On acetylation (OH group was changed to OCOCH3 group), the alpha
$ T3 T- y$ o4 P+ m" lcarbon will show a downfield shift while the belta carbon will show a1 C9 H# p4 j B3 g" Z$ T- ^
upfield shif. " |+ v- x3 f0 U
6 A. o: |7 D! d' S. \* g& a0 P* ASee the following article how to revise a structure reported before just using the above rule! 4 B1 C0 D0 T% }; @; [
; v O) x: u# x$ y& j0 ~
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. " K; W; d) M* n' b6 V: V, ^ - i* @" y# ?* L" k4 M2 b0 n! M# e
Another compound with one OH group acetylated: it could be C-6 OAc, C-7
K2 H, H m5 a" d) h( C; f tOH (compound I in the article) or could be C-6 OH, C-7OAc (compound II5 O5 n/ J( w& Q$ I5 c% s; S9 v
in the article). The chemical shifts of the two carbons are 78.1 and
; [- \, X" T+ u/ z80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
( x- u9 `, ? c3 U) F6 |0 d) o' i
0 d4 K2 r) }9 ~- ~5 r' c9 yThe auhors resovled the problem: # ^$ l# I0 @( D+ |! F6 I. p 8 U( j# [9 O0 @( O
If C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical3 J' k9 k* b- r' k& W) M
shifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.
0 x, \0 A# g x" }That assupmtion violates the above rule. . r# E, r# F1 c c7 J% N
- U- T' C/ _2 s2 Q% L$ Z% n" {7 R4 CSo 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 I7 B5 Q9 N" C5 e( G8 d. o 9 K, b: T% D8 G
C-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from: ]. `4 w* q3 H( z7 e7 @8 i! D* @
79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,2 |9 f! `8 B/ j' x. X' `
it was the C-7 with the OAC group (compound II), not the C-6 with the
! k4 R4 e5 h, \( @) u; W. EOAC group (compound I). - |7 X( V3 d8 L/ n I- L * O- _3 R) h& V0 f3 J4 ZSimple, logic structural revision published in 1981!
楼主: 上善若水
发表于 2009-5-28 16:27:12
[em79][em79]
