搜搜考试网英语翻译The resulting enolate 15 was then subjected to γ-deproto
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英语翻译
The resulting enolate 15 was then subjected to γ-deprotonation with potassium hexamethyldisilazide.This created an extended enolate that reacted with [+]-camphorsulfonyloxaziridine to produce 5 (see Section 6.4).The fact that the enolate and oxidant had 'matched' chirality clearly helped reinforce the stereoselectivity of this oxidation through double asym-metric induction.Hexamethylphosphoric triamide (HMPA) was added to the reaction mixture solely to enhance the reactivity of the enolate through coordination to the metal counterion.Typically,electrophilic hydroxylation proceeded in 90-94% yield.Smith now focused on detaching the O-isopropylidene group from 5 with acidic resin in aqueous acetonitrile.The resulting lactol 16was then oxidised to lactone 17 with diallyl carbonate and palladium(0).
\x05Since lactones are generally more susceptible to nucleophilic attack than ordinary alkyl esters,Smith found it possible to selectively ring-open 17 with ammonium hydroxide to form 18.A selective oxidation was then performed with the Parikh-Doering reagent [SO3-py/dimethylsulfoxide (SO3-py/DMSO)] to obtain 4.Selectivity was possible in this reaction because of the higher acidity of the amide α-hydroxyl in 18.This conferred much greater nucleophilicity on this OH towards the activated-DMSO reagent.Once generated,α-ketoamide 4 (see Scheme 6.2) underwent a spontaneous de Mayo-type retro-aldol fragmentation to relieve ring strain.This afforded the a-ketoamide enolate 3,which immediately cyclised to hemiacetal 19.Because hemiacetal 19 readily decomposed when treated with strong base,Smith hydrolysed its methyl ester with 3.6 N HCl (see Scheme 6.3).This furnished 1 in quantitative yield.The finale of Smith's(一)-echinosborin synthesis was his Mitsunobu lactonisation3 of hydroxyacid 1.Here the less-hindered hemiacetal hydroxyl was selectively converted to a reactive glycosyl phosphonium ion,which then underwent internal displacement by the pendant acid.It provided(一)-echinosporin in 28-30% yield.
\x05 Smith's synthesis of(一)-echinosporin stands out for its good stereo-and regiocontrol at every stage.Its large number of highly chemoselective reactions,and its use of an asymmetric [2+2]-photocycloaddition reaction for constructing the cis-fused 6,5-bicyclic core are also commendable.The latter reaction provides a beautiful illustration of how properly harnessed photochemistry can be decisively deployed for the stereocontrolled synthesis of complex natural products.
The resulting enolate 15 was then subjected to γ-deprotonation with potassium hexamethyldisilazide.This created an extended enolate that reacted with [+]-camphorsulfonyloxaziridine to produce 5 (see Section 6.4).The fact that the enolate and oxidant had 'matched' chirality clearly helped reinforce the stereoselectivity of this oxidation through double asym-metric induction.Hexamethylphosphoric triamide (HMPA) was added to the reaction mixture solely to enhance the reactivity of the enolate through coordination to the metal counterion.Typically,electrophilic hydroxylation proceeded in 90-94% yield.Smith now focused on detaching the O-isopropylidene group from 5 with acidic resin in aqueous acetonitrile.The resulting lactol 16was then oxidised to lactone 17 with diallyl carbonate and palladium(0).
\x05Since lactones are generally more susceptible to nucleophilic attack than ordinary alkyl esters,Smith found it possible to selectively ring-open 17 with ammonium hydroxide to form 18.A selective oxidation was then performed with the Parikh-Doering reagent [SO3-py/dimethylsulfoxide (SO3-py/DMSO)] to obtain 4.Selectivity was possible in this reaction because of the higher acidity of the amide α-hydroxyl in 18.This conferred much greater nucleophilicity on this OH towards the activated-DMSO reagent.Once generated,α-ketoamide 4 (see Scheme 6.2) underwent a spontaneous de Mayo-type retro-aldol fragmentation to relieve ring strain.This afforded the a-ketoamide enolate 3,which immediately cyclised to hemiacetal 19.Because hemiacetal 19 readily decomposed when treated with strong base,Smith hydrolysed its methyl ester with 3.6 N HCl (see Scheme 6.3).This furnished 1 in quantitative yield.The finale of Smith's(一)-echinosborin synthesis was his Mitsunobu lactonisation3 of hydroxyacid 1.Here the less-hindered hemiacetal hydroxyl was selectively converted to a reactive glycosyl phosphonium ion,which then underwent internal displacement by the pendant acid.It provided(一)-echinosporin in 28-30% yield.
\x05 Smith's synthesis of(一)-echinosporin stands out for its good stereo-and regiocontrol at every stage.Its large number of highly chemoselective reactions,and its use of an asymmetric [2+2]-photocycloaddition reaction for constructing the cis-fused 6,5-bicyclic core are also commendable.The latter reaction provides a beautiful illustration of how properly harnessed photochemistry can be decisively deployed for the stereocontrolled synthesis of complex natural products.
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