Stereochemistry Stereospecific reactions

The di-TT-methane rearrangement is a stereospecific reaction. There are several elements of stereochemistry to be considered. It is known that the double bond that remains uncyclized retains the E or Z configuration present in the starting material. This result excludes any intermediate with a freely rotating terminal radical. The concerted... 

Since the stereochemical course of a catalytic hydrogenation is dependent on several factors, " an understanding of the mechanism of the reaction can help in the selection of optimal reaction conditions more reliably than mere copying of a published recipe . In the first section the factors which can influence the product stereochemistry will be discussed from a mechanistic viewpoint. In subsequent sections the hydrogenation of various functional groups in the steroid ring system will be considered. In these sections both mechanistic and empirical correlations will be utilized with the primary emphasis being placed on selective and stereospecific reactions. 

Very few examples of singlet n-n dimerizations are reported, the reactions of 2-butenes in Eqs. 36 and 37 being examples with observable stereochemistry. The reactions are stereospecific in agreement with theory. The unsensitized reactions of styrene 115> and some stilbene derivatives 116> may also proceed via excited states, Eq. 47 and... 

Mortko, C.J. and Garcia-Garibay, M.A. (2006) Engineering stereospecific reactions in crystals synthesis of compounds with adjacent stereogenic quaternary centers by photodecarbonylation of crystalline ketones, in Topics in Stereochemistry, Vol. 25 (eds S.E. Denmark and J.S. Siegel), John Wiley Sons, Hoboken, NJ, pp. 205—253. 

Takano et al. have reported the first synthesis of ( )-antirhine (71) (Scheme 11),51 in which the problem of generating the desired, less stable (anti) stereochemistry at C-3 and C-15 was overcome by preparing the non-tryptamine fragment (72) from ( )-trinorcamphor (73) via a sequence of stereospecific reactions. Condensation of (72) with tryptamine, followed by cyclization and... 

The E2 is a stereospecific reaction, because different stereoisomers of the starting material react to give different stereoisomers of the product. This stereospecificity results from the anti-coplanar transition state that is usually involved in the E2. We consider more of the implications of the anti-coplanar transition state in Chapter 7. For now, Problem 6-38 will give you an opportunity to build models and see how the stereochemistry of an E2 elimination converts different stereoisomers of the reactants into different stereoisomers of the product. 

Stereospecific reactions lead to the production of a single isomer as a direct result of the mechanism of the reaction and the stereochemistry of the starting material. There is no choice. The reaction gives a different diastereoisomer of the product from each stereoisomer of the starting material... 

A lactone makes a good temporary tether because it can be hydrolysed or reduced to break the ring at the C-0 bond and reveal new stereogenic centres on the old structure. In this sequence a lactone, formed by iodolactonization, controls all the subsequent stereochemistry of the molecule in two ways it fixes the conformation rigidly in one chair form—hence forcing the iodide to be axial— and it blocks one face of the ring. The iodolactonization is very similar to one you saw on p. 872. Next, an alkene is introduced by E2 reaction on the iodide. This stereospecific reaction requires an... 

Stereospecific reactions—reactions where the mechanism means that the stereochemistry of the starting material determines the stereochemistry of the product and there is no choice involved... 

Both of these examples are very interesting because they show how, once we have some stereochemistry in a molecule, we can change the functional groups but keep the stereochemistry—this is the essence of a stereospecific reaction. In the second example, we change the bromide to a double bond, but we keep the stereochemistry (or stereochemical information ) because the geometry of the double bond tells us which bromide we started with. 

For a stereose/ect/ve reaction we can specify two different stereoisomers of the starting material and get the same product (first and third examples). In a stereospecific reaction, different starting material stereochemistry means different product stereochemistry. 

Once the strategy was selected, the validation of the relevant cyclization in solution and the determination of its stereochemical outcome and yield were carried out. The synthetic scheme is reported in Fig. 3.6. The commercially available allyl (3.3) and propargylglycines (3.7) were sequentially tosylated and alkylated with propargyl and allyl bromide, respectively, to give 3.5 and 3.9. The intramolecular Pauson-Khand cyclization produced the two isomers 3.6 and 3.10, with different stereochemistries, in a stereospecific reaction (the chiral allylglycine produced 3.6 as a single enantiomer. 

For this, stereospecific reactions are required that allow selective access to different stereoisomers by, for example, changing the stereochemistry of the catalyst or the chiral substrates. 

