Antiperiplanar influence effects

The diastereoselectivity observed can be explained by a synclinal transition state, probably influenced by chelation and/or stereoelectronic effects of the developing cation38. The minor product is formed via an antiperiplanar transition state. All compounds obtained are useful precursors for several spirocyclic natural products, such as terpenes like lubimine or acoradi-ene. 

Stereoelectronic effects in chemical reactivity The bond-lengthening and -weakening influence of an antiperiplanar lone pair leads to strong stereoelectronic effects on chemical reactivity.97 In molecule 28a with lone-pair-bearing atom D adjacent to an A—B bond, a vicinal nD—s-cab hyperconjugative interaction can be associated (cf. Example 1.4 and Section 3.3.1) with a partial admixture of the alternative resonance structure 28b,... 

Of all substituent effects, that caused by y-anti substituents has been the hardest to understand in terms of transmission mechanisms, notably because the resulting shifts are sometimes upfield and sometimes downfield. So, for example, y -SCS(OH) values range from about +6 (208) to —6 (209). Correspondingly, the discussion about transmission mechanisms is prone to speculation and controversy. At present it is impossible to arrive at a consistent interpretation of this substituent effect there is no choice but to compile a number of molecular properties that apparently affect the transmission of substituent influences on antiperiplanar carbon atoms. 

Vasella has suggested that the observed stereoselectivity is due to a stereoelectronic effect which strongly influences the stereochemistry of the transition state of the cycloaddition reaction. This stereoelectronic effect is caused by the nitrogen electron pair which is oriented antiperiplanar to the C-O bond of the... 

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