Retention of configuration, and

Figure 3-22 shows a nucleophilic aliphatic substitution with cyanide ion as a nucleophile, i his reaction is assumed to proceed according to the S f2 mechanism with an inversion in the stereochemistry at the carbon atom of the reaction center. We have to assign a stereochemical mechanistic factor to this reaction, and, clearly, it is desirable to assign a mechanistic factor of (-i-1) to a reaction with retention of configuration and (-1) to a reaction with inversion of configuration. Thus, we want to calculate the parity of the product, of 3 reaction from the parity of the... 

Cyclization of methyl (4R)-3-(2-diazo-3-oxobutanoyl)-thiazolidine-4-carboxylate (675) under basic conditions afforded 676 (91CC924). The reaction proceeded with retention of configuration and the structure was established by X-ray analysis (Scheme 140). 

In the presence of catalytic amounts of Pd(0), silicon-substituted vinyloxiranes can rearrange into the corresponding ot-silyl- 3,y-unsaturated aldehydes (Scheme 9.34) [151]. Treatment of 80 with Pd(OAc)2 and P(OPh)3 results in the formation of 7t-allylpalladium complex 81. Bond rotation to give 82, followed by migration of the silyl moiety, affords aldehyde 83, which is trapped in situ to provide the Felkin-Anh product 84. The reaction proceeds with retention of configuration and the ee of the starting material is retained in the product. The size of the silicon substituents is critical for the outcome of the reaction, as is the choice of ligands on palladium. 

Tin/lithium exchange on the a-alkoxy stannanes and subsequent addition of carbon dioxide led to optically active (7-protected a-hydroxy acids 18 with retention of configuration and without any loss of stereochemical information11. 

Both the Se2 (front) and Se2 (back) mechanisms are designated DeAe in the lUPAC system. With substrates in which we can distinguish the possibility, the former mechanism should result in retention of configuration and the latter in inversion. The reaction of allylsilanes with adamantyl chloride and TiCU, for example, gives primarily the antiproduct via a Se2 reaction. When the electrophile attacks from the front, there is a third possibility. A portion of the electrophile may assist in the removal of the leaving group, forming a bond with it at the same time that the new C—Y bond is formed... 

The reaction simply proceeds with retention of configuration, and the diastereomer ratio reflects that of the alkylation. 

Substitution of fi-nitro sulfides.2 (3-Nitro sulfides in the presence of a Lewis acid undergo displacement reactions with either allyl- or cyanotrimethylsilane. The reaction is considered to involve an episulfonium intermediate, which is then substituted at the more positive carbon (equation I). The reaction proceeds with retention of configuration, and anti (J-nitro sulfides react much more rapidly than... 

A suprafacial alkyl [1, 3] shift with retention of configuration and already discussed provides an example. The transition state contains four electrons and is of Huckel type and makes the reaction unfavourable in the ground state but many photo-chemical [1, 3] shifts do occur in the four numbered ring structure. 

Photolysis of the tosylhydrazone sodium salt (94) in diglyme gave, as the major product, 1,3-diene (95) by [l,2]-vinyl shift in the intermediate carbene. Vinyl migration occurs with retention of configuration and is postulated to occur in the singlet manifold. 

In summary (Scheme 15), 2-lithiopiperidines and 2-lithiopyrrolidines appear to be very versatile nucleophiles for the elaboration of these heterocyclic systems, affording a variety of 2-substituted heterocycles in excellent yields. The stereoselectivity of the reaction is near 100% in the piperidine series with most carbonyl electrophiles (retention of configuration) and alkyl halides (inversion of configuration). In the pyrrolidine series, the selectivity is also near 100% with carbonyl electrophiles (retention), but less selective (inversion predominates) with alkyl halides (less problematic with Af-aUylpyrrolidines). 

Assuming that the lithio intermediate possesses a chelated structure, the formation of two diastereomeric compounds 1 and 2 is likely. The stereochemical outcome of the reaction suggests that lithio compound 1 would have to be alkylated with retention of configuration, and lithio compound 2 with inversion of configuration. 

The stereochemical consequences of the cyclization of some 3-(2,5-dihydroxyphenyl)pro-pan-l-ols (247) have been investigated, with a view to optimizing the chiral economy of a tocopherol synthesis from (S)-chroman-2-carboxylic acid (81JOC2445). It was observed that acid-catalyzed dehydration occurred with retention of configuration and it was proposed (79JA6710) that the process involved the formation of a hemiketal through nucleophilic attack by the side-chain hydroxy group on the keto tautomer. 

Mandel, I., and A. L>. McLaren, J. Am. Chem.. Soe., 73, 1826 (1951). These authors noted that aspartic acid and alanine photolyzed with retention of configuration and suggested an a-Iactone intermediate. Such a possibiity could be accommodated by the present scheme. 

Since the oxidative addition occurs with retention of configuration and the transmetallation is also stereospecific with retention, the method is extremely valuable for the stereoselective synthesis of conjugated dienes. The... 

Whatever the method of control may be it seems evident that it is by no means completely efficient for the solid polymer is always a rather small fraction of the total. Price believes, presumably because any optical activity tends to be concentrated in the crystalline phase, that the amorphous and crystalline polymers are products of two different reactions, one in solution and the other heterogeneous (27). While this view is not impossible, it could be argued that solution reaction might be expected to lead to either inversion or retention of configuration and, hence, optically active polymer. Furthermore, some reports suggest rather strongly that the distinction between the two types of polymer is a rather arbitrary one based in part on polymer symmetry and in part on molecular weight. 

The thionation of 2-substituted-4-aryl-5,5-dimethyl-l,3,2-dioxaphosphorinanones with Lawessen s reagent proceeded predominantly with retention of configuration and a cyclic pentacoordinated intermediate was proposed.248 Both P—N bond cleavage in acidic solution and P—C and P—O bond cleavage in basic solution occur in the hydrolysis of A-(methoxycarbonyl)carbamoylmcthoxyphosphonyl)-a-amino acid ester. Only P—O bond cleavage occurs with the ATisopropy Icarbamoy I) compound.249... 

Both retention of configuration and inversion of configuration have been observed in these electrophilic substitutions. The kinetic form of substitutions proceeding as in reactions (4) and (5) should be second-order overall, first-order in substrate and first-order in electrophile, when the two reactants are present in dilute solution at the same order of concentration. 

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