E-crotyl boronate

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

Lactone synthesis through the reaction of 2-(alkoxycarbonyl)allyl metal species to carbonyl substrates holds a main position for the synthesis of the a-methylene derivatives. In 2(X)5, Hall et al. [30] obtained an interesting result in a TfOH-catalyzed reaction of a benzaldehyde derivative bearing electronically rich arene structure with (E)-crotyl boron reagent (E)-39 (Scheme 16). In this case, if the reaction proceeds via normal cyclic six-membered transition... 

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... 

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). 

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... 

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... 

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. "... 

Ramachandran and Pratihar [32] also reported interesting effects of Lewis acid and Bremsted acid toward the reaction with crotyl boron reagent 39. In the presence of 20mol% of In(OTf)3 or TfOH, the reaction of (E)-39 with aliphatic... 

Somfai and coworkers have developed a boron tribromide mediated [2,3]-sigmatropic rearrangement of allylic a-amino amides (59), which yields secondary amines (60) in appreciable yields (Equation 37) [37]. Here, the (E)-crotyl a-amino amide substrate exhibits excellent syn diastereoselectivity upon rearrangement. 

The use of chiral Br0nsted acids is illustrated in Eq. 93 as a method for catalyst-controlled double diastereoselective additions of pinacol allylic boronates. Aside from circumventing the need for a chiral boronate, these additions can lead to very good amplification of facial stereoselectivity. For example, compared to both non-catalyzed (room temperature, Eq. 90) and SnCU-catalyzed variants, the use of the matched diol-SnCU enantiomer at a low temperature leads to a significant improvement in the proportion of the desired anti-syn diastereomer in the crotylation of aldehyde 117 with pinacolate reagent (Z)-7 (Eq. 93). Moreover, unlike reagent (Z)-ll (Eq. 91) none of the other diastereomers arising from Z- to E-isomerization is observed. 

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... 

A definite improvement in the synthesis of A -methoxy aziridines was achieved by substituting boron trifluoride with trimethylsilyl triflate and diethyl ether with dichloromethane"9. In this way, the A -methoxy aziridines were obtained in good yields from a variety of linear and cyclic alkenes, e.g., 6-8. For comparison, the aziridine 8 was obtained in 50% yield by using boron trifluoride- diethyl ether complex in dichloromethane. Complex product mixtures were obtained with allyl and crotyl alcohols and with cyclohexenone. Further transformation of the A -methoxy aziridines into the N-H aziridines was possible using sodium/ ammonia reduction, e.g., 9. 

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. 

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