Putative intermediate

The proposed pathway for the biosynthesis of the avermectins (Fig. 3) has been described in a review (23). Some of the details are yet to be elucidated, although the steps, in general, are based on firm evidence from four types of studies incorporation of labeled precursors, conversion of putative intermediates by producing strains and blocked mutants, in vitro measurement of biosynthetic enzymes, and studies with enzyme inhibitors. The biosynthesis of the oleandrose units was elucidated from studies using and labeled glucose, which indicated a direct conversion of glucose to... 

The only synthesis which corresponds with this description formally, though not mechanistically, resulted in formation of (385) from a side reaction in the attempted esterification of the pyridinone acid (383) with diazoethane, via ring expansion of the putative intermediate (384) (76CPB1870). 

There is no published mechanistic study on the Auwers flavone synthesis. The mechanism may involve the nucleophilic addition of oxonium 7, derived from 1, with hydroxide to give 8. Base-promoted ring opening of 8 could provide the putative intermediate 9, which then could undergo an intramolecular Michael addition to form 10. Expulsion of bromide ion from 10 would then give flavonol 2. 

However the earliest example in the literature of a reaction that proceeds via the putative intermediates invoked in the Pfitzinger reaction can be traced to the formation of quindoline-5-carboxylic acid 10, a product of over-reduction of indigo. ... 

Hartwig and Whaley suggested that when substituents are placed in the 4-position of the ethylamine side chain, an oxazoline intermediate (7) is formed a side reaction that was first mentioned by Krabbe in 1940. However, Hartwig and Whaley did not isolate the putative intermediate. 

Although it may not be obvious, putative intermediate 12 could conceivably be fashioned in one step from lactol 13. Of course, 13 can be regarded as a latent aldehyde that should be amenable to an a-chelation-controlled carbonyl addition reaction21 with ethyl-magnesium bromide. This event could secure the formation of the indicated stereocenter in intermediate 12. It seems reasonable to suppose that the sequential action of aqueous acid and 1,1-... 

Figure 2.5 Catalytic cycle of cytochrome P450 including postulated structures of putative intermediates. RH represents the substrate and R(0)H the product. The porphyrin ring is abbreviated as a parallogram with nitrogens at the comers. Adapted with permission from Sato et ah, 1996. Copyright (1996) American Chemical Society.

Most researchers currently agree that the hydrido mechanism is more common than the alkoxycarbonyl path in the alkoxycarbonylation of alkenes with palladium systems. However, carbalkoxy complexes are putative intermediates in carbonylation reactions giving succinates and polyketone diesters, with metals like Co, Rh, or Pd.137... 

Earlier, Dunkin and Thomson had observed that matrix-isolated triplet 10a did not undergo photochemical ring expansion.83 However, Morawietz and Sander have recently provided evidence for photochemical conversion of 310a and 310b to the corresponding fluorinated azirines (Scheme 19).48d This represents a rare instance where an azabicyclo[4.1.0]heptatriene, the putative intermediate in the ring expansion of a phenylnitrene, has actually been observed. 

Boryl)(stannyl)palladium(n) complex, which is a putative intermediate of the stannaboration, is in fact formed by heating PdMe2(dmpe) and the stannylborane 9 (Equation (96)). The complex reacts with 1-octyne at 80 °C to afford the stannaboration product in 36% yield. 

Reuzeau C, Doolittle KW, McNally JG, Pickard BG. Covisualisation in living onion cells of putative integrin, putative spectrin, actin, putative intermediate filaments, and other proteins at the cell membrane and in an endomembrane sheath. Protoplasma 1997 199 173-197. 

Analogous to the annulation of the Sonogashira adducts (see Section 6.4.), a spontaneous cyclization via the intramolecular alkoxylation of alkyne 117 (the coupling adduct of o-bromophenol and phenyl acetylene) took place under the reaction conditions to give 2-phenylbenzofuran 119 [93]. Benzofurylpalladium complex 118 was the putative intermediate during the cyclization. 

Another occasionally troublesome process is related to the tendency of some substrates to preferentially, or competitively, undergo acid-base chemistry [43,44]. Of course, this is not surprising, considering the nature of the putative intermediates. One system where this process diminishes the efficiency of the electroreductive cyclization is that of compound 84. In this reaction, no more... 

Methoxyethyltosylamide also participates in the [3 -I- 2] addition reaction with 102, although it does not give any of the expected dihydropyrrole derivative 104. Instead, the major product was found to be pyrrole 105, which presumably results through ready elimination of methanol from the putative intermediate 104. Thus, this addition holds promise for the synthesis of 2-substituted tosylpyrroles (Scheme 29). In addition to 105, a minor product 106 (12%) is also formed in this reaction. 

Benzofurazans are thermally more stable but can be cleaved photolytically. For example, benzo-furazan itself in benzene affords cyanoisocyanate (17) and azepine (18), the latter being formed by reaction of the solvent with the putative intermediate acylnitrene (19) (Scheme 5) further supporting evidence for the proposed pathway is provided by trapping the nitrile oxide precursor with dimethyl acetylenedicarboxylate and isolation of the methylurethane derivative of the isocyanate <75JOC2880>. Photolysis of diphenylfurazan yields benzonitrile, diphenylfuroxan and 3,5-diphenyl-1,2,4-oxadiazole. 

Aryl substituted anhydro-5-thiolo-l,2,3,4-oxatriazolium hydroxides (56) are obtained by nitro-sation of arylhydrazinium dithiocarbamates (Scheme 14) <76CC306,79JCS(P1)732>. The putative intermediate nitrosoaryldithiocarbazates (55) have not been isolated. 

The MS of compound 40, a putative intermediate of the metabolic degradation of indol-3-ylacetic acid (39), shows the molecular peak at m e 163 and the parent negative peak at 162. This and other spectral and chemical evidence pointed to the compound being a hydroperoxide . 

The point of the change from a stepwise to a concerted mechanism for nucleophilic substitution at X-l-Cl may be detected as an upward break in the observed nucleophile selectivity nu/ s with decreasing stability of the putative intermediate X-1 (Fig. 2.2). Figure 2.5 shows that the position of this break and the change in mechanism shifts to more electron-withdrawing X as the reactivity of the nucleophile is decreased, from X = 4-F for... 

The hypothesis that the sex pheromone arises from oxidation of the preformed hydrocarbon was examined by topical application of radiolabeled synthetic 3,11-dimethylnonacosane and the putative intermediate 3,1 l-dimethylnonacosan-2-ol to vitellogenic B. germanica females. Radioactivity from the alkane was detected in... 

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