Stereospecific reactions Regioselectivity

The same regioselective and stereospecific reactions are observed in decalin systems. The 3/3-formate 605 is converted into the a-oriented (j-allylpalladium complex 606, and the hydride transfer generates the fra .s-decalin 607, while the cis junction in 610 is generated from the 3tt-formate 608 by attack of the hydride from the /3-side (609). An active catalyst for the reaction is prepared by mixing Pd(OAc)2 and BU3P in a 1 I ratio with this catalyst the reaction proceeds at room temperature. The reaction proceeded in boiling dioxane when a catalyst prepared from Pd(OAc)2 and BujP in a 1 4 ratio was used[390]. 

The formation of five-membered tetrahydrothiophene rings using nonsymme-trical reaction partners usually occurs with high regioselectively. In most cases, a stereospecific reaction was observed (9,27,94). Recently, a diastereoselective cycloaddition of the parent species la to various chiral a,p-unsaturated amides was also reported (32,95). 

N-Benzyl-N-methoxyraethyl-N-(triraethylsily1jmethylamine undergoes stereo-specific cycloaddition with dimethyl maleate and fumarate. The cycloaddition behavior of an unsymnetrically substituted a-methoxysilylamine has also been examined and found to occur with high overall regioselectivity. The stereospecificity and regioselectivity of the reaction is consistent with a concerted 1,3-dipolar cycloaddition reaction. 

For each class of pericyclic reactions two or more of the following characteristics will be discussed the typical reactions, regioselectivity, stereoselectivity, and stereospecificity. The discussions of typical reactions and stereospecificity will help you recognize when pericyclic reactions are occurring in a particular chemical reaction. The discussions of regioselectivity, stereoselectivity, and stereospecificity will allow you to predict the structures and stereochemistry of the products obtained from pericyclic reactions. 

Figure 14.5 Electrophilic 18F-fluorination of an organomercurial derivative of dihydroxyphe-nylalanine as a route to synthesis of6-[18F]fluoro-3,4-dihydroxyphenylalanine (6-[18F]fluoroDOF>A/ FDOPA). Reaction is stereospecific and regioselective but requires a step for deprotection of functional groups.

The Brown asymmetric crotylation is a highly regioselective and stereospecific reaction. Many organoboranes are now commercial available. Reviews (a) Denmark, S. E. Alrnstcad, N. G. In Modem Carbonyl Chemistry, Otera, J, Ed. Wiley-VCH Weinheim, 2000 Chapter 10 Allylation of Carbonyls Methodology and Stereochemistry,... 

Stereochemistry of Reactions Regioselective, Stereoselective, and Stereospecific Reactions... 

Mention should also be made of the addition of tervalent phosphorus compounds to alkynylphosphonic and to (alka-l,2-dienyl)phosphonic derivatives [in which addition occurs across the C(i)—C(2) bond]. The main product is of type 521 (X = Ph, OR or NR2), but is normally accompanied by smaller amounts of the isomers 522. Although this process is formally of a [3 + 2] format, a P NMR study of the kinetics suggests that the addition is not a synchronous addition but rather occurs in a stepwise manner . On the other hand, the phosphorylated Schiff base 523 undergoes regioselective and stereospecific reaction with a,j -unsaturated esters which, a kinetic study has shown, appears to be concerted. ... 

Enzymatic approaches to glycoside synthesis may circumvent these problems. Enzymes (i.e. nucleoside glycosyl transferases) are stereospecific and regioselective catalyzing glycosyl transfer reaction under mild conditions. Extensive protection-deprotection schemes are unnecessary and the control of anomeric configuration is managed. 

Two questions are of principal interest for predicting the structure of reaction products of 1,3-dipolar addition (1) Is the reaction stereospecific (2) Is the reaction regioselective The answer to the first question is yes with respect to the dipolarophile. Many specific recorded examples demonstrate that the cyclic product results from a stereospecific syn addition to olefins. The stereospecific addition that is observed is exactiy what would be expected on the basis of a concerted mechanism. 

The photocycloadditions show all the characteristics of concerted reactions, including stereospecificity and regioselectivity. Concerted 1,3-dipolar additions are known to proceed via a two-plane orientation complex. For the case of diphenylazirine and methyl acrylate, there are two possible orientation complexes (26 or 27). The interaction of substituent groups in the syn complex 26 can be of an attractive (tc overlap, dipole-dipole interaction) or of a repulsive nature (van der Waals strain). Both effects are probably negligible in the anti complex 27. The ratio of the two steric courses syn and anti) functions... 

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