2.4.4.6- Tetrabromo-2,5-cyclohexadienone

Synthesis of [1,2,3]triazolo[1,5-c]pyrimidines and [1,2,4]triazolo[1,5-c]pyrimidines A novel approach to [l,2,3]triazolo[l,5-c]pyrimidines is shown in Scheme 55. Batori and Messmer - in the course of their investigations on fused azolium salts - described a synthetic pathway to l,3-disubstituted[l,2,3]triazolo[l,5-c]-pyrimidinium salts <1994JHC1041>. The cyclization was accomplished by transformation of the hydrazone 436. This compound was subjected to an oxidative ring closure by 2,4,4,6-tetrabromo-2,5-cyclohexadienone to give the bicyclic quaternary salt 437 in acceptable yield. 

An unusual one-pot intramolecular sulfoxide alkylation-elimination reaction was found by Gibson et al. <2001SL712>. These authors found that treatment of 459 with potassium bis-trimethylsilylamide resulted in a ring closure to 460 in acceptable yield. Furthermore, Batori and Messmer found an effective method for preparation of [l,2,3]triazolo[l,5- ]pyrimidinium salts <1994JHC1041> oxidative cyclization of hydrazones 461 by 2,4,4,6-tetrabromo-2,5-cyclohexadienone gave rise to the quaternary salts 462. Under certain reaction conditions, the formation of 6-bromo-salts 462 (R6 = Br) was also experienced. As neither the starting compound nor the quaternary triazolopyridinium salt underwent bromination in this position, the authors assumed that this bromination process occurred on one of the intermediates in the course of the above-mentioned cyclization reaction. 

Treatment of (/, )-prelaurcatin with 2,4,4,6-tetrabromo-2,5-cyclohexadienone, a brominating reagent, provides two isomeric bromoallenes, which can be separated by HPLC. The major compound (23% yield) is identical with natural (+)-laurallene (Scheme 4) <1997T8371>. 

TETRAHYDROFURANES Iodine. Lead tetraacetate. Silverd) nitrate. 2.4,4,6-Tetrabromo-2.5-cyclohexadienone. 

Alkyl bromides and azides. Substitution of hydroxyl groups by bromine is readily performed with PhjP and 2,4,4,6-tetrabromo-2,5-cyclohexadienone. THP ethers and many silyl ethers are also converted to bromides directly. When zinc azide dipyridine complex is also present alcohols are transformed into azides. Inversion of configuration of the carbinolic center accompanies both transformations. 

Another Sml2-mediated domino process was reported by the Baran group in the synthesis of (+)-Cortistatin A 51 [22]. When the bromoketone 47 was treated with Sml2 in the presence of the additive DMPU (l,3-dimethyl-3,4,5,6-tetrahydro-2(lH)-pyrimidinone), a cyclopropane fragmentation occurred to form an a-bromo radical 48, which underwent subsequent reduction by Smij and then bromide elimination to give 49 with a newly formed C=C double bond and a Sm(lll) enolate. After a few minutes, the addition of 2,4,4,6-tetrabromo-2,5-cyclohexadienone (TBCHD) resulted in the a-brominated product 50, which was converted into cortistatin A 51 (Scheme 5.13). 

CClsH, CHal CCl CBr, CBrClg, CFgl, CCl BrCCl Br Hexachlorocyclo-pentadiene 2,4,4,6-Tetrabromo-2,5-cyclohexadienone Aryl iodides CgHgCl, CgHgBr o-Chlorotoluene Picryl fluoride, Picryl chloride... 

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