[3,3]-sigmatropic Processes. [2,3]-sigmatropic Processes. Ene Reactions. Generalized Sigmatropic Processes X,Y=C, N, O, S X,Y=C, N, O, S
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1 Generalized igmatropic Processes [3,3]-sigmatropic Processes 1 3,=C,,, 1 3 3,=C,,, 3 [2,3]-sigmatropic Processes 1 3,=C,,, 1 3 Ene eactions
2 Cope earrangement [3,3]- igmatropic earrangements Transition states: (+5.3 kcal/mol) disfavored ationalize: chair boat trans-trans 90% trans-cis 97% Tetrahedron, 1962, 67 ing train can be employed to drive the Cope process: Chem. Commun, 1973, 319 Annalen, 1958, 1.
3 Position of Equilibrium dictated by ring strain issues: JC, 1973, owever, Tautomerism can shift the equilibrium: Tet. Let. 1970, % Anion-Accelerated Cope earraangements: k 1 k 2 /k 1 = k C 3 C 3 C 3 K TF 66 C 3 C C 3 C 3 3 C
4 Examples: K! JAC, 1972, 4779 K, TF JC, 1981, K, TF JC 1982, 2268 K(TM) 2 JAC, 1988, 879.
5 The Claisen earrangement unsaturated carbonyl is thermodynamically favored: process driven by the formation of the more stable C= C 3 C C 12 h C Alkene tereochemistry: (E) (Z) E:Z 90:10 >99:1 >99:1 2 >98:2 Tet, Let.1969, 3243 JAC, 1973, 553 JAC, 1970, 741
6 Ireland Enolate Claisen A low temperature, mild variant of the Claisen process: TM TM LDA C C TMCl Enolates that do not fragment may be used without silyl trapping to afford the carboxylic acid products. Enolate rearrangements occur at 25 C. tereochemistry: 1 LDA TBCl E TB TB TB 1 94:6 E:Z LDA TBCl DMPU Z TB TB JC, 1991, 650. TB 1 7:93 E:Z Chelating ubstituents favor Z-Enolates: 1 PMB LMD/ TMCl 1 Li PMB 1 TM PMB 1 TM PMB TL 1993, 1103.
7 Johnson rthoester Claisen + cat. + 3 C 2 C C 3 C() 3 cat + 2 C 92% (E:Z = 98:2) JAC, 1970, 741 Eschenmoser Claisen + 2 xylene, 150 C 3 C 2 2 C 2 2 C 2 C 2 =C()( 2 ) 2 C 2 elv. Chim Acta, 1964, xylene, 140 C 2 2 E:Z= 99:1 2 xylene, 150 C
8 Boat Transition tates are more accessible in Claisen than in Cope earrangements TB boat preferred TB chair-destabilized boat favored TB owever, LDA, TBCl TB 1 1 TB chair-favored boat-disfavored (E) (Z) JAC, 1991, 3572 JC, 1981, C C 3 2 C C 3
9 Lewis Acid Catalysis of the Claisen earrangement LA LA LA Lewis Acids: 2 AlCl JC, 1970, 3166 TiCl 4 : Chem Lett. 1975, 1041 () 2 Al JAC, 1988, 7922; TL, 1989, 1265; JAC, 1990, 316 LiCl 4, Tet, 1993, 6025 disfavored =ibu allyl + LA Al 19 7 favored Al JAC, 1990, 316
10 Enantioselective Claisen earrangements i(tbu)ph 2 Ph Ph -1 Al Ar 2 B 2 Ar Br i(tbu)ph 2 amamoto, JAC, 1990, 7791 Corey, JAC, 1991, L 2 BBr ipr 2 BL 2 i 3 i 3 =Ph, 88%ee =C %ee L 2 BBr 3 BL 2 97%ee 96%ee
11 The Aza-Cope earrangement Ph 2 C Ph Ph Aza-Cope Mannich Process C 3 C C 3 C 3 3 ' C ' ' Mannich ' enol cyclization JAC, 1988, 4329 JAC, 1988, C C Ag 3, 3 C C 3 yield 78% 64% Ph 76% 37%
12 [2,3]-Wittig eaction C Base C C - TL, 1974, 2077 BuLi Chem. Commun, 1972, 757 Ph PPh 3 Ph Ph - Acc. Chem. es. 1974, C Bu 3 nc 2 I base 3 C Z T 3 C no 1,3-diaxial interactions nbu 3 vs. 3 C BuLi E T A-1,3-strain 3 C C 3 one isomer, Z - JAC, 1978, 1927 when A-1,3 strain is removed, E/Z selectivity is reduced.
13 Applications of the 2,3-Wittig in ynthesis BuLi JAC, 1975, 6878 JC, 1978, 1185 TL, 1978, C Ph C 2 JC, 1962, C C 2 C 3 TMTf 3 3 C C 3 C C C 3 C 2 Chem Lett. 1990, : 19
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