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ZnCl2, which results in a Zn-chelate 49 with fixed Z-geometry (Scheme 2.6). Rearrangement of
this intermediate via a chair-like transition state 49A, where the R 2 substituent is in a
pseudoeqatorial position, leads to the major allene diastereomer syn-50. On the contrary,
rearrangement via transition state 49B where the R 2 substituent is in a pseudoaxial position,
affords the minor diastereomer anti-50. The syn/anti notation for the allene products was
introduced by Hoppe in analogy of the aldol product terminology. 63
Scheme 2.6 Diastereocontrol in the ester-enolate Claisen rearrangement.
R 3
HO 2C
•
syn-50
"major"
R2 H
R
NHP
1
O
R2 O
H
R 1
NP
Zn
R 3
R 3
PHN
R 1
O
48
O
R 2
LDA
ZnCl 2
R2 O Zn
NP
O
H
R 1
49A 49B
syn-50 : anti-50 > 93 : 7
R 3
R 3
HO 2C
•
R
NHP
1
anti-50
"minor"
Notably, the reaction conditions were compatible with either carbamate (Boc, Cbz) or
sulfonamide (tosyl) protecting groups. All examples in this initial report by Kazmaier involved
terminally substituted alkynes in the propargylic esters (R 3 = alkyl, Table 2.2). Consequently, all
of the prepared allenes contained an alkyl group at the proximal allenic position (i.e.,
trisubstituted allenes).
21
R
H
2