Bromine, reaction with cyclic acetals

The oxidative reaction of furan with bromine in methanol solution or an electrochemical process using sodium bromide produces 2,5-dimethoxy-2,5-dihydrofuran (19), which is a cyclic acetal of maleic dialdehyde. The double bond in (19) can be easily hydrogenated to produce the corresponding succindialdehyde derivative. Both products find application in photography and as embalming materials, as well as other uses. 

BusSnH. Internal addition of amine to allylic acetates, catalyzed by Pd(PPh3)4, leads to cyclic products via a 8 2 reaction.Three-membered cychc amines (aziridines) can be prepared from chiral conjugated amides via bromination and reaction with an amine. Four-membered cychc amines (azetidines) have been prepared in a different way ... 

Palladium-catalyzed cyclizations of bromodialkenyl ethers, in which the key step is vinylic substitution of bromine, result in low to moderate yields of tetrahydrooxepines and methyleneoxepanes <83JOC3894>. y,5-Unsaturated seven-membered lactones are prepared in 50-60% yields from 2-phenylthio-3-butenyldiazomalonates <90CL613> on treatment of the latter with rhodium acetate the reaction involves a [2,3]-sigmatropic rearrangement of the generated cyclic allylsulfonium ylides. 

Bromoetheriflcation of aikenes can be achieved using NBS in the desired alcohol as the solvent. The reaction of 1,3-dichloropropene with NBS in methanol yields an a-bromo dimethyl acetal in the first step in a convenient synthesis of cyclopropenone. Using propargyl alcohol the reaction depicted in eq 22 has been extended to an annulation method for the synthesis of a-methylene-y-butyrolactones. Intramolecular bromoetheriflcation and bromoamination reactions are generally very facile (eq 23). In natural products synthesis, bromoetheriflcation has been used for the s)mthesis of cyclic ethers (by subsequent de-bromination, see tributylstannane) and for the protection of alkene appendages as cychc bromoethers (regenerated by reaction with zinc). ... 

Formation of Cyclic Ethers. Alcohols that have a hydrogen in the 5-position can be cyclized with silver acetate and bromine (eq 1). The ring closure occurs via an intermediate hy-pobromite. The reaction can be run under a variety of conditions (presence/absence of light, acidity, solvent, temperature). Because of this variety and the products formed, there has been some disagreement about the role silver plays in the hypobromite decomposition. ... 

Nitrobenzene reacts with the O-trimethylsilyl ketene acetal 663 in the presence of tris(dimethylamino)sulfur(trimefhylsilyl)difluoride (Me2N)3S(Me3SiF2) (TASF) to give the O-silylated adduct 1007 a, which can be oxidized in situ, e. g. by bromine, to give the 4-substituted nitrobenzene 1008 in an overall yield of 79% [87] (Scheme 7.28). With less hindered ketene-acetals, however, mixtures of ortho- and para-substituted nitrobenzenes are obtained. Yet, on reaction of 4-fluoronitroben-zene with the cyclic O-trimethylsilyl ketene acetal 1009 the ortho-substitution product 1010 is obtained in 79% yield [87]. 

The cyclic enol ethers 76 either form 1-alkoxy furans 77 by elimination44,1 or, more interestingly, they are oxidized by bromosuccinimide to brominated intermediates 78 which give a,J3-unsaturated y-oxoesters 79 after base treatment45). Reaction of the cyclic orthoesters 76 with LiAlH4 leads to y-oxoaldehydes 80 or their acetals 81 depending on the work-up procedure 46). 

The reaction of both open-chain and cyclic 2,3-epoxy alcohols with molecular Bromine or Iodine in the presence of Ti(0-i-Pr)4 at 0 °C leads to the regioselective formation of halo diols (eq 9). Interestingly, if these reactions are conducted at 25 °C a 1 1 mixture of the C-2 and C-3 cleavage products is obtained, and the same outcome is observed, even at 0 °C, when the acetate derivative of the 2,3-epoxy alcohol is involved as substrate. Dialkylamine hydrochlorides can be used as sources of halide nucleophiles in these types of epoxide ring-cleavage reactions. ... 

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