On acetylation (OH group was changed to OCOCH3 group), the alpha
" {5 z& w. }# ]1 z u4 ~5 k1 o' I. Mcarbon will show a downfield shift while the belta carbon will show a h( a# W0 U& [0 j& C: ~
upfield shif. 1 x' n3 a; |* [$ F2 d$ R) v/ ~+ ~
- _* Q0 G+ R: QSee the following article how to revise a structure reported before just using the above rule! ( [$ u8 T: ^# y1 N+ e ( y. m! [8 g( P
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. # d; b( J+ Y$ h
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Another compound with one OH group acetylated: it could be C-6 OAc, C-71 B, P5 C! g, `1 w
OH (compound I in the article) or could be C-6 OH, C-7OAc (compound II
/ m) b& ^2 Q' j0 u6 @& x3 \7 W$ bin the article). The chemical shifts of the two carbons are 78.1 and5 [7 p- T+ _4 h! ^8 D
80.5. How to determine which carbon (C-6 or C-7) has the OAC group?
+ Z8 C$ L' T$ q0 a1 Z7 _
' x) `$ r3 V, a" [7 n# [& aThe auhors resovled the problem: ( ^/ R" n8 w |: D4 g0 I ; L8 ^4 h( G) [" h9 @If C-6 (80.5) and C-7 (78.5) were right, meaning both the chemical
t4 L8 q' O% jshifts of C-6 (79.3 to 80.5) and C-7 (76.2 to 78.5) were increased.
- ^, ?7 K. p4 {That assupmtion violates the above rule. & ?1 X( ~5 T. Z0 M9 A; l
3 _" M' D# p; H! Y: G1 ]
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. 5 a0 |7 o) n# v2 q+ S
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C-6 (78.5) and C-7 (80.5) meaned that C-6 shifted to highfield (from
3 g G' @. L/ a+ j: T% x/ N79.3 to 78.5), C-7 shifted to lowfield (from 76.2 to 80.5). Logically,
9 T* M% E' ^, z0 n1 ?9 Qit was the C-7 with the OAC group (compound II), not the C-6 with the
9 C. g4 T5 z+ M/ b+ O( ?0 hOAC group (compound I). 7 D1 }" E& V& P% G# `2 d; x
! U' k o, H8 d+ a: p, d0 o Y KSimple, logic structural revision published in 1981!