PALLADIUM-COBALT-MEDIATED DOUBLE ANNULATION PROCESS - A NEW STRATEGY TO CHIRAL AND POLYSUBSTITUTED BIS-CYCLOPENTANOIDS ON CARBOHYDRATE PRECURSORS

摘要

The iodohydrins 2, 4, and 5 mere prepared by the ring opening of benzyl 2-O-p-tosyl-3,4-anhydro-beta-L-arabinopyranoside (1) or benzyl 2,3-anhydro-4-O-acetyl-alpha-D-ribopyranoside (3), respectively, using sodium acetate, sodium iodide, and acetic acid in acetone which on treatment with POCl3 in pyridine yielded the unsaturated sugars 6 and 7. After deacetylation of 7 with MeOH/H2O/Et(3)N (3:2:1) and treatment of 8 with tosyl chloride/pyridine at 50 degrees C 9 was obtained. The reaction of benzyl 2-O-p-tosyl-3,4-dideoxy-alpha-D-glycero-pent-3-eopyranoside (6) and benzyl 2,3,4-trideoxy-4-chloro-beta-L-glycero-pent-2-enopyranoside (9) with the sodium enolate of dimethyl propargylmalonate in the presence of catalytic amounts of tetrakis(triphenylphosphine)palladium(0) afforded the branched-chain sugars 10 and 11. The isomer 10 was obtained as a minor product from 6 with retention of configuration around C-2, and the major isomer 11 as a result of allylic rearrangement in a ratio of 1:9. On the other hand, compound 9 afforded 10 as a major product and its regioisomer 11 as a minor product in a ratio of 8:2; formation of the above mentioned isomeric mixture involves cis and trans diastereomeric intermediates during the reaction. Treatment of these precursors with Co-2(CO)(8) in benzene followed by oxidative decomplexation with DMSO yielded in a stereoselective manner the polysubstituted bis-cyclopentanoids 12 and 13. The stereochemistry of 13 was assigned with the help of X-ray analysis. Attempts were made to prepare the tetracyclic systems 15 and 17 using 12 and 13 with 3-acetoxy-2-[(trimethylsilyl)methyl]-1-propene (14); however, the alkylation products 16 and 18 were obtained. [References: 85]