Synthesis of fluorine analogues of vitamin E. II. Synthesis of 2-(3-chloropropyl)-2,5,7,8-tetramethyl-6-chromanol and its application for stereocontrolled Wittig reaction with trifluoromethyl ketones.

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As the extension of our research to find a biologically active analogue of vitamin E, a more convenient method for the synthesis of fluorine derivatives of 6-chromanol than our previous method was developed. Thus, 2-(3-chloropropyl)-2, 5, 7, 8-tetramethyl-6-chromanol (3) was synthesized by the reaction of 6-chloro-3-methyl-2-hexen-1-ol (2) and trimethylhydroquinone, and the phosphonium ylides (5 and 6) derived from 3 were condensed with trifluoromethylated ketones to give 2-(trifluoroprenyl)-6-chromanol compounds (1a, 1b and 1c) by means of the Wittig reaction as modified by Schlosser. By the use of this revised procedure, the total yields of the chromanols were much improved. Further, the modified Wittig reaction showed higher stereoselectivity than the previous syntheses of these compounds. The double bonds of the side-chain of the products were reduced to give fluorine derivatives of tocopherol analogues. Compound 3 was found to be a useful intermediate for the facile synthesis of vitamin E derivatives.