On acetylation (OH group was changed to OCOCH3 group), the alpha
' r+ L( Y8 M: g, q, qcarbon will show a downfield shift while the belta carbon will show a, f/ F* s2 V0 Z6 |
upfield shif. 2 J/ b u# W; A2 Q% J0 u8 t1 b; G# h 4 s) x C7 P4 i/ R$ g6 ]See the following article how to revise a structure reported before just using the above rule! 2 t$ C/ ]0 Y1 R" b5 u$ R( I0 J% d- e
! G; |& L% d" s" s" H
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. ! j. e; T L4 J7 N # d+ l/ D3 `) Y0 c8 K/ |Another compound with one OH group acetylated: it could be C-6 OAc, C-7: [1 Y {; { @5 R- _# B
OH (compound I in the article) or could be C-6 OH, C-7OAc (compound II
3 E5 Z. ?/ x9 O& sin the article). The chemical shifts of the two carbons are 78.1 and
3 }( a8 V2 D# S$ e! c3 w80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
3 @6 H6 Z* _0 s4 O6 I ; d& S( U c9 Q) z- e# _The auhors resovled the problem: % W2 Z, r' V# \7 ]0 w; }& D 6 b6 `* i/ ^1 V! x5 w3 X2 \" Y- O
If C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical
. N" e6 x3 L( I. C9 O: Jshifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.' x- R* r: o- C: l7 D: ?: M
That assupmtion violates the above rule. : Y# l) j: q% h8 J0 G
. T. ~# t3 w" b& f: R8 PSo chemical shift of C-6 was not 80.5 but 78.5, chemical shift of C-7 was not 78.5 but 80.5. 1 u; d6 V" f* @5 ]9 Y" L
8 C2 b; l: y4 P6 r. [& ~; a
C-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from. d8 \: t& }; i. C8 O, j
79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,. A) t+ e$ R6 \
it was the C-7 with the OAC group (compound II), not the C-6 with the
* x4 D0 N' C& x4 Y% HOAC group (compound I). ) k) K% _, V& S" i! ? 5 {1 f/ ?2 J" B/ n
Simple, logic structural revision published in 1981!
楼主: 上善若水
发表于 2009-5-28 16:27:12
[em79][em79]
