Preparahon of B-isoprenyldialkylboranes is achieved by adopting the Brandsma modification of the Schlosser procedure, namely metallation of isoprene with potassium 2,2,5,5tetramethylpiperidide followed by sequential treatment with B-methoxydialkylborane and boron trifluoride-etherate.
These reagents are used for the convenient isoprenylation of aldehydes. Reaction of isovaleraldehyde and g, p-dimethylacrolein with B-2'isoprenyldiisopinocampheylborane provides both ipsenol and ipsdienol, respectively in 65% yields and 96% ee.
The discovery of the hydroboration reaction has led to the creation of an array of versatile organoboranes providing a valuable synthetic tools for organic chemists. 2 Unlike many other organometallic reagents, boron reagents are both well behaved and easily controlled.
Chiral Allyl-and Crotylboration
A most important reaction that we have studied in detail over the past decade is allylboration.3a H. C. BROWP~ and R. S. RANDAD 1 2 3
Again, by preparing 3,3-dimethylallyldiisopinocampheylborane, 3 and condensing with aldehydes, we obtained 3,3-dimetbyl-1-alken-4-ols in >90% ee. 8 This provided a route to prepare "irregular" terpenes with nonhead-to-tail union of isoprene units. The reagent 3 was prepared very easily by direct hydmboration of 3-methyl-12-butadiene with diisopinccampheylborane.
We demonstrated the utility of this reaction for the first asymmetric synthesis of (-)-and (+)-artemesia alcohol, the acyclic monoterpene alcohol isolated from Arternisiu umuu L. and Arremisia herba-ah, respectively.89
We continued our success in various types of allylboration reactions with the synthesis of stereochemically stable allylic boranes such as B-2-cycloalken-1-yldiisopinocampheylborane, 4.10 We synthesized diastereomerically