Amides Boron group

The work by Armstrong was extended to include the room temperature resin capture of trisubstituted ethenes with a pendant boronate group.80 Modification of the amide linker above to a novel silyl-based one allowed for the traceless cleavage of superior analogues. 

Reduction of esters, nitriles, and amides. These groups are rapidly reduced by horanc-dimcthyl sulfide in refluxing THF (b.p. 67°) if the dimethyl sulfide (b.p. 38°) is removed as liberated. Under these conditions, the reagent is comparable to uncomplexed diborane. Reduction of secondary and tertiary amides is best effected in the presence of boron trifluoride etherate otherwise, excess reagent is utilized for formation of complexes with the products. 

Almost contemporaneously, B.D. Smith and co-workers reported the use of functionalized isophthalamide receptors for the coordination of anions [13]. Smith appended boronate groups to the peripheral aryl groups in order to form interactions between the Lewis acidic boron and the carbonyl oxygens of the amides therefore pre-organizing the receptor into the syn-syn conformation (preferable for anion coordination) and presumably increasing the acidity of the NH group. Proton COSY and NOE difference experiments indicated that the receptor did indeed adopt the desired syn-syn conformation in DMSO-de. NMR titration experiments in DMSO-de at 295 K showed that re-... 

Catalytic reactions and condensations with alkoxides, alkyl compounds, hydrides, and amides of group I and II metals can be modified by using the boron ester complexes of the above reagents. In combination with solvent variations, this makes possible the establishment of highly selective reaction conditions . a-N-Pyri-diniumdithioacetates have a remarkably high methylene activity, which allows aldol condensation even with a keto group . Tri-... 

Ketone Synthesis. In the Friedel-Crafts ketone synthesis, an acyl group is iatroduced iato the aromatic nucleus by an acylating agent such as an acyl haUde, acid anhydride, ester, or the acid itself. Ketenes, amides, and nittiles also may be used aluminum chloride and boron ttitiuotide are the most common catalysts (see Ketones). 

Note also the stereochemistry. In some cases, two new stereogenic centers are formed. The hydroxyl group and any C(2) substituent on the enolate can be in a syn or anti relationship. For many aldol addition reactions, the stereochemical outcome of the reaction can be predicted and analyzed on the basis of the detailed mechanism of the reaction. Entry 1 is a mixed ketone-aldehyde aldol addition carried out by kinetic formation of the less-substituted ketone enolate. Entries 2 to 4 are similar reactions but with more highly substituted reactants. Entries 5 and 6 involve boron enolates, which are discussed in Section 2.1.2.2. Entry 7 shows the formation of a boron enolate of an amide reactions of this type are considered in Section 2.1.3. Entries 8 to 10 show titanium, tin, and zirconium enolates and are discussed in Section 2.1.2.3. 

The directive effect of allylic hydroxy groups can be used in conjunction with chiral catalysts to achieve enantioselective cyclopropanation. The chiral ligand used is a boronate ester derived from the (VjA jA N -tetramethyl amide of tartaric acid.186 Similar results are obtained using the potassium alkoxide, again indicating the Lewis base character of the directive effect. 

The C(9)-C(14) segment VI was prepared by Steps D-l to D-3. The formation of the vinyl iodide in Step D-3 was difficult and proceeded in only 25-30% yield. The C(15)-C(21) segment VII was synthesized from the common intermediate 17 by Steps E-l to E-6. A DDQ oxidation led to formation of a 1,3-dioxane ring in Step E-l. The A-methoxy amide was converted to an aldehyde by LiAlH4 reduction and the chain was extended to include C(14) and C(15) using a boron enolate of an oxazo-lidinone chiral auxiliary. After reductive removal of the chiral auxiliary, the primary alcohol group was converted to a primary iodide. The overall yield for these steps was about 25%. 

There are several different routes to carboxamides. In most of these reactions, a carboxylic acid is converted to a more reactive intermediate, e.g. the acid chloride, which is then allowed to react with an amine. For practical reasons, it is preferable to form the reactive intermediate in situ. Arylboronic acids with electron-withdrawing groups such as (3,4,5-trifluorophenyl)boronic acid act as highly efficient catalysts in the amidation between carboxylic acids and amines. (3-Nitrophenyl)boronic acid and [3,5-bis(trifluoromethyl)phenyl]boronic acid are also effective eimidation catalysts and commercially available. 

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