On acetylation (OH group was changed to OCOCH3 group), the alpha
; Z+ S+ u2 T F, c, @) w ]carbon will show a downfield shift while the belta carbon will show a! W& u" R& T- p% L) q* U
upfield shif. p E, A* O+ p8 r3 Q- l& { $ s1 d- H( A! b$ U" d
See the following article how to revise a structure reported before just using the above rule! / |7 y8 b3 N4 b+ M" a2 l& M8 }
( |2 Y3 c1 R# A" \5 ^" WOne 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. " O3 d9 `; n2 U7 c. w) N, u & L5 B7 a$ o1 W* G% K7 j) t+ J* _Another compound with one OH group acetylated: it could be C-6 OAc, C-79 I8 q" Z8 F% A7 R; v, c. G0 N
OH (compound I in the article) or could be C-6 OH, C-7OAc (compound II, s D0 X2 S( a/ k
in the article). The chemical shifts of the two carbons are 78.1 and
! m- p" O8 C/ B- M) g80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
! D3 | v7 m. K ' @* {9 Z3 j( l+ G1 u6 PThe auhors resovled the problem: 2 F# Y9 |- [. e, c
1 o5 ?- T, U5 h" m+ {6 nIf C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical
j+ V) ~( ?2 z# _7 y( O; y2 z# oshifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.
) I+ }5 P$ b6 C8 P0 |4 KThat assupmtion violates the above rule. * z2 i# D1 s# j
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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. . b6 D8 W: r: m7 q1 P
! a9 O- h! f$ N) ]
C-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from
3 R6 w9 ^0 }. [7 I- S/ N5 X79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,
4 R5 e' A" ^$ _, q; v6 X2 |it was the C-7 with the OAC group (compound II), not the C-6 with the
* m$ U; s! ]5 Y) Y* iOAC group (compound I). % }8 r0 ~. f; e* g5 q) { g * O- B% m0 P5 c, }. j
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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