On acetylation (OH group was changed to OCOCH3 group), the alpha4 }' B9 E. h9 a6 V1 L- A- g4 v/ |& g
carbon will show a downfield shift while the belta carbon will show a
9 o" l& W) k3 m: ~8 _upfield shif. & I1 B! l3 p8 O* H8 C
& G/ b* U! C7 B' I5 Q, ?See the following article how to revise a structure reported before just using the above rule! 5 T3 y9 V X1 A* I) n9 |$ C7 y 6 V& K3 T1 v5 T# `& p) W; HOne compound with C-6 OH, C-7 OH (compound IV in the article): chemical shift values of C-6 79.3 ppm, C-7 76.2 ppm were known. 1 {8 c R* ?! ]7 Q7 G; G 7 H, E0 w( u' v% k1 p1 y
Another compound with one OH group acetylated: it could be C-6 OAc, C-7
+ k- N9 {( {3 C, x% OOH (compound I in the article) or could be C-6 OH, C-7OAc (compound II
& Y' D7 P% W |in the article). The chemical shifts of the two carbons are 78.1 and+ R$ X/ ]/ K# ^, M2 y$ W. b G1 ^
80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
' @. ^3 S1 H6 n, t 8 M$ s: M' E+ F3 x) I# zThe auhors resovled the problem: ! I- U& D$ ?* q6 V) u) h N 5 `5 y% M# D' V
If C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical) D1 E. ^. g8 h
shifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.* @8 T1 \# A0 p4 E) f- Y0 |
That assupmtion violates the above rule. * v, c: [+ E7 a9 z
( U: A g3 A+ k8 K8 bSo chemical shift of C-6 was not 80.5 but 78.5, chemical shift of C-7 was not 78.5 but 80.5. 5 M1 h( {, t6 E8 o- ~. W$ A
# l. {! \( i jC-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from: z7 ]% C. `+ s
79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,* ~+ l2 u5 I5 h
it was the C-7 with the OAC group (compound II), not the C-6 with the
' E. F( j1 U9 {2 dOAC group (compound I). $ }: B) @4 R# Z/ } # q0 _7 ?% g7 u! e) l/ g$ JSimple, logic structural revision published in 1981!
楼主: 上善若水
发表于 2009-5-28 16:27:12
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