Cleavage of acetals
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(1) Cleavage with bromodimethylboran.
The a-bromo ether 2 was used by Overman in the total synthesis of (+)-Laurencin.[Bratz, 1995] The a-bromo ether 2 was prepared from the acetal 1 by cleavage with bromodimethylboran.
See also.[Berger, 1997]
(2) TMSI in cleavage a 1,3-dioxolane
TMSI is used to cleave a 1,3-dioxolane to give an a-iodo ether.[Kim, 1996]
(3) TMSI with cyclic acetals
The reaction of TMSI with cyclic acetals and ketals at 20 oC gives bis(iodomethoxy)alkanes while at higher temp (above 80 oC) (iodomethoxy)iodoalkanes are formed. The latter reaction provides a convenient route to synthetically useful iodomethyl o-iodoalkyl ethers.[Voronkov, 1990]
(4) DAST in the preparation of 2,3-O-isopropylidene-5-O-methyl-b-D-ribofuranosyl fluoride
Treatment of methyl 2,3-isopropylidene-b-D-ribofuranoside with DAST gave a good yield of 2,3-O-isopropylidene-5-O-methyl-b-D-ribofuranosyl fluoride in which the methoxy group had migrated from C-1 to C-5 and been replaced with retention of configuration by fluorine.[Lloyd, 1993]
(5) DAST in the synthesis of glycosyl fluorides
Treatment of readily available phenyl 1-selenoglycosides and phenyl 1-telluroglycosides with DAST in the presence of halonium ion activators yields the corresponding glycosyl fluoride in high yield. The stereoselectivity of the conversion is affected by C-2 substituents, stereochemistry of starting glycoside and reaction solvent.[Horne, 1999]
(6) Selectfluor in the preparation of glycosyl fluorides
Glycosyl fluorides can be synthesized from anomeric hydroxy or thioglycoside derivatives by treatment with Selectfluor.[Burkart, 1997]
(7) TMSBr and glycosyl acetates
Stereoselective bromination of anomeric glycosyl acetates was achieved with trimethylsilyl bromide under mild conditions and in the presence of various protecting groups commonly employed in carbohydrate chemistry.[Gillard, 1981]
(8) Thioglycosides and fluorine-iodine mixtures
Thioglycosides react with elemental fluorine-iodine mixtures to give glycosyl fluorides in good yields.[Chambers, 1996]
(9) Thioglycosides and iodine monobromide
Iodine monobromide has been found to be an efficient reagent for the conversion of thioglycosides into glycosyl bromides.[Kartha, 1997]
(10) Thioglycosides with halogens
Thioglycosides with bromine.[Weygand, 1958]; Chlorine:[Wolfrom, 1963]
(11) O,S-acetals with sulfuryl dichloride
O,S-acetals with sulfuryl chloride.[Ducharme, 1995]
See also: [Benneche, 1983]; [Loubinoux, 1992] ; [Wu, 2000]; [Fors, 1998]; [Yong, 2005]; [Ohigi, 2005]; [Ali, 2010];
(12) Decanoyl chloride and dimethoxymethane
MOMCl is prepared in high yield from decanoyl chloride and dimethoxymethane in the presence of H2SO4.[Chong, 1998]
See also:[Linderman, 1994], [Kishore, 2006]. Other a-chloro ethers:[Zaidi, 1996] ,[Bailey, 1984]
(13) Thioglycosides with 4-methyl(difluoroiodo)benzene
Treatment of readily available thioglycosides with the reagent 4-methyl(difluoroiodo)benzene leads to the formation of the corresponding fluoroglycosides in moderate to good yields.[Caddick, 1996]
(14) Glycosyl chlorides with acid-labile protecting groups
Glycosyl chlorides with acid-labile protecting groups were prepared from their corresponding alcohols under basic conditions in good yields.[Hung, 1996]
(15) Thioglycosides with chlorosulfonium chlorides
Glycosyl chlorides are obtained by reaction of thiglycosides with chlorosulfonium chlorides.[Sugiyama, 2000]
(16) Cleavage with acetyl bromide
Treatment of 1,3-dioxolane with acetyl bromide gave (2-acetoxyethozy)methyl bromide in 88 % yield. [Robins, 1982]
See also: [Chen, 1996], [Edvardsen, 2000]
(17) Cleavage of an O,S-acetal with acetyl chloride
Heating the O,S-acetal 1 with excess acetyl chloride afforded the a-chloroether 2 in good yield [Trost, 1999]
(18) Cleavage off O,O-acetals with acid halides
Zinc(II) salts catalyze the reaction between acetals and acid halides to provide haloalkyl ethers in near-quantitative yield [Berliner, 2005]
(19) Preparation of (fluoromethoxy)benzenes by cleavage of O,S-acetals with Xenon difluoride.
