Difficulties Experienced with the Sigmatropic Shift in Cyclobutanated Species

5 Difficulties Experienced with the [1,5] Sigmatropic Shift in Cyclobutanated Species 

The following factors, however, make a clean realization of such a reaction more difficult than in the corresponding cyclopropane analog for the following reasons  

Fortunately, the problem arising from the secondary Cope rearrangement could be suppressed by the choice of a methoxy group as a stereochemical marker as in 22 (Ri=CH3, R2=OMe, R3=R4=H), 22 (R1=OMe, R2=CH3, R3=R4=H), 22 (R CIT, R2=OMe, R3=D, R4=CH3) and 22 (R,=OMe, R2=CH3, R3=D, R4=CH3). This is so because a hydrogen shift leads to an enol ether whose subsequent Cope rearrangement 

The epimerization at both the ring stereogenic centers causes the diastereomeric interconversion 22 30. Since the rates of [l,5]-hydride shifts in these two diastereomers are comparable, some of the product formed from the pyrolysis of 22 must actually arise from 30 and vice versa. After making corrections for the concurrent double epimerization by following an established procedure [9, 10], it was shown that the mechanistically significant ratio of the products /i,Z-31 and Z,Z-31 formed directiy from 22 was 220 1. Very clearly, the orbital overlap controlled the reaction of the stereochemically well defined reactant 22 and caused one symmetry-allowed pathway to take prominence over the other symmetry-allowed pathway. 

To sum up the above discussion, we have witnessed that the orbital overlap component of the stereoelectronic effect is indeed a very powerful tool as it controls both the stereochemistry and the rates of a range of pericyclic reactions by allowing exclusively one of the two possible symmetry-allowed pathways for the very simple reason of better overlap of the breaking bonds. 

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