On acetylation (OH group was changed to OCOCH3 group), the alpha0 w3 s$ p1 ?( M! H. i# k8 y% u8 C
carbon will show a downfield shift while the belta carbon will show a* H% G7 _; M- `
upfield shif. ! ^% w, N/ w$ n7 h; q- [ 8 ]5 N# U% F" N, k8 t DSee the following article how to revise a structure reported before just using the above rule! ' _2 |5 n( Z& J" P% e" ~0 n ( T7 f+ V. j+ ?. NOne 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. % t2 B7 b E9 C; v0 u" l 1 N: h, ^; l1 Z/ G! k& l' U: z. C
Another compound with one OH group acetylated: it could be C-6 OAc, C-7
1 V5 m. C" S& {2 `4 eOH (compound I in the article) or could be C-6 OH, C-7OAc (compound II
: {! Z8 ]. d8 S5 Y( _" ^in the article). The chemical shifts of the two carbons are 78.1 and
4 L% f5 `' w" a* Z7 U: D i y80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
! l( |( u6 d- ^, C 7 @' V( [5 f, t3 W4 W" Q6 VThe auhors resovled the problem: " a7 s) ?, i: w9 d1 j / ^- h' j5 Z$ Y+ RIf C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical( e+ \- O) Y6 \# L
shifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.( r6 l8 x* S; p) O* s5 C0 m
That assupmtion violates the above rule. ! f! {. l+ o% w# T9 u % x, x* i' G5 D" Q; d
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. + _! Y& W" p3 ~; W9 ?+ e% x8 I 8 z6 C- o, u6 y# d, M$ |: L" m. j& [C-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from
5 n) g3 G' X1 F; M79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,# o6 l6 H; h4 L
it was the C-7 with the OAC group (compound II), not the C-6 with the
9 e( c- s+ Z( ~9 S, {/ QOAC group (compound I). ) [: u( m2 \+ Y7 A! y9 { ! I( w; u, V+ |& n% c
Simple, logic structural revision published in 1981!
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楼主: 上善若水
发表于 2009-5-28 16:27:12
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