Stereochemistry Electrocyclic reactions, like all pericyclic processes, exhibit great stereospecificity. The stereospecificity of such reactions is demonstrated by thermal closure of fransjds,trans-2,4,6-octatriene (8.12) to ds-5,6-dimethyl-1,3-cyclohexadiene (8.13) and of the isomeric frans,cis,czs-octatriene (8.14) to frans-5,6-dimethyl-1,3-cyclohexadiene (8.15). 

The stereochemistry of reactions between carbenes and alkenes is determined by the states of the carbenes (when generated), whereby singlet carbenes react in a stereospecific one-step concerted process whilst triplet carbenes lead to a mixture of products via a diradical intermediate (Figure 6.58). Consequently, since fluorocarbenes are singlets in the ground state (Table 6.2), cyclopropanation of alkenes is often stereospecific [91] (Figure 6.59) (for more examples, see Sections A and B). 

The hydroalumination of alkynes was first observed by Wilke and Muller in 1955, when they demonstrated that dialkylaluminum hydrides added to disubstituted alkynes stereospecifically in a syn fashion (13b). Subsequently, Zakharkin and coworkers were able to add NaAlH4 to phenylacetylene by employing 5% of Bu 2A1H and diglyme as a solvent, but did not ascertain the stereochemistry of reaction. Some 10 years later, in 1966, Slaugh found that LAH in refluxing THF-diglyme mixtures, hydroalumi-nates such alkynes exclusively in an anti manner (14 equation 6). ... 

This experiment carries out an epoxidation reaction on cholesterol, which is a representative of a very important group of molecules, the steroids. The rigid cholesterol molecule gives products of well-defined stereochemistry. The epoxidation reaction is stereospecific and the product can be used to carry out further stereospecific reactions. 

Much of our present knowledge about the stereochemistry of reactions was developed from steroid chemistry. In this experiment the double bond of cholesterol is stereospecifically converted to the 5a,6a epoxide. The a designation indicates the epoxide is on the backside of the molecule. A substituent on the topside is designated j8. Study of molecular models reveals that the angular methyl group prevents topside attack on the double bond by the perbenzoic acid hence the epoxide forms exclusively on the back, or a side of the molecule. 

To show that double bond geometry can affect the stereochemistry of tetrahedral centres stereospecific reactions. 

All stereospecific reactions are necessarily stereoselective, but the reverse is not true. There are reactions from which one particular stereoisomer is the predominant product regardless of the stereochemistry of the reactant there are reactions in which the reactant cannot exist as stereoisomers, but from which one particular stereoisomer is the predominant product. Such reactions are stereoselective but not stereospecific. 

Polymer Stereochemistry and Optical Activity.—Although stereoregular products from carbocation propagations are not common, there are some notable examples, perhaps the most important being the isotactic materials from alkyl vinyl ethers. Recently, novel catalysts based on phosphoryl and thionyl chlorides with vanadium pentoxide have been added to those initiator systems capable of producing stereospecific reactions. 

For example, addition of bromine to cw-2-butene gives equal concentrations of (2 ,3 )-2,3-dibromobutane and (25,35)-2,3-dibromobutane (a racemic mix), whereas addition of bromine to rrans-2-butene gives the meso compound (2S,3R)-2,3-dibromobutane. Since the stereochemistry of the reactant is determining product stereochemistry, this reaction is stereospecific, and whatever mechanism is proposed must account for that. 

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,... 

Finally, carbenoid species can be used as the carbon donor in aldehyde epoxidations. Thus, the rhodium carbenoid derived from the cyclic diazoamide 49 and rhodium(II) acetate reacts stereo selectively with aryl aldehydes to provide spiro-indolooxiranes 50 with Z-stereochemistry. The reaction is believed to proceed via the formation of a carbonyl ylide 51, which undergoes stereospecific thermal conrotatory electrocyclization to form the observed epoxide <04SL639>. 

PROBLEM 8.15 The hydrolysis of sulfonate esters of 2-octanol is a stereospecific reaction and proceeds with complete inversion of configuration. Write a structural formula that shows the stereochemistry of the 2-octanol formed by hydrolysis of an optically pure sample of (S)-(+)-1-methylheptyl p-toluenesulfonate, identify the product as R or S, and deduce its specific rotation. 

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