On acetylation (OH group was changed to OCOCH3 group), the alpha j6 W. { q4 p& p
carbon will show a downfield shift while the belta carbon will show a" X/ f7 ?7 ~ ~" ? L# }* h
upfield shif. 8 A' h& t7 q) } ) G% c' H' x3 G9 x8 h) T, `
See the following article how to revise a structure reported before just using the above rule! 2 m1 R9 O0 Y1 S( m5 ~7 C2 a. E 1 Q" t) F$ M" b' E+ ~. QOne 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. * x# f/ N2 @/ d4 x; P * }& Y+ P+ E" n9 U3 p9 ~. G# WAnother compound with one OH group acetylated: it could be C-6 OAc, C-78 b0 Y5 l) q" u- ?5 s; R9 P
OH (compound I in the article) or could be C-6 OH, C-7OAc (compound II3 t7 G! k+ n9 P8 m- W" d7 p. e
in the article). The chemical shifts of the two carbons are 78.1 and! K* C8 ~$ k- Z* v
80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
( q; d9 _4 c7 h: h0 B; y7 m: Y , M9 I. p& a2 L4 R# u! i/ E
The auhors resovled the problem: ) ^- k k: u8 r0 P4 I7 w1 O 8 n0 R, D# f& SIf C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical
% Z3 L3 D7 \% @ W ^shifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased. J y: P. M' |' T1 b: ~5 y/ q+ E
That assupmtion violates the above rule. 0 [# ^/ A, \( H9 V2 T7 B; w ) C- X P/ X$ a. ?7 t, u. @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. ! S/ u* N5 x' S$ B
. ^6 K1 j) G/ f) Q0 e7 l9 i
C-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from
/ i" g/ o% q8 s5 a79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,$ T* ~2 M8 C8 ], s7 L
it was the C-7 with the OAC group (compound II), not the C-6 with the
/ i4 S/ f5 d5 o3 _3 N% y1 a4 g, uOAC group (compound I). 6 m) n: y0 _2 f% {; M7 w6 z . l& H8 [% y6 N- B8 W" b
Simple, logic structural revision published in 1981!
楼主: 上善若水
发表于 2009-5-28 16:27:12
[em79][em79]
