Heterocyclic compound, formation

RING SYNTHESIS FROM NON-HETEROCYCLIC COMPOUNDS FORMATION... 

The low-temperature generation of o,o -dibromobiphenyI from BuLi and o-BraCgH4 can be effected in yields of up to 75% (376). The reactions of o,o -dilithiobiphenyl with various organometallic halides (Scheme 6) provide an example of the utility of dilithio reagents in heterocyclic compound formation. Reaction of organoboron derivatives... 

Among condensed heterocyclic compounds formation of radical ions have been proved for the following compounds radical anions of acridine [117], phthalazine [126], phenazine [70, 126], 1,4,5,8-tetraazanaphthalene [131], 9,10-diazaphenanthrene [129], 2,2 -biiso-benzimidazolidine [129], and phenothiazone [108] radical cations for... 

In the presence of a double bond at a suitable position, the CO insertion is followed by alkene insertion. In the intramolecular reaction of 552, different products, 553 and 554, are obtained by the use of diflerent catalytic spe-cies[408,409]. Pd(dba)2 in the absence of Ph,P affords 554. PdCl2(Ph3P)3 affords the spiro p-keto ester 553. The carbonylation of o-methallylbenzyl chloride (555) produced the benzoannulated enol lactone 556 by CO, alkene. and CO insertions. In addition, the cyclobutanone derivative 558 was obtained as a byproduct via the cycloaddition of the ketene intermediate 557[4I0]. Another type of intramolecular enone formation is used for the formation of the heterocyclic compounds 559[4l I]. The carbonylation of the I-iodo-1,4-diene 560 produces the cyclopentenone 561 by CO. alkene. and CO insertions[409,4l2]. 

Once formed, 7 and 8 undergo a Michael reaction that gives rise to ketoenamine 9. Ring closure, to form 10, and loss of water then afforded 1,4-dihydropyridine 11. The presence of 9 and 10 could not be detected thus ring closure and dehydration were deduced to proceed faster than the Michael addition. This has the result of making the Michael addition the rate-determining step in this sequence. Conversely, if the reaction is run in the presence of a small amount of diethylamine, compounds related to 10 could be isolated. Diol 20 has been isolated in an unique case (R = CFb). Attempts to dehydrate this compound under a variety of conditions were unsuccessful. Stereoelectronic effects related to the dehydration may be the cause. In related heterocyclic ring formations, it has been determined that dehydration (20 —> 10) is about 10 times slower than diol formation (19 —> 20). Therefore, one would expect 20 to... 

A.4 Reactions with heterocyclic compounds and formation of heterocycles... 

Identification of the product(s) resulting from the reaction of heterocyclic compounds with diazomethane has been used in attempts to elucidate their tautomeric composition (for summaries, see references 7 and 41). This work was based on the assumption that if a compound which is capable of existing in both an —NH and an —OH form produced only the =NMe derivative when it w as treated with diazomethane, it existed entirely in the =NH form. On the other hand, formation of the —OMe derivative was interpreted to mean that a finite amount of the compound existed in the —OH form. In some cases the tautomer present in the solid state w as concluded to be different from that present in solution for example, 41 42 gave a higher proportion of the 3,4-dimethoxy derivative when ethereal diaz-... 

In spite of the fact that the vast majority of quaternizations of amino-heterocyclic compounds are reported as occurring on the ring nitrogen atom only, it seems quite likely that salt formation may also take place on the exocyclic nitrogen in other cases but that it has been overlooked in the absence of a test such as was available for 4. 

Detailed kinetic studies of the substitution reactions of anions with heterocyclic compounds to include, for example, the effects of solvent, added salts, and ion pair formation have not been made as yet. 

Solvent effects also depend on the ground-state structure of the substrate and on the transition-state structure, as is shown below. Here let us merely note that A-heterocyclic compounds tend to form a hydrogen bond with hydroxylic solvents even in the ground state. Hydrogen-bond formation in this case is a change in the direction of quaternization of the aza group, as demonstrated by spectral evidence. Therefore, it is undoubtedly a rate-enhancing interaction. 

Miscellaneous reactions in neutral ionic liquids Kitazume et al. have also investigated the use of [EDBU][OTf as a medium in the formation of heterocyclic compounds [58]. Compounds such as 2-hydroxymethylaniline readily condense with... 

Dipolar cycloaddition reactions of thioisoraunchnones (l,3-thiazolium-4-olates) have not been as extensively studied as those of munchnones (l,3-oxazolium-5-olates) despite offering rapid access to novel heterocyclic compounds. The cycloaddition of the thioisomunchnone (52) with trans-P-nitrostyrene results in the formation of two diastereoisomeric 4,5-dihydrothiophenes (53) and (54) via transient cycloadducts. These cycloadducts then undergo rearrangement under the reaction conditions <96JOC3738>. 

The reductive alkylation of DAP with acetone led to high conversions and selectivity to the dialkylated product over Al, Bl, and BS2 catalysts. The ASl catalyst, which typically has lower activity than the Al or Pt-based catalysts showed greater formation of heterocycles. These results indicate that a more active catalyst, a shorter reaction time, a higher operating temperature, or sterically hindered amines/ketones will help minimize the formation of the heterocycles. Similar high selectivities were obtained with DAP-MIBK reaction over BSl and BS2 catalysts with no heterocycles being formed. However, over Al, the undesired heterocyclic compound was over 15%. This indicates that the reaction between diamines and ketones has a significant potential to form heterocyclic compounds unless the interaction between these is kept to a minimum by the use of a continuous flow reactor as proposed by Speranza et al. (16) or by other methods. 

In Chapter 10 of Part A, the mechanistic classification of 1,3-dipolar cycloadditions as concerted cycloadditions was developed. Dipolar cycloaddition reactions are useful both for syntheses of heterocyclic compounds and for carbon-carbon bond formation. Table 6.2 lists some of the types of molecules that are capable of dipolar cycloaddition. These molecules, which are called 1,3-dipoles, have it electron systems that are isoelectronic with allyl or propargyl anions, consisting of two filled and one empty orbital. Each molecule has at least one charge-separated resonance structure with opposite charges in a 1,3-relationship, and it is this structural feature that leads to the name 1,3-dipolar cycloadditions for this class of reactions.136... 

In practical terms, it is invariably a nitrogen atom that is protonated in salt formation. This always leads to a downfield shift for protons on carbons both alpha and beta to the nitrogen concerned. In alkyl amines, the expected shifts would be about 0.7 and 0.3 ppm respectively. Remember that some heterocyclic compounds (e.g., pyridine) contain nitrogen atoms that are basic enough to protonate and comparable downfield shifts can be expected (Spectrum 5.9). 

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