Precursors aromatic

Original routes involving the direct oxidation of aromatic precursors (14,15) iato quiaols (16,17) followed by a thermal transformation of the latter have been patented for the synthesis of methyLhydroquiaone [95-71-6] (10) andphenyLhydroquiaone [1079-21-6] (11) (80,81). 

Benzyl-type carbanions and their metallo compounds, derived from aromatic or hetero-aromatic precursors, bearing carbon- or hetero-substituents, are readily available with variable substitution patterns due to their mesomeric stabilization (see Section 1.3.2.2)2. Even dicarbanions are accessible without difficulty3,4. The equilibrium acidities of many aromatic hydrocarbons have been determined5-7. The acidities of a-hetero-substituted toluenes8 are similar to those of the corresponding allylic compounds and can usually be generated by the same methods. 

FIGURE 6.16 ortho-Quinone methide 3 stabilization of the zwitterionic rotamer in a complex with /V-methyImorpholine /V-oxide (17). The zwitterionic, aromatic precursor 3a affords the common quinoid form of the o-QM 3 by in-plane rotation of the exocyclic methylene group. 

The reduction of 1,2,5,6-tetrahydropyridine (THPY) with D2 in the presence of [Rh(NCMe)3Cp ]2+, yielding exclusive deuterium incorporation in the C3 and C4 carbon atoms, and the independent synthesis of [Rhfz/ fNJ-THPYfNC-Me)2Cp ]2+ showed that 1) (NJ-THPY complexes are not intermediate to piperidine production and 2) partially hydrogenated N-heterocycles are easily dehydrogenated to their aromatic precursors [55]. 

Changing to a trisubstituted aromatic precursor such as 1,3,5-tribromobenzene (153) yields the trisallene 155 when l-bromo-2-butyne (154) is used as the coupling partner. When leaving groups of different reactivity are present in the aromatic substrate, such as in l-bromo-4-iodobenzene, different propargyl halides can be connected to the aromatic core, resulting in the formation of arylallenes with different allene substituents. 

An example of the method described is the synthesis of saphenic acid (47) that has recently been reported by Nielsen et al. [81]. Starting from properly substituted aromatic precursors 92 and 93, the naturally occurring 1,6-disub-stituted phenazine was synthesized in racemic form. Here, the first major step involves an intermolecular nucleophilic aromatic substitution that, due to the substitution pattern, has proved to be relatively unproblematic and after hydrolysis of the acetal yields the o-nitrodiphenylamine 94. Much more difficult is the ring formation leading to the final phenazine, which can best be achieved through a high excess of NaBH4, accompanied by reduction of the methyl ketone. But at 32%, the yield is still rather poor. 

Alternatively, if only a single cycloalkene is released from the surface, then at least 40-50% of the aromatic precursor must be reduced via this alkene. 

Cyclohexadienones 61 and 64 are readily available from monoprotected hydro-quinones or para-substituted phenols, respectively. Conjugate additions to these symmetrical dienones result in desymmetrization of the prochiral dienone moieties, providing access to multifunctional chiral synthons in two steps from the aromatic precursors (Scheme 7.17) [72]. 

TABLE 3 3 Relative Importance of Aliphatic and Aromatic Precursors ... 

A detailed account of the chemical and spectroscopic evidence for the structures of cochlioquinone-A (18a) and -B (18b) has been published. These compounds are metabolites of Cochliobolus miyabeanus, a parasitic mould which grows on rice, and their unusual cyclofarnesane structure is probably derived in nature by introduction of a farnesyl unit into an aromatic precursor followed by cyclization of an intermediate bis-epoxide. 

The yields of secondary organic aerosols from a series of aromatic hydrocarbon-NOx oxidations have been measured by Odum et al. (1997a, 1997b). They showed that the total secondary organic aerosol formed from a mixture of aromatic hydrocarbons can be approximated as the sum of the individual contributions. Based on their experiments, the yield of secondary organic aerosols expressed as the total organic particle mass concentrations formed, AM, (in fxg m 3), divided by the mass concentration of aromatic precursor reacted, A (aromatic), is given by... 

Banwell MG, Edwards AJ, Harfoot GJ, Jolliffe KA, McLeod MD, McRae KJ, Stewart SG, Vogtle M (2003) Chemoenzymatic Methods for the Enantioselective Preparation of Sesquiterpenoid Natural Products from Aromatic Precursors. Pure Appl Chem 75 223... 

The aromatic precursor is 92 reduced and then alkylation of the enolate anion 93 gives 94, which was hydrolysed in aqueous HC1 to the ketone 95. In fact the NR2 group was chiral and 95 was indeed one enantiomer.17 So we have added a three-carbon side chain but in the wrong place. 

In summary, the metal can be readily removed from both 1//-pyrrole and 3-pyrroline (including azanorbomene) complexes to give a wide variety of highly functionalized molecules not readily obtained from the aromatic precursors without the use of osmium. The inherent instability of 2-pyrrolines prevents clean decomplexation unless quatemization or acylation of the nitrogen is carried out prior to oxidation of the metal. 

Fluorinated benzene anions were obtained238-242 by X-irradiating adamantane matrices containing the aromatic precursor and also Me3NBH3 as electron donor. 

The carbene insertion route (pathway (b)) has most frequently been implied to explain the thermal cydizations of various ethynyl-substituted aromatic precursors. Although the other two alternatives cannot be excluded at present, since in many cases (see below) no careful mechanistic studies have been undertaken, the carbene hypothesis has the advantage of... 

Mercury(II) is certainly an obvious choice for an electrophilic activation of an alkyne moiety. Nishizawa has very recently published a catalytic and biomimetic polycyclization leading to tricyclic derivatives 163, from aromatic precursor 162 [99]. After the Fricdcl Crafts step that generates 166 and TfOH, protonation of the alkene as in 167 is followed by regeneration of the catalyst (Scheme 51). 

The extension of an aromatic 7r-system by, for example, benzo-annelation or coplanar phenyl substitution, stabilizes the system overall, raises the energy of the HOMO, and decreases that of the LUMO. The consequent decrease in ionization potential and increase in electron affinity imply the formation of cation-radicals for lower energy cost, and of anion-radicals with greater energy gain, the more extensive the 7t-system of the diamagnetic aromatic precursor. 

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