Claisen rearrangements ynamine

C nitriles 269 were formed in 25 to 60% yield. This outcome was explained by an initial extrusion of Ph3P=0 to generate ynamines 273. The consecutive isomerization delivered iminoketenes 268, which underwent the usual iminoketene Claisen rearrangement to produce the nitriles 269. Detailed information is given in Table 18 (Scheme 51). 

To exploit the whole capacity of the Claisen rearrangement, appropriate methods for the preparation of the allyl vinyl ethers starting from allyl alcohols are necessary. The classical approach involves vinyl-ation with simple vinyl ethers or acetals. Unfortunately these methods fail with more complex systems and do not allow, except in the case of cyclic enol ethers, control of the stereochemistry of the substituted enol ether double bond. Until recently it was only possible to generate such substituted systems with appreciable stereocontrol via ketene N.O-acetals. Their preparation by addition of lyl alcohols to substituted ynamines can lead to adducts of either ( )- or (Z)-geometry, depending upon the conditions used (Scheme 60). 

The 3,3-sigmatropic shift of ketene A, 0-acetals, first developed by Eschenmoser, preceded studies of the ynamine-Claisen rearrangement, where the reactive intermediate is formed at ambient tempera-... 

Addition of Alcohols to Ynamines and Ynamides (Ficini-Claisen Rearrangement)... 

The Claisen rearrangement has been used to prepare j8,y-unsaturated amides, largely as the E-isomers, from 3-(trimethylsilyl)allyl alcohols and amide acetals (Scheme 48). Overall yields and stereoselectivities are excellent. Conditions have been found under which the stereochemical outcome of the related ynamine-Claisen rearrangement can be controlled. Thus, the kinetically favoured intermediate is the E-enamine (140) which leads to the "trans product (142), whereas the Z-isomer (141) is thermodynamically favoured and gives the cw ... 

Bartlett, P. A. Hahne, W. F. Stereochemical Control of the Ynamine-Claisen Rearrangement J. Orjf. Chem. 1979, 44, 882-883. 

Scheme 7.23 illustrates the diastereoselectivities observed under various conditions in the synthesis of 2,3-dimethyl pent-5-enamides from ( )-2-buten-l-ol [11, 14, 25, 26, 47]. The anti isomer usually predominates with the exception of the thermal Ficini-Claisen variant (Scheme 7.23, Eq. 2) [18]. In this case, slow addition of the allylic alcohol to the ynamine at elevated temperatures resulted in a 1 2 mixture of aniv.syn products. This result can be explained by assuming that addition of the alcohol to the ketene iminium intermediate (cf 16, Scheme 7.9) occurs from the less hindered side and results in the preferential formation of the (E)-ketene N,0-acetal. This kinetic intermediate then undergoes rearrangement...

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