Copper-catalysed, enantioselective desymmetrisation of meso cyclic allylic bis(diethyl phosphates) with organozinc reagents

A highly regio-, diastereo- and enantioselective desymmetrization of five-, six-, and seven-membered meso, cyclic allylic bis(diethyl phosphates) (3-5) with organozinc reagents was developed, using catalytic amounts (10 mol%) of [Cu(OTf)](2)(C6H6)-C-. and two different classes of chiral ligands: Schiff bases 1 and phosphoramidites 2. Good to excellent enantioselectivities were obtained for every substrate by a subtle balance of ligand structure and experimental conditions. In particular, ee's of up to 88% were obtained for the five-membered ring substrate 3 with ligands 1cjo and 1cjm using Et2Zn (94% ee with Me2Zn, 88% ee with nBu(2)Zn). Schiff-base ligands 1 were not effective with the six- and seven-membered substrates 4 and 5. The use of phosphoramidite ligands 2 afforded ee's of up to 94% (Et2Zn) for the six-membered ring product 7, and virtually only one enantiomer (ee greater than or equal to98%, with Et2Zn) for the seven-membered ring product 8. In addition, the desymmetrisation of the conduritol derivative 10 was obtained, with ee's of up to 86%. In this case, a fully functionalised cyclohexane derivative, containing four contiguous stereocentres and a double bond, was obtained as a single diastereomer and in high enantiomeric excess from an achiral starting material.