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

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)methylphenyl-magnesium 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-trimethyl-spiro[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.