Z,/?-unsaturated aldehydes

Z)-condenses with aldehydes in a (Z)-stereoselective Wittig reaction to afford diethyl acetals of (Z)-a,jS-unsaturated aldehydes in 57-86% yield. Hydrolysis of the acetals with p-TsOH as catalyst in acetone-water or with moist silica gel (2 days at 23°) affords the corresponding (Z)-unsaturated aldehydes in 47-98% yield. The product usually contains 4-14% of the (E)-isomer. These results contrast with those of Wittig reactions of formylmethylenetriphenylphosphorane and l,3-dioxolan-2-ylmethylenetriphenyl-phosphorane (5, 269), which afford (E)-unsaturated aldehydes. 

The aliphatic Claisen rearrangement of secondary allyhc ethers has been recognized to provide E double bonds (Eq. 3.1.20) [28, 29]. The Claisen rearrangement of C4 and C5 substituted vinyl ether provided a 90 10 ratio of ( )- to (Z)-unsaturated aldehydes (Table 3.1.1, entry 1). An increase in the steric bulk of the C5 substituent produced higher stereoselectivity (entry 3). 

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