Trimethylsilyl triflate catalysed Diels-Alder reaction of TMS ethers of conjugated dienols with cyclic enones: Evidence for an endo transition state,and first application to synthesis of enantiopure octalins

The AlCl3 catalysed 'ionic' Diels-Alder (DA) reaction of 6-methyl-2-cyclohe xenones with 3,5-hexadien-1-ol in dichloromethane gives a trans-fused octal in hemiacetal with a beta-equatorial methyl group and an axial hydroxyl. In contrast, trimethylsilyl triflate (TMSOTf, 5 mol%) at -20 degrees C in ace tonitrile catalyses the DA reaction to give the trans-fused adduct acetal, whose controlled hydrolysis gives the hemiacetal with an alpha-axial methyl group. The adduct is thereby differentiated from the AlCl3 DA adduct, and may be converted into the latter by treatment with p-toluenesulfonic acid i n aqueous THF or by AlCl3 in dichloromethane. In similar fashion, TMSOTf pr ovides full acetal adducts from 2-cyclohexenone, 2-cyclohexenone and methyl vinyl ketone. The optically active enone acetal derived from (1R,2R)-1,2-d iphenyl-1,2-ethanediol and 2-cyclohexenone gives a racemic acetal adduct de rived from the TMS ether of hexadienol. The TMS ether of a mixture enriched in the 3E-isomer of (2R,5E)-2-methyl-3,5-heptadien-1-ol in acetonitrile co ntaining TMSOTf (10 mol%) at -20 degrees C with 6-methyl-2-cydohexenone giv es the trans-fused acetal adduct hydrolysis which provides the correspondin g octalin hemiacetal as a single enantiomer. Similarly, the mixture enriche d in the 3E-isomer of (2S,5E)-2-methyl-3,5-heptadienol TMS ether is convert ed by way of the octalin acetal into the octalin hemiacetal, enantiomeric w ith the foregoing product. The reactions are thereby shown to proceed via e ndo transition states. (C) 1998 Published by Elsevier Science Ltd. All righ ts reserved.