Tandem transformations involving allylic silanes .2. Highly diastereoselective substitutions involving [(trialkylsilyl)methyl]cyclohexene derivatives with aldehydes. Synthetic studies on the problem of Lewis acid-promoted protodesilylation and enolization

Diels-Alder cycloaddition of 2-[(trialkylsilyl)methyl]-1,3-butadienes with a variety of dienophiles and substitution reactions between these allylsilane-containing adducts and aldehyde or acid chloride electrophiles have been combined into ''tandem sequential reactions.'' These tandem sequences proceed with equal or greater yield (50-80%) than the reactions performed separately with no decrease in regio- or stereoselectivity. The sequence produces cyclic compounds with three or four stereogenic centers with good to excellent diastereoselectivity from three simple, noncyclic, and achiral reaction partners. Unprecedented levels of diastereoselectivity (de > 93%) have been achieved in allylic substitution reactions involving substituted [(trialkylsilyl)methyl]cyclohexene derivatives with aldehyde electrophiles. During the course of these studies, protodesilylation of allylsilanes has been investigated in detail, and a cocatalyst system of TiCl4 and Me2AlCl has been developed that has eliminated silicon loss in the substitution reactions studied. Lewis acid-promoted enolization of ester and ketone substrates with chiral centers adjacent to the carbonyl moiety has been studied also. It has been shown that this event in our studies occurs primarily during catalyst quench. This isomerization is prevented by quenching the catalyst with a Lewis base, such as methanol or triethylamine, prior to aqueous workup.