On acetylation (OH group was changed to OCOCH3 group), the alpha
" m% o" b3 l: h+ _, p. n: {: Ocarbon will show a downfield shift while the belta carbon will show a
8 s4 }0 I/ C( b2 P5 j! Eupfield shif. - M: U5 p3 _5 C8 U. w8 U' E) U ; g, \% Z& m7 o6 H/ z, }, R) rSee the following article how to revise a structure reported before just using the above rule! + x6 Y( ]4 T" T3 X5 R- I 1 W% J% K5 S9 r' g( j' F
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. , p1 O: d# J/ k8 k 8 ~1 p: |2 H2 B; B3 A) }6 GAnother compound with one OH group acetylated: it could be C-6 OAc, C-75 D! c) k, g/ }" N" E2 s8 @
OH (compound I in the article) or could be C-6 OH, C-7OAc (compound II
0 F6 z e. J4 u; a0 Z: \in the article). The chemical shifts of the two carbons are 78.1 and' P& b7 D" k; U) w
80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
7 }7 K+ C2 ~5 K7 ?
, E z! a9 E8 p
The auhors resovled the problem: 5 t0 P5 L( }5 p4 |7 Z
' y7 @& k# y2 D) QIf C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical$ k, d1 H& d/ @0 j+ X( k9 p
shifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.
: x; z$ W0 N' e" l3 |! _That assupmtion violates the above rule. / \ W5 l6 V9 Z, f, V6 O " T' q9 M5 f3 `
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. 0 I# K& P' y5 ?! T5 ]$ S) _
! I7 g6 @0 B) z2 w/ k8 A
C-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from
( X' r* j; R% ?) U# c79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,
3 X! V( w/ [* D0 P+ c- zit was the C-7 with the OAC group (compound II), not the C-6 with the
' z# n; P, [- v7 i, ]OAC group (compound I). 8 a& i6 T1 }! n% ?% |- l0 Z( Q. o. i6 d . U9 a4 I& f, J. n6 i( c3 mSimple, logic structural revision published in 1981!
# m! w1 K( p+ B
- S- \8 M I4 H) S4 ~ / o+ b: i, j y/ O1 ?! ]7 \) B
楼主: 上善若水
发表于 2009-5-28 16:27:12
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