Chromene acid reaction

There is not sufficient evidence conclusively to prove or disprove one or two definite mechanisms for reaction of acid with active carbon. However, the chromene-acid reaction as herein described appears to be a logical and significant part of the overall reaction. Physical, electrostatic, or other mechanisms for acid sorption probably account for the remainder of the reaction. 

Several studies have been directed toward examination of the interaction of acids and bases with active carbons (I, 8, 10, 17, 18, 19). Boehm (3), Garten and Weiss (9), and Snoeyink and Weber (21) have presented reviews on the subject. Garten and Weiss (8, 9, 10) have shown that acid and alkali sorption can be related to surface functional groups which form during the preparation of the carbon. Alkali sorption occurs principally on carbons activated at temperatures near 400°C., and is attributed to the presence of phenolic and lactone functional groups on the carbon surface. Carbons which sorb acid usually are activated at temperatures near 1000°C. the acid reaction in this case is assumed to take place with chromene (benzpyran) structures on the surface. 

Benzaldehydes react with phenols and 3-oxobutanoates in the presence of a Lewis acid to afford 4-aryl-3-carboxylate-4//-chromenes. The reaction proceeds via formation and reaction of the ortho-quinone methide intermediate 115 with 3-oxobutanoates (Scheme 37) <2004GL1058>. 

More recently, Lee et al. have developed a new methodology for the preparation of a variety of benzopyrans by using ethylenediethylenediamine diacetate-catalyzed reactions of resorcinols to a,(3-unsaturated aldehydes, and applied these reactions to the synthesis of natural products ( )-cannabichromene 112a (Scheme 12.33), ( )-cannabi-chromenic acid 145 (Scheme 12.34), ( )-daurichromenic acid 113a (Scheme 12.35), and confluentin 147 (Scheme 12.36) [78,79]. 

H-Chromene, 2-ethyl-3-phenyl-synthesis, 3, 764 4H-Chromene, 2-phenyl-synthesis, 3, 763 4H-Chromene, 2,4,4-trimethyl-addition reactions, 3, 669 2 H-Chromene-3-carboxamide reduction, 3, 675 2H-Chromene-3-carboxylic acid methyl ester alcoholysis, 3, 668... 

Homofolic acid, 5,11-methenyl-tetrahydro-biological activity, 3, 327 Homofolic acid, tetrahydro-biological activity, 3, 327 Homoisoflavanones occurrence, 3, 722 thermoisomerization, 3, 722 thermolysis, 3, 728 Homolytic reactions heterocyclic compounds reviews, 1, 74 Homophthalic acid isocoumarins synthesis from, 3, 830 synthesis, 3, 830 Homophthalic anhydride isochroman-l-one synthesis from, 3, 860 20a-Homoporphyrin nomenclature, 1, 30 Homopterocarpin isolation, 4, 998 ( )- D- Homotestosterone synthesis, 1, 453 Homer-Emmons reaction chromene synthesis by, 3, 749 Hortiacine isolation, 3, 149 Hortiamine isolation, 3, 149... 

On the other hand, refluxing 9 in formic acid for 5 h afforded the N-formyl derivative 11 in high yield. Acetylation of 9 by refluxing in acetic acid, afforded acetic acid N -(2-(7-hydroxy-2-oxo-2H-chromen-4-yl)-acetyl)-hydrazide 12 in good yield. Compound 13 was also obtained by refluxing 9 with 3-(2-bromoacetyl)-4-hydroxy-2H-chromen-2-one in ethanol. Reaction of compound 9 with phenyl isothiocyanate in ethanol at room temperature gave 4-phenyl-1 - (7-hydroxy-2-oxo-2 H-chromen-4-acetyl- )thiosemicarbazide 14. 

Condensation of the aminochromenes 1039 and 1041 with 2-methyl-l,4-benzoquinone (841) afforded 2-(2,2-dimethyl-2H-chromen-7-ylamino)- (1042) and 2-(2,2-dimethyl-2H-chromen-5-ylamino)-5-methyl-l,4-benzoquinone (1043), along with the corresponding 6-methyl isomers. Finally, reaction of the benzoquinones 1042 and 1043 with stoichiometric amounts of palladium(II) acetate in acetic acid under reflux furnished pyrayaquinone A (175) and B (176) in 78% and 50% yield, respectively (623) (Schemes 5.157 and 5.158). 

In the regioselective total synthesis of edulane and its analogues, the acid-catalyzed reaction of the chroman 93 was studied, giving rise to a mixture of chromenes (Scheme 5) <1997J(P1)1875>. Nicolaou ef a/, showed that it is possible to constmct benzopyrans via an intramolectular 67t-electrocyclization, although a mixture of products was observed (Equation 31) <2004JA5493>. 

Isoprenylation of polyhydroxyacetophenones.1 Direct condensation of isoprene with some polyhydroxyacctophcnoncs is possible in this acid medium. The resulting 2,2-dimethylchromanes can be dehydrogenated to chromenes with DDQ. A typical reaction is outlined in equation (I). 

