On acetylation (OH group was changed to OCOCH3 group), the alpha: m+ e5 E% ~* d8 n8 }
carbon will show a downfield shift while the belta carbon will show a1 j7 S% \" X9 }7 y4 o9 p
upfield shif. ' j- D2 o U% W* I; z, z q' {
4 w* J) \, x0 E5 |* v; B. a$ vSee the following article how to revise a structure reported before just using the above rule! * Z2 m u' M2 o6 V1 @+ m7 ^* {6 z
% @2 G- y% o7 @& jOne 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. : L" u& Z( q7 r) Z H
6 |' v9 m( R4 K0 u' B" lAnother compound with one OH group acetylated: it could be C-6 OAc, C-73 C, p0 s# |- g5 B* W
OH (compound I in the article) or could be C-6 OH, C-7OAc (compound II/ }9 ^9 u1 B# E) Y% l$ l+ m
in the article). The chemical shifts of the two carbons are 78.1 and+ X0 _7 N3 o+ Z. O) k
80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
" E# }; T! n5 T, q
! @4 Z$ s" l1 x z7 [The auhors resovled the problem: g3 H- Y7 x) {; @! S/ b ; @9 x/ S6 r* J. r- EIf C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical
: U" a1 R4 O4 _# I# ^9 x3 x6 Ushifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.
* k7 p; {0 u' N2 j. }That assupmtion violates the above rule. 3 E1 l' j8 i+ ]% o, L8 m
6 }" a6 ^- t1 G& M0 A& x
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. - b4 G! F, V% t & {2 F; w' j3 @! B. b
C-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from E: n o2 d5 E3 U+ q+ a
79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,
: m w* d9 I5 Y! D! W9 ^( Yit was the C-7 with the OAC group (compound II), not the C-6 with the
+ R* b* z; a o1 ^; wOAC group (compound I). ' y5 w5 C, V7 W/ O & f, t8 v g8 k7 b+ _Simple, logic structural revision published in 1981!
1 ` Y3 u5 h: g5 L6 u
: f* z' u* j% ^3 {/ e2 i/ z' ~
0 [5 I; d- {0 e. o' H0 H
楼主: 上善若水
发表于 2009-5-28 16:27:12
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