On acetylation (OH group was changed to OCOCH3 group), the alpha
/ S' g& G1 o% J9 ucarbon will show a downfield shift while the belta carbon will show a+ q) b/ }# k4 e/ g- V3 D
upfield shif. $ P. Q, D8 [ \/ F- A2 U
0 K0 B8 }7 @- s2 G) f HSee the following article how to revise a structure reported before just using the above rule! , f2 ~9 o# Z% n, w" x) K
9 q& C. i- |7 n: Q$ D. Z
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! S) v$ |) F9 ~" Z4 s9 u! p+ O3 J
- x2 w8 K) v3 [/ wAnother compound with one OH group acetylated: it could be C-6 OAc, C-7
2 T& \6 Y, e# s7 P* r, l6 EOH (compound I in the article) or could be C-6 OH, C-7OAc (compound II
* S" t9 J+ H% F8 C; a# Sin the article). The chemical shifts of the two carbons are 78.1 and
8 ?9 R% _( f7 r2 l Y \' P% o80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
) B3 C3 G/ F. D! \ @7 D1 A7 W 4 J# m: r7 `9 Z5 i$ {$ m
The auhors resovled the problem: - [1 E# F$ y# M % U3 F/ u, ]6 y. ]If C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical7 E: E/ S2 I" T+ D# a8 a3 P, r9 E
shifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.% ?8 S. X# `: v6 @7 n3 J- o
That assupmtion violates the above rule. * g* \) Q/ o/ q9 k 3 x* |/ U/ F6 Q$ h+ O# RSo chemical shift of C-6 was not 80.5 but 78.5, chemical shift of C-7 was not 78.5 but 80.5. # ^4 `( }/ }; n' w1 q5 m# g
" ?. o! R" X8 L1 a: | _
C-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from1 X. N4 d. ]# Z" X, z6 Y- p
79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,
& Z/ M5 P: B p: m w7 o8 q/ R7 hit was the C-7 with the OAC group (compound II), not the C-6 with the
- N3 N6 z+ J. y2 q' i& UOAC group (compound I). $ p. Q( [) U3 k( | 0 I" T! q, F( f. v! @. D. I, u0 Y
Simple, logic structural revision published in 1981!
楼主: 上善若水
发表于 2009-5-28 16:27:12
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