Lewis acid-promoted, stereocontrolled, gram scale, Diels-Alder cycloadditions of electronically matched 2-pyrones and vinyl ethers: The critical importance of molecular sieves and the temperature of titanium coordination with the pyrone

(R)-(+)-Binol-titanium coordinates with commercial methyl 2-pyrone-3-carboxylate and promotes mild, highly enantiocontrolled Diels-Alder cycloadditions with electron-rich vinyl ether CH2=CHOCH2-1-naphthyl and vinyl silyl ether CH2=CHOSiMe(2)Bu-t leading to valuable 1 alpha,25-dihydroxyvitamin D-3 (calcitriol) intermediate (-)-2. Unexpectedly, two subtle variables were found to be critical for obtaining reproducibly over 90% enantioselectivities in gram scale cycloadditions: (1) the moisture content (15-17% is best) of the molecular sieves used to prepare the binol-titanium complex according to the Mikami protocol and (2) the temperature (50 degrees C is best) at which the pyrone ester is mixed with the binol-titanium complex. Unsubstituted 2-pyrone undergoes ytterbium-promoted, high-pressure, regioselective, and stereoselective Diels-Alder cycloaddition with benzyl vinyl ether to form versatile bicyclic lactone (+/-)-4 on gram scale.