developed a novel ligand-accelerated catalyst. The chirality of atropos, but achiral triphos ligand-Ru complex, can be controlled by chiral diamines. Using ( -dm-dabn as controller, the single diastereomeric triphos-Ru complex was achieved through isomerization of (i )-triphos-Ru complex in dichloroethane at 80 °G (Scheme l).44... 

The aforementioned observations have significant mechanistic implications. As illustrated in Eqs. 6.2—6.4, in the chemistry of zirconocene—alkene complexes derived from longer chain alkylmagnesium halides, several additional selectivity issues present themselves. (1) The derived transition metal—alkene complex can exist in two diastereomeric forms, exemplified in Eqs. 6.2 and 6.3 by (R)-8 anti and syn reaction through these stereoisomeric complexes can lead to the formation of different product diastereomers (compare Eqs. 6.2 and 6.3, or Eqs. 6.3 and 6.4). The data in Table 6.2 indicate that the mode of addition shown in Eq. 6.2 is preferred. (2) As illustrated in Eqs. 6.3 and 6.4, the carbomagnesation process can afford either the n-alkyl or the branched product. Alkene substrate insertion from the more substituted front of the zirconocene—alkene system affords the branched isomer (Eq. 6.3), whereas reaction from the less substituted end of the (ebthi)Zr—alkene system leads to the formation of the straight-chain product (Eq. 6.4). The results shown in Table 6.2 indicate that, depending on the reaction conditions, products derived from the two isomeric metallacyclopentane formations can be formed competitively. 

The general subject of asymmetric synthesis has been reviewed extensively (1-5). The term asymmetric synthesis has been defined in more than one way (1,4) however, a useful definition is the one given by Morrison and Mosher (1) a process which converts a prochiral unit [refs. 6 and 7] into a chiral unit so that unequal amounts of stereoisomeric products result. The stereoisomeric products may be enantiomeric or they may be diastereomeric. The substrate molecule must contain either enantiotopic or diastereotopic groups or faces (8,9), since the attack of a reagent at equivalent groups or faces cannot lead to isomeric products. 

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