Highly Regioselective Bromination of [3aR-(3aα,4β,7β,7aα)]-Hexahydro-7,8,8-trimethyl-4,7-methanobenzofuran-2(3H)-one via Nonclassical Bornyl Cations

摘要

Following our ongoing attempts to substitute [2R-(2α,-3aα,4β,7β,7aα)]-octahydro-7,8-trimethyl-4,7-methanoben-zofuran-2-ol with switchable catalytic functions in close proximity to the acetal reaction center, we had to tackle the challenge of regioselective substitution of the monot-erpene moiety. To this end, the well-aimed attack of bromine at specific regions (see Figure 1, left) of the precursor[3aR-(3aα,4β,7β,7aα)]-hexahydro-7,8,8-trimethyl-4,7-methanobenzofuran-2(3H)-one 1 appeared promising, in view of the arsenal of methods developed for the regioselective introduction of bromine into the corresponding methyl and methylene groups (Figure 1, right) of camphor 2 and its derivatives. In particular, elemental bromine in acetic acid is apt to replace either one or both hydrogens at C-3, in the α-position of the carbonyl group (route A in Figure 1, right).