The study of the generation of ortho-quinone methides and their cycloaddition reactionsSuttipol Radomkit
( M.Sc. )
The chroman or the benzopyran, the core structures of natural and synthetic compounds exhibiting a wide array of biological properties, can be furnished by the cycloaddition reactions between the o-QMs and appropriate olefins. As reported in literatures, there are a number of strategies used to furnish the chromans. However, there are some limitations in using those methods; for example, 1.1 equiv of the catalyst is necessary and the scope of the o-QM precursors is rather narrow. The aforementioned problems could be overcome by the use of the new catalyst PtCl4. The amount of catalyst could be 10 mol% to furnish the chroman products and almost all cases gave the products in good yields. Furthermore, the scope of the starting material was expanded. Variations of the substituent patterns on the aromatic ring of o-QM precursors potentially allow for access to natural product chromans. In addition, the leaving group of the o-QM precursor could be OTBDMS in appropriate o-QM precursors without detrimental effects on the yield. The α,β-disubstituted styrenes were used to investigate the diastereoselectivity between C2-C3 positions and the results showed a 1:1 mixture of diastereomers to single isomers favoring the trans-isomers of products in good to excellent. The o-QM precursors with substituents at the benzylic position were prepared to explore the diastereoselectivity between C2-C4 and C2-C3-C4 positions. With substituents at the benzylic position of the o-QM precursors, the cycloaddition reactions using simple styrene furnished the 2,4-cis isomer of the products. The electron-withdrawing group (p-OCF3 and p-CN) on aryl substituent gave moderate to good yields, but with moderate diasteroselectivity favoring the cis isomer between C2-C4, while between C2-C3 still favoring the trans isomer. In addition, 1,3-cyclohexadiene as the cis-conjugated double bonds gave a single isomer favoring cis between C2-C3 in moderate to good yields.