Journal of Organic Chemistry p. 9983 - 9990 (2007)
Update date:2022-08-11
Topics:
Yagi, Haruhiko
Jerina, Donald M.
(Chemical Equation Presented) We describe a novel trifluoroethanol (TFE) or hexafluoropropan-2-ol (HFP) mediated substitution reaction of the bay-region C10 acetoxy group in four stereoisomeric 7,8,9,10-tetraacetoxy-7,8,9, 10-tetrahydrobenzo[a]pyrenes (tetraol tetraacetates, two pairs of cis and trans isomers at the 9,10 positions) by the exocyclic N2-amino group of O6-allyl-3′,5′-di-O-(tert-butyldimethylsilyl)-2′- deoxyguanosine (3). The tetraacetates are derived from cis and trans hydrolysis of (±)-7β,8α-dihydroxy-9β,10β-epoxy-7,8,9,10- tetrahydrobenzo[a]pyrene (B[a]P DE-1) and of (±)-7β,8α- dihydroxy-9α,10α-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (B[a]P DE-2) at C-10 followed by acetylation. Excellent yields and high regioselectivity were observed. Similar cis/trans product ratios were observed for each set of cis and trans tetraol tetraacetates derived from DE-1 (~75/25) and from DE-2 (~67/33) in HFP. This strongly suggests that the substitution proceeds via an SN1 mechanism involving a carbocation intermediate that is common to the cis and trans tetraacetates. Since it is likely that the cis and trans products from 3 arise from different conformations of the carbocation, its lifetime must be sufficiently long to permit conformational equilibration before its capture by the purine nucleophile. The corresponding reaction of (±)-9α-bromo-7β,8α,10β- triacetoxy-7,8,9,10-tetrahydrobenzo[a]pyrene with 3 in HFP was highly regio- and stereoselective and gave exclusively trans 10β-adducts. This newly developed substitution reaction provides an attractive alternative synthetic strategy for the preparation of polycyclic hydrocarbon adducted oligonucleotide building blocks.
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