Alkenes, homoallylic hydroboration

Asymmetric hydroboration 2171 of prochiral alkenes with monoisopinocampheyl-borane in the molar ratio of 1 1, followed by a second hydroboration of non-prochiral alkenes with the intermediate dialkylboranes, provides the chiral mixed trialkylbo-ranes. Treatment of these trialkylboranes with acetaldehyde results in the selective, facile elimination of the 3-pinanyl group, providing the corresponding chiral borinic acid esters with high enantiomeric purities. The reaction of these intermediates with base and dichloromethyl methyl ether provides the chiral ketones (Eq. 130)2l8>. A simple synthesis of secondary homoallylic alcohols with excellent enantiomeric purities via B-allyldiisopinocampheylborane has been also reported 219),... 

The successful completion of this domino reaction encouraged us to check some further hydroformylation-allylboration-hydroformylation sequences. For instance, the homoallylic alcohol 58 was converted to the ynol ether 59. To convert the latter to the alkenyl boronate (55%) 60 a zirconium catalyst (30) had to be chosen for the hydroboration. With rhodium catalysts chemoselectivity between the alkyne and the alkene moieties was... 

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