1,5-Asymmetric induction in reactions between aldehydes and [(4S)-5-(tert-butyldimethylsilyloxy)-4-hydroxypent-2-enyl](tributyl)-stannane promoted by tin(IV) chloride

Abstract

[(4S)-5-(tert-Butyldimethylsilyloxy)-4-hydroxypent-2-enyl](tributyl)stannane 18 has been prepared from di-O-isopropylidene-D-mannitol 8. Oxidative cleavage of the mannitol derivative followed by condensation with triethyl phosphonoacetate and reduction gave the alcohol 10 which was converted into the xanthate 11. Deprotection gave the dihydroxy xanthate 12 which was protected as its bis-tert-butyldimethylsilyl ether 13. This rearranged on heating in toluene to give the dithiocarbonate 15 which reacted with tributyltin hydride under free radical conditions to give the [(4S)-4,5-bis(tert-butyldimethylsilyloxy)pent-2-enyl](tributyl)stannane 16 as an approximately 9:1 mixture of E- and Z-isomers. Deprotection and selective protection of the primary hydroxy group provided the [(4S)-5-(tert-butyldimethylsilyloxy)-4-hydroxypent-2-enyl]stannane 18. As a shorter route, the primary hydroxy group of the dihydroxy xanthate 12 was protected as its tert-butyldimethylsilyl ether 14 which underwent clean rearrangement into the dithiocarbonate 19 when heated in toluene. Reaction with tributyltin hydride under free radical conditions then gave the (5-tert-butyldimethylsilyloxy-4-hydroxypent-2-enyl)stannane 18. Treatment of this 4,5-disubstituted pentenylstannane with tin(IV) chloride generated an allyltin trichloride which reacted with aldehydes with excellent 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 be controlled by the 4-hydroxy substituent rather than by the 5-tert-butyldimethylsilyl group. Aspects of the chemistry of these products, in particular their conversion into 2,6-disubstituted 5,6-dihydro-2H-pyrans, was investigated.