On acetylation (OH group was changed to OCOCH3 group), the alpha8 X1 {3 \. Z. z6 C7 @9 I
carbon will show a downfield shift while the belta carbon will show a
9 _" c( o4 X; X3 Z. N jupfield shif. : p+ g6 ^# \1 y) E2 V6 D' c
" r, _# t/ Z4 ~* p6 T( qSee the following article how to revise a structure reported before just using the above rule! f8 v+ w, d0 J & H7 c' p; i5 p2 Y$ b. w; vOne 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. % g2 Z' c+ s' L- L, v* ~0 Q1 D
; e8 v1 v: Y' `' u" a* [7 q1 CAnother compound with one OH group acetylated: it could be C-6 OAc, C-7
4 d: \/ u% V: l: S, ~; o& UOH (compound I in the article) or could be C-6 OH, C-7OAc (compound II& u9 _: q: z, d O' r! w, r
in the article). The chemical shifts of the two carbons are 78.1 and
8 e: m$ y, Z% V, E80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
. B: }% F& \' w" P / R! Z! ^4 T( Y" c
The auhors resovled the problem: ( Q* p! [3 {$ x8 o/ u* s 6 O7 {8 n; w- P( tIf C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical+ w9 R9 t; k) i( y7 X9 A
shifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.: A$ S" ]2 Z1 U# Y7 I+ ~7 ?! m Z1 j
That assupmtion violates the above rule. ' _; q. W; u" a: V" g3 J7 u
, X& ` }: _' T# ^& ~& R
So chemical shift of C-6 was not 80.5 but 78.5, chemical shift of C-7 was not 78.5 but 80.5. ) u) B' ^$ o) w5 K5 Q( H, N ; [/ z. `* E. d K7 ]/ AC-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from: _8 o+ T6 y$ d
79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,( L" O- ]5 m* _9 H$ |; d
it was the C-7 with the OAC group (compound II), not the C-6 with the( _& u5 | a r, T
OAC group (compound I). 0 ^8 q: l+ k$ P5 [: R: I" h 4 \5 q; `. |8 n( {2 o( a
Simple, 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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