Acetals s. a. Dialkoxy

Accelerator s. Activator Acetalation preferential 24,187 Acetal group migration 24, 579 Acetals (s. a. Dialkoxy..., Hemiacetals)... 

TiCU-mediated additions of silyl ketene acetal (98) to chiral imines (99) and (100) (R = Et, Pr", Bu", Bu ) are described in equations (33) and (34) good diastereoisomeric ratios were obtained using imines (100), derived from (S)-valine methyl ester, which form with TiCU the chelated complex (101). Znh-catalyzed additions of acetate-derived silyl ketene acetals to chiral a. -dialkoxy nitrones (102 R = H) were reported to proceed with good yield (86-100%) and high diastereofacial selectivity ca, 90 10) in favor of the anti isomer (103 R = H, R = CHPh , R = Bu ) or of the syn isomer (104 R = H, R = CH2Ph, R- = Me) depending on the steric hindrance of R and (Scheme 8). Addition to nitrone (102 R = Me) gave the anti isomer (103 R Me, R = CHMePh, R = Me) in quantitative yield and 100% diastereofacial selectivity. This material was further elaborated to A/-benzoyl-L-daunosamine (Scheme S). ... 

S-Alkyl, 0-silyl ketene acetals can be derived from the corresponding thioesters in a manner analogous to the generation of dialkoxy ketene acetals... 

A 30% soln. of diazoacetophenone in dry benzene added in small portions to a 1 10 suspension by weight of Cu-powder in 1.5 equivalents of the ketene acetal previously heated in a bath at 90-100°, taking care that the N2-evolution subsides before each addition, and subsequently hqated at the same temp, for an additional 45 min. 5-phenyl-2,2-diethoxy-2,3-dihydrofuran. Y 87%. - The above method provides a route to previously unknown 2,2-dialkoxy-2,3-dihydro-furans. F. e. s. R. Scarpati et al., G. 96, 1164 (1966). 

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