Asymmetric Dearomatizing Spirolactonization of Naphthols Catalyzed by Spirobiindane-Based Chiral Hypervalent Iodine Species

This
report details the development of a spirobiindane-based chiral
hypervalent iodine reagent, especially focusing on its structural
elucidation for effective asymmetric induction of the chiral spiro
center during the oxidative dearomatizing spirolactonization of naphthols.
In this study we synthesized a new series of ortho-functionalized spirobiindane catalysts and demonstrated that the
enantioselectivity can be dramatically improved by the presence of
the substituents ortho to the iodine atom. The structural
elucidation of a spirobiindane-based hypervalent iodine catalyst has
led to further improvement in the stereoselective construction of
the spiro center during the oxidative dearomatizing spirolactonization
of naphthols. Thus, catalytic oxidation with the highest reported
level of enantioselectivity in hypervalent iodine chemistry has been
achieved with also an excellent level of asymmetric induction (92%
ee for substrate 3a). As a result, this study, dealing
with a series of modified iodine catalysts, can provide important
clues about the transition state and reaction intermediate to help
scientists understand the origin of the stereoselectivity. A plausible
transition-state model and intermediate in the reaction for the stereoselective
formation of spirolactone products are postulated by considering the ortho-substituent effect and the results of X-ray analysis.
In this reaction model, the high enantiomeric excess obtained by using
the spirobiindane catalysts could be well explained by the occupation
of the equatorial site and extension of the surroundings around the
hypervalent iodine bonds by the introduced ortho-substituent.
Thus, this study would contribute to estimation of the chiral hypervalent
iodine compounds in asymmetric reactions.