Baker-Venkataraman rearrangment

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

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. 

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). 

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>. 

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>. 

There is much interest in anthrapyran antibiotics. Synthesis of S-indomycinone indicates that a revision is required <07CEJ9939>. A total synthesis of racemic y-indomycinone 49 based on a Baker-Venkataraman rearrangement of the anthraquinone ester 47 and cyclisation of the resulting diketone to the chromone 48 has been reported <07EJ01905>. A total synthesis of the structurally simpler topopyrone C involves similar methodology <07TL1049>. In an alternative approach which is readily adaptable to the synthesis of other anthrapyrans, the... 

IS),125 has been obtained by the Baker-Venkataraman rearrangement of the o-acetophenyl ester of selenophene-2-carboxylic acid by the action of alkali in pyridine. This diketone, however, is unstable and turns easily into 2-(selenien-2-yl)chromone (19). 

Baker-Venkataraman rearrangement. This rearrangement of o-aroyloxyaceto-phenones (equation 1) is generally carried out with sodium or sodium ethoxide under... 

The base-catalyzed rearrangement of aromatic ort/70-acyloxyketones to the corresponding aromatic (3-diketones is known as the Baker-Venkataraman rearrangement. 3-Diketones are important synthetic intermediates, and they are widely used for the synthesis of chromones, flavones, isoflavones, and coumarins. The most commonly used bases are the following KOH, potassium fert-butoxide in DMSO, Na metal in toluene, sodium or potassium hydride, pyridine, and triphenylmethylsodium. 

In the laboratory of K. Krohn, the total synthesis of akianonic acid and its derivatives was undertaken, utilizing the Baker-Venkataraman rearrangement of ort/70-acetyl anthraquinone esters in the presence of lithium hydride. Using this method, it was possible to introduce ketide side-chains on anthraquinones in a facile manner. 

V. Snieckus and co-workers developed a new carbamoyl Baker-Venkataraman rearrangement, which allowed a general synthesis of substituted 4-hydroxycoumarins in moderate to good overall yields. The intermediate arylketones were efficiently prepared from arylcarbamates via directed ortho metallation and Negishi cross coupling. The overall sequence provided a regiospecific anionic Friedel-Crafts complement for the construction of ortho-acyl phenols and coumarins. 

Stigmatellin A is a powerful inhibitor of electron transport in mitochondria and chloroplasts. During the diastereo- and enantioselective total synthesis of this important natural product, D. Enders et al. utilized the Baker-Venkataraman rearrangement for the construction of the chromone system in good yield. ... 

A highly efficient and operationally simple domino reaction was developed in the laboratory of S. Ruchiwarat for the synthesis of benz[h]indeno[2,1-e]pyran-10,11-diones. The initial aroyl-transfer was achieved by the Baker-Venkataraman rearrangement by subjecting the starting material to KOH in pyridine under reflux for 30 minutes. 

---