(2,4,6-Triisopropylphenyl)selenium Bromide (TIPPSe-Br). An in Situ-Generated Reagent for Effecting Highly Selective Ring Closures of Homoallylic Alcohols to Substituted Tetrahydrofurans

Abstract

Ring closure reactions of homoallylic alcohols 1 can be effected by various electrophiles (E+), which commonly include protons, Hg+2,1 various sources of I+,2 and PhSeX (X = Cl, phthalimide).3 Stereoselectivities of cyclizations vary widely depending upon substrate structure (Figure 1) as well as choices of E*, temperature, and especially solvent.33 ® We now describe a new reagent for purposes of providing a general solution to tetrahydrofuranforming processes of the 5-endo-trig-like type.4 Moreover, in many cases, remarkably enhanced diastereoselectivities are to be expected relative to previously reported results.The reagent (2,4,6-triisopropylphenyl)selenium bromide, 4, derives from diselenide 3. This precursor is a known, stable, orange-yellow solid (mp 101-103 °C),5 prepared from commercially available 2,4,6-triisopropylbromobenzene (2)6 via lithiation and subsequent treatment with selenium (Scheme 1). Dissolution of 3 in CH2CI2 followed by cooling to -78°and dropwise addition of Br2 (1 equiv) generates 4 which is immediately ready for use.

Total Synthesis of Leucascandrolide A

“…3:1, anti:syn) prompted us to examine bulkier, substituted phenylselenyl halides. [24] In this respect, 2,4,6-triisopropylphenylselenyl bromide was the reagent of choice: Indeed 22 underwent clean cyclization in CH 2 Cl 2 to the desired 2,6-trans-substituted tetrahydropyran 23 (74 %, d.r. 88:12 by 1 H NMR).…”

Organoselenium Chemistry in Stereoselective Reactions

“…The size of the electrophilic reagent has a large influence on the diastereomeric ratio of the 39 and 40, which varies from 4:1 [46] to > 49:1 (Scheme 13). [47] OH Me Me C 6 …”

Kinetic resolution of 2-methoxycarbonylalk-3-enols through a stereoselective cyclofunctionalization promoted by an enantiomerically pure electrophilic selenium reagent

“…This selectivity is here reasonably increased by the higher steric requirement of the aryl moiety, like the effect described by Lipshutz using 2,4,6-triisopropylphenyl diselenide. 16 Consequently, based on the kinetic resolution, a pair of the diasteroisomeric tetrahydrofurans and enantiomerically enriched unreacted starting material were recovered after the reaction. As an example, starting from (±)-2a, reaction with half an equivalent of selenenylating reagent afforded mainly a pair of isomers in a ratio of 77:23 that is very close to the enantiomeric ratio measured on the kinetically resolved starting material (76:24).…”