Boron, thiol derivatives

The enolates of other carbonyl compounds can be used in mixed aldol reactions. Extensive use has been made of the enolates of esters, thiol esters, amides, and imides, including several that serve as chiral auxiliaries. The methods for formation of these enolates are similar to those for ketones. Lithium, boron, titanium, and tin derivatives have all been widely used. The silyl ethers of ester enolates, which are called silyl ketene acetals, show reactivity that is analogous to silyl enol ethers and are covalent equivalents of ester enolates. The silyl thioketene acetal derivatives of thiol esters are also useful. The reactions of these enolate equivalents are discussed in Section 2.1.4. 

Endothelin receptor antagonists 134 and 135 were prepared from the triflated oxicam derivative 136 (Scheme 18) <1998BMC1447>. Addition of aryl thiol 137 to the position gave product 134. Palladium-catalyzed Suzuki coupling of aryl boronic acid 138 and aryl triflate 136 affords the sulfonamide product 135. 

O-tert-Butyl trichloroacetimidate, prepared in 70% yield by reacting potassium rerr-butoxide with trichloroacetonitrile, reacts with carboxylic acids and alcohols in the presence of a catalytic amount of boron trifluoride etherate at room temperature in cyclohexane-dichloromethane [Scheme 6.35], 7 The method also converts alcohols to ferr-butyl ethers (see section 4.3.2). A very similar reaction that allows /erf-butylation under essentially neutral conditions on a large scale involves reaction of a carboxylic acid with 3-4 equivalents of JV,N -di-isopropyl-Orerf-butylisourea88 [Scheme 6,36].56S9 The reaction proceeds via a tertiary carbocation ion intermediate and since capture of the cation is inefficient, excess isourea is required. The presence of alcohols is tolerated but not thiols or unhindered amines. The reaction conditions are compatible with a range of acid sensitive groups such as AMrityl derivatives and cydopentylidene acetals.90... 

These reactions usually proceed more slowly than the reactions of the corresponding alcohols or phenols with boron halides. In some cases, where direct reaction is too slow, the lead or mercury derivatives of thiols are used. Attempts to use the reactions of thiols with diboron trisulfide to prepare trialkylthioboranes have usually been unsuccessful, although analogous reactions with oxygen compounds are commonly used to prepare trialkoxyboranes. Formation of B-S bonds... 

The formation of thioacetals from aldehydes and ketones involves the reaction with a thiol such as ethane-1,2-dithiol in the presence of a Lewis acid catalyst such as boron trifluoride eiherate. These derivatives have been described earlier. 

Acylation of a simple thiol with an alkyl carboxylate is not a very suitable method for preparation of S-alkyl thiocarboxylates. Transesterification is, however, possible if either the thiol or the carboxylic ester is activated. The enhanced reactivity of boron, aluminum and silicon thiolates has been utilized for the synthesis of a large variety of thiocarboxylic S-esters, including hydroxy derivatives (from lactones). a,P-Unsaturated thiol esters, e.g. cinnamoyl or 2-butenoyl derivatives, are also accessible. Michael addition, an undesirable side reaction of thiols, is completely avoided if alkyl trimethylsilyl sulfides ortris(arylthio)boranes are applied. ... 

Wagner and coworkers developed a traceless sulfur-based linker by applying the chemistry of benzylsulfonium salts [90]. Various benzyl bromides were attached to a thiol linker synthesized in four steps starting from Merrifield resin. Reaction of the resulting resin 96 with triethyloxonium tetrafluoroborate gave the solid supported sulfonium salt 97, which could be cleaved by reaction with boronic acids to afford biphenylmethyl derivatives 98 (Scheme 16.23). The alkylation can be regarded as an activation step and the linker as a safety-catch system. 

Cleavage of Benzyl Carbamates. The removal of the benzy-loxycarbonyl (Cbz) group from nitrogen can be achieved successfully using boron trifluoride etherate in the presence of either a thiol or dimethyl sulfide. The carbamates derived from secondary amines are cleaved more rapidly than those from primary amines using the ethanethiol method, even when using BF3-OEt2 as the solvent. The procedure allows reasonable selectivity, as shown in eq 11. ... 

Functionalized alkenyl diamino- and dialkoxyboranes have been produced regio-and stereoselectively through addition of carbon- or heteroatomgenerated from bromotrichloromethane, thiols, phosphines and tributyltin hydride) to ethynylbis(diisopropylamino)boranes. The synthetic utility of these reactions was illustrated by the preparation of stereodefmed (Z)- or ( )-alkenylboronic esters via palladium-catalyzed crossStille reaction and a Suzuki coupling under basic conditions can be further conducted (Scheme 9.14) [33]. 

The rearrangement of glycal derivatives to l-substituted 2-enose derivatives has been applied to the preparation of i) 2,3-dideoxy-i-thiohex-2-enopyranosides (9, trimethylsilylated thiols and boron trifluoride details of the complexities of the reaction are reported) ii) 2,3-dideoxyhex-2-enopyranosyl fluorides [10, pyridine poly(hydrogen fluoride)iii) 2, 3 -unsaturated nucleosides (11, purine or pyrimidine derivatives with trityl perchlorate) (Scheme 7). 

BSH contains a thiol group and the dodecahydro-cto o-dodecaborate anion [Bi2Hi2] as the boron cluster this cluster is easily prepared °B-enriched for use in BNCT, and it is readily mono-functionalized. However, further functionalization of the boron cluster in BSH is challenging due to lack of chemo-regioselectivity. Therefore, most derivatives of BSH prepared to date involve either deriva-tization via the thiol group, or the replacement of the boron cage by a more hydrophobic carborane cluster or the polyhedral anion which has nearly double the boron content. Carboranes... 

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