On acetylation (OH group was changed to OCOCH3 group), the alpha
+ {( I+ @) H, {2 e) @carbon will show a downfield shift while the belta carbon will show a6 T1 Y* t+ ^% |) O6 j: Q! I* [# c
upfield shif. . t& z& ~% T+ _5 c t
( F+ r- d7 Q% W1 x5 j
See the following article how to revise a structure reported before just using the above rule! & p2 {/ v! X* @$ R. U( f 9 w$ z' X9 Y" f. Q
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. d+ f: Y2 G: p* }" ~3 ] ; H3 e" o _4 x$ J YAnother compound with one OH group acetylated: it could be C-6 OAc, C-7& V D9 I1 ~4 H
OH (compound I in the article) or could be C-6 OH, C-7OAc (compound II
9 f/ f8 ?. d0 T# X2 Vin the article). The chemical shifts of the two carbons are 78.1 and5 R: A8 z. F1 l* W
80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
7 ?% C+ K* [$ l1 b+ l" V& P
9 V# ?/ a: i2 b5 z" _' aThe auhors resovled the problem: ( _$ Q& r( V1 k( f- T( c 1 b" b+ Q8 m3 `If C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical
" n- \3 t6 C4 s, a) `shifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.
! d U+ {( M, ?That assupmtion violates the above rule. 5 J9 V) J0 ^) H/ M/ x + `# r. x. h6 p7 K. aSo chemical shift of C-6 was not 80.5 but 78.5, chemical shift of C-7 was not 78.5 but 80.5. $ @& x* E, o* c" X6 v& } ' D) H0 ?7 v; q! M1 M6 ?3 s7 E
C-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from8 S, n* W7 O0 G! E
79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,& |6 P* c4 f' @ [) O6 N
it was the C-7 with the OAC group (compound II), not the C-6 with the" f1 F& Y, {# N6 y: Y
OAC group (compound I). ; Y1 g$ b! M% n( c+ ^6 _( f
. j; m( Q |4 O- v( S& b
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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