On acetylation (OH group was changed to OCOCH3 group), the alpha
& y( E+ F' ?' j! scarbon will show a downfield shift while the belta carbon will show a; D) y) S' G# `# x3 U
upfield shif. 8 L- l0 g# N% c/ i1 I. ?$ t ) N; G; J) [2 _' c1 A Q4 f( p
See the following article how to revise a structure reported before just using the above rule! * R* Z% c6 }) X* j 1 h7 r) U: a& w: d/ 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. ) D2 ~- z$ ~' R- ^0 ?& t 2 e# F: M9 |: z% E ^! j7 p/ m: a
Another compound with one OH group acetylated: it could be C-6 OAc, C-7) I! ?" F# T9 P* b0 g
OH (compound I in the article) or could be C-6 OH, C-7OAc (compound II
0 A/ Z/ g. U- |in the article). The chemical shifts of the two carbons are 78.1 and3 E+ c+ L) ^& N6 M3 A" F
80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
, U% a, l, t3 m9 K* a; Z
- K% n! g }6 O" ?# O9 n9 m0 L% _4 E: n
The auhors resovled the problem: 3 e# _( h' S. U
% x5 |' R( y% ?" h7 {0 ]( J
If C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical( `$ g/ Z9 }& G6 ?% n
shifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.
6 q( m2 l! Z% }2 X5 {That assupmtion violates the above rule. $ y; R' N0 {+ Z; \9 ~7 a! ?; i$ [
7 C1 |& O, m4 f
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. - m7 {6 w% q- Z, ?2 N
% b' M$ O2 _/ r9 o( {
C-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from
8 I2 I* K- \! e8 |& D79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,) O+ |1 S4 p' z9 M Y
it was the C-7 with the OAC group (compound II), not the C-6 with the7 d4 N) k( f- J1 k2 f- z
OAC group (compound I). 4 ?* P# A5 n F: R8 u0 {& s
$ q9 l* f% P. u4 O1 T
Simple, logic structural revision published in 1981!