On acetylation (OH group was changed to OCOCH3 group), the alpha& `3 e8 M0 n3 Y; `) |3 S/ [6 E) p
carbon will show a downfield shift while the belta carbon will show a$ d+ \6 Q) F0 A) O* C8 s$ W
upfield shif. / g% O* z5 \: p) m ! m6 D0 u0 O3 ^7 p
See the following article how to revise a structure reported before just using the above rule! , p, Y( G$ B' J; U% _
0 C0 [6 l! T- B1 TOne 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. ; f" I: h' y$ g& j! d: k ( C: S3 l7 B) f! t% u' t. A
Another compound with one OH group acetylated: it could be C-6 OAc, C-7
" X7 m; g3 g5 t% n4 ^9 vOH (compound I in the article) or could be C-6 OH, C-7OAc (compound II* \, X9 S" H0 P9 A1 T6 z$ W
in the article). The chemical shifts of the two carbons are 78.1 and8 o; @8 X% d. `1 U6 J3 T. D
80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
1 O) E. q4 r7 m( S! \6 t % e# r( P; ~4 P: f$ b7 s7 c! ~: [
The auhors resovled the problem: 2 ?: P: Y4 r' p: _& o. k 3 w! o1 ?( ^( `( J
If C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical
: f, l& ?: `2 e2 n1 w: ushifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.! m4 B% A4 G7 W7 _! y3 J: A
That assupmtion violates the above rule. : t# F% }8 L- L. t& c% O: B' e9 _ , {; g- b1 p% \( b/ f4 |8 fSo chemical shift of C-6 was not 80.5 but 78.5, chemical shift of C-7 was not 78.5 but 80.5. - y9 p2 |9 a" B: c" q
, Y; t* S7 a# w, sC-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from
( p% J: t( F8 E/ Q7 J5 E2 F79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,2 `9 P" u" P3 t" N& x% J
it was the C-7 with the OAC group (compound II), not the C-6 with the, y# F+ g9 |9 A, q) `9 r( e
OAC group (compound I). 3 g5 C) c/ `% V, y$ } $ \1 L3 ~+ _. V6 y) lSimple, 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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