(Fluoromethoxy)benzenes are prepared by cleavage of O,S-acetals with xenon difluoride in dichloroethane at 0 oC.[Lu, 1996] The reaction is clean when the aromatic ring contains an electron-donating or a weak electron-withdrawing group. With strong electron-withdrawing groups, e. g. 4-NO2, a-fluorination of the methyl group, instead of cleavage of the methylthio group, is seen. The amount of a-fluorination is dependent on the molarity of the reaction mixture: dilute solutions seem give less a-fluorination.
(20) Formation of a-fluoro ethers by cleavage of O,SO-acetals with DAST
a-Fluoro ethers can be prepared by treatment of
the sulfoxides with 1.5 equivalents of diethylaminosulfur trifluoride (DAST)
in dichloromethane at ambient temperature.[Ringom, 1999]
The a-alkoxy group is important for the cleavage
reaction to succeed, because without such a group, or anything else that can
stabilize a carbocation, sulfoxides give a-fluoro
sulfides in the reaction with DAST.[Hudliky, 1988]
(21) Cleavage of cyclic acetals with acetyl chloride.
The acetal (R1 =H; n 0 1) can be cleaved to the a-chloro ether by pure acetyl chloride.[Foye, 1982] It has, however, been found that adding a catalytic amount of zinc chloride was more efficient for the acylative cleavage.[Chen, 1996] High regiospecificity has been obseved in these reactions and the general rule is that a primary alkoxy group is cleaved off in preference to a secondary or tertiary alkoxy group. For example, 4-methyl-1,3-dioxane is converted to a single product in 86% yield by rupture of the C-O bond remote from the 4-methyl group.[Bailey, 1984]
(22) Cleavage of O,S-acetals with NCS and TMSCl
Cleavage of O,S-acetals containing acidic NH or OH bonds with sulfuryl chloride, is expected to be problematic. The O,S-acetal below can, however, ble cleaved in good yield with a mixture og N-chlorosuccinimide and chlorotrimethylsilane in dichloromethane.[Young, 2002]
(23) Cleavage of O,SO-acetals with acid chlorides
a-Alkoxy sulfoxides can be cleaved by acetyl chloride or thionyl chloride to give a-chloro ethers. [Antonsen 1988, Benneche 1991] The cleavage reaction with acetyl chloride is especially useful when you have unsaturated carbon-carbon bonds in the molecule.
(24) Reaction of bromotrimethylsilane with methoxytetrahydrofurans and methoxytetrahydropyrans.
Reaction of 2-methoxytetrahydrofuran and 2-methoxytetrahydropyran with bromotrimethylsilane gave the corresponding a-bromo ethers where ring is intact. The same reaction with 2,5-dimethoxytetrahydrofuran and 2,6-dimethoxytetrahydropyran resulted, however, in the ring opened a-bromo ethers.[Schank 1974]
(25) Reaction of bromotrimethylsilane with silylated hemiacetals
The silylated hemiacetal is selectively cleaved at the silyloxy group with bromotrimethylsilane.[Tompson 1990]
The 10a trimethylsilyl ether gives the 10b-bromide.Other methods to prepare the bromide, or other halides, did not succeed.[Haynes 2003]
(26) Cleavage of O,S-acetals with halogens.
On treatment of the O,S-acetals with one equivalent of bromine at room temperature in tetrachloromethane, the carbon-sulfur bond was selectively cleaved in the course of a few minutes to give the a-bromo ethers and the sulfenyl bromide.[Benneche, 1983] The highly reactive sulfenyl bromide can be trapped by the reaction with an olefin in situ to give a high boiling liquid from which the a-bromo ethers could be separated by distillation.
See also: [Sanchez, 2009]
(27) Cleavage of O,SO-acetals with bromotrimethylsilane
The carbon-sulfur bond in the sulfoxides was cleaved on treatment with bromotrimethylsilane at rt, the product being a-bromo ethers.[Antonsen, 1988] Products arising from a Pummerer rearrangement were not seen, but in one case was deoxygenation of the sulfoxides observed.
(28) In situ generation of glycosyl iodides
Peracetylated sugars can be converted to the corresponding iodides by I2/Et3SiH. [Iadonisi, 2003]