Silyl carboxylic acids, hydrolysis

Acetylsultam 15 is also used for stereoselective syntheses of a-unsubstituted /1-hydroxy-carboxylic acids. Thus, conversion of 15 into the silyl-A/O-ketene acetal 16 and subsequent titanium(IV) chloride mediated addition to aldehydes lead to the predominant formation of the diastereomers 17. After separation of the minor diastereomer by flash chromatography, alkaline hydrolysis delivers /f-hydroxycarboxylic acids 18, with liberation of the chiral auxiliary reagent 1919. 

Addition of a silyl substituent into a-position of the a-(benzotriazol-l-yl)alkyl ether brings additional possibilities. Thus, lithiation of silyl ether 770 followed by treatment with an aldehyde or ketone gives unstable P-hydroxy-a-silyl-a-(benzotriazol-l-yl)alkyl ether 771 that spontaneously eliminates silanol to give vinyl ether 772 (Scheme 121). Treatment with ZnBr2 followed by hydrolysis with a diluted acid removes both the benzotriazolyl and the methyl groups to furnish carboxylic acid 773. In this way, in a simple manner, aldehydes and ketones are converted to one-carbon homologated carboxylic acid <1996S1425>. 

Preparation of the donor 46 was started from 4,6-0-benzylidene protected thiomannoside 47 (Scheme 7.24). Alkylation with p-allyloxybenzyl chloride under phase transfer conditions78 was followed by 3-O-silylation and Pd(0)-mediated deallylation79 to give 48. The phenolic OH group was alkylated with ethyl 6-bromohexanoate and carboxylic acid, liberated by alkaline hydrolysis, was reacted with PEG monomethyl ether (MW -5000) under Mitsunobu conditions to afford 46. 

Miki and Hachiken reported a total synthesis of murrayaquinone A (107) using 4-benzyl-l-ferf-butyldimethylsiloxy-4fT-furo[3,4-f>]indole (854) as an indolo-2,3-quinodimethane equivalent for the Diels-Alder reaction with methyl acrylate (624). 4-Benzyl-3,4-dihydro-lfT-furo[3,4-f>]indol-l-one (853), the precursor for the 4H-furo[3,4-f>]indole (854), was prepared in five steps and 30% overall yield starting from dimethyl indole-2,3-dicarboxylate (851). Alkaline hydrolysis of 851 followed by N-benzylation of the dicarboxylic acid with benzyl bromide and sodium hydride in DMF, and treatment of the corresponding l-benzylindole-2,3-dicarboxylic acid with trifluoroacetic anhydride (TFAA) gave the anhydride 852. Reduction of 852 with sodium borohydride, followed by lactonization of the intermediate 2-hydroxy-methylindole-3-carboxylic acid with l-methyl-2-chloropyridinium iodide, led to the lactone 853. The lactone 853 was transformed to 4-benzyl-l-ferf-butyldimethylsiloxy-4H-furo[3,4- 7]indole 854 by a base-induced silylation. Without isolation, the... 

In this chemistry, pivotal final steps in the sequence are a dianion alkylation of the side-chain carboxylic acid, leading to erythro products only. Second, ozonolysis of this product is followed by silyl migration, hydrolysis to a hydroperoxyaldehyde, and, finally, after acidification, cyclization to product. 

Conversion of the isolated A-TBDMS carboxylic acid 111 to the active ester and reaction with amino acid esters yielded after desilylation the /3-sultam peptides 112 and 113. Removal of the protecting groups was also possible with benzylic esters but attempted hydrolysis of alkyl ester groups resulted in hydrolysis of the /3-sultam ring. Catalytic hydrogenation of the benzyl esters afforded the N-silylated /3-sultam peptides 114 in quantitative yields (Scheme 34) <2004M979>. 

The product of a Ireland-Claisen rearrangement is a silyl ester. However, silyl esters generally are so sensitive toward hydrolysis that one usually does not attempt to isolate them. Instead, the silyl esters are hydrolyzed completely during work-up. Thus, Ireland-Claisen rearrangements de facto afford carboxylic acids and, more specifically, they afford y,5-unsaturated carboxylic acids. 

