The stereochemistry of 3-arylideneflavanones has been established by the preparation of both cis (3a-c) and trans (2a-c) isomers. Configurations and conformations are assigned on the basis of nmr spectra. The products obtained by the acid-catalyzed condensation of flavanones and aromatic aldehydes have the trans configuration.Ultraviolet irradiation produces the cis isomers. Allylic coupling constants show that the trans isomers exist in the conformation with the 2-phenyl group axial. The cis isomers appear to exist in both conformations. In 2,2-diphenylchromanone (4) the steric effect of the second phenyl group causes formation of the cis product 2,2diphenyl-3-benzylidenechromanone (5). o-Hydroxybenzaldehyde with flavanone does not give the benzylidene derivative but forms o-hydroxybenzylflavone.Flavanones have been condensed with a number of aromatic aldehydes to form 3-arylideneflavanones (termed flavindogenides) in high yield.1 Flavindogenides have also been isolated as coproducts during the preparation of flavanones by the acid-catalyzed condensation of aromatic aldehydes and substituted o-hydroxyacetophenones.2 Interest in 6-nitro-3-benzylideneflavanones as bacteriostats led Széll and Zarandy to reinvestigate methods for their preparation.3 Recently two natural products, eucomin and eucomol, related in structure to flavindogenides, were isolated.4 Eucomin is the first member of the arylidenechromanone family to be found in nature. Two geometrical isomers are possible for the product from the condensation of aromatic aldehydes and flavanones since the /3-aryl groups of the flavindogenide may be either cis or trans to the carbonyl group. In the reported cases only one of the two possible geometrical isomers was obtained. The condensation of aryl aldehydes with methylene compounds normally yields unsaturated products which have the carbonyl function trans to the larger group at the /3-carbon atom.6•6 However, as shown below, there was some evidence to suggest that 3-(2-nitrobenzylidene)flavanone prepared in the normal way had the cis configuration. Cromwell and coworkers7 prepared ¿rans-2-(2-aminobenzylidene) -4,4-dimethyltetralone by the reduction of the corresponding nitro compound with iron and acetic(1) (a) A.

No abstract

Homoallylic alcohols can be dehydrated under exceptionally mild conditions by conversion into their thiobenzoate O-esters and irradiation between -70" and room temperature. A variety of substrates have been dehydrated by this method.SINCE O-cholestanyl thiobenzoate,2 unlike the corres-N -o ~i d e . ~~* The nitrile oxide was liberated 172 situ by ponding cholesteryl derivative, did not smoothly eliminate thiobenzoic acid on irradiation, it appeared that the transition state requires some conjugation of the newly forming double bond. O-2-Arylethyl thiobenzoates (I) should behave in a similar manner to 0-cholesteryl thiobenzoate, since the incipient double bond would be conjugated with the aryl residue. A series of parasubstituted phenethyl thiobenzoates were made and their behaviour on irradiation was studied.O-Fhenethyl thiobenzoate (11; R = H) was irradiated in cyclohexane with a medium-pressure mercury lamp to give styrene in 90% yield, and, on oxidative work-up, dibenzoyl disulphide (100%). The styrene was isolated as its dibromide or (better) as its I,%dipolar addition product (I11 ; R = H) with 9-nitrobenzonitrile Part XXXI, Part 11.

No abstract

No abstract