Cycloaddition reactions stereochemical modes

In a cycloaddition reaction, the two active n systems may approach each other in either of two orientations, for example, head to head or head to tail. If one combination dominates, the reaction is said to be regioselective. In the course of the reaction, 4 new saturated centers are formed. With maximum labeling, a total of 16 (=24) stereo-isomeric forms, consisting of 8 enantiomeric pairs of diastereomers if neither polyene is chiral, may be formed. In pericyclic reactions, the stereochemistry is determined by specifying the stereochemical mode in which each component reacts. Each of the two... 

With the hypothesis that allene-olefin cycloadditions and methylenecyclobutane rearrangements have a common intermediate comes the prediction that common stereochemical modes for the various rotations involved will prevail. The cycloaddition model outlined above involves disrotatory ring closure with the same sense of rotation about C(4)C(5) and C(5)C(2). For the methylenecyclobutane rearrangement, the same stereochemistry is anticipated for the reverse reaction. 

The li + j] refers to the number of ir electrons in each reactant that are active in the reaction. A more conprehensive system denotes the nature of the orbitals undergoing change (ir or a), and the stereochemical mode of cycloaddition (suprafacial (S) or antarafacial (a) - see Fig. 2), as well as the number of electrons in each reactant that are participating in the cydoaddition. 

In connection with cycloaddition reactions it is convenient to mention yet another interesting possibility of the exploitation of the Fukui s approach. This possibility concerns the rationalization of the so-called endo addition in Diels-Alder reactions. In order to demonstrate the preference of this specific reaction mode let us assume a simple addition of two butadiene molecules resulting in the formation of vinylcyclohexene. For this reaction there are two alternative 4s + 2s reaction paths differing in the stereochemical arrangement of the corresponding products. One of these products is of the so-called exo type, whereas for the other the name endo is reserved. 

The intramolecular thermal [5+2] cycloaddition of 3-alkoxy-4-pyrones with sulfur- (e.g., 416) or silicon- (e.g., 419) tethered alkenes has been shown to occur with complete regio- and stereochemical control to give adducts 417 and 421, respectively. The adducts can be converted by reduction and oxidation, respectively, to the bicyclic products 418 and 421 (Scheme 69) <1993JOC5585>. It should be noted that this thermal [5+2] cycloaddition has not been realized in a bimolecular mode <1977JOC3976>. This methodology serves as an alternative to the reaction of electron-deficient alkenes with pyrone-derived 4-methoxy-3-oxidopyrylium ylides <1992TL2115>. 

Although cycloadditions have frequently been observed in radical-cation chemistry, this reaction mode is apparently very rare in radical-anion chemistry because of the electron repulsion term. Few examples are known of Diels-Alder dimerizations [355], [2 -I- 2] cycloadditions [356], retro-[2 - - 2] cycloadditions [357], and cyclo-trimerizations [358]. Equally, little is known about electrocyclic reactions, despite their interesting stereochemical course [359]. 

According to the Woodward-Hoffmann rules, the concerted cycloaddition of two olefins to afford a cyclobutane is allowed photochemically as + 2J reaction and thermally as the [,2s + reaction. The different modes of addition give rise to products with different stereochemical structures as indicated in Figure 7.26. If the reaction does not follow a concerted pathway... 

With the achievement of high stereochemical control resulting from nearly exclusive endo topography in the Lewis acid catalyzed reactions interest next turned to the effect of chiral Lewis acid complexes on the enantiofacial selectivity of the cyclocondensation reaction. For example, consider the cycloaddition of benzaldehyde to diene (8) under the influence of a chiral catalyst. Assuming a pericyclic reaction mode and high endo selectivity, a mixture of two possible products can result. Compound (64) is desig-... 

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