Benzoin, Table

The dioxygenation of unsaturated a-diols (catechol and benzoin. Table 6-2) by the O2/Fe l(DPAH)2 system parallels that of the catechol dioxygenase enzymes, which are nonheme iron proteins. -l Hence, the reactive intermediate (1, Scheme 6-1) of the Feh(DPAH)2/O2 reaction may be a useful model and mimic for the activated complex of dioxygenase enzymes. ... 

Some commercial photoinitiatois (Table 11) undergo a Noirish Type I photocleavage to form two initiating radical fragments direcdy for a benzoin... 

This procedure is representative of a new general method for the preparation of noncyclic acyloins by thiazol ium-catalyzed dimerization of aldehydes in the presence of weak bases (Table I). The advantages of this method over the classical reductive coupling of esters or the modern variation in which the intermediate enediolate is trapped by silylation, are the simplicity of the procedure, the inexpensive materials used, and the purity of the products obtained. For volatile aldehydes such as acetaldehyde and propionaldehyde the reaction Is conducted without solvent in a small, heated autoclave. With the exception of furoin the preparation of benzoins from aromatic aldehydes is best carried out with a different thiazolium catalyst bearing an N-methyl or N-ethyl substituent, instead of the N-benzyl group. Benzoins have usually been prepared by cyanide-catalyzed condensation of aromatic and heterocyclic aldehydes.Unsymnetrical acyloins may be obtained by thiazol1um-catalyzed cross-condensation of two different aldehydes. -1 The thiazolium ion-catalyzed cyclization of 1,5-dialdehydes to cyclic acyloins has been reported. 

In contrast to the oxidation of prochiral esters and amides, which induces only moderate ee, sodium enolates of ketones give high stereoselectivity with (+)-147 or (—)-147 as the oxidant (Scheme 4-56 and Table 4-21). The highest stereoselectivity has been observed in the oxidation of the sodium enolate of deoxybenzoin 150, in which benzoin 149 can be obtained in over 95% optical purity. 

Ring closures of 3-mercapto-4-amino[l,2,4]triazole with benzoin or related compounds is also a well-established approach for the synthesis of the title compounds (with Table 5, entry 5, (tosyloxy)methyl 2-hetarylketones were used). These procedures are summarized in Table 5. 

Table 5 Ring closures to [1,2,4]triazolo[3,4-b][1,3,4]thiadiazines by reaction of 3-mercapto-4-amino[1,2,4]triazoles with benzoin or related reagents ...

When the source of initiation is altered from ionising radiation to UV, analogous additive effects to those previously discussed have been found. For reasonable rates of reaction, sensitisers such as benzoin ethyl ether (B) are required in these UV processes. Thus inclusion of mineral acid or lithium perchlorate in the monomer solution leads to enhancement in the photografting of styrene in methanol to polyethylene or cellulose (Table V). Lithium nitrate is almost as effective as lithium perchlorate as salt additive in these reactions (Table VI), hence the salt additive effect is independent of the anion in this instance. When TMPTA is included with mineral acid in the monomer solution, synergistic effects with the photografting of styrene in methanol to polyethylene are observed (Table VII) consistent with the analogous ionising radiation system. 

Aqueous NaOH (50%, 0.2 ml), TEBA-CI (15 mg, 0.06 mmol), and the cyanhydrin benzoate (1 mmol) in PhH (4 ml), are stirred at room temperature under argon for 10 min. The aryl aldehyde (1 mmol) in PhH (4 ml) is then added at 0°C and the mixture is stirred at room temperature for ca. 5 h (monitored by TLC). On completion of the reaction, the organic solution is separated and washed well with H20, dried (MgS04), and evaporated to yield the benzoin benzoate (Table 6.15). 

Table IV. Catalytic oxidation of benzoin by 1,4-benzoquinone in the presence of 4Fe-, 2Fe-, IFe-complexes...

In 2002, Enders and co-workers took advantage of the bicyclic restriction first introduced by keeper and Rawal to develop catalyst 20. Use of this catalyst provides a number of benzoin derivatives 22a-h in up to 95% ee (Table 1) [41]. The stereochemistry of the benzoin reaction catalyzed by thiazolium and triazo-lium pre-catalysts has subsequently been modeled by Honk and Dudding [42]. 

Table 1 Substrate scope of the asymmetric benzoin reaction [FXl]...

