3.4- Disubstituted 2//-benzopyrans

Keywords Propargylic aryl ethers, iodine, sodium bicarbonate, nitromethane, room temperature, electrophilic iodocyclization, 3,4-disubstituted 2//-benzopyrans... 

Harikrishnan LS, Showalter HDH. A novel s3mthesis of 2,3-disubstituted benzopyran-4-ones and application to the solid phase. Tetrahedron 2000 56 515-519. 

Dimedone and related compounds behave as carbon nucleophiles towards oxazines and oxazolidines leading to the octahydroxanthene derivative (3). In the presence of stoichiometric quantities of reactive C-nucleophiles, such as substituted acetonitriles, 2,3-disubstituted partially reduced 4ff-l-benzopyrans result <96T14273>. 

M. A. Halberstam and M. B. Gordin, Kinetics of reversible photochromic reactions in the series of 1,5-disubstituted 3,3-dimethyl-6 -nitro-8 -bromospiro-[(2 /7,l -benzopyran)-2,2 -indolines], Photo-... 

In the reaction of isatins with some cyclic ketones, such as 4-hydroxy-2H-benzopyran-2-one, the initial dioxindole formed reacts with a second equivalent of the ketone yielding a 3,3-disubstituted oxindole. 

The electrophilic cyclisation of aryl 3-arylprop-2-ynyl ethers to 3,4-disubstituted 2/7-1-benzopyrans 13 is achieved efficiently using IC1, h or PhSeBr. The reaction is compatible with a wide range of substituents and the 3-iodo group offers potential for the elaboration of the system <07JOC1347>. 

The value of 4-hydroxycoumarin in synthesis has been discussed <07THC(11)283> and its use in the synthesis of 3,4-disubstituted coumarins has been reported <07JOC7279>. The 3-arylation of 4-hydroxycoumarins, achieved using aryllead triacetates, sets up a substrate for a cascade of reactions which leads to the [2]benzopyrano[4,3-c][l]benzopyran system <07JOC3293> and the EDDA-catalysed reaction of 4-hydroxycoumarin with a,[3-unsaturated arylaldehydes produces pyrano[3,2-c]benzopyran-5-ones <07T10025>. Propargylation and... 

Intramolecular alkoxycarbonylation of secondary benzylic alcohols (e.g., 3) requires a shorter time (2 h) and a mixture of 1,3-disubstituted 3,4-dihydro-1 //-2-benzopyrans 4 is obtained in 82% yield and a 74 26 (cis/trans) diastereomeric ratio, as determined by HPLC. The stereochemistry of each isomer is determined by a comparison with known compounds84,85. 

Under similar conditions a low asymmetric induction is reported for the cyclization of the 2-[( )-pentenyl]phenol 3, which gives the 2,2-disubstituted 3,4-dihydro-2//-l-benzopyran 4 in 80% yield but only 10% ee60. 

A synthesis of benzopyrans 429 was performed by condensation of disubstituted a,[l-unsatura ted aldehydes 427 with 1 (75TL3407) the firstly formed dienones 428 underwent valence isomerization to give the stable 2H-benzopyrans 429. When the reaction was carried out at room temperature in pyridine, a small amount of 428 was isolated together with the major product 429 (75TL3407). When the monosubstituted... 

Draper, R.W., Hu, B., Iyer, R.V. et al (2000) An efficient process for the synthesis of trans-2,3-disubstituted-2,3-dihydro-4ii-l-benzopyran-4-ones (Chroman-4-ones). Tetrahedron, 56,1811-1817. 

The synthesis of heterocycles by means of Lewis bases has been carried out using the reactions described above. For instance, the dipole resulting from the reaction of pyridine with DMAD was intercepted with phenylisocyanate giving a 1,4 dipole that ultimately led to pyrimidindione derivatives. Wang and coworkers have described the synthesis of benzoxazoles by reaction of ynals with N-protected-2-aminophenols using pyrrolidine as catalyst. Salicylaldehyde has been the starting material for the synthesis of many benzoheterocycles. When using DBU as catalyst, it was transformed into benzopyran derivatives by reaction with 2,2-disubstituted allene esters. When DABCO is used as catalyst, the reaction of AT-tosylimines with ethyl... 

New rearrangements of 2-imino-2//-l-benzopyran-3-carboxamides, on their treatment with anthranilic acid, have been revealed. 4-Quinazolinones (72) have been prepared by rearrangement of 4-imino-4//-3,l-benzoxazines (71) via amidine carboxamides, while dihydrothiazines such as (73) which are disubstituted at C(7) have been observed to undergo a cyclization-ring contraction reaction with substituted acrylic acids to yield pyrroles (74). This interesting transformation has been accounted for by the route outlined in Scheme 23. 

Riva C, De Toma C, Donadel L, Boi C, Pennini R, Motta G, Leonard A (1997) New DBU (l,8-diazabicyclo[5.4.0]undec-7-ene) assisted one-pot synthesis of 2,8-disubstituted 4H-1-benzopyran-4-ones. Synthesis 1997 195-201... 

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