Crotyl boronate

Homoallyl alcohol (3) Metalation of (E) butene (1 05 equiv) with n BuLI (t equiv) and KOtBu (1 equiv) in THF at SO C for 15 mm followed by treatment of (E)-crotyl potassum salt with B(OiPr)3 at 79°C gave after quenching with 1 N HCI and extraction with EtjO containing 1 equiv of diisopropyl tartarate. the crotyl boronate 2 A solution of decanall (156 mg 1 mmol) was added to a toluene solution of 2 (1 1 15 equiv) (0 2 M) at 78 C containing 4A molecular sieves (15-20 mg/L) After 3 h at -78°1 N NaOH was added, followed by extraction and chromatography to afford 208 mg of 3 (90%), anti syn 99 1... 

Over the past two decades, chiral allyl- and crotyl-boron reagents have proved to be extremely valuable in the context of acyclic stereoselection. The development of superior allyl-boron reagents, which can give enantio- and diastereoselectivities approaching 100%, has become both challenging and desirable.68... 

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). 

With this method, that is, the reaction of tartaric acid ester-modified crotyl-boronates with chiral 2-methyl aldehydes, the C-19 to C-29 fragment of rifa-mycin has been constructed similarly.76... 

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. 

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 low temperature is a testament to the reactivity of the crotyl boronates and also helps minimize any isomerization of the reagents while maximizing the effect of the energy differences between the favoured and disfavoured transition states. 

The rate of interconversion of crotyl boron reagents varies with the nature of the R groups on boron crotyldialkylborane (crotyl-BRj) > crotylalkylborinate (crotyl-BR(OR) > crotylboronate (crotyl-B(OR)2). 

Stevastelins are depsipeptides exhibiting immunosuppressant activity. The first total synthesis of stevastelin B was described by Y. Yamamoto and co-workers. To construct four consecutive stereocenters, the Evans aldol reaction and the Roush asymmetric allylation were utilized. In the allylation step, the authors used (S,S)-diisopropyltartrate-derived ( )-crotyl boronate. The anti homoallylic alcohol product formed as the only diastereomer. 

Figure 11-6. Transition states of (E)-crotyl-boronate with a-methyl chiral aldehydes.

Excellent double diastereoselection has also been realized in the reactions of (151) and chiral crotyl-boron reagents (Table 7). Interestingly, the best selectivity for diastereomers (153) and (156) is obtained by using the tartrate crotylboronates (S,S)-(18) and () , )-(19), respectively (entries 2 and 3), while Masamune s 2,5-dimethylborolane reagents (HJl)-(221) and (5,5)-(222) provide the greatest selectivity for diastereomers (152) and (157 entries 7 and 10). Comparative data for the diastereoselectivity ol tained with the achiral crotylteronates (1) and (2) appear in the last two entries of Table 7. 

Table 20 Syn-Anti Selectivity in the Reactions of Crotyl Boronates (87) and (88) with Oximes (89 Equation 20) ...

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... 

Scheme 5.3. (a) Stereospecificity (within experimental error) of crotyl borane additions to aldehydes, R = Me, Et, i-Pr, Ph [26]. (b) Transition structures for stereospecific addition of crotyl boron compounds to aldehydes. 

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