Complementary diastereoselectivity in the intermolecular addition of titanium and magnesium naphtholates to asymmetric lactaldehydes
Addition of 7-benzyloxy-4,5-dimethoxy-1-naphthol 3 as its triisopropoxytitanium naphtholate to (2R, 1′R or S)-2-(1′-ethoxyethoxy)propanal 4 afforded solely (1S, 2R, 1″R or S)-1-(7′-benzyloxy-4′,5′-dimethoxy-1′-hydroxy-2′-naphthyl)-2-(1″-ethoxyethoxy)propan-1-ol 5, being the erythro product arising from anti addition. Complementary reaction of the naphthol 3 as its bromomagnesium naphtholate with aldehyde 4 gave rise solely to the alternative (1R, 2R, 1″R or S) diastereomer 6 . The naphthol 3 was prepared through the completely regioselective addition of 2-methoxyfuran 9 to 5-benzyloxy-3-methoxydehydrobenzene 8. This differentially protected bisalkoxybenzyne was conveniently prepared from vanillin.