Stereoselective debromination

One-electron reduction of organic halides is convenient for generating radicals. gem-Dibromocyclopropancs are reduced by such a system [77]. Using an excess of MoH2Cp2, stereoselective debromination is successful, possibly because of... 

Stereoselective denomination. A key step in a notably efficient synthesis of shi kimic acid (3) from 1,4-dihydrobenzoic acid (see also, 8,84-86) is the stereoselective debromination of 1 with Bu3SnH to give 2 in 72% yield. When BujSnD is used for this reduction, 6/J-deuterioshikimic acid is obtained after alkaline hydrolysis with... 

More recently organotellurides have been used for the stereoselective debromination of vic-dibromides. ... 

Debromination has also been achieved in the presence of an ionic liquid as catalyst and solvent. 1-Methyl-3-pentylimidazolium fluoroborate, [pmIm]BF4, an ionic liquid, is a suitable solvent for the stereoselective debromination under microwave irradiation. ... 

Z)-I-Bromo-l-alkenes/ The stereoselective debromination of the readily accessible 1,1-dibromoalkenes under mild conditions is rapid it also allows the synthesis of enediynes and dienynes on subsequent coupling with alkynes, because the latter process is also catalyzed by Pd. 

In the dehalogenation reactions, ILs used for debromination can be achieved by using a metal like Zn, Mg, or In, in an organic solvent, such as tetrahydrofuran (THF) or methyl alcohol (MeOH). Ranu et al. [21, 22] reported that IL [pmim] [BFJ can be used as a catalyst as well as a reaction medium for the stereoselective debromination of vicinal dibromides to the corresponding ( )-alkenes under MW irradiation. 

Ranu and Jana(2005) observed that ionie liquid, [pmim][BFJ, ean be used as an effective catalyst as well as solvent for the stereoselective debromination of a broad... 

Analogously, for preparation of racemic carba-a-glucopyranose 49 from 52, esterification of (—)-52 furnished the ester 95, which was transformed into compound 96 by debromination with zinc dust and acetic acid. Stereoselective hydroxylation of 96 with osmium tetraoxide and hydrogen peroxide, followed by acetylation, gave compound 97. Lithium aluminum hydride reduction of 97, and acetylation of the product, gave pentaacetate 98, which was converted into 99 by hydrolysis. ... 

Treatment of 122 with (R,R)-tartrate crotyl-boronate (E.R.R)-W 1 provides the alcohol corresponding to 123 with 96% stereoselectivity. Benzylation of this alcohol yields 123 with 64% overall yield. The crude aldehyde intermediate obtained by ozonolysis of 123 is again treated with (Z,R,R)-111 (the second Roush reaction), and a 94 5 1 mixture of three diastereoisomers is produced, from which 124 can be isolated with 73% yield. A routine procedure completes the synthesis of compound 120, as shown in Scheme 3-44. Heating a toluene solution of 120 in a sealed tube at 145°C under argon for 7 hours provides the cyclization product 127. Subsequent debromination, deacylation, and Barton deoxygenation accomplishes the stereoselective synthesis of 121 (Scheme 3-44). 

Enantiomerically pure tetrahydro-l//-pyrrolo[2, l -acrylamides derived from proline (see Section 11.11.7.4), are versatile intermediates for the synthesis of natural products or drugs. Compound 86a was submitted to debromination with Bu3SnH followed by ring opening in KOH and further reduction with BHj to give diol 89 that was then easily transformed into (A)-4-(2,2,4-trimethyl-l,3-dioxolan-4-yl)-lT>utanol 90, a key intermediate for )-frontalin, <2002TA155>,... 

A study of debrominations of vtc-dibromides promoted by diaryl tellurides and din-hexyl telluride has established several key features of the elimination process the highly stereoselective reactions of e/7f/tro-dibromides are much more rapid than for fhreo-dibromides, to form trans- and cw-alkenes, respectively the reaction is accelerated in a more polar solvent, and by electron-donating substituents on the diaryl telluride or carbocation stabilizing substituents on the carbons bearing bromine. Alternative mechanistic interpretations of the reaction, which is of first-order dependence on both telluride and vtc-dibromide, have been considered. These have included involvement of TeAr2 in nucleophilic attack on carbon (with displacement of Br and formation of a telluronium intermediate), nucleophilic attack on bromine (concerted E2- k debromination) and abstraction of Br+ from an intermediate carbocation. These alternatives have been discounted in favour of a bromonium ion model (Scheme 9) in which the role of TeArs is to abstract Br+ in competition with reversal of the preequilibrium bromonium ion formation. The insensitivity of reaction rate to added LiBr suggests that the bromonium ion is tightly paired with Br. ... 

Tartaric acid has been used as a chiral auxiliary in a patented route to (S)-naproxen (20) (Scheme 23.3).45" 9 In initial studies, the acetal 21 was used to allow a stereoselective bromination that resulted in a 91 9 ratio of the (RRS)- and (RRR)-bromo derivatives 22 and 23. The bromo acetal diesters could be completely separated. Debromination of 22, followed by acid hydrolysis, led to formation of (S)-naproxen (20) in 80% yield, >99% ee, and recovery of the auxiliary. Conversely, debromination and hydrolysis of 23 gave only 12% yield of (R)-naproxen and 86% ee. In this case, the hydroxy acetal 24 was the major product (68%). However, the auxiliary was recovered in enantiomerically pure form. 

Stereoselective synthesis of (+ )-botryodiplodin was carried out by a radical cyclization of dibromoacetal (203) containing an allene group, with Bu3SnH initiated by Et3B, through the 5-exo-trig cyclization, and the subsequent debromination with bulky... 

Debrominatton of ic-dibromides. v/r-Dibromides are reduced to alkenes by treatment with Na,S or NaSH in benzene/HiO in the presence of catalytic amounts of meth-yltrioctylammonium chloride (Adogen 464). The debromination occurs by stereoselective / t/-climination. The phase-transfer catalyst is not required with water-soluble substrates. Example ... 

Mechanistic studies indicate that carbenoid intermediates occur in the reductive debrominations. If chiral educts are applied, the reactions usually run stereoselectively. 

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