On acetylation (OH group was changed to OCOCH3 group), the alpha! }' C$ D# z) m
carbon will show a downfield shift while the belta carbon will show a' r. m# }$ N5 d1 J: Z( Q! E. z4 r
upfield shif. ' k1 N2 ]0 r! _. J% J
/ ^) _7 I# i0 E: rSee the following article how to revise a structure reported before just using the above rule! - }! t! r% C: Q4 L2 @8 l
3 z$ R5 d2 { \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. & j4 u( X' C. j( }# s | ; t9 S$ z% H" i" PAnother compound with one OH group acetylated: it could be C-6 OAc, C-7
- O8 D! l9 B/ iOH (compound I in the article) or could be C-6 OH, C-7OAc (compound II
) a& X' z- f$ uin the article). The chemical shifts of the two carbons are 78.1 and
6 L" W5 w H/ G" s+ E" b80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
* _2 Y+ z2 ?, \2 S; n - S3 N7 |, c. p1 TThe auhors resovled the problem: % y) N$ w$ O' D1 P
" h. s0 p6 } q' B* M; A( u7 J9 K- IIf C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical: Z, u, ]" h# [) z- S9 {+ u
shifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.
& l/ b+ ?* a3 f" h' q4 ZThat assupmtion violates the above rule. ( R5 l$ {# z# o% q5 r 2 q" V# s' j! p6 J4 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. + e3 t8 g5 p7 {3 N2 A1 w- l - |7 P) A/ D6 C/ ?. W, ]C-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from. x% L& I/ w6 J% v x. t
79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically, J; l9 `, N. _1 v( V% t2 b
it was the C-7 with the OAC group (compound II), not the C-6 with the
% G& q% W, x+ iOAC group (compound I). 2 E% g3 w9 i* o, w ( K. S/ ?$ [' }
Simple, logic structural revision published in 1981!
( K1 M( S8 S8 w0 s
# `2 S( U: K' l5 y8 S p' P% m$ G $ B+ _! m+ T3 `' ^
楼主: 上善若水
发表于 2009-5-28 16:27:12
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