BNAH

It was shown that dibenzothiophene oxide 17 is inert to 1-benzyl-l,4-dihydro nicotinamide (BNAH) but that, in the presence of catalytic amounts of metalloporphyrin, 17 is reduced quantitatively by BNAH. From experimental results with different catalysts [meso-tetraphenylporphinato iron(III) chloride (TPPFeCl) being the best] and a series of substituted sulfoxides, Oae and coworkers80 suggest an initial SET from BNAH to Fe1 followed by a second SET from the catalyst to the sulfoxide. The results are also consistent with an initial coordination of the substrate to Fem, thus weakening the sulfur-oxygen bond in a way reminiscent of the reduction of sulfoxides with sodium borohydride in the presence of catalytic amounts of cobalt chloride81. 

BNAH may be coupled to reducing sugars without reduction, since the linkage formed between a hydrazide and an aldehyde is much more stable than that with an amine. 

Another advantage of the BNAH derivative is that the conjugation reaction with reducing sugars can be done in aqueous conditions and in an environment that permits carbohydrate and biotinylation reagent solubility. Modified carbohydrates may be stored for at least 1 year at —20°C in a solution of water/methanol (9 1 v/v) without degradation. 

Figure 11.21 BNAH contains a hydrazide group that can be used to label the reducing end of released glycans through the formation of a hydrazone bond.

Dissolve 100 nmol of the carbohydrate to be modified and 500 nmol of BNAH in 25 pi of methanol (20 mM BNAH solution). 

Purification of the conjugates may be done by reverse phase HPLC separation. Dry the reaction solution under a nitrogen stream and reconstitute in a minimum volume of acetonitrile/water (1 1, v/v). Apply the sample to a 5 pm Cig-silica HPLC column (250 X 4.6 mm, Nucleosil). Elute with a gradient of water to acetonitrile at a flow rate of 1 ml/minute over a time course of 30 minutes. Free BNAH and BNAH-glycan derivatives can be monitored by absorbance at 275 nm. The conjugate peak also will be positive for carbohydrate by reaction with orcinol, which can be detected by spray after spotting a small eluted sample on a TLC plate. 

The reduction of xanthylium cations by BNAH apparently involves rate-determining electron transfer followed by fast hydrogen atom abstraction. Conformational studies... 

The photocatalytic system is shown in Scheme 5, where BNAH is oxidized by the ZnP + moiety in the radical ion pair ZaP -Ceo (ki) produced upon photoirradiation of ZnP-Ceo, whereas HV " is reduced to HV by the Ceo" moiety of ZnP +-C6o ki). These individual electron-transfer processes compete, however, with the BET in the radical ion pair (/cbet)- This pathway was experimentally confirmed by photolysis of the ZnP-Ceo/BNAH/HV and ZnP-H2P-C6o/BNAH/HV + systems with visible light (433 nm) in deoxyge-nated PhCN [70], For instance. Fig. 4 depicts the steady-state photolysis in deoxy-genated PhCN, in which the HV absorption band (X ax = 402 and 615 nm) increases progressively with irradiation time. By contrast, no reaction occurs in the dark or in the absence of the photocatalyst (i.e., ZnP-Ceo or ZnP-H2P-C6o) under photoirradiation [70]. Once HV+ is generated in the photochemical reaction, it was found to be stable in deoxygenated PhCN. The stoichiometry of the reaction is established as given by Eq. (3), where BNAH acts as a two-electron donor to reduce two equivalents of HV [70] ... 

Scheme 5 Photocatalytic system of ZnP-Ceo for uphill photo-oxidation of BNAH by hexyl viologen. (From Ref. 70.)...

Scheme 8 Photoalkylation of RacrH+ with Bu-BNAH viaphotoinduced electron transfer. (From Ref. 79.)...

Table 1 Electron-Transfer Rate Constants (ked Determined from the Fluorescence Quenching of RAcrH" by BuTlNAH and R Sn, Determined from the Dependence of the Quantum Yields on [BuTlNAH] and [R Sn] in the PhotoaUtylation of RAcrH with Bu BNAH and R Sn in MeCN, CHCI3 and Benzene at 298 K, and the Driving Force of Back-Electron Transfer (—AGbet) from RAcrH" to Bu BNAH and R Sn in MeCN...

Determined from the fluorescence quenching of RAcrH by Bu BNAH and R Sn. 

Eine Mischung aus I0 mmol a>Nitro-sulfon und 30 mmol BNAH in N2-freicm DMF werden bet 20 unter Stickstoff geriihrt b Ausbeute durch GC besttimmt B isolierte Ausbeute... 

Fig. 18. Aerobic oxidation of BNAH with immobilised flavin...

The presence of a base is also essential for the efficient reductive dehalogenation of RX by 1-benzyl-1,4-dihydronicotinamide (BNAH) via photoinduced electron transfer [121,122], Since the one-electron oxidation potential of the singlet excited state of BNAH ( BNAH ) is —3.1 V (vs. SCE) [50], which is more negative than the one-electron reduction potential of benzyl bromide (PhCH2Br), photoinduced electron transfer from BNAH to PhCH2Br occurs efficiently with the diffusion-limited rate [122]. This fast process needs no base catalyst to accelerate the electron transfer rate further. However, the photoinduced electron transfer results in... 

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