Chrom-3-enes reactions

Another example illustrates the utility of ethyl acetoacetate in the synthesis of chrom-2-enes. Reaction of the /3-keto ester with the benzyl halide (173) affords the hydroxyphenylpropanone (174) which forms the chromene with acetic anhydride (Scheme 27) (42JA435). 

The photochemical ring opening of chrom-3-enes (29) has been shown to produce o-quinoneallides (30). These have been trapped as the phenols (31) and (32) by their in situ reduction with lithium aluminum hydride (67JPC4045). The reverse process, thermal electrocyclic ring closure of the allide, is a facile reaction and hence any synthesis of an o -quinoneallide is effectively a synthesis of a chrom-3-ene. Many chromene syntheses are considered to proceed through the intermediacy of a quinoneallide and all of these are... 

The reaction of several amines with 7-methoxy-3-phenylbenzopyrylium perchlorate has been shown to yield the corresponding 2-substituted 3-phenylchromene (78JCS(P1)88), although the reaction of secondary amines with simple benzopyrylium salts gives mixtures of chrom-3-enes and chrom-2-enes (72JOC1069). 

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). 

By analogy with the conversion of tetrahydrofurfuryl alcohol into dihydropyran, the action of heat on 2,3-dihydro-2-hydroxymethylbenzofuran might be considered a potential source of chrom-2-ene. Whilst the reaction is indeed successful, 2-methylbenzofuran is also produced and separation of the two compounds is difficult (54MI22400). It thus appears that there is competition between proton loss before and after a Wagner-Meerwein rearrangement. 

The base-catalyzed reaction of dihydrocoumarin with ethyl formate leads to the 3-formyl derivative (183), which rearranges in the presence of methanolic hydrogen chloride to the chroman ester. Upon heating with polyphosphoric acid, the chrom-2-ene is produced (Scheme 32) (60CB1025). 

Bcnzo[/)]furan and derivatives, recent advances in chemistry of, Part 1, occurrence and synthesis, 18, 337 Benzo[c]furans, 26, 135 recent advances in the chemistry of, and related compounds, 73, 1 Benzofuroxans, 10, 1 29, 251 2//-l-Benzopyrans (chrom-3-enes), 18, 159 Bcnzofc] pyrylium salts syntheses, reactions, and physical properties, 50, 157 1,2-and 2,1-Benzothiazines and related compounds, 28, 73... 

Chichibabin reaction, advances in, 44, 1 Chiral induction using heterocycles, 45, 1 Chrom-3-ene chemistry, advances in,... 

Chichibabin reaction, advances in, 44, 1 Chiral induction using heterocycles, 45, 1 Chrom-3-ene chemistry, advances in, 18,159 Claisen rearrangements in heteroaromatic systems, 42, 203 in nitrogen heterocyclic systems, 8, 143 Complex metal hydrides, reduction of nitrogen heterocycles with, 6, 45 39, I Concept of aromaticity in heterocyclic chemistry, 56, 303... 

Chloroperoxidase catalysis by, 58, 302 in chlorination of pyrazoles, 57, 337 Chlorophyll, thioaldehyde synthetic intermediate to, 55, 3 Chlorosulfonyl isocyanate, reaction with 2-arylhydrazono-3-oxobutanoate, 59, 148 Chromatography, of [l,2,4]triazolo[l,5-a]-pyrimidines, 57, 106 Chrom-3-enes, see 2//-l-Benzopyrans Chromium tricarbonyl complexes of 3,5-diphenyl-l-(alkyl- or oxido-)-thiabenzenes, 59, 206, 227 indoles, lithiation of, 56, 181, 184 of pyridine, 58, 160 pyridines, lithiation of, 56, 230, 239 of 2f/-thiopyrans, 59, 227 Chromones, see l-Benzopyran-4-ones Cinnamonitrile, a-cyano-, condensations with thio-, seleno-amides, 59, 184, 186 Cinnoline, nitration, MO calculation, 59, 302... 

The reaction of o-acyloxybenzyl bromides (175) with an excess of a phosphorus ylide leads to high yields of chrom-2-enes (Scheme 28) (79TL2995). The process is considered to... 

Although strictly outside the reactions of prenyiphenolic compounds the extensive work on the regiospecific general synthesis of chrom-3-enes (2H-benzo[b]pyrans), in particular2,2-dimethylchromenes, from the reaction of metal phenoxides with 3-methylbut-3-enal is very noteworthy. In the general procedure a solution of titanium tetraethoxide (0.025 mol) in toluene was treated with the phenol (0.1 mol) in toluene and the ethanol formed removed by distillation over... 

A direct procedure, partly based in earlier knowledge, giving the chrom-3-ene (2H-benzo[b]pyran) structure in one-step has been referred to earlier. In this method titanium or magnesium phenolates formed from the phenol with the metal ethoxide and removal of ethanol, are then reacted with a,p-unsaturated aldehydes or ketones in toluene solution at 110 C over 8 hours (ref. 40). In this way precocene 2 was derived from the reaction of the magnesium salt of... 

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