Collidine as reagent

If the choice is between a seven- and an eight-membered lactone, the seven-membered ring 151 is preferred to the medium sized ring as in a recent series of experiments24 using the collidine-Br+ reagent 150. 

The following materials may be obtained as reagent or other similar quality grade, and used without further purification chloroform, 2,4,6-collidine, dichloromethane, dicyclohexylcarbodiimide, l-(3-dimethyl-aminopropyl)-ethylcarbodiimide,4-dimethylaminopyridine, ethanol, 1 -methylimidazole, ninhydrin.p-nitrophenol, pentachlorophenol, piperidine, succinic anhydride, trichloroacetic acid, triethylamine, and toluene. The following reagents may be used as purchased, but must be stored tightly sealed to avoid undue exposure to moisture acetic anhydride, 3-aminopropyltriethoxysilane, and trimethylsilyl chloride. 

Note The reagent can be employed on silica gel and cellulose layers and the dipping solution can also be employed as a spray solution. The addition of collidine... 

Electrophilic iodine reagents are extensively employed in iodocyclization (see Section 4.2.1). Several salts of pyridine complexes with 1+ such as bis-(pyridinium)iodonium tetrafluoroborate and b/.s-(collidine)iodonium hexafluorophos-... 

Bis(5> m-collidine)iodine(I) tetrafluoroborate in DMSO has been found to be a convenient reagent for the conversion of alkanes to a-iodocarbonyl compounds. When dihydrofuran (305a) and dihydropyran (305b) are the substrates, this reaction affords the corresponding a-iodolactones 306 (Scheme 77). This method converts certain glycals such as 307 to their corresponding a-iodo-a,/3-unsaturated lactones 309, presumably because of elimination of a molecule of acetic acid from the initially formed lactone 308 (86S727) (Scheme 78). 

In the case of 2,3,5-tri-O-benzoyl-D-ribofuranose (27) pyridinium poly(hydrogen fluoride) was added to a solution of in anhydrous dichloromethane and the solution was shaken at room temperature for 10 h in an atmosphere of dry argon. Anhydrous acetone was found to be equally effective in most reactions. The reaction of compound with Olah s reagent required the use of c.nhydrous acetone or anhydrous dichloromethane—collidine [1 1 (v/v)] in the case of compound 30 the addition of collidine was disadvantageous, whereas in the case of compound best results were obtained using anhydrous acetone—collidine [1 1 (v/v)] as the solvent. Compounds 32 and 33 were treated using pyridinium poly(hydrogen fluoride) as the only solvent. Reaction times varied from 2 h for compound 31 to more than 12 h for 32 and 33. 

Electrophilic iodine reagents have also been employed in iodocyclization. Several salts of pyridine complexes with I+ such as bis(pyiidinium)iodonium tetrafluoroborate and bis(collidine)iodonium hexafluorophosphate have proven especially effective.61 y-Hydroxy- and d-hydroxyalkcncs can be cyclized to tetrahydrofuran and tetrahydropyran derivatives, respectively, by positive halogen reagents.62 (see entries 6 and 8 in Scheme... 

The conditions using 2,4,6-collidine/Tf20 and having the reaction warm from -78 °C to 25 °C were chosen mainly to avoid using more expensive reagents such as 2,6-di-r-butyl-4-methylpyridine or A-phenyl triflimide. (a) Stang, P. J. Hanack, M. C. Subramanian, L. R. Synthesis 1982, 85. (b) McMurray, J. E. Scott, W. J. Tetrahedron Lett. 1983,24,979. 

Cyclization of enynones to methylenecyclopentenones. This cyclization can be effected under thermal conditions, particularly in the case of activated (Z)-enynones, but in only moderate yield. Some acceleration is observed with reagents known to promote enolization (LiCl and collidine p-toluenesulfonate), but dramatic effects can be obtained with phenols such as 1. Vitamin E is marginally more... 

Positive halogen complexes with pyridine bases are known as versatile halogenating reagents. Bis(sym-collidine)iodine(I) tetrafluoroborate (59) in dimethyl sulfoxide is a potential reagent for the direct conversion of alkenes to a-iodo carbonyl compounds (Scheme 6). The oxidation involves the... 

---