Stereoselective and also stereospecific condensation at C-4 of flavan-3,4-diols of known absolute configuration with phloroglucinol and resorcinol in acid medium proceeds at ambient temperatures with partial retention of configuration for 2,3-trans-isomers and with inversion for 2,3-cis-analogues. Circular dichroism spectra of the resultant 4-arylflavan-3-01s all exhibit multiple Cotton effects. The sign of high intensity Cotton effects to low wavelength, contributed by aryl chromophores at C-4, may almost invariably be correlated with the absolute configuration at this chiral centre of 2,3-trans-3,4-trans, 2,3-trans-3,4-cis-, and 2,3-cis-3,4-trans-isomers. * This also gave low yields of full C-3 O-methyl ethers [( 10, (13), (17), and (26)] as side-reactions. t Magnetic non-equivalence of H-3 and H-5 of the phloroglucinol D-rings of the 2,3-trans-3,4-trans-analogues (7) and ( 15) at ambient temperatures is due t o chirality at C-4 coupled with restricted rotation, as evidenced by their coalescence at higher temperatures.
Out with convention! The use of borosalicylic acid, derived from boric and salicylic acids, as the acid promoter in the methoxycarbonylation of ethylene to give methyl propionate has been investigated (see scheme). Not only was moderate catalyst activity observed, but much lower formation of phosphonium salts occurred than with conventional acids.
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