Ynamines with allylic alcohols

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). 

Bartlett and Hahne have achieved selective synthesis of the diastereomeric amides by the reaction of allylic alcohols and ynamine. When the reaction was carried out at ambient temperature in the presence of BF3, the ketene Af,C -acetals have the thermodynamically favored Z-configuration. If the reaction was carried out by adding the alcohols slowly to a refluxing solution of an ynamine in xylene, the rearrangement proceeded via formation of a kinetically controlled intermediate with -configuration. Thus alcohol 110 reacted with ynamine 111 using BF3 at 25 °C to give 114 and 115 in the ratio of 1 10. The product ratio was reversed (114/115 = 10 1) when the reaction was conducted under eonditions of kinetic control (Scheme 9). 

Early extensive accounts of the 4v participation of a,/)-unsaturated carbonyl compounds in [4 + 2] cycloadditions detailed their reactions with electron-deficient dienophiles including a,/3-unsaturated nitriles, aldehydes, and ketones simple unactivated olefins including allylic alcohols and electron-rich dienophiles including enol ethers, enamines, vinyl carbamates, and vinyl ureas.23-25 31-33 Subsequent efforts have recognized the preferential participation of simple a,/3-unsaturated carbonyl compounds (a,/3-unsaturated aldehydes > ketones > esters) in inverse electron demand [4 + 2] cycloadditions and have further explored their [4 + 2]-cycloaddition reactions with enol ethers,34-48 acetylenic ethers,48 49 ke-tene acetals,36-50 enamines,4151-60-66 ynamines,61-63 ketene aminals,66 and selected simple olefins64-65 (Scheme 7-1). Additional examples may be found in Table 7-1. 

The previously unknown (+ )-(lS,2S,4R)-isodihydrocarveol (157) has been made from (+ )-limonene epoxide (158) as a component of a mixture of isomers, either with lithium in ethylamine or with the stoicheiometric amount of lithium aluminium hydride. Dihydrocarveol (159) has been synthesized from 4-acetyl-1-methylcyclohexene by conventional means.A method that is said to convert allyl alcohols into the corresponding chlorides without allyl rearrangement has been applied to carveol. The chloride was indeed obtained, but since the rotations of the compounds were not recorded it is unfortunately impossible to draw any conclusions about rearrangement. An ingenious synthesis of pure stereoisomers of carvomenthone-9-carboxylic acids involves a [2 -I- 2]-type cycloaddition of an ynamine to 2-methylcyclohex-5-enone (160). This leads... 

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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