Baker-Venkataraman

Baker-Venkataraman synthesis, 2, 76 Balsoxin, 6, 232 Bamicetin isolation, 3, 147 Band theory semiconductors, 1, 355 Ban thine... 

BAKER VENKATARAMAN Flavone synthesis Rearrangement of aromatic keto esters ot phenols to tiavones... 

Scheme 2.130. Domino reaction involving a Baker-Venkataraman rearrangement to afford 2-579.

An improvement in the Baker-Venkataraman route to flavonols (2-aryl-3-hydroxychromones) which avoids a final oxidation step of the flavone involves formation of... 

One-pot conversions of 2-hydroxy(acylbenzenes) with anhydrides or acid chlorides to produce coumarins [52-54] and flavones [54-58] under mild liquiddiquid or solidtliquid two-phase conditions via a Baker-Venkataraman type reaction (Scheme 6.19) are catalysed by quaternary ammonium salts. 3-Substituted coumarins are produced from salicylaldehyde and malonodinitrile, or substituted acetonitriles, in high yield (>85%) in a one-pot catalysed sequential aldol-type reaction and cycliza-tion in the absence of an added organic solvent [59]. When 2 -hydroxychalcones are reduced under catalytic two-phase conditions with sodium borohydride, 2,4-cis-flavan-4-ols are produced [60] (see Section 11.3). 

A new synthetic protocol consisting of sequential directed ortho metallation, crosscoupling and a carbamoyl Baker-Venkataraman rearrangement has been applied to an efficient construction of coumarins (see Scheme 10). The formation of o-nitrosobenzaldehyde during the hydrolysis of o-nitrobenzyl tosylate in aqueous acetonitrile has been presented as a strong indication that the nitro group participates in the departure of the tosylate group as shown in Scheme 11. 

DMSO resulted in an intramolecular ester condensation (Baker Venkataraman-type rearrangement) to give a Cl 5 phenol the cyclization of which furnished the 0-benzyl-protected flavone-C-glycoside. After deprotection, isoembigenin was obtained. 

Method 1 is a new method for the direct production of flavone from o-benzoyloxyacetophenone and has been successfully applied to the synthesis of other flavones. Method 2, which involves the Baker-Venkataraman transformation, is recommended because of its high over-all yield and the reproducibility of the results. Mozingo and Adkins method 8 is satisfactory but the yield of 0-hydroxydibenzoylmethane is variable. 

Work on the Mills-Nixon effect has been reviewed by Badger [2]. Most of the experimental work done in this field has involved a study of relative chemical reactivities and hence is subject to the severe limitations mentioned earlier [ lc]. From a study of the ease with which the benzoates of isomeric hydroxy-5,6,7,8-tetrahydroacetonaphthones underwent the Baker-Venkataraman transformation O Farrell et ah [3] recently concluded that the 1,2-bond of 5,6,7,8-tetrahydro-naphthalene has a higher bond order than the 2,3-bond. Similar work also led these authors to conclude that the 4,5-bond in indan is of higher order than the 5,6-bond. These workers did not attempt to assess the extent of the difference in bond order presumably exhibited by the bonds in question. 

A ring closure that is often associated with the reaction is called the Baker-Venkataraman Reaction. N. Thasana, S. Ruchirawat, Tetrahedron Letters 2002, 43, 4515... 

A facile Baker-Venkataraman synthesis of flavones (39), using PTC, has been recently proposed by Jain et al.66... 

The Kostanecki-Robinson reaction proceeds through O-acylation followed by a Baker-Venkataraman rearrangement to the 1,3-diketone. Cyclization then yields the chromone. Early evidence was based on the observation that both of the 1,3-diketones (448) and (449) yielded the same chromone on reaction with the appropriate acid anhydride (33JCS1381). Thus, the reactions were considered to proceed through the common intermediate (450 Scheme 159). 

It has since been shown that the enol ester (451) is an intermediate in the synthesis (69T715). Indeed such esters readily form chromones on treatment with alkali and the ortho acyloxy group becomes part of the pyranone ring as a result of a Baker-Venkataraman rearrangement (Scheme 160) (69T707). 

The coumarin is probably derived from the initial acylated hydroxyacetophenone which, in addition to undergoing a Baker-Venkataraman rearrangement, may cyclize through an intramolecular aldol condensation. Elimination of water then gives rise to the coumarin (Scheme 161). 

The base-catalysed acyl transfer (Baker-Venkataraman reaction) of enantiopure 2-acetyl-l-hydroxyanthraquinone esters of 2-methylbutanoic acid or of O-allyllactic acid produces the respective 1,3-diketones, precursors of anthra[l,2-fc]pyran antibiotics, without any racemization.9... 

Flavanols are similarly accessed from 2-(2-(benzoyloxy)acetyl)phenyl benzoates 752 via a Baker-Venkataraman rearrangement to form 3-benzoyloxy flavones followed by deprotection of the hydroxyl group (Scheme 194) <2000JOC583>. 3-Aroyl flavones are prepared form or/ o-hydroxyacetophenones and aroyl chlorides in modest yield, using a Kostanecki-Robinson approach (Equation 305) <2005SC315>. 

A Baker-Venkataraman rearrangement of 2-acetylphenyl methyl phthalate 753 forms the (3-diketone intermediate 754, which undergoes successive ring closures leading to the formation of benzo[ ]indeno[2.1-< ]pyran-l-,ll-dione 755 in high yield (Scheme 195) <2002TL4515>. 

Trifluoromethyl derivatives of thiochromone are also available from thiosalicylic acid by conversion to alkyl 2-mercaptophenyl ketones followed by a Baker-Venkataraman reaction with trifluoroacetic anhydride in triethyl-amine (Scheme 205) <2005PS(180)1315>. 

Benz[6]indeno[2,l-e]pyrandiones are readily available by the acylation of o-hydroxy-acetophenone with monomethyl phthalate and subsequent Baker-Venkataraman rearrangement <02TL4515>. 

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