Homoallylic alcohols Boron trifluoride etherate

The first total synthesis of ( )-lycoridine margetine (33), a member of the non-basic alkaloids which possess antimitotic activity has been achieved (Scheme 3). The homoallylic alcohol (26) served as a latent diene in a Diels-Alder reaction with ethyl acrylate to give a diastereomeric mixture of adducts which upon base equilibration and hydrolysis provided the tmns-acid (27). Modified Curtius reaction on (27) afforded the corresponding isocyanate which was cyclized to the lactam (28) in 89% yield by a new method using boron trifluoride etherate. Compound (28) was converted into an N-acetyl derivative which upon basic hydrolysis gave the acid (29). Treatment of (29) with NBS followed by reflux in pyridine solution gave the lactone... 

The allylation of aldehydes with allylstannanes is effected with Lewis acids such as boron trifluoride etherate or titanium(IV) chloride (eq (104)) [99], in which diastereo-selectivity is dependent on the catalyst employed the most striking feature is that. n77-homoallyl alcohols are stereoselectively produced using boron trifluoride etherate. Although this allylation occurs without a catalyst, high reaction temperature must be used. 

In the absence of Lewis acids, allylsilanes react only with very electrophilic ketones like hexafluo-roacetone, when they undergo ene reactions rather than electrophilic replacement of the silyl group. In the presence of Lewis acids, allylsilanes react with aldehydes and ketones with clean allylic transposition (Scheme 1) and the formation of homoallylic alcohols. The range of Lewis acids used is wide, but titanium tetrachloride and boron trifluoride etherate are the most common. Typically, reaction takes place somewhere between -78 °C and 0 C in dichloromethane solution, and a molar proportion of Lewis acid is used. 

Note that the aldehyde approaches the alkene from the direction anti to the silicon atom. Therefore, when a chiral allylsilane or allylstannane with a substituent in the a-position is used, chirality transfer takes place, to generate the homoallylic alcohol with essentially no loss in enantiomeric purity. For example, reaction of the aldehyde 157 with the chiral allylsilane 158, using boron trifluoride etherate as the catalyst, gave predominantly the syn product 159 (1.151). The absolute stereochemistry can be determined by using a model in which the hydrogen atom on the a-carbon of the allylsilane eclipses the alkene (the so-called inside hydrogen effect ) in order to minimize steric interactions (1.152). 

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