Miiller and co-workers have developed an enantioselective enzymatic crossbenzoin reaction (Table 2) [43, 44], This is the first example of an enantioselective cross-benzoin reaction and takes advantage of the donor-acceptor concept. This transformation is catalyzed by thiamin diphosphate (ThDP) 23 in the presence of benzaldehyde lyase (BAL) or benzoylformate decarboxylase (BFD). Under these enzymatic reaction conditions the donor aldehyde 24 is the one that forms the acyl anion equivalent and subsequently attacks the acceptor aldehyde 25 to provide a variety of a-hydroxyketones 26 in good yield and excellent enantiomeric excesses without contamination of the other cross-benzoin products 27. The authors chose 2-chlorobenzaldehyde 25 as the acceptor because of its inability to form a homodimer under enzymatic reaction conditions. 

Table 2 Asymmetric enzymatic cross-benzoin reaction...

Table 3 Acyl Silane cross-benzoin reaction catalyzed by lithiophosphites...

In concurrent and independent work, Suzuki and Enders found that tethered keto-aldehydes undergo highly enantioselective cross-benzoin reactions using tria-zolium based catalysts [50, 51], The scope includes various aromatic aldehydes with alkyl and aryl ketones (Table 4). Additionally, aliphatic substrate 39a is cyclized in excellent enantioselectivity, albeit in 44% yield. 

Table 4 Suzuki and co-workers enantioselective intramolecular cross-benzoin]...

The synthesis of y-lactams has been achieved under similar reaction conditions (Table 18) [124]. Initially, Bode and co-workers screened a variety of acyl imines in order to find suitable electrophiles. Control experiments provided evidence for carbene addition to the acyl imine, yielding a stable complex with complete inhibition of the desired reactivity. Reversibility of this addition was key to the success of the reaction. A -4-Methoxybenzenesulfonyl imines 212 proved to be the most efficient partners for lactamization with cinnamaldehydes 228 to provide y-lactams 229 in moderate yields and good diastereoselectivities. Notably, no benzoin or S tetter products or their corresponding derivatives were observed during this reaction. 

Table 2.2.1.3 Synthesis of (R)-benzoin catalyzed by benzoylformate decarboxylase variants.

In contrast to the large variety of aromatic, olefinic, and aliphatic aldehydes which can be used as donor substrates, wild-type BFD does not tolerate a modification of the methyl group of acetaldehyde in the case of aliphatic acceptor aldehydes. Apart from acetaldehyde, BFD shows activity with aromatic and heteroaromatic aldehydes as the acceptor substrate, forming enantiomerically pure (R)-benzoin and derivatives (Table 2.2.7.3, entries 6-8) [55]. 

It should, however, be pointed out that - where applicable - product composition can be significantly different. For example, whereas thiazolium catalysts afford exclusively dihydroxyacetone with formaldehyde as substrate, the triazolium systems afford glycolic aldehyde (plus glyceraldehyde and C4 and C5 sugars as secondary products) [246], Catalyst-dependent differences in the relative rates of the partial reactions within the catalytic cycle (Scheme 6.105) most probably account for this phenomenon. A subsequent study by Enders et al. on chiral triazolium salts identified the derivative 233 as a first catalyst for the asymmetric benzoin condensation that affords substantial enantiomeric excesses (up to 86%) with satisfactory chemical yields (Table 6.3) [247]. 

In the reduction of an optically active ketone, the reaction results in the creation of an additional asymmetric center, and two diastereoiso-mers are possible. In many cases it has been found that both isomers are formed, often in comparable amounts. For example, reduction of camphor (XY) gives a mixture of bomeol and isobomeol (XVI).6 Numerous other examples may be noted in the tables at the end of this chapter. However, the reduction of benzil (XVII) or benzoin (XVIII) is reported to give 90% of wieso-hydrobenzoin (XIX). 6... 

Membrane-casting Techniques. Uhtil recently, PVC membranes have been exclusively formed by solvent casting techniques but which are not well-suited to the fabrication of ISFET devices. Membrane components in tetrahydrofuran are difficult to manipulate on a micro scale and are prone to absorb atmospheric moisture, thus weakening the adhesion at the sensor-ISFET interface. Che innovation which dispenses with the tetrahydrofuran casting stage is based on an in situ photolysis of the model calciun sensor cocktail admixed with monobutyl methacrylate + benzoyl peroxide + benzoin methyl ether at 340 nm (3). Hie resultant matrix adhered well to the ISFET gate and its potentiate trie response compared favourably with the analogous P and P2-MPMA ISE (Table II). 

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