Stereochemistry configurational stereoisomerism

A reaction is stereospecific if, starting from compounds of different configurations, stereoisomerically distinct products are formed. The stereochemistry of the products is determined by that of the starting materials. A stereospecific reaction is necessarily stereoselective. 

The general trend is that boron enolates parallel lithium enolates in their stereoselectivity but show enhanced stereoselectivity. There also are some advantages in terms of access to both stereoisomeric enol derivatives. Another important characteristic of boron enolates is that they are not subject to internal chelation. The tetracoordinate dialkylboron in the cyclic TS is not able to accept additional ligands, so there is no tendency to form a chelated TS when the aldehyde or enolate carries a donor substituent. Table 2.2 gives some typical data for boron enolates and shows the strong correspondence between enolate configuration and product stereochemistry. 

Since vinyl anions generally retain configuration 39> while isomeric vinyl radicals rapidly interconvert 40) these results constitute evidence that reductions of alkyl iodides do proceed via radical intermediates. Isomerization of stereoisomeric vinyl anions is ruled out by the lack of effect of phenol on the stereochemistry of the products (Scheme III). Since cis and trans-3-hexene are formed in differing proportions from the two halides, it may be concluded that the stereoisomeric vinyl radicals are being intercepted by electron trans-... 

Stereochemistry Coordination Polymerization. Stereoisomerism is possible in the polymerization of alkenes and 1,3-dienes. Polymerization of a monosubstituted ethylene, such as propylene, yields polymers in which every other carbon in the polymer chain is a chiral center. The substituent on each chiral center can have either of two configurations. Two ordered polymer structures are possible — isotactic (XII and syndiotactic (XIII) — where the substituent R groups on... 

A polymer such as -[-CH=CH-CH(CH3)-CH2-hr which has two main-chain sites of stereoisomerism, may be atactic with respect to the double bond only, with respect to the chiral atom only or with respect to both centres of stereoisomerism. If there is a random distribution of equal numbers of units in which the double bond is cis and trans, the polymer is atactic with respect to the double bond, and if there is a random distribution of equal numbers of units containing the chiral atom in the two possible configurations, the polymer is atactic with respect to the chiral atom. The polymer is completely atactic when it contains, in a random distribution, equal numbers of the four possible configurational base units which have defined stereochemistry at both sites of stereoisomerism. 

Arcus, C. L. Stereoisomerism of Addition Polymers. Part I. The Stereochemistry of Addition and Configurations of Maximum Order. J. chem. Soc. [London] 1955, 2801. The Stereoisomerism of Addition Polymers. Part II. Configurations of Maximum Order from Altering Copolymerisation. The Requirements for Optical Activity in Polymers. J. chem. Soc. [London] 1957, 1189. 

Pyrrolizidine derivatives with at least one substituent, and particularly the pyrrolizidine alkaloid components, have one or more asymmetric carbon atoms. The stereochemistry of pyrrolizidine was clarified for the most part in the course of investigation of the naturally occurring pyrrolizidine alcohols. Here, the problems of relative and absolute configuration and of stereoisomeric transformations will be considered. 

The true stereochemistry of 1, 3-chloropalladation is revealed in the reactions of cis-9-methylenebicyclo[6.1.0]nonane to give a single allylic complex (equation 324), and of trans-9-methylenebicyclo[6.1.0]nonane to give selectively a 4 1 mixture of syn and anti stereoisomeric allylic complexes (equation 325)389 393. In contrast to the noncyclic allylic complexes described above which interconvert under the reaction conditions, these mono-cyclic allylpalladium complexes are configurationally stable even under reflux in benzene for 8 h in the presence of 5 mol% PPh3. 

Recently, direct evidence for the stepwise nature of the cycloaddition has been obtained85 from the study of the stereochemistry of the addition of diphenylacetylene to the Z and E isomers of 1,2-dimethyl-1,2-diphenyldisilene, generated from 20 and 21 by pyrolysis at 300 °C. Although the two isomeric disilenes yield different mixtures (63 37 and 38 62) of the two expected stereoisomeric products, whose structures could not be assigned with certainty, the retention of configuration is much less complete than was the case in the 4 + 2 addition of anthracene, discussed in Section II.B.l.a. The incomplete... 

The products of the sodium borohydride reduction of (-)-mecambrine (20b) and of the catalytic hydrogenation of the same alkaloid and (—)-roemeramine (22a) were studied and, on the basis of the ORD curves and H NMR spectra, the relative configurations of these two alkaloids and of four stereoisomeric dihydromecambrinols 22b and 22c were established (418). Casagrande et al. (203) studied the synthesis of ( )-glaziovine (20a) and the stereochemistry of partially reduced proaporphine products (20, 412,446). 

It is of interest that the optical rotations of FTF, FTH, and FTL are opposite (levorotary) to those of FTJ, FTE, and FTG (dextrorotary), respectively, though their chemical structures are identical except for the stereochemistry at C-12. Yamazaki et al. (156) found that FTF, FTH, and FTL are obtained upon treatment of FTJ, FTE, and FTG, respectively, with 0.1% KOH in MeOH. This fact indicates that the metabolites of the former group are the C-12 stereoisomers of the latter group (S configuration), respectively. The possibility that the compounds identified as FTF, FTH, and FTL are artifacts remain. FTM (89) is also obtained from nortryptoquivaline by the KOH-MeOH reaction, demonstrating their stereoisomerism. 

We now wish to consider how these elements of stereochemistry come into play in synthesis. It is important to know how reaction stereochemistry is controlled by structural features of the reactant molecules. This topic can be broadly covered by the term asymmetric synthesis, which has been defined as a reaction in which an achiral unit in an ensemble of substrate molecules is converted by a reactant into a chiral unit in such a manner that the stereoisomeric products are produced in unequal amounts. Thus, we will be dealing with methods for controlling the configuration of newly formed chiral centers. As will be seen, these methods often depend on the fact that reagents attack molecules along the less hindered path. 

The situation with alkaloids containing the dihydro-corynantheane skeleton is similar there are four stereoisomeric types, normal (e.g. corynantheine YAAA, pseudo, alio (e.g. corynantheidine K9.5) and epiallo. Their stereochemistry has been fully discussed [2]. On the basis of this work a corynantheine type alkaloid can be allotted to one of the four classes by spectroscopic measurement, making the same assumption about Cys) configuration [2]. 

The water molecule formed in the elimination step is evidently captured primarily from the front side, leading to retained configuration for the alcohol. The ester product can be formed by solvent collapse from the front or back side or by capture of the acetate ion. It is clear that the two stereoisomeric amines do not form the same intermediate, even though a simple mechanistic picture would show the 2-decalyl ion as a common intermediate. The product composition is very significantly different for the two starting materials. Similar results have been found for the cis-and 4-t-butylcyclohexylamines. Some of the data are summarized in Table 5.16. The general picture which arises from these and other diazotization studies then is one of a very rapid collapse of the cationic intermediate with the product ratio and stereochemistry determined by the immediate solvation environment. 

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