On acetylation (OH group was changed to OCOCH3 group), the alpha$ h' u5 Z& p) |) e; |! J
carbon will show a downfield shift while the belta carbon will show a1 P( |, k5 f0 p3 O
upfield shif. ( y8 v. L9 v; }: M ! `! k3 |# B% _' Z; PSee the following article how to revise a structure reported before just using the above rule! ( l5 M# N8 v1 y( @# C1 W/ t
3 G2 \2 @3 A. Y' ~One 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. 3 u' U) x( l$ | i' f) B) l# L
- |/ \: L& w3 lAnother compound with one OH group acetylated: it could be C-6 OAc, C-7* Z% R- R& t% @# B N( U
OH (compound I in the article) or could be C-6 OH, C-7OAc (compound II
' ^! ^$ r$ R3 A$ |' ^in the article). The chemical shifts of the two carbons are 78.1 and
% F- c/ [4 ?3 ^) X1 f1 {8 c80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
; [ I2 `0 m1 v3 V" a$ O 5 J. N( L* y& P. |4 k5 d3 V+ U) U
The auhors resovled the problem: 1 l* f( f5 X7 d& C; t W4 m( ^) g8 o# E, Z4 JIf C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical- ], h, x+ N# Q* @5 G2 A' r- V
shifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.
. N* h V5 t( MThat assupmtion violates the above rule. 1 T/ G1 c- h7 _* P4 w5 V4 a: t$ b
+ {" B3 W" D: f W8 i) `- O9 ASo chemical shift of C-6 was not 80.5 but 78.5, chemical shift of C-7 was not 78.5 but 80.5. 8 t* H# \" Z1 n$ |1 ?0 B; s! ^7 m
* A1 Z) d9 {8 U' f2 d$ D
C-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from9 v- K: @7 ^& ?- p# I2 h0 L& H
79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,
* w* o' a! U: E! g' Sit was the C-7 with the OAC group (compound II), not the C-6 with the/ T/ n0 O- |5 [0 n0 n+ P S1 i
OAC group (compound I). V5 `9 @- W, b" l
% }6 U- U3 G6 \
Simple, logic structural revision published in 1981!
2 _7 ^/ r3 g& d6 a+ d5 `* K
. O4 `( ~8 h. Y* I/ T9 t E4 H" v- r
" t7 B E: [8 z* w+ r) ~
楼主: 上善若水
发表于 2009-5-28 16:27:12
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