Angular Alkylation of cis-Decalin Epoxides with C-nucleophiles: Mechanism,scope, and limitations of a novel bridgehead functionalisation

The angular alkylation of cis-decalin epoxides like 5 or 7 can be achieved at C(8a)

1 For convenience, the arbitrary numbering given for 5 (Scheme I) is used throughout the General Part; for systematic names, see Exper. Part.

in good yield by using CuI and a large excess of Grignard reagents without an sp³ centre at C(2). The reaction proceeds via a carbenium-ion intermediate which is stabilised by homoconjugative interaction with the adjacent double bond. Due to 1,3-diaxial strain in the alkoxides resulting from alkylation or reduction at C(4a) of the epoxides 5 or 7 , the nucleophile is delivered selectively to C(8a). Grignard reagents possessing H-atoms at C(β), transfer a hydride to the epoxide yielding the trans-decalol 11 (Grignard reduction). The angular alkylation of 5 with allylic and benzylic Grignard reagents proceeds with good yield.