Selenyl triflates

On the other hand, higher 1,2-asymmetric induction is observed for the cyclization of 5-aIkenols which bear an oxygenated function at C-4. Thus, treatment of 5-hexen-1,4-dioI (3) with phenyl-selenyl triflate in dichloromcthanc at —78 °C gives the corresponding tetrahydro-2/f-pyran 4 in 52% yield as a 91 9 (cis/trans) mixture. The preferential 2,3-cis relationship can be explained by analogy with the halocyclization of 4-pentene-l,3-diols (see Section 4.6.2.1.1.3.)19. 

An interesting application of the asymmetric alkoxyselenenylation of alkenes to natural product synthesis was reported recently by Wirth, who described a short procedure to obtain some furofuran lignans 147]. The total synthesis of (+)-Samin 53 [47 a] is shown in Scheme 7. The protected allylic alcohol 50 was treated with the selenyl triflate derived from diselenide 29 in the presence of 2,3-butadien-l-ol, and afforded the addition product 51 in 55% yield and with a diastereomeric ratio of 15 1. The favored 5-exo-trig radical cyclization of the major isomer afforded the tetrahydrofuran derivative 52 from which the final product was obtained through few classical steps. 

Scheme 35) with the chiral selenyl triflate derived from the diselenide 23, in methylene chloride and methanol, afforded a 1 1 mixture of compounds 231 and 232 [114]. These compounds are derived from the capture of intermediate... 

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). 

Phenylsclenenyl triflate shows highly electrophilic character since it reacts even at — 78 °C with unsaturated carboxylic acids to afford the corresponding lactones. In some cases, the reaction of selenyl halides in the presence of the nucleophilie halide anion causes unfavorable side reactions, such as addition of the halogen atom, and decreased stereoselectivity. On the other hand, reactions of Ar-(phenylseleno)imides, which are not effected by the nucleophile, require a strong acid catalyst because of their lower reactivity. 

Thio- and selenoacetals and esters are excellent substrates for mild Friedel-Crafts reactions, because of the affinity of sulfur and selenium for copper (Sch. 23). Anisole was readily acylated with methylselenoesters 94 at room temperature with activation by CuOTf to affordpnra-substituted (> 95 %) derivatives 95 [50,51]. Mercury(II) and copper(II) salts, which were effective for the activation of selenyl esters for reaction with alcohols, amines, and water, were not effective for the Friedel-Crafts reaction. Aromatic heterocycles 96 could be acylated in high yields, and the alkylation product 100 was obtained from dibutylthioacetal 99 and anisole. Vedejs has utilized this methodology in the cyclization of 101 to afford 102 in 77 % yield [52]. This intramolecular variant did not require the use of the more reactive bis copper triflate-benzene complex. 

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. 

Related to the activation of thioglycosides by sulfenylation are selenylation type reactions (E = SeR) (Scheme 4.15). Arylselenylating type promoters used with thioglycosides include benzeneselenenyl triflate [199,200] or the Wphenylselenylphtalimide-trimethylsilyltriflate [201] and Wphenylselenylphtalimide-magnesium perchlorate [202] systems. 

Areneselenenyl triflates. Conversion of selenenyl bromides to the triflates by AgOTf makes it possible to avoid introduction of the halogen atom to the alkene substrates on their selenylation. For example, stereoselective selenomethoxylation is readily achieved. ... 

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