On acetylation (OH group was changed to OCOCH3 group), the alpha
+ W3 a n! O6 V( h" i; i: Jcarbon will show a downfield shift while the belta carbon will show a3 u7 L5 i( S& y0 b0 s
upfield shif. . q9 B1 ~* ?1 F& C6 s
- ^7 S7 N" m r5 f9 sSee the following article how to revise a structure reported before just using the above rule! # l; F8 I; a, r- b) T `
' Q. u( q- K- ?8 x$ A# E3 y7 sOne 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. % W. f! q1 a% x( K
' {7 ~* C5 A1 X( W( DAnother compound with one OH group acetylated: it could be C-6 OAc, C-7* v/ n( r' u6 w: U4 D0 ~) n
OH (compound I in the article) or could be C-6 OH, C-7OAc (compound II
- S. B* N: V2 F. A) f3 fin the article). The chemical shifts of the two carbons are 78.1 and) p8 L4 i1 I9 g! h
80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
, K4 r9 P# l1 P' U 3 f1 P! b7 x- o3 a) ^The auhors resovled the problem: : V8 ]3 d7 r& ^5 c, Z* l
. U, A! n' y# r6 G5 ~! LIf C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical
7 ]6 @$ e8 g- U, vshifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased. i, v* }* g" h8 L1 w
That assupmtion violates the above rule. * A% N& t) T B; a4 C2 q' j. n 7 u& N$ \$ A1 GSo chemical shift of C-6 was not 80.5 but 78.5, chemical shift of C-7 was not 78.5 but 80.5. 9 D% w( j8 N0 y( d$ `# j& |$ y
) R: c' G- J1 N% A. Q7 I1 K
C-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from5 X4 G! j: z) f3 w9 j c1 S) t
79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,! [# u6 [4 V* m, z# k( g( x
it was the C-7 with the OAC group (compound II), not the C-6 with the
. F0 K; q0 G1 U9 \+ G" sOAC group (compound I). - D0 M0 q) f0 ~% |
$ g& \6 h+ e/ @0 c1 U4 h8 xSimple, 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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