Z-crotyl boronates

The enantioselective addition of ally organometallics to carbonyls has become one of the workhorses of organic synthesis. Dennis Hall of the University of Alberta reports (J. Am. Chem. Soc. 125 10160, 2003) the scandium triflate catalysis chiral allylboronic acids become more effective tools. The best of these, the Hoffmann camphor derivative 2, adds to aldehydes under Sc(OTf), catalysis with excellent enantiomeric excess. The reaction works equally well for methallyl, and for the E and Z crotyl boronic acids. The crotyl derivatives react with the expected high diastereocontrol. A limitation to the boronate additions is that branched chain aldehydes give low yields. 

Treatment of 122 with (R,R)-tartrate crotyl-boronate (E.R.R)-W 1 provides the alcohol corresponding to 123 with 96% stereoselectivity. Benzylation of this alcohol yields 123 with 64% overall yield. The crude aldehyde intermediate obtained by ozonolysis of 123 is again treated with (Z,R,R)-111 (the second Roush reaction), and a 94 5 1 mixture of three diastereoisomers is produced, from which 124 can be isolated with 73% yield. A routine procedure completes the synthesis of compound 120, as shown in Scheme 3-44. Heating a toluene solution of 120 in a sealed tube at 145°C under argon for 7 hours provides the cyclization product 127. Subsequent debromination, deacylation, and Barton deoxygenation accomplishes the stereoselective synthesis of 121 (Scheme 3-44). 

IR,2S, 3R,4S )-2,3-0-l(Z)-2-Butenylboryl]-2-phenyl-l,7,7-trimethyIbor-nanediol [Preparation of the (Z )-Crotyl Camphordiol Boronate Reagent]. ... 

A number of optically active allylic boron compounds have been used, including380 B-allylbis(2-isocaranyl)borane (28),381 E- and Z-crotyl-(/ ,/ )-2,5-dimethylborolanes (29),382... 

The coupling occurred at the less-hindered terminal carbon giving thermally stable ( )-allyl boronates. Thus, the (.E)- and (Z)-crotyl chloride, and 3-chloro-l-butene, all afforded an (A)-crotyl boronate. The reaction with benzyl halides gave benzyl boronates.333... 

The six-member ed transition state for the reaction of an allylic borane or boron ate is very reminiscent of the cyclic transition state for the aldol reaction you met in Chapter 34. In this case the only change is to replace the oxygen of the enolate with a carbon to make the allyl nucleophile. The transition state for the aldol reaction was a chair and the reaction was stereospecific so that the geometry of the enolate determined the stereochemistry of the product aldol. The same is true in these reactions. -Crotyl boranes (or boronates) give anti homoallylic alcohols and Z-crotyl boranes (or boronates)... 

When there is a substituent on the allyl double bond, geometric isomers are possible and two new stereocenters are formed. The transition structures in Scheme 5.3 illustrate how the -crotyl boron compound affords racemic anti addition product and the Z-crotyl compound affords the syn product. For the E isomer, the... 

Figure 5.1 lists a number of auxiliaries for asymmetric allyl addition to aldehydes. Substituted allyl boron compounds have also been used in reactions with achiral aldehydes. Table 5.1 lists several examples of 2- and 3-substituted allyl boron compounds, and the products derived from their addition. Note that for the E- and Z-crotyl compounds, the enantioselectivity indicated is for the isomer illustrated. In some cases, there was more than one of the other three possible isomers found as well. 

Hoffmann has published full details of his diastereoselective synthesis of homoallylic alcohols from crotyl boronates (c/. 4, 149) and aldehydes (Scheme 24). In results consistent with the rationalization presented above, -boronates gave rhreo-alcohols and Z-boronates gave the erythro-diastereomer with virtually complete diastereoselectivity, i.e. threo erythro ratios in the products mirroring the E Z ratio in the starting boronates. A full account has also appeared of work by the same group on addition of chiral allylboronates such as (33) to aldehydes to produce optically active homoallylic alcohols (Scheme 25) cf. 3, 143), with approx. 70% e.e. in the case of saturated aldehydes and having the configumtion as illustrated. "... 

The additions of allyl-, crotyl-, and prenylborane or -boronate reagents to aldehydes are among the most widely studied, well developed, and powerful reactions in stereoselective synthesis. The additions not only display excellent levels of absolute induction in enantioselective synthesis, but also exhibit superb levels of reagent control in diastereoselective additions. The additions of ( )- or (Z)-crotyl pinacol boronates to aldehydes have been observed to give predominantly 1,2-anti- and 1,2-syn-substituted products, respectively (Scheme 5.3) [31, 50]. The inherent stereospecificity of the reaction is consistent with a closed, cyclic Zimmerman-Traxler transition state structure [51], In the accepted model, coordination of the aldehyde to the allylation reagent results in synergistic activation of both the electrophile and the nucleophile... 

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