A Facile Synthesis of 2,4-Dialkoxy-, 2-Alkoxy-4-phenoxy- and 2,4-Diphenoxy-6-trifluoromethyl-3,4-dihydro-2H-pyrans. Hetero-Diels-Alder Reactions of trans-β-Trifluoroacetylvinyl Ethers with Various Vinyl Ethers