WCC 3 |
| We have investigated annulation reactions leading to tetrahydropyrans. Homoallylic enol ethers react with α,β-unsaturated ketones by a Mukaiyama-Michael addition in the presence of titanium tetrabromide to form an intermediate oxocarbenium ion, which then undergoes a 2-oxonia-Cope rearrangement. The zwitterion then collapses to form a tetrahydropyran, but when the allyl group of the initial enol ether is replaced by a crotyl group, sigmatropic rearrangement does not occur and only a diastereomeric mixture of the bromine-trapped tetrahydropyran forms. We have also investigated the formation of tetrahydropyrans from the rearrangement of dihydropyrans. The dihydropyran forms a mixture of endo products from a hetero Diels-Alder reaction of an á-keto ester with a homoallylic alcohol. In the presence of scandium (III) triflate the dihydropyran opens to form an oxocarbenium ion, which then undergoes a sigmatropic rearrangement. The resulting zwitterion collapses to form a diastereomeric mix of tetrahydropyrans. The development of these annulation reactions will be useful new tools for the synthesis of complex natural products. |
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Many Faces of Chemistry: The Merck Index Women in Chemistry Award Symposium
9:15 AM-11:50 AM, Monday, August 20, 2007 Westin Boston Waterfront -- Revere, Oral
Women Chemists Committee |