On acetylation (OH group was changed to OCOCH3 group), the alpha* l! w% O1 S, W2 b- Y2 C
carbon will show a downfield shift while the belta carbon will show a
- o: ^& t [8 h. Q/ r6 M Tupfield shif. ' l1 J/ ?5 _ ?9 \/ l/ U9 I, Y) i ( l+ l4 c9 o9 B- ~% L2 A
See the following article how to revise a structure reported before just using the above rule! % [; L! m& D' ?; W. ?& Z m # b. G# T2 }. ] T/ E5 i
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. # [- M, ^' O5 e
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Another compound with one OH group acetylated: it could be C-6 OAc, C-7$ |, r8 Z. o4 w* E/ U- E* G7 n3 P
OH (compound I in the article) or could be C-6 OH, C-7OAc (compound II
& o2 U6 u. Y1 s; U( W2 E- k4 Qin the article). The chemical shifts of the two carbons are 78.1 and
7 J9 a' ~: T$ U+ a80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
1 Z& M7 V% x" ?+ `: b" _9 w
( G* ^+ ]; v4 I0 J4 l6 V
The auhors resovled the problem: $ m! U- B. h% O/ S) U# N ! Y" `8 | x( A
If C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical
. ~0 @$ A4 a+ w% M0 [9 o- p- [shifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.. b, Z! T3 J2 m/ s$ v
That assupmtion violates the above rule. 6 J# e' d& w3 i ^* |
4 B7 W8 u- O' Z* h5 O( E! QSo chemical shift of C-6 was not 80.5 but 78.5, chemical shift of C-7 was not 78.5 but 80.5. , a' m$ \/ W. T7 h+ S! k 3 N- o9 K/ P" i. a* f3 ]C-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from
6 E6 {: ]& ^# n F @1 v) D79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,% g! n" i) p% B7 @$ K; N
it was the C-7 with the OAC group (compound II), not the C-6 with the; Y0 N5 U- ^, d4 T. a$ F h
OAC group (compound I). 4 ^7 q0 n5 M+ E m* a8 B3 P
1 s6 _* s9 A- ISimple, logic structural revision published in 1981!
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/ b# e& K1 _# U+ U+ [1 c) b
楼主: 上善若水
发表于 2009-5-28 16:27:12
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