8‐Prenylflavanones through Microwave Promoted Tandem Claisen Rearrangement/6‐endo‐trig Cyclization and Cross Metathesis

Abstract

Prenylated flavanones were obtained from ortho‐allyloxy chalcones through a one‐pot sequence of Claisen rearrangement and 6‐endo‐trig cyclization, followed by olefin cross metathesis of the intermediate allyl flavanones with 2‐methyl‐2‐butene. The synthetic utility of this route is illustrated for the synthesis of several naturally occurring prenyl flavanones.

Total Syntheses and Antibacterial Evaluations of Neocyclomorusin and Related Flavones

“…Due to the structural similarities between 23 and 24 , distinction between the two natural compounds was a challenge and may lead to misassignment. Therefore, the structures of the regioisomers 23 and 24 were further confirmed by 2D NMR (HMBC and HMQC) data analysis . The HMQC data established all 1 J ( 1 H– 13 C) connectivities.…”

“…Therefore, the structures of the regioisomers 23 and 24 were further confirmed by 2D NMR (HMBC and HMQC) data analysis. 27 The HMQC data established all 1 J ( 1 H− 13 C) connectivities. As shown in Figure 1, the HMBC correlations from 5-OH [δ H 13.14] (due to the hydrogen bond) with C-4a [δ C 104.7] and C-6 [δ C 99.3, δ H 6.17 On the other hand, based on the above spectroscopic data analyses, the main product of aldol-type electrocyclization between 19 and 1,1-diethoxy-3-methyl-2-butene (20) was the angular isomer 21.…”

Scope and Applications of 2,3-Oxidative Aryl Rearrangements for the Synthesis of Isoflavone Natural Products

“…For the synthesis of 8-allyl flavanones 31 , we pursued a sequence previously developed in our group . To this end the o -allyloxy acetophenones 29a and 29b were condensed with benzaldehydes 26 .…”

“…Isobutene, which has also been used for this cross metathesis reaction, has the disadvantage of being a gas at ambient temperature but gives the prenylated products in comparable yields and selectivity . Originally introduced by Grubbs and co-workers, CM reactions with 2-methyl-2-butene have been used for the synthesis of prenylated flavanones, flavones, isoflavones, , coumarins, ,− and other natural products. − The cross metathesis reactions of allyl isoflavones 32g – i and their flavone isomers 33g – i with 2-methyl-2-butene in the presence of 5 mol % second generation Grubbs’ catalyst A at ambient temperature furnished the respective prenyl isoflavones 12 , 35h , i , and prenyl flavones 36g – i in high yields and selectivity. The MOM-ethers 35h and 36h could be deprotected in methanol in the presence of aqueous HCl without concomitant acid catalyzed addition of methanol to the prenyl substituent (Scheme ).…”

The Suzuki–Miyaura Cross-Coupling–Claisen Rearrangement–Cross-Metathesis Approach to Prenylated Isoflavones

“…Compound 18 was subjected to microwave irradiation to induce a Claisen rearrangement to the 3′-allylisoflavone 19 . We previously successfully conducted similar microwave-promoted Claisen rearrangements in toluene, e.g., with O -allylchalcones. ,,, As an unpolar and moderately high-boiling solvent, toluene can be conveniently removed by evaporation, but it has the disadvantage of a very low dielectric loss tangent (tan δ = 0.040) when used in microwave reactions. A low tan δ means that a solvent is nearly transparent for microwave irradiation and that energy transfer mainly occurs via the absorption of the microwave irradiation by the starting materials, which therefore need to be sufficiently polar.…”