C—N bond, cleavage

Photofragmentation of chloromethyldiazirine in the presence of several gases was assumed to involve C—N bond cleavage to form a diradical, which undergoes a second C—N bond cleavage to form the carbene (8OMI508OI). 

Important additional evidence for aryl cations as intermediates comes from primary nitrogen and secondary deuterium isotope effects, investigated by Loudon et al. (1973) and by Swain et al. (1975 b, 1975 c). The kinetic isotope effect kH/ki5 measured in the dediazoniation of C6H515N = N in 1% aqueous H2S04 at 25 °C is 1.038, close to the calculated value (1.040-1.045) expected for complete C-N bond cleavage in the transition state. It should be mentioned, however, that a partial or almost complete cleavage of the C — N bond, and therefore a nitrogen isotope effect, is also to be expected for an ANDN-like mechanism, but not for an AN + DN mechanism. 

Kaneko C., Katagiri N., Nomura M., Sato H. A New Method for the Stereoselective Synthesis of Nucleosides by Means of Sodium Borohydride Mediated Reductive C-C or C-N Bond Cleavage Reaction Isr. J. Chem. 1991 31 247-259 Keywords carbohydrates... 

DHQ is an important intermediate in the HDN network of quinohne only C-N bond cleavage is needed to remove the nitrogen atom from DHQ. However, since dehydrogenation of DHQ proceeds quite fast, THQ5, THQl, and Q are usually present as well, thus making the study of the HDN of DHQ difficrdt. 

The conversion of DHQ to C-N bond cleavage products as a function of space time (Fig. 3A) demonstrates that PCHA is a primary HDN product and that PCH is a secondary HDN product. 

The crucial reaction intermediate PCHA in the HDN network of quinohne-type compounds has been clearly observed. Formation of cis-PCHA was faster than that of trans-PCHA, but isomerization was relatively rapid. The presence of H2S in the reaction stream favours the cleavage of the first C-N bond in DHQ, but slows down the C-N bond cleavage in PCHA. The presence of H2S decreases the adsorption constants of DHQ and NH.i. It is concluded that 40% of the HDN reaction of DHQ takes place through the reaction path of DHQ- THQl->OPA —>HC at 593 K and 3.0 MPa in the absence of H2S, while less than 10% takes place in the presence of H2S. 

The C—N bond cleavage is a fundamental and key reaction in skeletal rearrangements of protoberberines. The remaining two cleavages, that is, of the C—C and C—O bonds, are not always essential to transformation. 

The Hofmann degradation is the most well-known C—N bond cleavage reaction, and its value to structural elucidation of alkaloids has been demonstrated (76). Hofmann degradation of tetrahydroberberine methohy-droxide (1) led to two products base A (2), the C-14—N bond cleavage product, and base B (3), the C-6—N bond cleavage product (Scheme 2) (17,18). The former was the major product when 1 was heated under reduced pressure, but the latter, the thermodynamically controlled product, predominated when the reaction was carried out at atmospheric pressure or in an alkaline medium because base A recyclized back to the starting quaternary base through the transannular reaction. In fact, 2 was heated in aqueous alcohol to afford 1. The mechanism of this recyclization reaction was discussed by Kirby et al. (19). 

The von Braun reaction (Scheme 6) is another basic reaction for C—N bond cleavage 32). Tetrahydroberberine (26) was heated under reflux with cyanogen bromide in benzene to afford the bromocyanide (28) and the unsaturated cyanide (29) through C-6—N and C-14—N bond cleavage, respectively. The C-8—N bond cleavage product was not obtained because of the steric hindrance of the methoxyl group at C-9 in SN2-type reactions 33). The... 

Scheme 8. C—N Bond cleavage of protoberberine /V-oxides. Reagents a, Na, air, liq NH3 b, fev, MeOH.

Scheme 9. C—N Bond cleavage with ethyl chloroformate. Reagents a, ClC02Et, Nal, acetone b, ClC02Et.

O-acetylophiocarpine (381) with ethyl chloroformate afforded the C-8—N cleaved urethane 382 in quantitative yield. Sequential treatment of 382 with silver nitrate, PCC, sodium hydroxide, and p-toluenesulfonic acid in ethanol furnished acetal 384, which was reduced with lithium aluminum hydride followed by hydrolysis to afford the hemiacetal 385. Oxidation of 385 with PCC provided (+ )-a-hydrastine (369). Similar treatment of O-acetylepi-ophiocarpine (386) afforded ( )-/J-hydrastine (368) however, in this case, C—N bond cleavage of 386 with ethyl chloroformate proceeded without regioselectivity. 

Holland et al. (214) reported a conversion of /Miydrastine to the corresponding indenobenzazepines. On treatment with p-nitrophenyl chlorofor-mate, /J-hydrastine (368) was converted to the ene lactone 425 through C—N bond cleavage (Scheme 84). Treatment of 425 with sodium methoxide in methanol afforded the indanedione 426a. Basic hydrolysis of 426a and... 

As can be seen, in both the 5,6- and the 7,8-dihydroxy-2(lH)quinolinone pathways, after initial hydroxylation adjacent to the N-heteroatom, the benzene moiety of the quinoline ring is transformed to a dihydroxy derivative 5,6- or 7,8-, respectively, which subsequently undergoes ring cleavage. However, neither of them involves C—N bond cleavage and consequently do not lead to denitrogenated products. 

The proposed pathways for methyl-substituted quinolines differ from those shown in Fig. 23, even for the same culture, and most particularly, the fact that no C—N bond cleavage has been observed in most of the strains. A limited number of methylquinolines can be hydroxylated due to the inhibiting and blocking effect of the methyl group, particularly at position 2. So, neither P. aeruginosa QP nor P. putida QP could metabolize 2-methylquinoline however, a new strain of Pseudomonas (MQP) isolated by Grant and Al-Najjar [328] was reported to be able to transform 2-methylquinoline, yielding... 

Oxaziridines 381 have been reacted with chlorosulfonyl isocyanate (Scheme 65), this time forming 1,2,4-oxadiazolidin-3-ones 382 via C-N bond cleavage. The structure of the product was established by X-ray crystallography as the 3-one rather than the 5-one that might be expected on the basis of the chemistry shown in... 

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