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isomerically pure B-(E)- and B-(Z)-crotyldiisopinocampheylboranes (Scheme 13). 14 The reaction of these derivatives with aldehydes achieved the synthesis of the four possible isomers of β-methylhomoallylic alcohols with remarkable enantiomeric and diastereomeric efficiencies. It is important, however, to maintain the reaction temperature below -45 o C to avoid scrambling the crotylboranes. Since the reagent controls the diastereoselectivity in reactions with chiral aldehydes, it is possible to prepare all eight diastereomeric homoallylic alcohols at will by the appropriate choice of reagent and chiral aldehyde (Scheme 14). 50 5.8. B-2’-Isoprenyldiisopinocampheylborane The success with crotylpotassium led Brown and Randad to the preparation of B- 2’-isoprenyldiisopinocampheylborane from 30 VOL. 35, NO. 1 • 2002 Scheme 12. Asymmetric Synthesis of Vicinal anti Amino Alcohols. Scheme 13. Preparation of Crotyldiisopinocampheylboranes. Scheme 14. Crotylboration of α-Substituted Chiral Aldehydes. isoprenylpotassium and B-methoxydiisopinocampheylborane. Condensation of this reagent with aldehydes provided isoprenylated chiral alcohols, including the pheromones of the bark beetle Ips paraconfusus Lanier, ipsenol and ipsdienol (Scheme 15). 51 5.9. Higher Crotylboranes A series of higher crotylboranes were synthesized by the hydroboration of appropriate allenes with Ipc2BH. The hydroboration produced essentially pure E crotylboranes, which provided the anti alcohols in high de’s and ee’s (Scheme 16). 23 5.10. γ-Chloroallyldiisopinocampheylborane Oehlschlager and coworkers prepared a series of chlorohydrins in very high de’s and ee’s, by utilizing an α-pinene-based chloroallylborane. The chlorohydrins were readily converted into chiral vinyl epoxides (Scheme 17). 52 5.11. B-2-Cyclohexen-1yldiisopinocampheylborane Brown and coworkers reported the synthesis of the title compound by hydroboration of 1,3-cyclohexadiene. “Allyl”boration using this reagent provided 1-(2-cyclohexenyl)-1-alkanols in high de’s and ee’s (Scheme 18). 53 This procedure was successfully applied to the synthesis of the C2-symmetric chiral auxiliary, dicyclohexyl- 1,2-ethanediol, 30 via reaction with glyoxal and hydrogenation (Scheme 19). 54 5.12. Other Chiral Dialkylallylboranes Brown and coworkers tested several other terpenes as chiral auxiliaries for allylboration (Figure 6). 55 Of these, the reagents prepared
from 2- and 3-carene turned out to be superior. 55,56 Other known chiral dialkylallylborating agents derived from synthetic chiral auxiliaries are also shown in Figure 6. 57 6. Applications of Allyl- and Crotylboration Asymmetric “allyl”boration is a very useful reaction for the synthesis of complex natural products and biologically active molecules. Both tartrate- and pinane-derived Scheme 15. Asymmetric Synthesis of Ipsenol and Ipsdienol. Scheme 16. Higher Crotylboration. Scheme 17. Asymmetric Synthesis of Vinyl Epoxides via Chloroallylboration. Scheme 18. Preparation of B-2-Cyclohexen-1-yldiisopinocampheylborane. Scheme 19. Synthesis of the Chiral Auxiliary Dicyclohexyl-1,2-ethanediol. reagents have been widely exploited in synthesis. Schemes 20–23 depict representative applications of pinane-derived reagents as reported in the literature. 58-62 6.1. Allylboration–Ring-Closing- Metathesis Reactions for the Synthesis of α-Pyrones Optically pure α-pyrones (5,6-dihydro- 2H-pyran-2-ones) are structural features in several biologically active natural products. 63 Our protocol for the synthesis of α-pyrones involves the esterification of homoallylic alcohols with acryloyl chloride, followed by ring-closing metathesis (RCM) 64 using Grubbs’s ruthenium catalyst. 65 Representative examples of our approach are shown in Scheme 24. 66 Similar approaches were utilized for the synthesis of argentilactone (Scheme 25), 67 tarchonanthuslactone (Scheme 26), 68 and umuravumbolide (Scheme 27). 69 VOL. 35, NO. 1 • 2002 31
Page 1 and 2: DEDICATED TO PROFESSOR H. C. BROWN
Page 3 and 4: Aldrich Chemical Co., Inc. 1001 Wes
Page 5 and 6: Boron-Based Reducing Agents for the
Page 7 and 8: Figure 3. Stoichiometric Boron-Base
Page 9 and 10: orane reductions 29,30 of 1,4-diphe
Page 11 and 12: α-Hydroxy Acetals R Reduction Cond
Page 13 and 14: and 5 provided moderate ee’s in t
Page 15 and 16: Other reductions using 14, 64c,d 16
Page 17 and 18: Masuda, H.; Ito, K.; Hirao, A.; Nak
Page 19 and 20: The synthesis of biaryl compounds v
Page 21 and 22: 52,396-8 [87199-18-6] 3-Hydroxyphen
Page 23 and 24: Josiphos is a family of chiral phos
Page 25 and 26: Equipment for Air-Sensitive Chemist
Page 27 and 28: Equipment Equipment for Air-Sensiti
Page 29 and 30: Equipment for Air-Sensitive Chemist
Page 31 and 32: Glassware for Air-Sensitive Chemist
Page 33 and 34: Pinane-Based Versatile “Allyl”b
Page 35 and 36: B-O bond. 15 The mechanism involves
Page 37 and 38: Scheme 4. Tartrate Ester Auxiliary
Page 39: eagent with aldehydes at low temper
Page 43 and 44: Scheme 22. Application of Crotylbor
Page 45: Abstr. 1968, 68, 29743j. (11) Refer
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Page 52 and 53: These distillation heads are design
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