On acetylation (OH group was changed to OCOCH3 group), the alpha
1 g' S; ^1 Q: W# G* r& t& ]- vcarbon will show a downfield shift while the belta carbon will show a
Q. U" D% W7 p4 eupfield shif. , c7 s% W$ e0 v
, a6 M5 M) F8 E& H* E KSee the following article how to revise a structure reported before just using the above rule! ( }+ k+ y" w: }: v: s" ?
8 F( J+ F4 }2 ~3 i9 P8 V- O
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. 9 S9 t, O: t7 E
$ m( t1 |7 i9 r: O; H3 v! ]Another compound with one OH group acetylated: it could be C-6 OAc, C-7
" l) x5 X5 `+ ^OH (compound I in the article) or could be C-6 OH, C-7OAc (compound II: B5 D% D, x2 ?- y8 ]" `/ I
in the article). The chemical shifts of the two carbons are 78.1 and
1 ?$ v; d! N" ]" \. l! O80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
3 W$ S9 E' K ?1 A" W" a
' Z4 M$ f8 n8 C" o1 j. [0 h. zThe auhors resovled the problem: / C# ?1 ]- }( N0 ^0 j 9 K" c7 S4 |3 I6 i( g8 mIf C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical: w8 K0 `. a3 K* T/ m
shifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.
2 B3 _9 s4 ]4 m2 u% n% iThat assupmtion violates the above rule. 2 ~' E* i7 A3 w. |5 A: @# y; B
; C" V. t1 E# T: }
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. ( c: a" |0 f7 c
% P6 L( [. P- R) _C-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from
9 p( G& ?3 _% o/ V4 U: @79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,$ ^ Z5 s( w' f) A) i4 T; t. r0 I
it was the C-7 with the OAC group (compound II), not the C-6 with the; d5 M$ L% `; a3 ]2 j+ X! G
OAC group (compound I). 4 g3 a( ]# L1 \/ n! W
4 \" j9 Z+ j) Q+ a( N3 x/ LSimple, logic structural revision published in 1981!
: B# z& x* w9 N2 C. o j
/ T5 u. j+ A( q/ K; n, x . S# m$ \$ b- Q: g- x! v" O
楼主: 上善若水
发表于 2009-5-28 16:27:12
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