Formal Synthesis of Both Atropomers of Desertorin C and an Example of Chirality Transfer from a Biphenyl Axis to a Spiro Centre and its Reverse

Abstract

In connection with the synthesis of 4,4′,7,7′-tetramethoxy-5,5′-dimethyl-6,8′-bicoumarin (desertorin C) (11) in enantiopure form, the diastereomeric ratios of the products of the reactions between 2-isopropyloxy-6-methoxy-4- methylphenylmagnesium bromide (24) and (4S)-4-isopropyl-2-(2,3,5-trimethoxyphenyl)-4,5-dihydrooxazole (23), between 2,4-dimethoxy-6-methylphenylmagnesium bromide (37) and (4S)-4-isopropyl-2-(2,3-dimethoxy-5-methylphenyl)-4,5-dihydrooxazole (36), and between 2,4-dimethoxy-6-(t-butyldimethylsilyloxy)methyl-phenylmagnesium bromide (46) and the oxazole (36) were explored. The major product of the last mentioned reaction was converted into (S,4S)-4-isopropyl-2-(2′-hydroxymethyl-4′,6,6′-trimethoxy-4-methyl-1,1′-biphenyl-6-yl)-4,5-dihydroxazole (49), the axial configuration of which was confirmed by single crystal X-ray structural determination. The similar product (S,4S)-2-(2′,4′,6-trimethoxy-4,6′-dimethyl-1,1′-biphenyl-6-yl)-4,5-dihydrooxazole (43) was converted into (S)-1-(2,4′,6′-trimethoxy-4,6′-biphenyl-2-yl)ethanone (57) which furnished (S)-1-(2′,4′,6-trimethoxy-4,6′-dimethyl-1,1′-biphenyl-2-yl)acetamide (58) (43%) and (S)-2,7′-dimethoxy-3′,5′,6-trimethylspiro[cyclohexa-2,5-diene-1,1′-(1H)isoindole]-4-one (61) (30%) on Schmidt rearrangement. The dienone (61) on reduction and methylation regenerated the ketone (57). The methodology of Lipschutz was adapted for the synthesis of both enantiomers of 1,1′-(2′,4-dihydroxy-6,6′-dimethoxy-2,4′-dimethylbiphenyl-3,3′-diyl)bisethanone (32) and (83) which constitutes a formal synthesis of both enantiomers of desertorin C.