Synthesis of Chiral Intermediates of Quinine Alkaloids and (+)- Dihydroantirhine.

(4R,5R)-2-Ethoxy-5-ethyl-4-methoxycarbonylmeth yl-3,4,5,6-tetrahydro-2H-pyran 3, which has been enantioselectively prepared, is converted into the cis-substituted lactone 11 by treatment with propanedithiol in the presence of boron trifluoride–diethyl ether. The product 11 is converted into the synthetic intermediate 7 of quinine alkaloids and the synthetic precursor 19 of (+)-dihydroantirhine 10.