TMS production involves one specific functional group (-OH, -COOH, =NH, -NH2, or -SH), which loses an activated hydrogen and is replaced by a trimethylsilyl group (Proestos et ah, 2006). To achieve silylation, some authors have used BSTFA (N,0-hA(trimethyl-silyl)trifluoroacetamide) and TMCS (trimethylchlorosilane) successfully in several matrices (e.g. aromatic plants, cranberry fixiit) (Zuo et ah, 2002 Proestos et ah, 2006). Using silylated derivatives is advantageous for several reasons phenols and carboxylic acids are prone to silylation, these compounds can be derivatized in the same part of the process, and the minor products do not impede analysis and are well documented (Little, 1999 Stalikas, 2008). A two-step methylation procedure was used to analyze catechins and tannins in plant extracts. The first step used trimethylsilyl diazomethane (TMS-diazomethane) to pre-methylate the sample, and the second step used thermally assisted hydrolysis and methylation (THM). The pre-methylation step with TMS-diazomethane stabilized the dimer molecule m/z 540) by minimizing isomerization and reducing reactivity. (Shadkami et ah, 2009). 

The diols (97) from asymmetric dil droxylation are easily converted to cyclic sii e esters (98) and thence to cyclic sulfate esters (99).This two-step process, reaction of the diol (97) with thionyl chloride followed by ruthenium tetroxide catalyzed oxidation, can be done in one pot if desired and transforms the relatively unreactive diol into an epoxide mimic, ue. the 1,2-cyclic sulfate (99), which is an excellent electrophile. A survey of reactions shows that cyclic sulfates can be opened by hydride, azide, fluoride, thiocyanide, carboxylate and nitrate ions. Benzylmagnesium chloride and thie anion of dimethyl malonate can also be used to open the cyclic sulfates. Opening by a nucleophile leads to formation of an intermediate 3-sidfate aiuon (100) which is easily hydrolyzed to a -hydroxy compound (101). Conditions for cat ytic acid hydrolysis have been developed that allow for selective removal of the sulfate ester in the presence of other acid sensitive groups such as acetals, ketals and silyl ethers. 

Danishefsky and coworkers have demonstrated the conversion of lactones to carbocycles by the 3,3-sigmatropic shift of silylketene acetals. Jq the total synthesis of the Fusarium toxin equisetin, for example, keto lactone (138) was converted to its bissilyl derivative (139) by reaction with 2 equiv. of LDA and an excess of TMS-Cl. In situ thermolysis of ketene acetal (1 ) led to a very smooth transformation into ester (140), which was carried on to equisetin (Scheme 26). This methodology was also applied by Schreiber and Smith in the preparation of the cyclohexyl moiety of the immunosuppressive agent FK-506. Ireland-Claisen rearrangement of silylketene acetal (142), prepared by treatment with TBDMS-OTf and triethylamine at low temperature, provided, after hydrolysis of the silyl ester, the carboxylic acid (143) in 71% overall yield (Scheme 27). The strict translation of configuration via a boatlike transition state is typical for this permutation. 

In some cases, the requisite cinnamic acid was commercially available and could be reduced after esterification. Partial reduction was seen when the carboxylic acid was used as the substrate. After hydrolysis and silylation, the desired intermediate ester was obtained. 

Recently, it has been shown that nucleophilic addition of neat dimethyl trimethylsilyl phosphite to serine-derived P-lactonc at I ()() C for 24 h leads specifically to the carboxylic trimethylsilyl ester by preferential transfer of the trimethylsilyl group. A simple aqueous workup induces hydrolysis of the silyl ester to give the N- and P-protected free carboxylic acid ready for activation and coupling. 

With a bountiful supply of 69 at our disposal, the synthesis was continued by PCC oxidation to the aldehyde level and application of the Corey-Fuchs procedure [35] for chain homologation via dibromo olefin 70 to the acetylenic ester 71. Since amide bond construction next had to be implemented, this ester was saponified under mild conditions and the resulting carboxylic acid was activated by formation of a mixed anhydride with mesitylenesulfonyl chloride in advance of in situ condensation with 59. In order to preclude hydrolysis of the silyl ether functionality in 72, deacetalization had to be performed under anhydrous conditions in dry acetone containing a catalytic quantity of p-toluene-... 

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