On acetylation (OH group was changed to OCOCH3 group), the alpha) v9 r/ I* y- c9 w
carbon will show a downfield shift while the belta carbon will show a
5 T* P) \- k3 [/ `4 Qupfield shif. k1 ?4 n5 [" Q. D9 s
' p6 R7 R) ?$ |4 C/ p9 FSee the following article how to revise a structure reported before just using the above rule! " d9 j- r# F) O1 ~& Y) q
9 J/ |0 E9 e( N D/ f2 o) COne 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. 1 d! u) }' _9 y; }9 R5 b; v5 H : A7 } P$ X3 x) c. @
Another compound with one OH group acetylated: it could be C-6 OAc, C-7
- [) [% q$ a) m m( F; L/ ^OH (compound I in the article) or could be C-6 OH, C-7OAc (compound II' n( J$ I' U* |* X8 Y0 P
in the article). The chemical shifts of the two carbons are 78.1 and2 W$ V! |" q0 J2 O
80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
5 ^' X& T. w9 P + B+ ^' x7 s0 w6 q0 m3 ^8 d0 n
The auhors resovled the problem: ' D3 T d5 F7 A. i
+ E% j) q& P, r0 y( F9 U! nIf C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical, _7 l% Y5 O+ o* S3 w
shifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.7 ?" l4 e% @" ^! O. ^1 h7 B) w
That assupmtion violates the above rule. , j: i$ J* p0 @& X8 j" d9 E2 J
; l4 ^% o* U* x: r4 SSo chemical shift of C-6 was not 80.5 but 78.5, chemical shift of C-7 was not 78.5 but 80.5. + v3 T) K; R; _0 n
) s& W3 i; y( }4 r, o. g2 t5 iC-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from
( I4 N; w% w& P79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,
7 o( l( b9 ?; C0 V7 r! i# U5 Z6 bit was the C-7 with the OAC group (compound II), not the C-6 with the" t3 [# w4 c% k% w
OAC group (compound I). ) a+ O4 a2 b# e4 C5 F% \5 P8 G M( K
0 f: H$ y& D+ @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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