3- Trifluoroacetyl-3,4-dihydro-2/7-pyran

Alkyl bromides have been shown to add to 5-trifluoroacetyl-3,4-dihydro-2/7-pyrans in the presence of zinc powder (Scheme 31). When BrCH2C02Me is employed as the alkyl bromide, after the initial addition, the ester may easily be cleaved under acidic conditions and the product undergoes spontaneous cyclization to afford a bicyclic lactone <1999JEC141>. 

Diethoxv-5-(trifluoroacetyl)-6-(trifluoromethyl)-3,4-dihydro-2//-pyran Typical Procedure 51... 

Hojo, M. Masuda, R. Okada, E. A convenient synthetic route to functionalized 5-trifluoroacetyl-6-trifluoromethyl-3,4-dihydro-27/-pyrans hetero-Diels-Alder reaction of p,P-his(trifluoroacetyl)vinyl ethers with electron-rich alkenes. Synthesis 1990, (3), 347-349. 

Barrett at al. ° found that 2,2,2-trichloro-2,2,2-trichloroethoxysulfonyl, and trifluoroacetyl isocyanate reacted with 1 to give the respective unsaturated amide 3, no p-lactam being isolated in all three cases. Similar results were reported by Chan and Hall for monosubstituted dihydro-2H-pyrans. 

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