On acetylation (OH group was changed to OCOCH3 group), the alpha
: h' R" U5 Q2 i5 m* u/ A( gcarbon will show a downfield shift while the belta carbon will show a
/ Y) _8 C( c1 u6 @) C$ n2 V/ \/ Y9 f: ~# Qupfield shif. 3 u [# O8 @/ v$ Y/ ?) W) c8 P
5 _9 G* k n# W- s$ i% x3 @7 G/ i* o
See the following article how to revise a structure reported before just using the above rule! ' {1 G/ H6 I$ S
$ T4 R- s3 G) u* X6 pOne 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. & _: W4 Z2 @+ e- G6 ^( Z" d. ?/ q
6 ~* F; K8 \+ p$ D& n G
Another compound with one OH group acetylated: it could be C-6 OAc, C-7
6 `& S/ ]3 I5 J0 j- xOH (compound I in the article) or could be C-6 OH, C-7OAc (compound II6 T6 E0 d8 O9 {3 f1 U9 t
in the article). The chemical shifts of the two carbons are 78.1 and' k! O7 S" J) Y! ?) J7 g
80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
0 {9 R# `5 x8 P% W
- e9 i& ? A S
The auhors resovled the problem: + | x" a! S$ c( {, F" s
6 Z U: c8 }5 ZIf C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical
5 K; O6 H! y4 }- `4 [" ^, N1 ushifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.. q9 K" L A9 Z& r* j/ C' L, ^0 Q
That assupmtion violates the above rule. . e: o1 T# K* J9 B+ n; g0 y 6 ^1 _7 r4 E, [6 w eSo 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 o0 f' m, _8 m# A8 g
' w7 L2 u% A5 ~2 j' U* XC-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from
" U0 X! p% ^0 B; p* Z79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,$ c2 L9 ^0 |4 e! X5 a X
it was the C-7 with the OAC group (compound II), not the C-6 with the
9 t, W' {$ P3 z* WOAC group (compound I). 3 |7 S% D2 }5 r% X. P3 \" `" L , ]9 w0 `' n; t6 r8 B0 lSimple, 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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