Uracil, silylated

Keywords glucopyranosyl bromide, silylated uracil, silver trifluoroacetate, N-glycosylation, A-glucosid... 

The best solvent, temperature, and Lewis acid conditions for the Sn reactions of CRS)-l-(2-mtrobenzenesulfonyl)- and (7 5 )-l-(4-nitrobenzenesulfonyl)-3-methoxy-l,2,3,5-tetrahydro-4,l-benzoxazepines with silylated 5-fluorouracil and uracil129 proved to be SnCL at 50 °C in MeCN. The more nucleophilic silylated uracil reacts faster at its N(3) atom giving mainly the cyclic 0,N-acetal (by C-OMe cleavage). The silylated 5-fluorouracil, on the other hand, reacts at its less sterically hindered N(l) atom and gives mainly an acyclic product (by ring C-0 cleavage). Calculations at the HF/6-31G level of theory support the experimental observations. 

For the preparation of /V-2-methacryIoyloxyethyl-5-fluorouracil (MAOFU), which is a typical polymerizable monomer containing 5-FU, the silylated 5-FU (1) was allowed to react with 2-bromoethyl methacrylate in acetonitrile at refluxing temperature the decomposition of the silylated compound followed in a manner similar to that described previously [27]. MAOFU was obtained in 20% yield. When no solvent was used, the yield was extremely low, as is the reaction of silylated uracil with 2-bromoethyl methacrylate [27],... 

The synthesis of 132, starting from S-benzyloxy propanal (131), involved the ring opening of an optically active epoxide 133 with a xanthate anion (Scheme 37)J22 Stereoselective synthesis of 133 by Sharpless epoxidation allowed preparation of the 2-deoxy-4-thio-D- and L-ezyr/zro-pentoses, " which were transformed into the corresponding pyrimidine nucleosides with silylated uracil and McaSiOTf. and then deprotected with Bu NF. 

N-glycosylation reactions of glycopyranosyl halides and silylated uracil or thymine in the presence of silver trifluoroacetate gives exclusively one ano-mer (in solid-solid conditions), while the fusion method lead to an anomeric mixture [Eq. (43)] [75]. 

Synthesis of nucleosides. Reaction of shylated heterocycles with a protected 1-0-acyl or 1-0-alkyl sugar in 1,2-dichloroethane with either trimethylsilyl perchlorate or trimethylsilyl trifluoromethanesulfonate, (013)3810802-CF3, as catalyst gives nucleosides in high yield. The reaction is formulated for the reaction of silylated uracil (1) with l-0-acetyl-2,3,5-tri-0-benzoyl- 3-D-ribofuranose (2) either at 22 for a prolonged period or for 4 hr. at reflux temperature. The function of the catalyst is to convert the sugar into the glycosyl... 

The DL-serine uracil derivative 580 was prepared from OL-serine methyl ester hydrochloride by V-benzoylation to give 578 chloromethylation gave 579, which then was substituted by the silylated uracil (88ZOB2404). 

Condensation of 2-(benzyloxy)ethanol with a mixture of the hydrate and methyl hemiacetal of methyl glyoxalete 638 gave a hemiacetal, which was converted directly with methanesulfonyl chloride to a-chloro ether 639. It condensed with silylated uracil, 5-fluorouracil, and A -acetylcytosine,... 

The mechanism of the Friedel-Crafts-catalysed Hilbert-Johnson reaction has been reviewed, with particular reference to the reaction of silylated uracils and stannic chloride or trimethylsilyl trifluoromethanesulphonate. ... 

Diacetyl-L-rhamnal underwent a Ferrier-type reaction with silylated uracil and thymine to give the 2, 3 -unsaturated nucleosides. ... 

The [2- C]nucleosides of ribose and 2-deoxyribose have been prepared by incubation of the appropriate ribofuranosyl 1-phosphate and pyrimidine with E. coli. [4- C]Uridine has been synthesized from the labelled bis(trimethyl-silyl)uracil by standard means. 

Condensation of 2-azido-2-deoxy-a,j8-D-arabinofuranosyl chlorides (see Chapter 7) with silylated uracil gave a mixture of the a- and j8-uracil derivatives, which were separated. The jS-anomer was reduced to give the 2 -amino-2 -deoxy-ara6mo-uridine. ° ... 

Reaction of peracetylated 2-acetoxyglycals with silylated uracil in the presence of SnCU affords the a-anomers of nucleoside enones such as (84)... 

The influence of 5- and 6-substituents on silylated uracils in the stannic-chloride-catalysed reactions with l-0-acetyl-2,3,5-tri-0-benzoyl-jS-D-ribofuran-ose and 1,2,3,4,6-penta-O-acetyl-jS-D-glucopyranose has been investigated. Electron-releasing substituents e.g, OMe and NHg) at C-5 of uracil promoted... 

The doubly-branched pyrimidine nucleosides 102 have been prepared by coupling of silylated uracil with a sugar unit (Chapter 14), and branched pyranosyl systems 103 have been reported. 

It was found that the amount of the undesiied threo azide (12) to the desired odiyro azide (13) could be controlled by the use of a bulky protecting group at the 5 position. The t-butyldiphenylsilyl group was found to be the best in that no threo azide was formed. Reduction with Dibal-H and acetylation gave 14. On condensation of silylated uracil with 14 (Scheme 5), the a to P ratio (15a 16a) varied from 7 3 for SnCU/acetonitrile to 1 1 for TMSOTf/acetonittile to 3 7 for TMSOTf l -dichloroethane. However, for the condensation of silylated thymine with the same sugar the ratio of 15b to 16b was found to be 1 1 for all of the above reaction conditions. 

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