On acetylation (OH group was changed to OCOCH3 group), the alpha& S }0 }2 @1 p$ r$ C
carbon will show a downfield shift while the belta carbon will show a( f5 s ~! r. z$ G ]0 P u
upfield shif. . r8 X6 ^1 j+ n4 r$ w. e) m4 W
0 F, H0 t. p2 {" t& o2 n& K% j
See the following article how to revise a structure reported before just using the above rule! 8 L2 G* y8 |" _5 o5 f$ r ; _7 g2 T; ~) b- T8 L9 [; IOne 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. 4 F G! ?! s& \+ _( |
: m3 D$ h/ X3 `8 h; c
Another compound with one OH group acetylated: it could be C-6 OAc, C-7; N: b: j( N) n) {5 I* F
OH (compound I in the article) or could be C-6 OH, C-7OAc (compound II$ v- k6 y, ?8 N5 i d
in the article). The chemical shifts of the two carbons are 78.1 and
2 W! U2 D8 ^" K- m7 |80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
6 H, R, I8 b3 H , v0 X% Y. h, Q8 cThe auhors resovled the problem: , e3 U9 e, `1 ]. a( I4 h. h) u 9 q6 `6 _2 Y( p% q8 M0 g6 U7 w" g" FIf C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical4 R" Q( E- Y g. ~, d! W$ @; y* p$ F7 M
shifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.
: H# i0 v8 p8 X6 K+ GThat assupmtion violates the above rule. - N0 k& j! h/ Z* n
9 ^* e" V+ y3 X, ?4 l
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. % } X2 B9 _+ L* I 3 q6 l& q6 \- O G
C-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from4 ^/ Z s7 ]. S& T& D. u, P
79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,
' @" x) k+ H; W+ {9 ~( Dit was the C-7 with the OAC group (compound II), not the C-6 with the
1 O- h% s, _; c1 j! a! G# K3 h- q) cOAC group (compound I). ( w e9 f2 |. j1 B% d
6 i, t# Q0 ~# }5 G; aSimple, 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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