Retentive Solvolysis by a New Neighboring Group Effect between Cage B-atom and Nucleophiles (J. Am. Chem. Soc. 2000)
Organic Reaction Mechanism and Synthetic Method
A New Neighboring Group Effect Based on Electronic Property of Carborane
Icosahedral closo carboranes have been described as three-dimensional aromatic systems, and the implications for electronic interaction with substituents have been of particular interest since the first synthesis of these compounds. Concerning the electronic effect of icosahedral carboranes on a substituent outside the cage showed that the icosahedral carboranes behave as strongly electron-withdrawing groups in the sequence ortho >> meta > para towards carbon substituents. These investigations have also shown that the electron-withdrawing inductive effect of the carborane cage is similar to that of halogens, and that ground-state cage-ring-pai interaction is not important.
In our kinetic investigations, we have reported that the order of rates in the acetolysis of (o-, m- and p-carboranyl)benzyl tosylates was consistent with the electron-withdrawing effect of the icosahedral carboranes. However, we found that the hydrolysis of o-carboranylbenzyl tosylate, bearing what is thought to be the most electron-withdrawing group among the carboranes, was significantly accelerated compared with those of m- and p-forms. Furthermore, in the hydrolysis of (+)-o-carboranylbenzyl tosylate, the reaction afforded the retentive product, with an enantiomeric purity of 71%, whereas the products formed from m- and p-forms were racemic. Therefore, we suggested a mechanism involving interaction between the oxygen atom of the nucleophile and the 3-position boron atom (a new neighboring group effect) in the o-carborane cage to explain the characteristic reaction of o-carborane.
Retentive Solvolysis by a New Neighboring Group Effect between Cage B-atom
and Nucleophiles (J. Am. Chem. Soc. 2000)
Development of Chiral Building Blooks and Application for Medicinal Chemistry
Whether it may be prepared enzymatically or chemically, an enantiopure material that allows a variety of transformations in stereocontrolled manner is of use a versatile chiral building block for the construction of biologically active chiral compounds. From this viewpoint, we have demonstrated the preparation and the practical uses of designed chiral building blocks, which are not depended on natural chiral sources such as D-sugars and L-amino acids.
Recently, we have investigated to prepare both enantiomers of 1,3-dioxycyclopent-4-ene 1 from dicyclopentadiene in enantiomerically pure form. On the basis of the C2 symmetry and allylic trans-dioxy functionalities in 1, we have established to synthesize (+)-mannostatin A 2 which exists powerful mannosidase inhibition, and to synthesize cyclopentenone 3 which has been an useful intermediate of prostagrandins.
On the other hand, we also have established to prepare enantiopure tricyclic lactone 4 and have investigated that the lactone 1, in spite of its enantiomeric form, is a synthetic equivalent of both
enantiomers of chiral gamma-substituted butenolides 5 with tertiary and quartenary asymmetric center. According to our findings, we have achieved enantiodivergent synthesis of (+)- and (-)-trans-quercus lactones 6, which were isolated from oak woods and aged spirits. We have just demonstrated
more effective exploitations of the prepared chiral building blocks to
synthesize pharmacologically important natural or unnatural compounds.
