On acetylation (OH group was changed to OCOCH3 group), the alpha
( ?; E4 @/ a& Z$ a4 ycarbon will show a downfield shift while the belta carbon will show a
9 j5 v" Y' H; _/ W7 H: P8 rupfield shif. ' n7 i# Q; K5 e* g! C 3 G& T& F- B7 G2 u7 S1 ]& ^7 m+ TSee the following article how to revise a structure reported before just using the above rule! # s0 l* ~/ v' e+ x' d $ Y4 c. {2 _2 }1 ^1 p- F, C+ XOne 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. # x. G( U$ M2 ?
" f9 |6 t' `, Q/ q& M* DAnother compound with one OH group acetylated: it could be C-6 OAc, C-7" n* v% X4 }3 H7 p9 d8 G& b9 G
OH (compound I in the article) or could be C-6 OH, C-7OAc (compound II
/ u" @" B8 f0 A- l+ e* r1 N4 G* \in the article). The chemical shifts of the two carbons are 78.1 and
( Q' D: v0 q0 @5 {7 o2 R80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
' T# V6 g" t* S+ i ; P3 q& Z/ z2 c4 V. tThe auhors resovled the problem: ) d, P% x% q" {" ^ 4 W% G6 S3 e' E; l6 oIf C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical
J9 j& t9 l7 S. t( Fshifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.
& i0 p. T# I/ W% I/ c/ r- VThat assupmtion violates the above rule. ' }! b/ n# Q; J. l
4 ~# W* b2 s- i( V ]) rSo chemical shift of C-6 was not 80.5 but 78.5, chemical shift of C-7 was not 78.5 but 80.5. 6 M; X% K% Z$ q7 w7 ~* B 1 V( A) A, o- }* ]- v& `' x/ x
C-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from
5 L" u( D1 x5 V9 A$ k79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,8 g2 _7 Z2 v7 f q$ P" O* O* y
it was the C-7 with the OAC group (compound II), not the C-6 with the
1 }) f7 [/ A$ X; FOAC group (compound I). # U8 B: F8 P" I" ~- j' ~
# H" _; y9 x* i" S9 H# U$ T
Simple, logic structural revision published in 1981!
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7 m1 ]" t3 ~4 ^' s2 A: s , C1 o/ I1 `. Z- Z- B: s6 H- Q
楼主: 上善若水
发表于 2009-5-28 16:27:12
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