On acetylation (OH group was changed to OCOCH3 group), the alpha
; @* V8 |0 R6 z; v5 k& @. }carbon will show a downfield shift while the belta carbon will show a
: \/ o0 E% K: O) q Bupfield shif. ! Z7 y, Q1 t* Q9 { ^9 @2 u " \/ X+ w3 ~) J: [See the following article how to revise a structure reported before just using the above rule! 0 @$ _3 P1 t( E' U/ X+ H
8 p! p1 y& f3 N& T) A& N
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. 7 l" m( Q5 h% ^ , b: l' \" k% u1 m: z- \& \# ^
Another compound with one OH group acetylated: it could be C-6 OAc, C-7
) t! j( m, D$ TOH (compound I in the article) or could be C-6 OH, C-7OAc (compound II
( Q0 w" V* `0 S! ^1 c1 O* kin the article). The chemical shifts of the two carbons are 78.1 and2 O# x0 j! X5 R/ Q* I& i5 z3 A
80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
$ _+ S6 s, y5 n * \" a7 G8 n" i0 B( v! Q
The auhors resovled the problem: ) N, U0 e7 V$ y0 H6 V0 q' f
) a3 ]! K& w/ ]If C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical3 |* y" O9 `$ L+ N+ @- s; v% F
shifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.' I7 @9 T; t# Z4 i; I8 P
That assupmtion violates the above rule. 0 \/ _5 d9 C3 e
( ]" g+ X6 f* e% j
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. 6 b. G& {+ r+ A% C6 o- I/ k
" O6 \8 v% F, j7 {
C-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from, O- k( X% Z8 ]) K# h: L& x2 c8 h0 i- S
79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,- o1 n( s/ w& f& {8 x
it was the C-7 with the OAC group (compound II), not the C-6 with the
1 s+ Q k/ f( k# @8 g6 F0 FOAC group (compound I). 4 F' g B S* f8 K : b7 S- h+ H) N; x2 E( fSimple, logic structural revision published in 1981!
楼主: 上善若水
发表于 2009-5-28 16:27:12
[em79][em79]
