Circular dichroism and absolute configuration of Aza‐ and Thiaflavanones

Abstract

The correlation between the absolute configuration and CD of aza-and thiaflavanone (1, 2) is discussed: the 2R configuration for the laevorotatory enantiomers has been established by a comparison of their CD data with those of ( S ) -( -) -3 . For the Cotton effect within the n + I[* band of 1 and 2 the same helicity rule is valid as for the homochirally analogous flavanones (3). Starting from (S)-( -)-flavanone (3) and cholesterol Recently, the optical resolution of azaflavanone[l3I (1) and thiaflavanone[I41 (2) has been described but the absolute configuration of their enantiomers has not been elucidated. Since the laevorotatory flavanones possess, without exception, S configuration it seemed to be also interesting to examine the validity of this empirical rule for their aza and thia analogs. In order to obtain chiroptical and stereochemical information we have carried out CD measurements in the UV absorption region of 1-3 in solvents of different polarity.(8-( -)-Flavanone ["] has been selected as a suitable reference compound, whose CD spectra have been investigated respectively. The lLb band of the chromane chromophore has been unequivocally identified in the range between 255 and 290 nm in the CD spectra of 4 and 20. For this Cotton effect the opposite helicity rule is valid as compared with the homochirally analogous tetralins: the P/M helicity of the hetero ring leads to a negative/positive CD within the 'Lb band. The modification of the helicity rule can be explained with the help of Petruska's "q values". in the range from 400 to 200 nm and compared with those of (-)-1 and (-)-2. In order to assign the origin of the Cotton effects of n + R* and 'Lb in the above-mentioned spectra unequivocally, we have synthesized the chromane derivatives 4 and 20 with known absolute configuration. In this paper we describe these syntheses and discuss the respective CD spectra. SynthesesThe synthesis of (S)-(-)-flavan (4) has been achieved by Clemmensen reduction [16] of the carbonyl group of (a-(-)-

Cytotoxic cardiac glycosides and coumarins from Antiaris toxicaria

“…The S -configuration of C-2 was determined by a positive Cotton effect observed at 312 nm and a negative one at 286 nm in the circular dichroism (CD) spectrum of 43 . 22 Based on these observations, 43 was established as (2 S )-3′,5,8-trihydroxy-4′-methoxy-6,7-methylenedioxy-2′-prenylflavanone, and named antiarone L.…”

Chiroptical properties of 2,3‐dihydrobenzo[b]furan and chromane chromophores in naturally occurring O‐heterocycles

“…According to our rule this should lead to a negative 1 L b band CD, which was found experimentally. 37 The chiroptical properties of model compounds (−)-10, (+)-14, and (−)-18 showed that P/M helicity of the Oheterocyclic ring in the chromane chromophore is reflected by a negative/positive Cotton-effect within the 1 L b band transition of the benzene ring when the Oheterocyclic ring adopts a half-chair conformation and there is no substituent on the fused aromatic ring. This rule can be safely used for 2,3-substituted chromane derivatives and it is also valid for the (2R;4R)-4-hydroxyflavan, 20, whose heterocyclic ring has half-chair conformation and its helicity is governed by the equatorial orientation of the phenyl group.…”

“…35,36 For the preparation of 10, 5 was treated with 2-(benzyloxy)-benzaldehyde to obtain the 2-arylidene ketone 6 in 60% yield (Scheme 1). 37 Next, the bulky 2-(benzyloxy)-benzyl group at C-2 of the cholestane skeleton was fixed in 2␣ configuration, namely, in the thermodynamically more stable equatorial position (6→7) in four steps. A mesyloxy group, a good leaving group, was introduced into C-3 in a stereocontrolled manner (7→8, 7→9) to obtain the suitable precursors for the S N 2-type ring closure.…”

“…According to our expectations, the reaction of 8 with sodium methoxide afforded the cis 2␤,3␤-dihydro compound 10 with the inversion of configuration at C-3, while the ring closure of 9 did not give the trans 2␤,3␣-dihydro-derivative 11 because the preferred elimination of the axial mesyl group took place instead to give the 2-(2-hydroxybenzyl)-5␣-cholest-2-ene as the sole product. 37 The chromane derivative 14 with P-helicity of the heterocyclic ring was produced from the ring closure of the dihydroxy compounds 12 and 13, which were synthesized in seven steps from cholestanone 5. 38 After treating 12 with BF 3 ·OEt 2 , the ring closure took place by multiple hydride shifts resulting in 14 besides the cis-2␣,3␣-dihydrobenzofuran 15 and the unsaturated benzofuran 16.…”

Three new compounds from the bark of Antiaris toxicaria

“…The absolute configuration of C-2 was determined to be S configuration by a negative cotton effect at 289 nm and a positive cotton effect observed at 331 nm in the circular dichroism (CD) spectrum of 2(Fig. 5)(Antus et al, 1994) (Shi et al, 2014). Thus, compound 2 was established as (2S)-4,5,7-trihydroxy-3 0 -methoxy-2 0 -prenylflavanone named antiarone M.Compound 3 was obtained as colorless oil.…”