Z-Crotyldiisopinocampheylborane

Z)- and (E)-Crotyldiisopinocampheylboranes, Ipc2BCH2CH=CHCH, (1). Brown and Bhat1 have prepared the two (Z)-crotyldiisopinocampheylboranes [derived from (-)- and (+ )-pinene] by reaction of (Z)-crotylpotassium with the methoxy-diisopinocampheylboranes. The (E)-crotyldiisopinocampheylboranes are prepared in the same way from (E)-crotylpotassium. 

The reaction of the (Z)-crotyldiisopinocampheylborane derived from (+-)-a-pinene with aldehydes at -78 °C, followed by oxidative workup, furnishes the corresponding j yn-P-methylhomoallyl alcohols with 99% diastereoselectivity and 95% enantioselectiv-ity. Use of (Z)-crotyldiisopinocampheylborane derived from (-)-a-pinene also produces 5yn-alcohols with 99% diastereoselectivity but with opposite enantioselectivity, an example of reagent control. 

The synthesis of 108 began with homoallylic alcohol 114. Protection of the alcohol was followed by ozonolysis to give aldehyde 115. Reaction of the aldehyde with Z-crotyldiisopinocampheylborane, a chiral crotylborane, gave 116 in high yield with excellent diastereo- and enantioselectivity (see Calcimycin-13... 

The use of enantiopure allylic boranes in reactions with achiral aldehydes results not only in high diastereoselection, but also in high enantioselectionP Pure (Z)- and )-crotyldiisopinocampheylboranes can be prepared at low temperature from (Z)- or E) crotylpotassium and S-methoxydiisopinocampheylborane, respectively, after treatment of the resultant ate-complexes with BF3 OEt2. The R-methoxydiisopinocampheylboranes are prepared by reacting (-)-diisopinocampheylborane, derived from (+)-a-pinene, or (+)-diisopinocampheylborane, derived from (-)-a-pinene, with methanol. 

Crotyldiisopinocampheylborane. IpC2BCH2CH=CHCH3 (1). The (Z)-isomer is prepared by reaction of (Z)-crotylpotassium with methoxydiisopinocampheylbo-rane [obtained from (- )-a-pinene] to form an ate complex, which is converted into (Z)-l on reaction with BF3 etherate at -78 . The (E)-isomer is prepared similarly from (E)-crotylpotassium. Reaction of acetaldehyde with (Z)-l at - 78° results in (2S,3S)-3-methyl-4-pentene-2-ol (2) in 99% diastereoselectivity and 95% enantioselectivity (equation I). A similar reaction with (E)-l provides (2S,3R)-2. 

Allylation of aldehydes and ketones and related electrophiles using allyldialkylboranes continues to stimulate interest, especially within the context of asymmetric synthesis. All four possible diastereomeric 3-methylhomoallyl alcohols corresponding to (149) and (150) may be prepared in a highly selective manner using either Z- or E- crotyldiisopinocampheylboranes (147) and (148) and by controlling... 

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