Benzo pyrans Chromenes

Almost every class of natural phenolic compounds contains examples of substances with a 2,2-dialkylchromene ring, and the number of those that are discovered increases every year. It would be difficult to give an exhaustive list of these compounds. Their chemistry will be discussed here only when it is related to some particular behavior of the benzo-pyran ring. Examples include simple chromenes substituted in the aromatic ring,3,4,35-44 benzodipyrans,3,39 dimers of chromenes,45 naphthopyrans,46,47 quinones,48 flavones,49 flavonols,49 chalcones,49 flavanones,49 isoflavonoids,50 rotenoids,50 pterocarpans,50 couma-rins,17,51-53 3-arvl-4-hydroxycouniarins,50 4-phenylcoumarins,54 chroma-... 

The use of coumarin in Diels-Alder reactions is, by comparison to pyran-2-ones, still largely unexplored. This is likely due to the low reactivity of the double bond at positions 3 and 4. Diels-Alder reactions with 3-substituted coumarins are rare, despite the potential to synthesize tetrahydro-6//-benzo[c]chromen-6-ones, important precursors to functionalized biphenyls and several natural products. Electron-withdrawing substitution at C-3 does not serve to efficiently activate the dienophilic system thus, it has been found that high temperatures and pressures are necessary to effect the reaction (e.g., Equation 25) <2006JOC70>. 

Methylbut-2-enyl)oxy]-7H-furo[3,2-g][1]benzo-pyran-7-one. AI3-61725 Ammidin 5-Benzofuranacryllc acid, 6-hydroxy-7-((3-methyl-2-butenyl)oxy)-, 8-lactone CCRIS 4346 EINECS 207-581-1 7H-Furo(3,2-g)(1)benzopyran-7-one, 9-((3-methyl-2-butenyl)oxy)- HSDB 3497 6-Hydroxy-7-(3-methyl-2-butenyioxy)-5-benzofuranacrylic acid ro-lactone 8-lsoamylenoxypsoralen 8-Isopentenyl-oxypsoralene Imperatorin Marmelosin 9-((3-methyl-2-butenyl)oxy)-9-(3-Methylbut-2-enyloxy)-7H-furo(3,2-g)-chromen-7-one 9-((3-Methyl-2-butenyl)oxy)-7H-furo(3,2-g)(1)benzopyran-7-... 

Recently, Brahmachari and his group [133] have developed a simple, facile, and convenient practical method for one-pot syntiiesis of pharmaceutically interesting diverse kind of pyran-annulated heterocycles such as 4H-pyrano[3,2-c]coumarins (14), 4H-pyrano[3,2-c]-a-pyrones (12), 5,6,7,8-tetrahydro-5-oxo-4H-chromenes (9), and benzo[//]-4H-chromenes (8) in the presence of ammonium or sodium formate as catalyst via three-component... 

J.M. Khurana, B. Nand, P. Saluja, DBU a highly efficient catalyst for one-pot synthesis of substituted 3,4-dihydropyrano[3,2-c]chromenes, dihydropyrano[4,3-fc]pyranes, 2-amino-4H-benzo[/t]chromenes and 2-amino-4H-benzo[ ]chromenes in aqueous medium. Tetrahedron 66 (2010) 5637-5641. 

All of the compounds discussed are based on three molecules 2/f-pyran (1), 4//-pyran (2) and the pyrylium cation (3). Names which have been used for the benzologue (4) of 2//-pyran include 2H- 1-benzopyran, benzo-a-pyran, chrom-3-ene and 2//-chromene. A similar situation exists for the corresponding derivative (5) of 4/f-pyran. The unambiguous and simplest name chromene is used in the present work. The benzologue (6) of pyrylium is known both as benzopyrylium and chromylium the former name is preferred here. Higher benzologues are referred to as naphthopyrans, such as 2H-naphtho[ 1,2-6 jpyran (7), but the names xanthene and xanthylium are used for (8) and (9). 

The simple pyrans are rather unstable compounds and of little biological or industrial significance but some of their benzo derivatives are of considerable interest. Many 2H-1-benzopyrans (chromenes) are found in plants, for example evodionol (126), lapachenole (127), lonchocarpin (128), rottlerine (129) and edulan (130). A few of the constituents of marijuana (hashish) belong to this class, for example cannabinol (131). 

Six-membered unsaturated oxygen heterocyclic compounds are based on three molecules 2/7-pyran 1,4/7-pyran 2, and the pyrylium ion 3. Based on this, the benzo analog of 2/7-pyran is named 277-1-benzopyran (commonly 2H-chromene) and the benzo analog of 477-pyran is called 4/7-1-benzopyran (commonly 477-chromene). The benzo analog of 3 is known as 1-benzopyrylium 6 (sometimes chromylium). Related naphthyl analogs are exemplified by 2/7-naphtho[l,2,A]pyran 7, the xanthylium ion 8, and xanthene 9. 

The detailed structure of the clathrate compound formed between 2,5,5-trimethylhex-3-yn-2-ol and 4-p hydroxyphenyl-2,2,4-trimethylthio> chroman has been determined by A -ray crystallography. tra/i5 -3,4-Dibromothiochroman and analogous dibromo-naphthothio-pyrans, prepared by addition of bromine to the appropriate A -thio-chromenes, may exist in sofa conformations. The dibromo-compounds undergo ring-contraction reactions with alkalis, yielding benzo[ ]thiophen derivatives. ... 

M. Khoobi, L. Ma mani, F. Rezazadeh, Z. Zareie, A. Foroumadi, A. Ramazani, A. Shafiee, One-pot synthesis of 4H-benzo[b]pyrans and dihydropyrano[c]chromenes using inorganic-organic hybrid magnetic nanocatalyst in water, J. Mol. Catal. A Chem. 359 (2012) 74-80. 

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