Stereoselective Synthesis ofHighly Substituted Annulated Dihydropyrans Based on γ-Lithiationand Homoaldol Reaction of an Enantiopure α-Silylated VinylCarbamate

The N,N-diisopropylcarbamate of (―)-myrtenol is converted into the corresponding [1-(trimethylsilyl)vinyl] carbamate. Its γ-lithiation by means of sec-butyllithium/TMEDA, titanation, and addition to aldehydes leads with high stereoselectivity to enantiopure anti-homoaldol products. These add, under the influence of boron trifluoride―diethyl ether complex, a second equivalent of aldehyde and form, in an intermolecular silyl-Prins reaction, the title compounds, again with essentially complete stereoselectivity. After desilylation, the reaction takes the regular course with formation of an annulated five-membered ring.