Stereochemistry suprafacial process

The principles of orbital symmetry conservation establish that concerted suprafacial [3, 3]-sigmatropic arrangements are allowed processes. Based on these principles and the results of numerous experiments their stereochemistry is highly predictable. 

Although the stereochemistry could not be seen in the last example, it is evident in the migration of the hydrogen atom in the zwitterion intermediate 5.46, which is symmetry-all owed as a suprafacial [1,4] shift. Although the first step is a photochemical disrotatory 6-electron process, the second step is... 

As shown in the biosynthesis of granaticin, a hydride shift occurs intramolecularly. This process is mediated by an enzyme-bond pyridine nucleotide. A concerted abstraction of H-4 as a hydride in la and a C-5 deprotonation in 2a leads to the 4,5-enol ether 3a. The reduced form of the pyridine nucleotide transfers the hydride to C-6, simultaneously releasing a hydroxide to give 4a. Final tautomerization yields the dTDP-4-keto-6-deoxy-sugar in v-xylo configuration 4a. In other enzymes of the oxidoreductase type, the active site may show a different configuration. Thus, the intermediate 3a can be protonated from above at C-5 to yield the l-arabino isomer of 4a [2]. The stereochemistry of this mechanism was demonstrated by double labelling (cf. l-4b series), and as a net result proved a suprafacial 4—>6 hydride shift. 

Several cases of photochemical reactions, for which the thermal equivalents were forbidden, are shown below. In some cases the reactions simply did not occur thermally, like the [2 +2] and [4 +4] cycloadditions, and the 1,3- and 1,7-suprafacial sigmatropic rearrangements. In others, the photochemical reactions show different stereochemistry, as in the antarafacial cheletropic extrusion of sulfur dioxide, and in the electrocyclic reactions, where the 4-electron processes are now disrotatory and the 6-electron processes conrotatory. In each case,... 

The Diels-Alder reaction is one of the most powerful and efficient processes for formation of six-membered rings with the potential of controlling the relative and absolute stereochemistry at four newly created stereogenic centers [1]. Relative stereochemistry is usually well-defined because of the formation of a cyclic transition state arising from suprafacial-suprafacial interaction, with endo approach [2]. The reaction can be accelerated by Lewis acids, high pressure, or radical cations. Diels-Alder reactions catalyzed by Lewis acids are generally more regio- and stereoselective than their thermal counterparts [3]. 

A cycloaddition reaction can be classified not only by the number of electrons in the individual components, but also by the stereochemistry of the reaction with regard to the plane of the tt system of each reactant. For each component of the reaction, there are two possibilities the reaction can take place on only one side of the plane or across opposite faces of the plane. If the reaction takes place across only one face, the process is called suprafacial if across both faces, antarafacial. The four possibilities are shown in the following diagram ... 

These terms resemble the familiar ones syn and anti, but with this difference. Syn and anti describe the net stereochemistry of a reaction. We have seen anti addition, for example, as the overall result of a two>step mechanism. Suprafacial and antarafacial, in contrast, refer to actual processes the simultaneous making (or breaking) of two bonds on the same face or opposite faces of a component. 

The discovery of the [47t -i- 2tu] cycloaddition reaction by Otto Diels (Nobel Prize, 1950) and Kurt Alder (Nobel Prize, 1950), a landmark in synthetic organic chemistry, permits the regio- and stereoselective preparation of both carbocyclic and heterocyclic ring systems. Its application can result simultaneously in an increase of (1) the number of rings, (2) the number of asymmetric centers, and (3) the number of functional groups. The reaction controls the relative stereochemistry at four contiguous centers. The Diels-Alder reaction can be depicted as a concerted -1- (suprafacial) cycloaddition. While depicted as a concerted process, the reaction has been proposed to proceed in a nonsynchronous manner via an unsymmetrical transition state. °... 

In no case is an exo-endo isomerization at C-5 observed. This finding rules out a migration of C-5 with retention ( suprafacial retention, sr) in the bicyclopen-tene systems, a process calculated by semiempirical molecular orbital (MO) methods to be the favored one (28). The observed stereochemistry corresponds to an inversion at C-5 ( suprafacial inversion, si) and is in accord with the stereochemical requirements for an orbital symmetry controlled process. 

We may further extend the analysis of pericyclic reactions by considering that a single p orbital, denoted by the symbol m, can be a participant in a pericyclic reaction. In this analysis, one lobe of the p orbital makes up the top face of a one-atom n system, while the other lobe makes up the bottom face. The participation of a single p orbital is suprafacial if both cycloaddition processes involve only one of the two lobes of the p orbital, and it is antarafacial if the cycloaddition involves both. We may thus predict that the conrotatory opening of the cyclopropyl anion to an allyl anion (Figure 11.72) should take place via an -F 2 ] pathway. Conversely, the opening of the cation would be a -F 2 ] process, giving the opposite stereochemistry in the product." ... 

Sc(OTf)3 was proved to be an effective catalyst for Diels-Alder reactions of a new type of 1-hydrazinodiene with many dienophiles (Scheme 12.16) [32]. The obtained Diels-Alder adducts could be converted to six-member ring alkenes via suprafacial 1,5-sigmatropic rearrangements of hydrogens after the loss of dinitrogens. This process enabled a 1,3-transfer of stereochemistry to a new position on the six-member ring system. 

Among the cycloaddition reactions that have been shown to have general synthetic utility are the [2 + 2] cycloadditions of ketenes and alkenes. The stereoselectivity of ketene-alkene cycloaddition can be analyzed in terms of the Woodward-Hoffmann rules. To be an allowed process, the [ 2 + 2] cycloaddition must be suprafacial in one component and antarafacial in the other. Figure 6.5 illustrates the transition state. The ketene, utilizing the low-lying LUMO, is the antarafacial component and interacts with the HOMO of the alkene. The stereoselectivity of ketene cycloadditions can be rationalized in terms of steric effects in this transition state. Minimization of interaction between the substituents R and R leads to a cyclobutanone in which these substituents are cis. This is the stereochemistry observed in these reactions. 

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