Carboxylic esters selenylation

Sulfenylation, Sulfonation, and Selenylation of Ketones and Carboxylic Esters... 

The /V-phenylselenophthalimide (NPSP)-tributylphosphine (Bu3P) system is also a useful reagent for the facile transformation of carboxylic acids into seleno esters, and of alcohols into alkyl phenyl selenides (including some carbohydrate derivatives).102 The utility of iV-phenylselenophthalimide (N-PSP) as a selenylating agent has been reviewed by Nicolaou and co-workers.103... 

Achiwa reported a short synthesis of pyrrolizidine derivatives by the cycloadditions using a nonstabilized azomethine ylide 23 (m = 1) (82CPB3167). When the trimer of 1-pyrroline is treated with a silylmethyl triflate, N-alkylation of the 1-pyrroline takes place. Then the resulting iminium salt is desilylated with fluoride ion in the presence of ethyl acrylate to give ethyl pyrrolizidine-l-carboxylate 295 as a mixture of stereoisomers (28%). After the epimerization of 295 with LDA, the ester moiety is reduced with lithium aluminum hydride in ether to provide (+ )-trachelanthamidine (296). A double bond can be introduced into 295 by a sequence of phenyl-selenylation at the 1-position, oxidation with hydrogen peroxide, and elimination of the selenyl moiety. The 1,2-dehydropyrrolizidine-l-carboxylate 297 is an excellent precursor of (+ )-supinidine (298) and (+)-isoretronecanol (299). Though in poor yield, 297 is directly available by the reaction of 23 with ethyl 3-chloropropenoate. 

Following the original report that the oxazoline carboxylic acid 7.5.12 can serve as an excellent side chain precursor 281), several syntheses of this compound have been reported. Thus racemic oxazoline was prepared from protected cinnamyl alcohol by addition of phenyl-selenyl triflate and azide ion to give the adduct 7.5.9, which was then converted to oxazoline 7.5.10 and ester 7.5.11 282). A similar but more efficient route used diphenyldiselenide and ammonium persulfate to give the racemic oxazoline 7.5.10 in 95% yield from cinnamyl acetate 283). 

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