1,5-ASYMMETRIC INDUCTION IN REACTIONS BETWEEN ALDEHYDES AND SILYLOXY)-4-HYDROXYPENT-2-ENYL](TRIBUTYL)-STANNANE PROMOTED BY TIN(IV) CHLORIDE

lsilyloxy)-4-hydroxypent-2-enyl](tributyl)stannane 18 has been prepare d from di-O-isopropylidene-D-mannitol 8. Oxidative cleavage of the man nitol derivative followed by condensation with triethyl phosphonoaceta te and reduction gave the alcohol 10 which was converted into the xant hate 11. Deprotection gave the dihydroxy xanthate 12 which was protect ed as its bis-tert-butyldimethylsilyl ether 13. This rearranged on hea ting in toluene to give the dithiocarbonate 15 which reacted with trib utyltin hydride under free radical conditions to give the S)-4,5-bis(t ert-butyldimethylsilyloxy)pent-2-enyl] (tributyl)stannane 16 as an app roximately 9:1 mixture of E- and Z-isomers. Deprotection and selective protection of the primary hydroxy group provided the [(4S)-5-(tert-bu tyldimethylsilyloxy)-4-hydroxypent -2-enyl]stannane 18. As a shorter r oute; the primary hydroxy group of the dihydroxy xanthate 12 was prote cted as its tert-butyldimethylsilyl ether 14 which underwent clean rea rrangement into the dithiocarbonate 19 when heated in toluene. Reactio n with tributyltin hydride under free radical conditions then gave the 5-tert-butyldimethylsilyloxy-4-hydroxypent-2-enyl) stannane 18. Treat ment of this 4,5-disubstituted pentenylstannane with; tin(IV)chloride generated an allyltin trichloride which reacted with aldehydes with ex cellent 1,5-asymmetric induction to give 1,5-syn-products, e.g. 20, 29 -31 and 41. The stereoselectivity of these reactions would appear to b e controlled by the 4-hydroxy substituent rather than by the 5-tert-bu tyldimethylsilyl group. Aspects of the chemistry of these products, in particular their conversion into 2,6-disubstituted 5,6-dihydro-2H-pyr ans, was investigated.