The Lewis acid-catalyzed reaction between tertiary chloroalkanes and o-hydroxybenzaldehydes has been used to synthesize chromenes with a long alkyl chain at C-3. The chloroalkane undergoes ready dehydrohalogenation and the route parallels those above (79CJC1377). 

The acid-catalyzed cyclization of 3-(2-hydroxyphenyl)propan-l-ones leads to chrom-2-enes. This reaction accounts for the formation of 2,4-diphenylchrom-2-ene from the reaction of phenylmagnesium bromide on coumarin. The organometallic reagent ring-opens the coumarin to the ketone which cyclizes to the chromene on boiling with acetic acid (Scheme 26) (63T839). In a similar manner, various benzocoumarins afford propanones which cyclize readily in acetic acid to the naphthopyran (70JCS(C)1758). 

Treatment of 2/Z-chromenes which have at least one hydrogen atom at C-2 with acid gives a mixture of chroman and benzopyrylium salt (68T949). Reaction of a chromerte with triphenylmethyl salts gives essentially only the benzopyrylium salt (67AC(R)1045, 72CR(C)(274)650>. 

Reactions of the hydronium ion with porous active carbon have been investigated in aqueous systems. Hydronium-ion activity, specific-anion concentration, and carbon dosage have been among the major variables studied. Rates of reaction have been found to be limited by pore diffusion, as partially verified by activation energies of —(2 to 3) kcal. per mole-deg. The results can be interpreted partly in terms of a reaction of the hydronium ion and dissolved oxygen with a surface benzpyran (chromene) group to produce hydrogen peroxide and a surface benzopyrylium (carbonium) ion with a sorbed anion, and partly in terms of physical sorption of the acid on the carbon surface. 

Resorcinols. Condensation of this diene with the ketal of a fi- keto acid derivative results in a resorcinol with complete regiocontrol. Thus, the TiCl4-catalyzed reaction of 1 with ketal ester 2 results in the resorcinol 3 in 72% yield. However, use of the acid chloride 4 corresponding to 2 in the same reaction results in the isomeric resorcinol, methyl olivetolate (5). The regiocontrol is based on the reactiv ity order acid chloride > ketal > ester. The resorcinol 5 was used in a biomimetic synthesis of the chromene A1 -tetrahydrocannabinol (6), a component of marijuana. 

It has been thought that the use of 3-methylbut-2-enal derivatives in the synthesis of chromenes was restricted to the reaction with electron rich phenols. Not so The reaction is efficiently catalysed by pyridine and 3-methylpyridine and is most successful with the more acidic phenols, indicating the need for a careful balance between acid and base catalysis (95JOC3397). 

The reaction of chromenes with mono- and bis- imides of 1,4-benzoquinones (6, X = O or NCOPh) is promoted by Lewis acids and affords pterocarpans and azapterocarpans, respectively (95TL2713). A total synthesis of the pterocarpan, neorautenane, involves a chemoselective coupling of a benzodipyran with o-chloro-mercuriophenol (95JCS(P1)949). 

Acyl-2//-chromenes can be synthesized from O-propargylic salicylaldehydes in the presence of a Bransted acid (HBF4). The reaction proceeds via a formal [2+2]-cycloaddition of the alkyne and carbonyl group to form intermediate oxete 38, which undergoes cycloreversion to afford 3-acyl-2//-chromenes in excellent yield (Scheme 12) <2005OL2493>. 

A simple synthesis of 2-methyl-2-trifluoromethylchroman-4-ones is possible upon heating iV-benzyl-2-trifluoro-methyl-477-chromen-4-imines 1115 in the presence of malonic acid. The reaction proceeds through ring opening and recyclization of the intermediate 1116 (Scheme 277) <2002S2341>. 

Figure 4. Synthesis of precocene. Reaction of an appropriate phenol with dimethyl acrylic acid and polypnosphoric acid (PPA) on the steam bath gives the chromanone in quantitative yield. Reduction with lithium aluminum hydride (LAH) and brief treatment with 4N hydrochloriic acid gives the chromene.

Another approach to ring closure reaction is the o-arylation of substituted phenoxide ions by o-bromobenzonitrile followed by Si02-catalysed lacto-nization. The phenoxide ions of the amino acid (S)-tyrosine, protected as A, O-diacetyl methyl ester, does not racemize under the standard SRN1 conditions and can be used to obtain the optically active benzo[c]chromen-6-one (the O-acetyl is hydrolyzed in the reaction media to furnish the phenoxide ion) (Sch. 43). Racemic dibenzopyranones are obtained by the reaction of the anion from the TV-acetyl methyl ester of (R)-hydroxyphe-nylglycine with o-bromobenzonitrile 2-cyano-4,5-dimethoxybromobenzene (65 and 79% respectively) [110]. 

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